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PRACTICALMARINEELECTRICALKNOWLEDGE Second Edition Dennis T. Hall B.A. (Honsl, G. Eng., M.|.E.E.' M.l.Mar.E 15l t o l r9 r T 1n . / U v / MAIN GENERATORS t l Ar"-*n.. I \$y'ceH I ) ) ; ; A q r r a r r r c r s g s l g a - ! t r - - - r E - l t ! a E a t r ! r - t t t t t t g l E & r c a r g r g g E E E g E E l E E S t ! a a t t ! l l t r t r l l t E t a q g E t l ! a E t a r t t E B E E E r E l B E E E 6 E e E E s E t g t t a c - F ! atsle'qa'tln*mra.|{l$4@t{ 6.6 kV 60 Hz i/tqlN S\ /BD r.if;*f.:::,:::*: WITIIERBY
20 PRACTICAL MARINE ELECTRICAL KNOVVLEDGE Dennis T. Hall BA (Hons), CEng MIEE, MIMaTE PruAF t?ot{Pu "r1 /t/zo o'r AA*J--,.._*N4 Stolt-Niclsel Atlantic Flcet :r!?rffir*
First Published 1984 SecondEdition 1999 ISBN 1 85609182 1. @ Dennis T. Hall 1999 Witherby & Co Ltd 32-36 Aylesbury Street London ECIR OET Tel No: 02072515341 FaxNo: 02072511296 International Tel No: +44 2O7 251 5341 Internationaf Fax No: +44 2O7 251 1296 Email: books@witherbys.co.uk www.witherbys.com British Library Cataloguing in Publication Hall, Dennis T. Practical Marine Electrical Knowledge - Second Edition 1. Title ISBN 1 856091821 WITHERBY I mn-l twlPUBUSHERS : All rights reserved, Nq.p-aft-of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher and copyright owner. While the principles discussed and the details given in this book are the product of careful consideration, the author and the publisher cannot in any way guarantee the suitability of recommendations made in this book for individual problems or situations, and they shall not be under any legal liability of any kind in respect of or arising out of the form or contents of this book or any error therein, or the reliance of any person thereon. Data t l II II I) i I$ . B I! &,ts F I r : - ' J ' l ' ' _ . n J ; : ' t , . ; ; -
This book is designed to assist sea-going personnel in their understanding of testing and maintenance of ships electrical equipment and services. The publication also supports a series of eight film/video cassettes (with the which examine practical electrical maintenance and fault-finding procedures on types. Further details of the film/video cassettescan be obtained from the producers: Videotel Productions. 84 Newman Street, London W1P 3LD, U.K. Telephone: +44 207 299 1800 Fax: + 44 207 299 1818 E-mail:mail@videotelmail.com Website:http://www.videotel.co.uk the safe operation, same chapter titles) board various ship ; t i Videotel Productions and Witherby Publishers would like to thank the following organisations for their contribution and assistancein the production of Practical Marine Electrical Knowledge: South Tyneside College P & O Cruises (UK) Ltd. Atlantic Power PGS Offshore Technology BP Shipping Ltd. Shell Tankers (UK) Ltd. Mobil Shipping Co. Ltd. Lothian Shipping R & B Switchgear Services Ltd. The Institute of Marine Engineers International Maritime Organisation (IMO) We wish to thank the following authors and publishers for permission to use some of the illustrations in this book: M.L. Lewis, Electrical Installation Technology 2 (Hutchinson) M. Neidle, Electrical Installations and Regulations (Macmillan) M. Neidle, Basic Electrical Installation Principles (Macmillan)
v Preface This book describes up-to-date electrical practice employed in international shipping. The chapters have the same titles as eight electrical training videos within a series also entitled Practical Marine Electrical Knowledge. The content of the book has been designed to be complete in itself but is also arranged to give training support to the practical video material. It has been particularly written to assist marine engineer and electrical officer personnel in their understanding of electrical systems, equipment and its maintenance. A ship's electrical power system is explained in terms of its main and emergency generation plant and the distribution network. Electrical safety and safe working practice is stressed throughout. The types and significance of circuit faults are examined together with the various forms of protection methods and switchgear operation. An appreciation of generator construction and its control is followed by a guide to its protection and maintenance. Motor and starter construction, operation and protection are explained. A survey of variable speed control methods for motors applicable to ships is also included. A wide range of ancillary electrical services for ships lighting, catering, refrigeration, air- conditioning, laundry equipment and cathodic protection are described together with battery support, care and maintenance. The special design and maintenance for electrical equipment used in potentially hazardous areas is reviewed in relation to oil, gas and chemical tankers. Various explosion-protected (Ex) methods are outlined along with electrical testing in hazardous areas. Specific parts of the electrical network together with its UMS requirements, are examined in relation to the electrical survey by a classification society. correct operation and safety, including standards to be met for a successful The application and operation of electrical propulsion for ships is explained, together with high voltage practice, safety procedures and testing methods. About the author: Dennis Hall has a long experience with the marine industry. His initial training in shipbuilding was followed by practical experience in the merchant navy as an electrical officer. This was followed by design and inspection work for large power industrial electrical systems around the world. Further experience and knowledge was acquired in the Royal Navy where he was introduced to the requirements and effective delivery methods for the training of engineering personnel. At South Tyneside College, as lecturer and manager, his cumulative knowledge has been very usefully applied to the training of merchant navy electrical and engineering candidates from cadet to senior officer level. As Head of Electrical Power Systems at the college, he has examined many ship types and visited many marine colleges in Europe, USA and fapan in his drive to meet the training and education needs of the marine industry.
V Contents Chapter Two Chapter One Ships' Electrical Systems, Safety and Maintenance Introduction - Ships' Electrical System- Circuit Calculations- Electrical Diagrams - Electrical Safety - Electric Shock - Insulation Resistance- Circuit Testing - Insulation Testing - Continuity Testing - Multimeters - Diode Tests- Current Clampmeters- Live-line Testers- General Electrical Maintenance - Fault Finding. Electrical Distribution Introduction - Power Distribution System- Insulated and Earthed Neutral Systems- Significance of Earth Faults- Distribution Circuit Breakers- Transformers- Instrument Transformers- Shore Supply Connection - Circuit Protection - Electric Cables. Generators and Main Circuit Breakers Introduction - AC Generator Operation - Generator Construction and Cooling - Excitation Methods - Automatic Voltage Regulation- Generators in Parallel- Emergency Generators- Generator Protection - Generator Maintenance - Main Switchboard - Main Circuit Breakers. Motors and Starters Introduction - Motor Construction - Enclosures and Ratings - Induction Motor Operation - Control Equipment - Direct on-Line Starting - Reduced Voltage Starting - Speed Control - Motor Protection - Single PhaseMotors - Maintenance. Ancillary Electrical Services Introduction - Ships' Lighting - IncandescentLamps - Discharge Lamps - Voltage Effects on Lighting - Navigation and Signal Lights - Emergency Lighting - Maintenance of Lighting Fittings - Refrigeration and Air Conditioning - Galley and Laundry - Cathodic Protection- Battery Supplies. Page L ttg Chemical Tankers l4g Introduction - Tanker Classification - Hazrdous Zones - Electrical Igintion of Gas - Apparatus Gas Groups - Temperature Class - Types of Explosion Protection - Exd Flameproof Enclosure - Exi Intrinsic Safety - Exe Increased Safety - Exn Non-Sparking - Exp Pressurised Enclosure - Exs Special Protection - Certification and Identification - Electrical Testing in Hazardous Areas - Maintenance of Ex-protected Apparatus. 25 57 85 Chapter Three Chapter Four Chapter Five Chapter Six Special Electrical Practice for Oil, Gas and
VI Contents Chapter Seven Electrical Survey Requirements Introduction - SOLAS - Classification Societies- Main Electrical Survey Items - Generators and Governors- Circuit Breakers- Switchboards and Fittings - Cables- Insulation Resistance- Motors and Starters- Emergency Power and Associated Equipment - Parts of Steering Gear - Navigation Light Indicators - tlMS Operation - Tankers. Page 167 Chapter Eight Electric Propulsion and High Voltage Practice Introduction-Electric Propulsion Scheme-Power Supply Network- Review of Motor Operation - Controlled Rectification and Inversion - Converter Types - Propulsion System Operation - Harmonics - Propulsion Auxiliaries and Protection - High Voltage on Ships - High Voltage Safety- High Voltage Equipment Testing. Index
Chapter One Ships' Electrical Systems, Safety and Maintenance L.0 1..1 1.2 1.3 7.4 1,.5 't.6 1.7 1..8 1..9 1.10 1..1L 1..12 1.L3 1.14 1,.15 Introduction Ships' Electrical System Circuit Calculations Electrical Diagrams Electrical Safety Electric Shock Insulation Resistance Circuit Testing Insulation Testing Continuity Testing Multimeters Diode Tests Current Clampmeters Live-Line Testers General Elechical Maintenance Fault Finding Page 1. 1. 3 4 9 L0 1L 72 13 1.5 76 18 18 t9 20 22 1-.0.Introduction L.1.. Ships' Electrical System An overview of a ship's electrical system is-presented and describesvarious types of circuit diagrams used in electrital work. Electrical calculations, safety prec_autions,circuit diagrams and testing methods are outlined together with a description of general electrical maintenance and fault finding techniques. Auxiliary services on board ship range from engine room pumps, compressors and fans, deck winches and windlasses, to general lighting, catering and air conditioning. Electrical power is used to drive the majority of these auxiliary services. The electrical power system on board ship is designed to provide a
2 Ships'Electrical Systems, Safety and Maintenance secure supply to all loads with adequate built-in protection for the equipment and operating personnel. The general scheme of a ship's electrical power system is common to nearly all ships. The main a.c. generators (sometimes called alternators) produce the electrical power. It is supplied to the main switchboard and then distributed to the various auxiliary services comprising the electrical load. An emergencygeneratorand emergencyswitchboardmaintain supplies in the event of a main power failure. Compare this general layout in Fig. 1.1 with the system on your ship. Note the great similarities and also note the differences - all ships' systems differ in some respect. The generators may be driven by u diesel engine, by a steam or gas turbine, or by the main propulsion engine as a shaft generator. The type of prime mover is determined by the design of the ship and by economic factors. The combined power rating of the generators is determined by the overall demand of the ship's electrical load. FI ffi/-:\/ \ r \ l v trH KH A [\V ffi/G\ [v ffi/G\ [\v A \/ ') 60 Hz ECR SWBD 440 V 60 Hz EMERGENCYSWBD. Fig. 1.1 Electric power system.
Circuit Calculations 3 Large passenger ships usually have four large generatorsrated at L0 MW or more to supply the electric propulsion motors and the extensive hotel services on board. A cargo ship may have two main generatorstypically rated from 350 to 1000kW which are sufficient to supply the engine room auxiliaries while at sea and the winches or cranes for handling cargo while in port. The limited load required during an emergency requires that an emergency generator may be rated from about 10 kW for a small coaster to about 300 kW or more for a cargo liner. The shipbuilder must estimate the number and power rating of the required generators by assessing the power demand of the load for all situations whether at sea or in port. Electrical power on board ship is commonly generated at 440 V, 60 Hz (sometimes 380 V, 50 Hz). Ships with a very large electrical power demand will require generators that operate at a high aoltage(3.3 kV, 6.6 kV or 11 kV) to limit the size of normal load current and the prospective fault current. The British Standard (BS) and Inter- national Electrotechnical Commission [EC) definition of.Iow aoltageis 50 V a.c. to 1000V a.c. (the IEC give this definition to harmonise British and European standards). Lighting and other low power ancillary services usually operate at 110 V or 220 V, single-phase a.c. Transformers are used to reduce the 440 V svstem voltage to these lower voltage leveis. Where portable equipment is to be used in dangerous, hot and damp locations, it is advisable to operate at 55 V or even 24 V supplied again by a step-down transformer. Occasionally, transformers are also used to step-up voltages, e.g. supplying a large 3.3 kV bow thruster motor from a 440 V switchboard supply. Batteries for various essential services operate at 1.2 V or 24 V d.c. but sometimes higher voltages are used if such loads require a large power supply. L.2. Circuit Calculations The following gives d.c. and a.c. circuits a brief revision of and calculations. d.c. circuit Rr = Rr + R2 + Re * . .., (in series) 1 1 * _ * - +. .. (in parallel) R 2 R 3 Y: /.R (OhmsLaw) Zemfs: Zpd's(Kirchhffi Xlnv: Elour Kirchhffi P : V . l : 1 2 . R l = 1 , + 1 , Example: Using the above circuit with a 110 V d.c. supply and R1 : 6 Q, Rz : 5 O, R3 : 5.5 O: Calculate all currents, supply power and p.d. across the 6 O resistor. Determine as. 11: 11.0/(6+ 5) : 70A andlz : 110/5.5: 20 A so supplycurrenti, f- SOA,. Supplypoweris P : V.I : 1"L0. 30 : 3.3 kW f c h e c k u t i t h P : Z ( I ' R ) ] p.d. across6 Cl resistoris 11.6:1-0 . 6 : 60 V
4 Ships' Electrical Systems, Safety and Maintenance Example: Using the above circuit with a 220 V, 60 Hz a.c. supply and R1 : 6 A, Rz : 5 O, L : 0.1 H, C : 100 lF: Calculate all currents, supply power, overall power factor and p.d. across the 6 O resistor. Determine as, X y : 2 . n . f . L = 3 7 . 7Q a n d X 6 = 7 / 2 . n . f . C : 2 6 . 5A Then 21 : 38.2 Q at 81" Aagging) and 22: 27 A at 79.3" (leading) So, 11 : 220/38.2 : 5.76 A lagging V by 81" and l, = 220/27 : 8.1,5A leadingV by 79.3" The total suryIy current is the phasor sum of 11 and 12 which must be resoktedinto "in-phase" (hoizontal) and "quadrature" (aertical) componentsbeforeadding, the result (for you to check)is l: 3.34 A at 43.8" leading SupplyPoweris P : 220.3.34.cos43.8": 531W lcheckwith P: Z(l2R)] OaeraII power factor is cos43.8" : 0.72 leading p.d. across6O : Ir . 6 : 5J6 . 6 -- 34.56 V three phase a.c. circuit V7 : .l3.Vpp and ly : lpu 0n ST14R) V t : V p u a n d I y : ' / 3 . 1 p 1 1G n D E L T A ) P pu = Vp11.Ip11.cosS: lr&.R Balanced3-phase:P :'/ 3.VL.ly.cosg Example: Using the above circuit with a M0 V, 3-phase, 60 Hz a.c. supply and Za1: 10 O at p.f. = 0.8 lagging (balanced load) Calculate phase and line currents and supply power when connected as: (a) STAR and (b) DELTA Determine as, (a) in Star, Vpu : 440/J3 : 254 V so Ippl'J: 254/1-0: 25.4 A and ly: lpu : 25.4 A also P : J 3 . 4 4 0. 2 s . 4. 0 . 8: 1 s . 4 9k w (b) in Delta, Vpn: VL: 440 V so lpp: 440/10 : 44 A a n d l 7 : ' / S . l E : 7 6 . 2A P : "/3 . 440 . 7G.2. 0.8 : 46.46 kw (notice this power is three times the aalue in star) L.3 Electrical Diagrams of diagram an electrical Single phase a.c. circuit I = l, ! l, (phasor addition) Xy: 2nfL @) X6: 1/2nfC@) z : J R 4 x J o r z : J N + x j l = V / Z powerfactor : cosQ: R/Z : PlS (lag or lead) P : V.l.cos6or P : l2R (W) Q : V . l . s i n fo r Q : l 2 x U A r ) S : V . l o r S : l 2 Z U A ) Iyr,l V I ' t v , l I There are various types which attempt to show how
Electrical Diagrams 5 circuit operates. Symbols are used to representthe various items of equipment. The shipbuilder provides a complete set o f s h i p s ' e l e c t r i c a l d i a g r a m s . I t i s important that you study these diagrams to be able to read and understand them competently, and to use them as an aid in locating electrical faults. A blockdiagramshows in simplified form the main inter-relationships of the elementsin a system, and how the system works or may be operated. Such diagrams are often used to depict control systems and other complex relationships. The block diagram in Fig. 1.2 describes the main functions of an overcurrent relay (OCR) used for protection. lts circuit diagram shows one way of realising the overall OCR function. Diagrams like this state the function of each block but usually do not give any information about the components in each block or how the blocks are actually interconnected. A systemdiagram,as in Fig. 1.3, shows the main features of a system and its bounds, without necessarily showing cause-to-effect.Its main use is to illustrate the ways of operating the system. Detail is omitted in order to make the diagram as clear as possible, and s0, easily understood. A circuit diagram shows, in full, the functioning of a circuit. A1l essential parts and connections are depicted by means of graphical symbols arranged to show the operation as clearly as possible but TRIP& ALARM RLA INSTANTANEOUSTRIP ELECTRONTCOVERCURRENTRELAY(BLOCKDTAGRAM) Ro D3 R7 - c 3 Trip & Alarm t l \ - rt l +ve -ve ELECTRONICOVERCURRENTRELAYCIRCUITDIAGRAM Time Setting R3 Fig. 1..2 Block and circuit diagrams.
6 Ships'Electrical Systems, Safety and Maintenance II A I M a in Di s t r ib u t io n T r an s f or m er H a r m o n i c rirterll ?I II E x c i t a t i o n S u p p l i e s 3 - P H ,6 0 H z , a . c . Fig. 1.3 Power system diagram. STAR DELTA C I R C U I TD I A G R A ML 3 L2 Fig. 1.4 Power and control circuit diagram.
Electrical Diagrams 7 without regard to the physical layout of the various items, their parts or connections. The electrical connections in Fig. 1.4 for a motor starter are clearly shown in the simplest possible way. A most important point is that no attempt is made to show the moving contacts of a relay or contactor alongside the coil that operates them (where they are actually physically located). Instead, the coil and its related contacts are identified bv a common number or letter. Although there are international agreements as to the symbol to be used to represent electrical components you must be prepared to meet various different symbols represent- ing the same component. The use of a circuit diagram is to enable the reader to understand the operation of the circuit, to follow each sequence in the operation from the moment of initiating the operation (e.g. by pressing a start button) to the final act (e.9. starting of the motor). If the equipment fails to operate correctly, the reader can follow the sequence of operations until he comes to the operation that has failed. The components involved in that faulty operation can then be examined to locate the suspect item. There is no need to examine other components that are known to function correctlv and have no influence on the fault, io the work is simplified. A circuit diagram is an essentialtool for fault finding. A wiring diagram shows the detailed connections between components or items of equipment, and in some cases the routeing of these connections. An equipment wiring diagram shows the components in their approximate positions occupied within the actual enclosure.The component may be shown complete (e.9. a contactor coil together with all the contacts it drives) or may be simply represented by a block with the necessary terminals clearly marked. A different'thickness of line cln be used to differentiate between power and control circuit connections. The wiring diagram in Fig. 1.5 is of the same starter shown for the circuit diagram of Fig. 1.4. A wiring diagram may be of a fairly simple circuit, but its layout makes it quite difficult to use and to understand the sequential operation of the circuit. The purpose of a wiring diagram is mainly to instruct the wiring installer how to construct and connect the equipment. It is of little use in trouble shooting apart from identifying the exact position of suspect components, terminals and wires.
8 Ships' Electrical Systems, Safety and Maintenance STAR DELTA S T A R T E R WIRING D I A G R A M (on door pffid) t - - - - - - - - - r S T O P S T A R T 3 - P H A S EA . C . 4 l 5 V . 5 0 H z A M M E I E RC T O C R C T s 3 6 O C R U N I T t 0 l l 0 7 0 8 0 9 L I L 2 L 3 c l T o M o T o R Fig. 1.5 Power and control wiring diagram.
Electrical Safety 9 ANSWER It may well save time and trouble to convert the wiring diagram into a much simpler and more useful circuit diagram. When converting a wiring diagram into a circuit diagram certain basic rules and conventions should be followed. o Every sequence should be drawn from left to right and from top to bottom (where possible). o Each stage should be in order of occurrence from left to right. . A11 contacts and components which are in series should be drawn in a straight line (where possible) with the component they control. All contacts and components which are in parallel should be drawn side by side and at the same level to emphasisetheir parallel function. All major components operating at bus-bar voltage should be drawn at the same level (or aligned horizontally) to help identify the required components quickly. All contacts should be shown open ot closedas in their normal or de- energised condition. QUESTION What are vou to do if difficulties arise in locating a'fault on an item of equipment and only a wiring diagramis available? There are other conventions but these cover the main points of good systematic diagrams. Block,system,circuit and wiing diagrnms are the main types in general use for electrical work. Other types of diagram are sometimes used to give information for which the basic types are unsuitable (e.g. a pictorial view of a component). You should study the ship's electrical diagrams to gain an understanding of equipment operation prior to carrying out maintenance or fault finding. Diagrams should be regarded as an essential tool when carrying out work on electricalequipment. 1,.4.Electrical Safety Large power equipment and processes utilise high forces. Electrical,mechanical, thermal and chemical changes produce the desired operation. Very high values of voltage, current, power, temperature, force, pressure etc. create the possibility of danger in an engineering system. To minimise the safety risk to personnel and equipment a system must be designed and manufactured to the latest high standardsand be correctly installed. During its working life the equipment must be continuously monitored and correctly maintained by professionally qualified personnel who understand its operation and safety requirements. Before attempting any electrical work, there are some basic safety precautions you must bear in mind. The possible dangers arising from the misuse of electrical equipment are well known. Electric shock and fire can cause loss of life and damage to equipment. Regulations exist to control the construction, installation, operation and maintenance of electrical equipment so that danger is eliminated as f.ar as possible. Minimum acceptablestandards of safety are issued bv various bodies including national governments, inter- national governmental conventions (e.9. SOLAS), national and international standards associations(e.g. BS and IEC), learned societies(e.9. IEE), classification societies(e.g. Lloyds), etc. Where danger arises it is usually due to accident, neglect or some other contravention of the regulations.
10 Ships' Electrical Systems, Safety and Maintenance Ships' staff must operate equipment in a safe manner and maintain it in a safe condition at all times. Failure to do so will cause danger with serious consequencesarising. Keep in mind an essential list of DO's and DO NOT's when working with electricalequipment: ,/ DO get to know the ship's electrical system and equipment. Study the ship's diagrams to pinpoint the location of switches and protection devices supplying distribution boards and essential items of equipment. Write down this information in a note book. Become familiar with the normalindications on switchboard instruments so that abnormal operation can be quickly detected. / DO operate equipment according to the manufacturer's recommen- dations. DO maintain equipment according to the manufacturer's recommendations or the shipowner's maintenance procedures. DO ensure that all guards, covers and doors are securely fitted and that all bolts and fixings ire in place and tight. DO inform the Officer of the Watch before shutting down equipment for maintenance. DO switch off and lock-off supplies, remove fuses, and display warning notices before removing covers of equipment for maintenance. DO confirm that circuits are DEAD (by using an approved voltage tester) before touching conductors and terminals. x DO NOT touch live conductors under any pretext x DO NOT touch rotating parts. x DO NOT leave live conductors or rotating parts exposed. x DO NOT overload equipment. x DO NOT neglect or abuse equipment. You should think SAFETY at all times and so develop a safetyconsciousattitude. This may well save your life and the lives of others. Most atcidents occur due to a momentary loss of concentration or attempts to short-circuit standard safety procedures. DO NOT let this happento YOU. L.5 Electric Shock Nearly everyone electric shock at it is an unpleasant it is fatal. has experienced an some time. At best experience, at worst DAN.'G.ERr:fla6l1pig Fig. 1.6 Electricalsafety warning.
Insulation Resistance 11 Anyone who has accessto live electrical equipment must be fully aware of first-aid and safety proceduresrelated to electric shock as described in relevant safetv acts. Copies of these safety procedurei should be displayed on board ship. Electric shock is due to the flow of current through your body. This is often from hand to hand or from hand to foot. A shock current as low as 15 mA a.c. or d.c. mav be fatal. Obviouslv the size of shock current is related to tlie applied voltage and your body resistance. Unfortunately, your body resistance goes down as the applied voltage goes up. This means that the shock current is further increased at high voltages. The size of your bodv resistance also depends on other factors such as your state of health, the degree of contact with live wires and the perspiration or dampness on your skin. Typical dry full-contact body resistance is about 5000 O at 25 V falling to about 2000 O at 250 V. QUESTION What would the equivalent shock current levels be at 25 V and 250 V? ANSWER 5 mA and 125 mA. Voltages of about 60 V and below are regarded as reasonably safe for portable hand tools. This is why special step- down isolatingtransformers are used with portable tools and handlamps. These transformers supply the tool or lamp at l,L0 V a.c. but becausethe secondary winding is centre-tapped to earth, the maximum shock voltage to earth is 55 V a.c. Electric shock is often accompanied by falling, which may cause additional physical injury and require first-aid action. If the shock victim is unconscious, resuscitation must take priority over first aid methods. Check the resuscitation techniques described on the electric shock posters displayed on your ship. L.6. Insulation Resistance A11 electrical equipment has insulation. The purpose of the insulation is to keep electric currents in the conductors and to prevent contact with live wires. The electrical resistance of insulation must be very high (MO) to prevent current leakingaway from conductors. Insulation resistanceis measured between: o Conductors and Earth o Conductors. The insulation resistance includes the resistance of the insulation material and also the resistanceof any surface deposits of dirt, oil, moisture, etc. Surface deposits can reduce the insulation resistance. The flow of leakage currents through such surface deposits is called tracking which is also affected by the creepagennd clearancedistances between terminals as shown in Fig. 1,.7. Equipment must be maintained in a clean condition to prevent tracking and to maintain a high value of insulation resistance (usually at least 1,MO). Insulation materials are non-metallic and have very few of the generally good physical properties associated with metals. Insulation is adverselv affected by many factors such as iumidity, temperature, electrical and mechanical stress, vibration, chemicals, oil, dirt and, of course, old age. Traditional insulation materials include cotton, silk, paper, etc. They may be either dry or treated with suitable varnishes or resins to exclude moisture and other harmful substances. Other
12 Ships' Electrical Systems, Safety and Maintenance C r e e p a g e C l e a r a n c e D i s t a n c ea c r o s ss u r f a c e o f i n s u l a t i n gm a t e r i a l D i s t a n c ei n a i r Fig. 1.7 Creepage and clearance distances. materials include mica, glass fibre, etc., and more modern materials such as PVC and other plastics and compounds. An extensively used medium not normallv considered as an insulation material is the air surrounding the electrical components. . The majority of insulation materials in common use cannot withstand temperatures much in excess of 100"C. All electrical equipment heats up when carrying load current with the consequent rise in temperature. This temperature rise is above that of the ambient cooling air temperature. All marine electrical equipment is con- structed and rated to work satisfactorilv in a maximum ambient air temperatur-e of 45"C (Lloyds). Under these conditions the expected temperature rise will not exceed the permitted temperature limit set for the insulation material. It is therefore the insulation material that dictates the maximum permitted operating temperature of the electrical equipment. For this purpose, insulation is classified according to the maximum temperature at which it is safe to operate. Various classes of insulation are listed in British Standards (BS) and classesA, E, B and F are used for marine electrical equipment. The maximum temperature allowed for each of these classesis: These are steady surface temperatures measured with equipment stofped and no flow of cooling air. Hot-spot temperatures of 105'C (Class A) and 130'C (Class B) are generally accepted as normal at the centre of coils and windings of machines with these surface temperatures. A machine operating continuously with these hot-spot temperatures would have an expected life of 15 to 20 vears before the insulation failed compleiely. However, the life expectancy would be halved for every 10'C above these allowed hot-spot temperatures. 1.7. Circuit Testing This section looks at the various electrical circuit testing operations you lnsulation Class A E B F H C Max. Temp. 55"C 70"c 80"c 105t 130t >130t
Insulation Testing 13 Insulation Resistance (IR) Using a (megger) tester (at 500 V d.c. for a 440 Y circuit) Do not use a multimeter for this task Continuity Resistance (Low O) Typically using a multimeterComponent Resistance (O or kO) Voltage (a.c. or d.c.) Current Using a clampmeter (or multimeter for small currents) may need to carry out, and at the instruments you will need. The main tests are for: f..8. Insulation Testing A measurement of the insulation resistance (IR) gives one of the best guides to the state of health of electrical equipment. The resistance should be measured between insulated conductors and earth, and between conductors. An insulation tester is a high reading resistance meter using a high test voltage - usually 500 V d.c. The test voltage is produced either by an internal hand-driven generator or by u battery and electronic voltage charger. A test voltage of 500 V d.c. is suitable for testing ships' equipment rated at M0 Y a.c. Test voltages of 1000 V and 5000 V are used for high voltage (Fry systems on board ship. There are several manufacturers of insulation testers available but the Megger trade name is known worldwide. To prove the basic operation of the tester, short the two probes together, TEST Fig. 1.8 Insulation resistance(IR) tester. switch to t'lll{{2" and press the test button or rockerswitch. The pointer should indicate approximately "0e2". Before applying the test, the equipment to be tested must be disconnectedfrom the live power supply and locked-off according to standard safety procedures. A meggertype IR tester can be used to check whether the circuit to be tested is liae. Switch the instrument to "MO" and connect the probes to pairs of equipment terminals. DO NOT press the button. The meter will now indicate that the circuit is liae or not. If the circuit is dend it is then safe to press the test button. Confirm that a reliable earth connection is obtained by connecting the probes to two separate earth points on the equipment frame while testing for low resistance continuitv. For an IR test on a - three-phase machine, measure and log the phase- to-phase insulation resistance values. Three readings should be measured as U-V, V-W, W-U as shown in Fig. 1.9.
1,4 Ships' Electrical Systems, Safety and Maintenance Fig. 1.9 IR test connections. AC Compressor Motor No. 1 IR TREND ER cold(dry-dock) Fig. 1.10 IR log and trend. Measure and log the phase-to-earth insulation resistance values. Three readings should be measured as U-E, V-E, W-E: Note: Insulation resistance decreaseswith increaseof temperature. QUESTTON Whv should the measurement of the insulation resistance of a machine ideally be made while the machine is hot?
Continuity Testing 15 ANSWER Insulation becomes more leaky (its IR value falls) at high temperatures. So testing while hot shows the realistic IR value dt, or near, its working temperature. Insulation resistance can vary considerably with changing atmos- pheric conditions. A single reading gives little information. However, the regular recording of test results may show a downward trend which indicates impending trouble which canbe remedied by preventive maintenance. An example of an IR log for a motor is shown in Fig. 1.10 together with its graphical trend. 1.9. Continuity Testing An insulation tester normally also incorporates a low voltage continuity test facilitv. This is a low resistance instrument-for measuring the continuity (or otherwise) of conductors. It can be used to measure the low resistance of cables, motor windings, transformer windings, earthing straps, etc. The procedure for use is similar to that for the insulation tester. 'z PROVE the correct operation of the instrument. r ISOMTE and lock off the equipment to be tested. ,z PROVE the equipment to be dead. ,/ Switch the instrument to "Cl" or "continuity". ,, Connect the probes to the circuit. r Operate the test the indication on all readings. switch and check the "C2" scale. Log Fig. 1. L1 Continuity test connections. In the case of three-phase motors and transformers, etc. the comparisonbetween readings is usually more important than the absolute value of the readings. All readings should be identical. If one reading is significantly smaller than the others this could indicate the possibility of short-circuited turns in that winding. Conversely, a high continuity resistancevalue indicatesa high resistance fault or an open-circuit (e.9. a loose connection). Some models of insulation/continuitv testers also provide facilities to measur-e resistance in the ttk(2" range and "a.c. voltage" (acV). To measure verv low continuitv resistance values such as those betwe'en bus-bar joints and circuit breaker contacts it is necessaryto use a micro-ohmmeter.This type of iester drives a set d.c. current, e.g. l-0 A, through the circuit while measuring the resulting volt-drop across it. A set of four test leads are used - two to apply the current and two to measure the volt-drop directly at the current injection points. The meter then calculates R: V/I (Ohms Law), and displays the result as a digital readout in milli-ohms (mO) or micro-ohms (pO).
16 Ships'Electrical Systems, Safety and Maintenance L.L0. Multimeters Routine electrical test work involves measuring current, voltage and resistance i.e. Amps, Volts and Ohms. This is most conveniently done using a multimeter with all the necessarv functions and ranges. The instrument may be the traditional switched-range analogue type (pointer and scale) or the more common digital type with auto-ranging and numerical display. Digital meters have a clear numeric readout which may be supported by a bar-graph display. Where distorted voltage waveforms are likely (e.g. with variable frequency motor drives) it is necessaryto use a "true-rms" meter for accuracy. Digital meters are also available which display the test voltage waveform shape with a storage oscilloscopefacility on the LCD screen. In all instrument models an internal battery is fitted for use when measuring resistance. Before measuring the resistance of a component it is essential that the circuit is switched off, locked off, and any capacitorsdischarged. The instrument is likely to be damaged otherwise. r l l l r l l l r l l The multimeter should be proaed for correct operation before use. The manufacturer's instructions should be carefully followed for this but a general procedure is as follows: Use the correctprobe leads and insert into the correct socketson the meter. If the multimeter is an analoguetype: Ensure the pointer indicates zero - adjust if necessary.Set selector switches to "{2" and connect probe tips together. Pointer should deflect to indicate 0 O. If not at the zero point adjust trimming controls. Check each resistancerange in this way. Set selector switch to " acY" (highest range). Connect probes to a suitable known live supply (with CARE) such as the electricalworkshop test panel. Pointer should indicate correct voltage. Very special care is necessary when using a multimeter to check for a live voltage. If the multimeter has been accidentallv set to the current or resistance range the instrument acts as a low resistance across the live supply. The resulting short-circuif current may easily cause the meter to explode with local fire damage and very serious consequences for the operator. Fused probe leads are therefore highly recommended for use with a multimeter. Instrument batterv failure is checked when the instrument is set to read "C2" with the probe tips connected together. If the pointer fails to reach "0 {1" after adjustment of the resistance range trimmer, the battery must be replaced. The instrument should be switched-off when not in use to preserve battery life. If the multimeter is a digital type: Switch on and connect the two probe tips together. Set selector switches to "dcY" (highest range). Display should indicate zero (000). Repeat for all " dcY" selector switch positions and note the shift of the decimal point. Separate the probe tips. Set selector switches to "C2" (highest range). Fig. 1.12 Digital multimeter.
Multimeters 17 "0L" (over-range) model). Connect display should Repeat for all "C)" selector switch positions and note movement of the decimal point. Set selector switches to " acY" (highest range). Connect probes to a suitable known live supply. Display should indicate correct voltage. Test the d.c. voltagerange alsoand note the polarity indication on the meter. Instrument battery failure is usuallv indicated by the numeric display. Thi) display may include "BT" or the decimal- point may blink, or some other display effect mav be used. The initrument should be switched off when not in use to preserve battery life. These simple proving tests should be performed el)ery time bef.ore using the instrument for real. It is obviously very dangerous to touch conductors beiievini; them to be dead having checked them with a faulty instrument. o To measure resistance: Display should indicate ot "100" (depends upon probe tips together - indicate zero (000). r PROVE the correct instrument r ISOMTE and lock to be tested ,z CONNECT the probes to the terminals being tested. Takegrentcarenot to touch the probe tips and rememberthat the equipmentbeing testedis LIVE. r NOTE the voltage reading. lf a lower voltage range would give a more accurate reading, adjust the selector switches accordingly to shift the decimal point. However, most digital meters have an auto-ranging facility. No harm will be caused to the instrument by operating the selector range switches while still connected to a live supply. But GREAT CARE must be taken not to switch into either the current or resistancemode. This would almost certainly operate the instrument overload device and mav causeseveredamage to the instrument and danger to yourself. Take your time to operate the selector switches during the operation and THINK about what you are doing. Fused probe leads are highly recommended. ,z Disconnect the probes and switch the instrument to OFF. r To measure current: Most test instruments can only measure up to a few amps (usually 10 A maximum). The current measuring facility is intended only for small-current components, and in particular, for electronic circuits. The instrument will almost certainly be damaged if it is used to measure the current to motors and other Dower circuits. The basic current range can be extended by using external shunts (d.c.) and current transformers (a.c.). These accessories are generally purchased separately from the instrument manufacturers. The procedure to be used to measure current in a small-current circuit: r PROVE the correct instrument operation. operation of the off the equipment ,z PROVE the equipment to be dead ,z SWITCH the instrument to the appropriate resistancerange, connect the probes to the equipment and note the resistancevalue ,z Disconnect the probes and switch the instrument to OFF. o To measure ooltage: r PROVE the correct instrument operation r SWITCH the instrument to the highest voltage range (either acV or dcV as appropriate)
18 Ships'Electrical Systems, Safety and Maintenance ,z SWITCH the instrument to the highest current range (either acA or dcA as appropriate). ,z TURN OFF the power to the circuit to be tested and discharge all capacitors. 'z OPEN the circuit in which current is to be measured - removing a fuse-link often gives a convenient point for current measurement. Securely connect the probes in SER/ES with the load in which current is to be measured. Turn ON the power to the circuit being tested. Note the current size on the meter display. Turn OFF the power to the circuit being tested and dischargeall capacitors. Disconnect the test probes and switch the instrument to OFF. Reconnect the circuit that was being tested. Often, the most convenient way to measure current is to use a clamp-meter which is simply clamped around an insulatedconductor. through the diode/p-n junction while it also acts as a voltmeter to measure the volt-drop acrossit. ,/ CONNECT the two probes across the diode. READ the forward volt-drop across the diode. This should be between 5 0 0 m V a n d 9 0 0 m V ( 0 . 5 - 0 . 8 V ) for a healthy silicon diode or p-n junction. REYERSE the probe connections and the display should indicate oaer-range. If the display indicates over-range in both directions the diode is open- circuit faulted. If the display indicates less than 1 V in both directions, the diode may be short-circulffaulted. The associated diode circuitrv mav be giving false readings so the"diod"e must be disconnectedfrom the circuit then re-tested. 1..12.Current Clampmeters I L.LL.Diode Tests -++-I Electronic diodes, and other semi- conductor devices with p-n junctions (e.g. the base-emitterof a transistor) can be tested using a digital type instrument using the following procedure: ,z PROVE the correct instrument operation. 'z SWITCHthe instrument to diode test. If the diode is still in circuit, turn off the power to the circuit, dischargeall capacitors and remove fuses. In this test the instrument drives a small d.c. current (a few mA) Power currents (a.c.) can be measured simply by means of a clampmeter which acts as a current transformer. The instrument tongs are clipped round a single insulated conductor - the circuit is not interrupted. The value of current is obtained from the magnetic flux strength around the conductor and is usually displayed on a digital display. Direct current (d.c.) measurement is also available with clampmeters having a flux-voltage transducer known as a "Hall-effect" device. Many modern clampmeters are virtually multimeters with the addition of facilities to measure voltage and resistance as well as measuring currents up to 1000A.
Live-Line Testers 19 which is produced by the current. In a balanced3-core(or 2-corefor that matter) cable, the net flux is zero - hence no indication. This is why the clampmeter is only connected around a single conductor. L.L3. Live-Line Testers When equipment is to be inspected for maintenance it is important that supplies be switched OFF and locked OFF. The equipment must then be PROVED to be dead to eliminate the danger of electric shock. A live-line (or voltage) tester is a simple device to check only whether or not a voltage exists at terminals. Live-line testers, up to 500 V, are of various types. Some light up (e.g. screwdriver type with a neon indicator), some make a noise, others operate LED's or mechanical indicators (flags) to indicate the approximate value of voltage. It is important that voltage testers themselves be PROVED to operate correctlv before use. This can be convenientlv carried out at the electrical workshop t-estpanel. Home-made test lamps should not be used as they can be dangerous because protective equipment, e.g. fuses and finger guards, are not fitted. Great care is required with high voltage circuits where a special HV test probe must be used, see Chapter Eight. Fig. 1.13 Current clampmeter. CARE must be taken when measuring the current in uninsulatedconductors. More advanced clamp-type meters can indicate power and power factor in single and three phase a.c. circuits by using additional connections to measure voltage. QUESTION What would a clampmeter indicate if clipped around a 3-core cable which is known to be carrying 100 A a.c. to a motor? ANSWER Zero. This is because the clampmeter monitors the magnetic flux around the cable
20 Ships'Electrical Systems, Safety and Maintenance LED INDICATORS Fig. 1.14 Live-line testers. 1..14.General Electrical Maintenance and optimum economy in order to help keep operational costs to a minimum to maintain financial competitiveness. Nearly all equipment needs main- tenance. An efficient maintenance engineer must get to know the power system and its equiprnent. The ship's drawings and circuit diagrams must be checked and updated to relate them to the actual equipment. Electrical services and equip- ment must be kept under continuous observation so that normal healthy operating conditions become known, and abnormal operation becomes quickly apparent. Faults can then be pin-pbinted and corrected before a breakdown occurs. Maintenance can be classified as: o Breakdownmaintenance o Planned maintenance o Condition monitoring All equipment is subject to wear and tear, eventually reaching the end of its useful life when it must be replaced. As equipment nears the end of its safe working life its condition can deteriorate to such an extent as to be a danger to personnel and other plant. The purpose of maintenance, therefore, is to extend the useful life by repair and/or replacement of defective p-arts and to maintain it in a safe and serviceablecondition. The marine environment is particularly arduous for electrical equipment du-e to the damp, salt-laden atmosphere, extremes of temperature and constant vibration. Shipboard equipment is in particular need of correct maintenance. The continuous operation of equipment on board ship demands high effitiency
General Electrical Maintenance 21 Breakdownmaintenance(corrective main- tenance) is when equipment is left untouched until a breakdown occurs. At this time the equipment is repaired or replaced and any other specified maintenanceprocedure carried out. Planned maintenance(preventive main- tenance) is when equipment is regularly inspected and maintained according to a fixed timetable and set of procedures specifying the actual work to be done to prevent equipment failure. Condition monitoring (another form of preventive maintenance) is when equipment is regularly monitored and tested. When monitoring indicates that a breakdown is imminent, the equipment is repaired or replaced and any other specified maintenance procedures are carried out. Regular insulation testing and aibration testins are two forms of condition monitoring. There are several disadaantagesin breakdown maintenance: X A serious breakdown of equipment may cause sufficient down-time to put the ship out of commission until it is repaired. X If several breakdowns occur simul- taneously the available manpower on board ship may not be able to cope adequately, resulting in delays. X Some items of equipment may need the specialist services of the manu- facturer to carry out repairs which may cause further delays. Planned maintenance is carried out at fixed regular intervals whether the equipment needs it or not and the aim is to prevent breakdown. This type of maintenance has the following adaantages: z Fewer breakdowns and reduced down time produces higher levels of operating efficiency. Maintenance is carried out at times favourable to the operation of the plant. More effective labour utilisation because maintenance is carried out at times favourable to the ship's staff. Replacement equipment can be ordered in advance. Equipment is maintained in a safe condition with reduced possible dangers. Where a specialist manufacturer's services are required these can be obtained at convenient times to suit the ship operation. z Replacement of short-life components at scheduled intervals. Condition monitoring is also carried out. at fixed regular intervals. The aim is to forestall breakdown by predicting probable failure from the TREND shown by the monitoring results. The advantage of this type of maintenance is that equipment is not subjected to unnecessarymarntenance. Equipment is regularly condition- monitored according to a monitoring schedule. Measurements are taken of insulation resistance, temperature and vibration (of motors). Contacts and other parts subject to deterioration are inspected. All findings are recorded in an historical record file. No maintenance is carried out until the trend of test results indicate that it has become necessarv. The equipment is then either replacei, repaired or subjectedto a major overhaul as specified on a job card. A maintenance records svstem is required. The recorded measurements of insulation resistance may show a falling trend indicating a piogressive degradation of insulation. The equipment should be inspected and repaired before
22 Ships'Electrical Systems, Safety and Maintenance the insulation resistance falls to a dangerously low value. Hot-spot temperatures emitted from live electrical equipment can be monitored from a safe distance using an infra-red detector or camera. The recorded measurements of the vibration of a motor may follow a rising trend indicating progressive bearing deterioration.Bearingsshould be replaced before failure occurs. Immediate repair or maintenanceis probably not necessary but should be put in hand at the earliest convenient moment. L.L5. Fault Finding Generally, fault finding is not an easy task. It is essential to have a good under- standing of the operation of the particular n equipment and general insight into some of the diagnostic skills used to solve the problem. Here is a list of the general techniques used: z Planning A good fault-finder has a mentally planned strategy. The evidence is carefully considered before deciding what action to take. In contrast, the "muddler" acts on impulse. A good diagnosticianwill use most of the following mental abilities: I Memory I Logical thinking I Percepliog._.,, . ^ ' ; r Y l, t - . - . t : : . ' - - tr Spatiahnrec,lqrieatiUitity\' f, Social skills;, _, ,,' I Persistence.,,i.:-. . Background (underpinning)knowledge Together with the mental abilities above, knowledge and experienceare essential. This is wide ranging and includes knowledge of components, methods and systems together with their operational characteristics.The combination of knowledge and direct practical experience with the equip- ment is a powerful aid to fault finding. Diagnostic performance In addition to the necessarv skills of the diagnostician, systematic use of " job aids" will improve fault finding method. Examples are: Fault charts A list of typical symptoms and faults for a particular equipment plus suggestedremedies. These lists should be updated according to experience to show the most probable faults. FACERAP The seven letters of the mnemonic "FACERAP" are the key steps to logical fault finding: D F (fault) A (appearance) C (cause) E (effect) the name and classi- fication of a fault; the description of the fault or its related symptom; the operational reason for the fault; the consequential effect of the fault; R (responsibility)the correct person to take remedial action; A (action) the standard procedure adopted to rectify the fauLt; (preoention) the procedure to avoid reoetition of the fault.
Fault Finding 23 z Search strategy Once the diagnostician can visualise the circuit or machine as a series of functions and/or use a job aid, a search strategy can be applied to locate the fault in the minimum time. A "six step approach" is sum- marised as: 1. Collect evidence (stop and think). 2. Analyse evidence (check assumptions). 3. Locate fault (inspect and test). 4. Determine and remove cause. 5. Rectify fault. 6. Check system. Conclusion? Fault finding is not easy! However, a logicalapproach supported by knowledgeand experiencewill certainly help.
25 Chapter Two Electrical Distribution 2.0 2 . 1 2.2 2 . 3 2.4 2.5 2.6 2.7 2.8 2.9 Introduction Power Dishibution System Insulated and Earthed Neutral Systems Significance of Earth Faults Distribution Circuit Breakers Transformers Instrument Transformers Shore Supply Connection Circuit Protection Electric Cables Page 25 25 29 3L 36 37 39 4't 43 51 This chapter examines a ship's electrical distribution network in detail. Particular attention is paid to earth faults and their detection together with a survey of the ralg,e and purpose of the various types of electrical switchgear and proteition equipment. Ships' electric cables with their glanding, terminations and testing are reviewed. 2.0. Introduction 2.1-.Power Distribution System The function of a ship's electrical distribution system is to safely convey the generated electrical power to every item of consumerequipment connected to it. Probably the most obvious element in the system is the main distribution centre, i.e. the ship's main switchboard. The main board supplies bulk power to qotor group starter boards (often part of the main board), section boards -and distribution boards. Protection, e.g. circuit-breakers and fuses, strategically placed throughout the system auto- matically disconnects a faulty circuit within the network. Transformers inter- c-onnectthe high voltage and low voltage distribution sections of the svstem. The operational state of a- distribution system is indicated by the monitors for power, voltage, current and by protection relays for overcurrents and earth-faults at each main control centre. Study the electrical power diagrams for your own ship to see if you can relate them to the actual equipment they represent. The vast majority of ships have an alternating current (a.c.) distribution
26 Electrical Distribution system in preference to a direct current (d.c.)system. The required electrical services are broadlv considered as main and emetgehcy supplies. o Main supply An a.c. network is cheaper to install and operate than a d.c. system. In particular, d.c. offers a higher power/ weight ratio for the generation, distribu- tion and utilisation of electricity. Simple transf ormers efficiently step-up or step-down a.c. voltages where required. Three-phasea.c. is effectively converted into rotary mechanical power in simple and efficient induction motors. A ship's electrical distribution scheme generally follows shore practice. This allows normal industrial equipment to be used on board ship after being "mtrinised", where necessary,to withstand the rigours of a sea-life (e.g. it must withstand the vibration, humidity, high temperature/ ozone, sea-water, etc. encountered in various parts of the ship). The majority of ships have a 3-phase d.c., 3-wire, 440 V insulated-neutral svstem. This means that the neutral pbint of star-connectedgenerators is not earthedto the ship's hull. For continental European vessels, a 380 V, 3-phase system is common. Ships with very large electrical loads have generatorsoperating at high voltages (HV) of 3.3 kV, 6.6 kV and even 11 kV. Such high voltages are economically necessary in high power systems to reduce the size of current, and hence reduce the size of conductors and equip- ment required. Operating at such high voltages is becoming more common as ship size and complexity increase,e.g. for large cruise liners. Offshore oil and gas production platforms operate at up to 13.8 kV, where equipment weight saving is important. Distribution systems at these high voltages usually have their neutral points earthed through a resistor or earthing transformer to the ship's hull. The frequency of an a.c. power system can be 50 Hz or 60 Hz. In Europe and most of the world the national frequency is 50 Hz but is 60 Hz in North America and in a few other countries. The most common power frequency adopted for use on board ships and offshore platforms is 60 Hz. This higher frequency means that motors and generators run at higher speeds with a consequent reduction in size for a given power rating. Lighting and low power single-phase supplies usually operate at the lower voltage o1220V a.c. although 1L0 V a.c. is alsb used. These voltages are derived from step-down transformers connected to the M0 V system. The distribution system is the means by which the electrical power produced by the generators is delivered to the various motors, lighting, galley services, navigation aids, etc. which comprise the ship's electricalload. The electrical energy is routed through the main switchboard, then distributed via cables to section and distribution boards then ultimately to the final load consumers. The circuit-breakers and switches are the means of interrupting the flow of electric current, and the fuses and relays protect the distribution system from the damaging effects of large fault currents. Fig. 2.1 shows an HV/LV layout of a ship's distribution system. The system is called a radial or branchirg system. This distribution system has a simple and logical structure. Each item of load is supplied at its rated voltage via the correct size of cable and is protected by the correctly rated protection device. The main electrical load is divided into essentialand non-essentialservices. Essential services are those required for the safety of personnel and for the safe navigation and propulsion of the ship. They include certain supplies to navigational aids, communications, machinery spaces, control stations and
Power Distribution System 27 fH A [\V l_ E l L+-JHarmonic Filter aY') 2 2 O V 6 0 H z E C R S U B ffi fa ffi [a A \/ ') 440 V 60 Hz EMERGENCYSWBD. Fig.2.1 HV/LV power system. teering gear. The essential services breaker which is typically high set at 150"/" with a 20 seconds delay. In addition, each generaior has its 9wn preferenceoverload trip, this being low set generally at 110"/" current, instantaneousoperation. If a generator overload condition dev,elops, its preference overload trip will operate to energise the timing relay. The timing relay then operates to disconnect non-essential services in a definite order at set time intervals, e.g. o 1st trip - air conditioning and ventilation - 5 seconds may be supplied directly from the main switchboard or via section boards or distribution boards. Emergency supplies are necessaryfor loads r,inic6 are required to handle a potentially dangerous situation. To maintain generator operation during an overload, a preferential load shedding arrangement is employed. This is ac-bie-vedby u special overload relay, called a preferencetrip relay. If a generator overload develops, the preference trip relay sets an alarm and acts to trip selectednon-essentialloads. This reducesthe generator load so that it may continue to supply essential circuits. Each generator has its own normal overcurrent relay to trip its own circuit- o 2nd trip - refrigerated cargo plant - l-0 seconds o 3rd trip - deck equipment - L5 seconds
28 ElectrirralDistribution This order of tripping obviously varies with the ship type. When sufficient non-essentialload has been disconnected, the preference overload trip resets and no further load is disconnected. The generator preference trip system can also be initiated by low generator frequency or by low speed at the generator prime-mover. In many cases the preference trip protection is incorporated in a combined electronic relay which also monitors generatorovercurrent and reversepower. To maintain the preference relay trip settings as originally specified they must be periodically tested by calibrated current injection. Preferential load shedding, generator scheduling and load sharing is usually part of an overall pout)ermanagementsystem (PMS) under computer control. o Emergency supply An emergency electrical power service must be provided on board in the event of a main power failure. Such a supply is required f.or emergency lighting, alarms, communications, watertight doors and other services necessary to maintain safety and to permit saf-e evacuation of the ship. Regulations require that the emergency power source be a generator, or batteries, or both. The emergencypower source must be self-contained and not dependent upon any other engine room power supply. A battery when fully charged is obviously self-contained. An emergency generator must have an internal combustion engine as prime mover and have its own fuel supply tank, starting equipment and switchboard in the near vicinity. The emergencypower source must come into action following a total mains failure. Emergency batteries can be arranged to be switched into service immediately following a main power failure. Emergency generators can be hand cranked, but are usually auto- matically started by compressed air or a battery to ensure immediate run-up following a main power failure. Although regulations may permit a battery to be the sole source of emergency power, in practice a suitable battery may be physically very large and hence a diesel driven generator is usually installed with its own small starting battery or air-start supply. Other small batteries may also be installed to locally supply control and communication equipment. On passenger ships, regulations require that the primary emergency power supply be provided by a diesel driven generator for up to 36 hours (18 hours for non-passenger vessels). In addition, an emergency transitional battery must also be installed to maintain vital services (mainly lighting) for a short period - typically a minimum of 3 hours. This emergency battery is to ensure that a total blackout cannot occur in the transitional period between loss of main power and the connection of the emergency generator. A typical ship's distribution system is shown in Fig. 2.2. The system in- corporates emergency power supplies. There is no standard electrical supply arrangement, all ships differing in some respect. It will be seen that both the main and the emergencyconsumers' are supplied by the main service generators during normal operating conditions. In the event of an emergency, only the emergency services are supplied by the emergency generator. The emergency power system must be ready and available at all times. Such reliability requires special care and maintenance. At regular intervals it must be tested to confirm that it does operate correctly. The testing is normally carried out during the weekly emergency fire and boat drill practice sessions.The main generators are not shut down but the emergency power sources are energised and connected to supply the
Insulated and Earthed Neutral Systems 29 MAIN440 V SUPPLYINPUTS tEMERGENCY GENERATOR EMERGENCYSWBD. Qsp"re !unit 440 V 60 Hz ECR SWBD. {( 220 l ) ) ) ) l TO 220V EMERGENCYCONSUMERS ) t t ) l TO 440 V EMERGENCYCONSUMERS Fig. 2.2 Emergencypower supplies. emergency servicesfor the period of the practice session. The regulations governing the emer- gency source of power are detailed in International Conventions, e.g. SOLAS -(Safety of Life_1t Sea), Nati6nal regu- lations, e.g. IEE Regulations for t"he Electrical and Electronic Equipment of Qhips (UK) and in the regulitidns of the Classification Societies such as Lloyds, Det Norske Veritas, etc. 2.2. Insulated and Earthed Neutral Systems An insulafedsystem is one that is totallv electrically insulated from earth (shipis hull). An earthedsystem has the supply neutral point connected to earth. Shipboard main LV systemsat M0 V a.c. are normally insulated fuom earth (ship,s hull). Similar systems ashore are normally earthed to the ground. HV systems ( > L000 V) are usually earthed to the ship's hull via a neutrai earthing resistor (NER) or through a high im-pedance transformer to limit eartli fault-current. The priority requirement on board ship is to maintain continuity of the electricil supply to essential eiluipment in the event of a single earth-fault occurring. The priority requirement ashore is the immediate isolation of earth-faulted equipment which is automatically achieved by an earthedsystem. A circuit consistsessentiallyof two parts: o Conductor, the part which carries current through the circuit; o Insulation, the part which keeps the current inside the conductor. Three basic circuit faults can occur: o An open-circuitfault is in the conductor, as current cannot flow. due to at A, a break so that
30 Electrical Distribution L 1 LOAD -9- sTNGLE-PHASEA.c. = INSULATEDNEUTRAL e -:- THREE_PHASEA.C. INSULATEDNEUTRAL I SINGLE_PHASEA.C. EARTHEDNEUTRAL THREE_PHASEA.C. EARTHEDNEUTRAL Fig. 2.3 Insulated and earthed neutral systems. Fig. 2.4 Circuit faults. An earth fault is due to a break in the insulation, as at B, allowing the conductor to touch the hull or an earthed metal enclosure. A short-circuitfault is due to a double break in the- insulation, as at C, allowing both conductors to be connectedso that a very large current by-passesor "short-circuits"the load. The sizeof fault current that will occur depends on the overall impedanceleft in the circuit under fault conditions. QUESTION A 10 A motor operates from a 220 Y insulatedsystem. The supply cableshave a total impedance of 0.01 O. If: (a) an open-circuit fault, (b) an earth fault and (c) a short-circuit fault occurred, what circuit current would flow in each case?
Significance of Earth Faults 31 ANSWER (a) the open-circuit fault has infinite impedance, so: - 22ov : ZER}o Q (b) the earth fault has NO effect on the circuit current, so I remains at 10 A. (becausethis is an INSULATED system) (c) the short-circuit fault impedance is limited onlv by the 0.01 O of the cables, so: - . v 220v lsc : : 22.000A or 22 kAlt z 0.01f) The majority of earth faults occur within electricalequipment due to an insulation failure or a loose wire, which allows a live conductor to come into contact with its earthed metal enclosure. To protect against the dangers of electric shock and fire that may result from earth faults, the metal enclosures and other non-current carrying metal parts of electrical equipment must be earthed. The earthing conductor connects the metal enclosureto earth (the ship's hull) to prevent it from attaining a dangerous voltage with respect to earth. Such earth bonding of equipment ensures that it always remains at zero volts. 2.3. Significance of Earth Faults If a single earth fault occurs on the live line of an earthed distribution svstem it would be equivalent to a short-circuit fault across the generator through the ship's hull. The resulting large earth fault current would immediately cause the line protective device (fuse or circuit breaker) to trip out the faulty circuit. The faulted electrical equipment would be immediately isolated from the supply and so rendered safe. However, the loss of power supply could create a hazardous situation, especially if the equipment was classed essential,e.g. steering gear. The large fault current could also cause arcing damage at the fault location. In contrast, a single earth fault " A" occurring on one line of an insulated distribution system will not cause any protective trip to operate and the system would continue to function normallv. See Fig. 2.5. This is the important point: equipment continues to operate with a single earth fault as it does not provide a complete circuit so no earth fault current will flow. If a second earth fault at "B" occurred on another line in the insulatedsystem, the two earth faults together woirld be equivalent to a short-circuit fault (via the ship's hull) and the resulting large current would operate protection devices and cause disconnection of perhaps essential services creating a risk to the safety of the ship. An insulated distribution systemtherefore requires two earth faults on tuto different lines to cause an earth fault current to flow. In contrast, an earthed distribution system requires only one earth fault on the line conductor to create an earth fault current which will trip out the faulty circuit. An insulatedsystem is, therefore, more effective than an earthed system in maintaining continuity of supply to essentialservices.Hence its adoption for most marine electrical systems. Note: Double-poleswitches with fuses in both lines are necessary in an insulated single-phasecircuit. High voltage systems (3.3 kV and above) on board ship are normally earthed. t : !Z
32 Electrical Distribution FUSES LOADS FUSES LOADS -=. FAULT - \w-l Fig. 2.5 Double earth faults in an insulated system. Fig.2.6 Neutral earthing in HV system. Such systems are usually earthed via a resistor connecting the generator neutrals to earth as shown in Fig. 2.6. The ohmic value of each earthing resistor is usually chosen so as to limit the maximum earth fault current to not more than the generator full load current. Such a Neutral Earthing Resistor (NER) is usually assembledfrom metallic plates. The use of such an earthed HV svstem means that a single earth fault will cause current to flow in the neutral connection wire. This is monitored by an earth fault (E/F) relay to create aiarm and trip functions. QUESTTON What would be the ohmic value of an NER to limit the earth fault current to the full load rating of a 2 MW, 0.8 pf, 3.3 kV, 3-phase a.c. generator? ANSWER In a 3-phasesystem; P :,'/5.Vulr.cos{ where V7 is line voltage (3.3 kV), /r is the line current and cos@is the power factor.
Significance of Earth Faults 33 The generator full load current is: t _ I L _ 2,000,000 : 4 3 7 A EXAI\,IPLE INDICATION o o oEARTH FAULT ON LINE 3 F \ t t r 5 l L- -] lwtrcH Fig.2.7 Earth fault monitoring with lamps. Earth indication lamps in a 3-phase a.c. system are arranged as shown inFig. 2.7. When the system is healthy (no earth faults) then the lamps glow with equal half brilliance. If an earth fault occurs on one line, the lamp connected to that line goes dim or extinguished. The other lamps experience an increased voltage so will glow brighter than before. Earth indication lamps have been the most common method used for many years, being an inexpensive installation which is easy to understand. Their major disadvantage is that they are not very sensitive and will fail to indicate the presence of a high impedance earth fault. This has led to the development of instrument type earth fault indicators which are being increasingly used. One common type of earth fault instrument-type monitor connects a small d.c. aoltage to the distribution system. Any resulting d.c. currentis a measure of the insulation resistanceof the system. The injection-type instrument limits the maximum earth fault monitoring current to only 1 mA (compared with about 60 mA for earth lamps), and the meter indicates insulation resistance directly in kO or MO. The monitor triggers an alarm when its set value is reached. This type of arrangement has been developed to meet regulations which demand that on tankers, for circuits in or passing through hazardous zones, there must be continuous monitoring of the system insulation resistance.Visual and v 3 . 3 , 3 0 0. 0 . 8 Under EiF conditions a phase voltage of: VpH : =q : 1905Vdrives the fault current V 3 through the NER. So its ohmic value has to b"r 1905v : 4.4 Q 437A Certain essentialloads (e.g. steeringgear) can be supplied via a transformer with its secondary unearthed to maintain security of supply in the event of a sinele-earth fault. Regulations insist that tankers have ly insulated distribution syslems. This intended to reduce danger from earth ult currents circulating in the hull ithin hazardous zones which may use an explosion of the flammable stem does not extend gine room bulkhead area. ical supplies forward of the engine bulkhead are usually 3-phaseM0 V'r.oomDulKneaqare usuaily J-pnase ++u v insulated and obtained from a 3-phase 3.3 kVlM0 V transformer. Regulations require that an earth fault monitor is fitted to the main switchboard to indicate the presence of an earth fault on each isolatedsection of a distribution system, e.g. on the 440 V and 220 Y sections. An earth fault monitor can be either a set of indicator lamps or an instrument (calibrated in kf) or MO) to show the system IR value to earth. forward of the and into the
34 Electrical Distribution Measurement of the in an earthed system varlous means; one in Fig. 2.10. earth fault current can be provided by method is shown Fig. 2.8 Earth fault monitoring by d.c. injection. audible alarms are given if the insulation resistancefalls below a pre-set value. An HV system (1 kV - 11 kV) is usually earthed at the generator neutral point via a neutral earthingresistor(NER). This arrangement allows the neutral (and hence earth fault) current to be monitored for alarm/trip by a current transformer (CT) and E/F relay. 3-PHASE BUS-BARS Fig,2,9 NER circuit. STARTER MAGNETIC CORE Fig. 2.10 Core-balanceCT. Here the current transformer (CT) measures the phasor sum of the 3 line currents supplied to the motor. If the motor is healthy (no earth faults) the phasor sum of the currents measured by the CT is zero.- If an earth fault (E/F) occurs in the motor, an earth fault current flows and the phasor sum of the currents is now not zero. The current monitored bv the E/F relay is used to trip the coniactor in the starter to isolate the faulty motor circuit. The earth fault monitor on the switch- board shows the presence of an earth fault on the distribution svstem. It is up to the maintenance staff to trace (searchfor) the exact location of the fault and then to clearit as quickly as possible. Fig. 2.11 Earth fault monitors in a distribution system. Alternatively, a special three-phase earthing transformeris connected to the HV systembus-bars.This high impedance earthing transformer is arranged to limit the maximum permitted E/F current and initiate an alarm/trip voltage signal to a connectedprotection relay. 'l \u_ ad ll/-*-' Arocnr- Llt_ ?ALARM t o f i \ , l , l T o (mn)\)/-l--lexrenul CURRENTrlR | | ALARM L I M I T E RU ' ' S H I P ' S LOAD V 6 O H Z M A I N S W E D ) I 2 2 O V l o l ' T O I t c l o l t E/F MONITOR E/F MONITOR
Significance of Earth Faults 35 I ] I t An apparently simple method would be to open the circuit-breakers feeding loads A, B, C, etc. one at a time and by watching the earth fault monitor while observing which circuit-breaker, when tripped, clears the earth fault. The earth fault must then be on that particular circuit. In practice, circuits cannot be dis- connected at random in this way. Some vital service may be interrupted causing the main engines to stop ... perhaps in dangerous narrow waters. Tracing the earth fault must be co-ordinated with the operational quirements of the ship's electrical rvices. The method of earth fault rance will be described fullv for a ting distribution circuit shown in 2.12. 2.12 Three phase to single phase distribution. the earth fault monitor on the lighting distribution board (d.b.) the presence of an earth fault. witches A, B, C, are sequentially pened and closed in turn until the earth t monitor indicates the earth faulted circuit. Suppose this is switch B. Circuit B supplies a distribution fuse- board (d.f.b.) located near its lighting circuits. Here there is no earth fault monitor so an IR (megger) tester must be used. Fig. 2.13 TR testing at distribution fuse board. At this d.f.b. fuse-pairNo. L is removed to isolatethe supply to the load. (Fig. 2.13) The IR tester (megger) is now connected with one lead to earth (hull) and the other lead to "b" (the outgoing terminal as shown), and a test applied. If healthy (IR > 1 MO), connect the test lead to " a" arrd repeat the test. If both " a" arrd "b" are healthy, circuit 1 is healthy and fuse-pair 1 can be replaced. Fuse-pair2 is now removed and tested at " a" and "b" . If an earth fault is indicated (IR : low) then the faulted circuit has been located All fuse-pairs are checked in turn to confirm whether healthy or faulted. At the faulted circuit, the fuses should be removed, all switches should be opened, and all lamps taken out as shown in Fig. 2.14. ccr 2 FUSES REMOVED SWITCH ' EFcleaLcdr Fig. 2.14 IR test on a lighting circuit. This breaks the circuit into several isolated conductor sections.
36 Electrical Distribution At the supply distribution board, test a! "a" and then at "b". If both have an IR> 1 MO then the conductors connected to "a" and "b" are clear and healthv. Close the switch and re-test at " a". If the IR is low then the earth fault lies on the conductors bevond the switch. At lamp L remove the fitting and disconnect the conductors as shown to further break down the circuit. Use the IR tester on each of these disconnected leads. If one conductor is indicated as having an earth fault (suppose it is the conductor between Lr and L) then the earth fault lies at lamp 1. or lamp 2 or on the conductor. Both lamp fittings must now be opened and visually inspected to trace the exact location of earth fault. The method of tracing the earth fault is essentially that of continually breaking down the circuit into smaller and smaller sections until it is finally located. When located, the damaged insulation must be repaired. The method of repairing the earth fault depends upon the cause of the earth fault and this is determined bv visual examination. A lamp fitiing that is damaged must be replaced. Dampness in insulation must be dried out by gentle heat and then some precaution must be taken to prevent the future ingress of moisture. Insulation that has been mechanically damaged or weakened by overheating must be made good again. If surface dirt is the cause, a thorough cleaning will probably cure the fault. 2.4. Distribution Circuit Breakers Details of. main circuit-breakers for main generators and main feeder circuits are included in Chapter 3. The function of anv circuit-breaker is to safely make onto and break open the prospectiaeshort-circuit fault current expected at that point in the circuit. The main contacts must open rapidly while the resulting arc is transferred to special arcing contacts above the main contacts. Arc chutes with arc-splitters quickly stretch and cool the arc until it snaps.The CB is open when the arc is quenched. Feeder and distribution circuits are usually protected by the moulded-case (MCCB) type or the miniature (MCB) type of circuit-breakers. o MCCBs These are small, compact air circuit- breakers fitted in a moulded plastic case. They have a lower normal current rating (50-1500 A) than main breakers and a Iowerbreakingcapacity. See Fig. 2.15. Fig. 2.15 MCCB outline construction. They usually have an adjustable thermal overcurrent setting and an adjustable or fixed magnetic overcurrent trip for short-circuit protection built into the case. An undervoltage trip coil may also be included within the case. Operation to close is usually by a hand operated lever but motor-charged spring closing can also be fitted. MCCBs are reliable, trouble free and require negligible maintenance. If the breaker operates in the ON position for long periods it should be tripped and closed a few times to free the mechanism and clean the contacts.Terminals should be checked for tightness otherwise overheating damage will develop. G iNd vWithdrawableversion MFixedversion Plug -in version
Transformers 37 e e ) The front cover of larger MCCBs (around 1000 A rating) can usually be removed for visual inspection and MCBs must be replaced if faults develop - no maintenance is possible. Electrical generation on board ship is typically at 3-phase a.c./ M0 V, 60 Hz, while fixed lighting and other low power loads are supplied with 220 V a.c. single-phase from very efficient (typically > 90"/") static transformer units. Ships with HV generation require 3-phase transformers to supply the LV engine-room and accommodation sub- switchboardse.g. using 66001M0V units. See Fig. 2.17. The principle of operation of a single- phase transformer is simple. An applied a.c. voltage Vr to the primary winding sets up an alternating magnetic flux in the laminated steel core. The flux induces an emf in the secondarywhose size is fixed by the ratio of primary and secondary turns in the pair of phase windings (N1 and N2) to aning. Following tripping under a rt-circuit fault, the breaker should be ted for damage,checkedfor correct 2.5. Transformerstion, and its insulation resistance ured. A test result of at least 5 MO usually required. Any other faulty eration usually requires replacement overhaul bv the manufacturer. MCCBs can be used for every appli- ion on board ship from generator to small distribution breakers. g I limited breaking capacitymay demand back-upfuses be fitted for very prospective short-circuit fault levels. MCBs are very small air circuit-breakers in moirlded plastic cases. See 2.16. They have current ratings of 100 A and generally thermal over- rent and magnetic short-circuit tection. Thev have a verv limited *ing capaciiy (about 3006 A) and*ing capacity (about 3000 A) and commonlv used in final distribution rds instead of fuses. The d.b. is lied via a fuse or MCCB with the ired breaking capacity. The secondary voltage Vz is available to drive current through a load. It is the load connected to the secondary that sets the size and power factor angle of the load current 12. This is matched on the primary side from: Transformers are rated in apparent power (VA or kVA) units. giu",fi : N l Nz tl e )r e o a d is e !r t d n d e V t _ l z V2 11 QUESTTON A 4401110V single supplies a load of 5 factor load. phase transformer kW at 0.8 power lnH'. Fig.2.16 MCB outline construction.
38 Electrical Distribution Calculate secondary and primary currents (ignoring transformer power losses). ANSWER F r o m : P 2 : V 2 . l 2 . c o s { , I r : - J 2 -- V't.cos6 - 5ooo : s6.BzA 1 1 0x 0 . 8 : I 1 : 1 2 x Vz V1 / . l 1 n \ = 56.82t | "' l: '14.2 A lM}) : or, checkfrom P1: V1.I1.cosQ The transformers are generally air cooled, being mounted in sheet steel enclosures which are often located adjacent to the main switchboard. Alternatively, they may be fitted within the switchboard sb transformer enclosuresare not required. Three-phase 4401220V lighting trans- formers are usually composed of three separate single-phase units inter- connectedto form a 3-phasearrangement. This enables easy replacement of a single-phase unit if it develops a fault. The alternative is to use a single 3-phase unit with all windings mounted on a common magnetic core. This type has to be completely isolated in the event of a fault on one phase only. Transformersfor use on 3-phaseinsulated systems are generally interconnected in a delta-deltacircuit configuration using copper links between the phasewindings. SeeFig. 2.18. If a fault develops on one phase of such an arrangement, the faulty unit can be disconnected (via the links) creating an open-deltaor "V" connection and a 3-phase supply will still be available, although at a reduced power capacity. This is obviously a useful safeguard. In some cases,a spare 4th transformer is available to replace the faulty unit. Transformers for use on 3-phase HViLV earthed systems ashore are generally connecteddelta-starto provide a 3-phase, --r- _JIE- S I N G L E PHASE t f u'{Du, TypicalLaminated MagneticCores PrimaryandSecondary PhaseWindings Mountedon CoreLimbs Alternative Transformer Symbols ffim u,€&u, IHISEu,€&u, Sample3-PH Connections Fig. 2. L7 Transformer arrangements.
lnstrument Transformers 39 3-PH 220V \- -\ \'--\ \ \ ilt flt ilr flr tll ||lul ul |]i rlr rlr rlr t t t t t li t t i i i ----1'NGL;;F.AS, u."l- L I G H T I N GA N D LOWPOWERSUPPLIES 3 x 1-PHASE TRANSFORMERS C O N N E C T E D A S :A - A R 3-PH440 V MAINSWITCHBOARD Fig. 2.18 Delta-deltatransformer connection. 4-wire LV supply, e.g. a 6600/400V ratio gives a secondary line voltage of 400 V plus a line-neutral phasevoltage of 400/V3 :230 V. An earth fault occurring on a such neutral-earthed system will immediately operate the prot-ective fuse or circuit-breaker. This interruption of supply leads to rapid identification of the faulty circuit. Transformers are static items of equip- ment which are usually very reliable and trouble-free. However, like all electrical equipment, transformers must be sub- jected to the usual maintenance checks. At regular specified intervals, trans- formers must be disconnected, covers removed and all accumulated dust and deposits removed by " vacuum cleaner and suitable brushes. Windings must be inspected for any signs of damage or over-heating. Winding continuity resistancevalues are measured, recorded and compared with each other for balance. Anv differences in continuitv readings will-indicate winding faults such as short-circuited turns. The insulation resistance of all windings must be measured both with respect to earth and to the other phase windings. The cause of any low insulation resistance reading must be investigated and rectified. Cable connections must be checked for tightness. Covers must be securely replaced and the transformers re- commissioned. All test results and observationsshould then be recorded for future reference. 2.6. Instrument Transformers Transformers are used to supply instruments and protection relays with proportionally smallcurrents and voltages derived from the large currents and voltages in a high power network. See F i g . 2 . 1 9 .
40 Electrical Distribution MAIN BUS-BARS C-T_e.g.1000/5A e . g . 6 6 0 0 / 1 1 0 V Fig.2.L9 Instrument connectionswith CT and W. Voltage transformers (Ws) supply volt- meters and the voltage operated coils of instruments and relays. A standard secondary voltage of 110V is used. Current transformers (CTs) supply ammeters and the current operated coils of instruments and relays with a s t a n d a r d i s e d 5 A o r l A . The use of VTs and CTs allows standardised instruments and relays to be used. They also improve safety by providing low voltage and low current isolated supplies for monitoring instru- ments and protection relays. VTs are built like small power transformers.They are not normally used at voltages less than 3 kV. CTs can be of the wound primary or bar primary tyPe. The bar primary type CT is used with very high primary current ratings the wound primary type being used for small step-down ratios, e.g. 1000/5 A bar primary; 50/5 A wound primary. The ratio specified on a VT details its input and output voltages, e.g. 3.3 kV/ 110 V is used on a 3.3 kV mains circuit Fi1.2.20 Bar primary CT. and steps the voltage down to 110 The associated instrument will have scale calibrated 0-3.3 kV and will marked "3.3 kV/110 V VT ratio". The ratio specified on a CT similarly details its input and output currents, €.8. 150/5 A CT is used on a 150 A mains circuit and steps the current down to 5 A. The associatedinstrument will have its scale calibrated "0-L50 A" and will be marked "15015A CT ratio". The use of instrument transformers does not eliminate danger to operators. The 110 V output from a VT will apply a severe, possibly lethal shock to un- suspecting fingers! The secondary circuit of a CT must neuer be opened while mains primary load current is flowing. Excessiveheating will be developed in an V . its be
Shore Supply Connection 41 ope-n-circuitedCT with an extremely high voltage, ar_isingat the open sec6ndaiy terminals. If an ammeter is to be removed from circuit, the CT secondary output terminal must be first short-circuiied, with the primary circuit switched off. The secondary short circuit will not damage the CT when the primary current is switched on. For further s#ety, one end of the secondary winding of a CT or VT is connected to earth. Status indicator lamps on switchboards are .commonly of the transformer type, having a small transformer built into the lamp fitting. The transformer provides a 6 Y or 12 V output. The lamp is of !9ry watla,ge with small bayon-et cap fitting. Although not an accuiate instrui- ment transformer, the lamp transformer is similar in function to a VT. the shore supply cable. A voltmeter is fitted to indicate polarity of. a d.c. shore supply. For an a.c. shore supply a phase-sequence indicator is fitted to indicate correct supply phase sequence. This indicator may be arranged as two lamps connected as an unbalanced load across the three phases via resistors and capacitors. The sequenceis "right" (or correct)when the ight side lamp is bright and the other is dark. An alternative P.S.I. indicator is a rotary pointer driven by u small 3-phase induction motor. At the main switchboard an indicator is provided, usually a lamp, to indicate that the shore supply is available for connection to the bus-bars via a con- necting switch or circuit-breaker. It is not normally possible to parallel the shore supply with the ship's generators. The ship's generators must, therefore, be disconnected before the shore supply can be connected to the main switchboard. Normally, the shore supply switch on the main switchboard- is interlocked with the generator circuit- breakers so that it cannot be closed if the generators are still connected. QUESTTON \A/hy is it essential to know if the phase sequence of the incoming shore supply is "correct"? ANSWER By "correct" we mean that it is the same sequence as the ship's supply (red- yellow-blue). A reversed phase sequence (red-blue-yellow) will produce a reaersed shaft rotation in all 3-phase motors because the direction of their rotating magnetic fields will be reversed with disastrous results. This fault is remedied by interchanging any_two conductors of the shore supply cable at the connection box. A shore-supply is required so that the ship's generatorsand their prime-movers can be shut down for major overhaul during a dry-docking period. There must be a suitable connection box conveniently located to accept the shore supply ca6le. The connection box is often located at the entrance to the accommodation or in the emergency generator room. The connection box must have suitable terminals to accept the shore supply cable, including an earthing termihdl to earth the ship's hull to the shore earth. The connection box must have a circuit- breaker or an isolator switch and fuses to protect the cable linking the connection box to the main switchboard, with a data plate giving details of the ship's electrrcal system (voltage and frequency) and showing the method for connecting
42 Electrical Distribution VOLTMETER [r;{ l ( . \ l l fo{HoRE o'l I SUPPLYI lcoNNEcTtoNI G___q9r_3 Fig. 2.21. Shore connectionbox and indicators. Fi9.2.21,shows a typical shore connection arrangement but some variations occur. For example, the shore supply may be connected directly to the emergency board which then back-feedsto the main switchboard. The shore supply may have a different frequency and/or voltage to that of the ship's system. A higher frequencywill cause motors to run faster, be overloaded and overheat. A higher aoltage will generally cause equipment to take excess current and overheat. It will also cause motors to accelerate more rapidly and this. may overstressthe driven loads. A lower voltage is generally not so serious but mav cause motors to run slower and overheat, and may cause motors to stall. If the shore supply frequency differs from the ship's normal frequency then, ideally, the shore supply voltage should differ in the same proportion. QUESTTON If your ship is designed for 440 V - what value should supply voltage be if operating 60 Hz at the shore at 50 Hz? ANSWER Supply voltage should be reduced to about 380 V.
Circuit Protection 43 2.8. Circuit Protection Many forms of electrical protection are availablewhich are designed to protect the distribution system when a fault occurs. Protection relays are used to monitor overcurrent, over/under voltage, over/ under frequency, earth leakage, un- balanced loading, over-temperature, reverse power (for generators) etc. The HV power system shown in Fig. 2.22 lists typical protective relay functions. As most protection relays monitor current and/or voltage, we will limit our examination to ooercurrentand under- voltage protection together with an appreciation of protectiaediscrimination. (Reverse power protection is included with generator protection in Chapter Three) No matter how well designed and operated, there is always the possibility of faults developing on electrical equip- ment. Faults can develop due to natural wear and tear, incorrect operation, accidental damage and by neglect. The breakdown of essential equipment may endanger the ship, but probably the most serious hazard is FIRE. Overcurrent (I2R resistive heating effect) in cables and equipment will cause overheating and possibly fire. The size of conductor used in cables and equipment is such that with rated full load current flowing, the heat developed does not raise the temperature beyond about 80'C (i.e. 35'C rise above an ambient of 45'C). A copper conductor can withstand very high temperatures(melts at 1083"C), but its insulation (generally organic materials such as cotton or plastic com- pounds) cannot withstand temperatures much in excess of 100'C. At higher A A[\V [\v A[v 440V 60 Hz ECRSWBD. t I t d l I ? Overcurrent(lnst.) Overcurrent(l/t) Undervoltage UnbalancedLoad Reverse Power Earth Leakage Overcurrent(lnst.) Overcurrent(l/t) Undervoltage UnbalancedLoad EarthLeakage Overcurrent(lnst.) Overcurrent(l/t) EarthLeakage Overtemperature 6.6 kV 60 Hz MAINSWBD 7 t t + .l. dOvercurrent(lnst.) Undervoltage Overfrequency Underfrequency. Overcurrent(lnst.) Thermal Overload Locked Rotor Earth Leakage { { LI A / M \ * F-,, Fig. 2.22 HV protection scheme.
M Electrical Distribution QUESTION Suggest three reasons equipment is essential distribution svstem. why protection in an electrical temperatures the insulation suffers irreversible chemical changes, loses its insulation properties and becomesburnt out. Short-circuit and overload currents must, therefore, be detected and rapidly cleared before damage occurs. fault current. An overcurrent relay detects the fault current and initiates the trip action. The circuit-breaker or fuse must be capable of safely and rapidly inter- rupting a short-circuit current. They must be mechanically strong enough to withstand the thermal and magnetic forces produced by the fault current. The size (strength) of the circuit-breaker or fuse is specified by its breaking capacity which is the maximum fault current it can safely interrupt. For example, an MCCB may be con- tinuouslv rated at 440 V with a rated current -of OOOA. Its breaking capacity mav be 12.5 MVA which means it can safely interrupt a fault current of 1.6,400A (from 12.5x106143.440: 1,6,400A). The prospectiaefault current level at a point in a circuit is the current that arises due to a short-circuit at that point. See Fig. 2.23. The size of this short-circuit fault current is determined by the total impedance of generators,cablesand transfoimers in the circuit between the generator and the fault. See Fig. 2.24. This total impedance is generally very small so the maximum fault current (called the prospectioefault current) can be very large. ANSWER t/ To disconnect and isolate faultv equipment in order to maintain th-e power supply to the remaining healthy circuits in the system ,/ To prevent damage to equipment from the thermal and magnetic forces that occur during short circuit and overload faults ,/ To protect personnel from electric shock The protection scheme consists of circuit- breakers, fuses, contactors, overcurrent and undervoltage relays. A circuit- breaker, fuse or contactor interrupts the r TRANSFORMER GENERATOR r - - - - - - l MAIN SWITCHBOARD Fig. 2.23 Short-circuit fault location.
Circuit Protection 45 I 0.025o i r Shortcircuit I fault location i oqqr I Fig. 2.24 Fault circuit. A 440 V, 5 kW, 0.8 pf 3-phase load is supplied as shown in Fig. 2.24. The normal full load PO\AtrERis P: Ji.Vr.ly.cosgwatts Example So the load full load current is 5,000T _ T L _ rEvr.cosd G.++o . o.s Suppose now a short-circuitfault occurs at the load terminals The total impedance is Z r : 0 . 0 2 5 + 0 . 0 1 +0 . 0 1 5: 0 . 0 5O and the prospective short-circuit fault current is tr: ! : Mo v : 8.800A' Z P 0 ' 0 5O : ' - - So the prospective fault current level at the load is 8800 A and, for a short-circuit at the d.b. the fault level is: M 0 v (0.025+ 0.01)c):2,fl1 ! Note that the fault level increases, the nearer the fault occurs to the generator. The circuit-breaker or fuse must have a breaking-currentcapacityin excessof the prospectioefault current level expected at the point at which it is fitted. If less, the circuit breaker (or fuse) is liable to explode and cause fire. The ability of a protection system to disconneci onlv 'the faulted circuits and to maintain the electrical supplies to healthy circuits is called proleitiae discimination. Discrimination is achieved bv co- ordinating the current ratings and- time settings of the fuses and overcurrent relays used between the generator and the load as shown in Fig. 2.25. The protective devices nearest the load having the lowest current rating and shortest operating time. Those nearest the generator having the highest current rating and longest operating time. If a short-circuit fault occurs in the lampholder in Fig. 2.25, the fault current will be large enough to operate all protection devices from the generators to the fault. However, the 5 A fuse protecting the lamp circuit has the lowest current rating and shortest operating time in the system so will be the quickest to operate. This action will clear the f.ault and leave all other healthv circuits still connected. : 8 . 2 A for a short-circuit at the main switchboard the fault level is: .yo' :17.600 A 0.025f,
46 Electrical Distribution Fig. 2,25 Protective discrimination scheme. In the case of fuses, it is generally accepted that discrimination will be achievedif consecutivefuses have a ratio of about 2:1. The shipbuilder specifies the current ratings of fuses, together with the current and time settings of relays, in the protection scheme. It is important that the original settings are maintained to achieve correct discrimination. D OvercurrentProtection The general term "overcurrent" applies to a relatively small increase over the full load current (FLC) rating (e.g. due to mechanical overloading of a motor) rather than the massive current increase caused by a short-circuitfault. Generally, an overcurrent, supplied from a CT, is detected by a relay with an appropriate time-delay to match the protected circuit. Short-circuitfaults in LV distribution circuits are mainlv detected and cleared almost instantaneoitslybyfuses, MCCBs or MCBs. Main supply feeders are usually protected against short-circuits by circuit breakers with instantaneous magnetic trip action. Overcurrent relay types: Magnetic Thermal Electronic Fig. 2.26 Inverse current/time (I/t) curve. All relay types have an inverse current- time characteristic called OCIT (over- current inverse time), i.e. the bigger the current the faster it will operate. See Fig. 2.26. The basic inoerseI/t curve would tend towards zero time for the highest currents. To make the relay action more precise at very high fault currents the action is arranged to operate at a definife minimum time which is fixed by the design. This type is called an OCIDMT (overcurrent inverse and
Circuit Protection 47 definite minimum time) relay action. The OCIDMT can also be combined with an instantaneozs(high set) trip to give the fastest action agiinst extremely high currents due to I short circuit fault. A magneticrelay, as shown in Fig,.2.27, directly converts the current into an electromagnetic force to operate a trip switch. One type is tfie attracted armature action -similar in construction to a ,simple signalling relay but with an adjustment for the current setting. Th" time of operation is fixed at -a definite minimum time which'is usuallv less than 0.2 seconds.This is regarded as instantaneousi.e. with no deliberate time-delay. To obtain a magnetic inverse-time action, 9.8-. for motor overload protection, an induction disc movement is usuallv employed. This construction is similar tb a kWh energy meter used in a house but the disc movement is constrained by a spring so is not allowed to actually rotate. The disc travel is verv small but sufficient to_operate a set of trip switch contacts. Bolh current and tim^e settings are adjustable.A combined relay including an attracted armature element Adjusting Screw Contacts Attracted Armature Fig. 2.27 Magnetic overcurrent relay (instantaneousaction). and induction disc element will give an instantaneous action (high set current) and an inverse/time characteristic. Fig. 2.28 shows a thermal rcIay which utilises the bending action of a bimetallic b.ar (gn9 p^erphase) to open a normally- closed (NC) contact whith then trips-a Fig. 2.28 Bimetallic thermal relay
48 Electrical Distribution contactor or circuit-breaker. A small circuit current will be allowed to flow directly through the bimetallic strip but larger -currents will be directed through a lieater coil surrounding the strip. The three bimetal strips in a three phase relay, all bend in the same direction with balanced overcurrents to cause a trip. A mechanical bell-crank trip arrarigement can also operate with unbalinced (differential)currents. This is particularly effective with a single- phising motor fault. In this case, two of^the Uimetal strips bend further in the normal direction with increased line current, while the other cools down allowing this strip to move relatively backwards (differential action). The time iaken to heat the bimetal strip to cause sufficient bending fix-es the^required time to trip. Resetling the relay cin only be achieved after the_strip has- cooled down back to the ambient temperature. The inaerseI/t overcurrent characteristic of a thermal relay is very useful for the indirett temperature protection of motors. Its thermal time. delay is, however, far too long for a shori-circuit fault so back-up instantaneous protection must also be used in the form-of fuses or a circuit breaker. An electronic overcurrent relay usually converts the measured current into a proportional voltage. This is then compared with a set voltage lfvel within the -monitoring unit which may be digital or analogue. In an_analogueunit (aJ shown in F1g. 2.29) the time delay is obtained by the time taken to charge up a capacitor. This type of relay has separate adjustments for overcurrent and time settings together with an instantaneous trip. The electronic amplifiers within the relay require a low voliage d.c. power supply, e.g.24 V d.c. deriv:ed from a 110 V a.c. auxiliary supply. Here, the input from a line current transformer (CT) is rectified to produce a d.c. voltage which is proportional to the line curlent. This voltage charges capacitor C2 at a rate sel 4 conjunction wilh potentiometer R5 which determines the inverse-time characteristic for the relay. When this capacitor voltage exceeds the predetermined level (set by R2) the detector circuit drives Power tiansistor T2 to operate the output electromagneticrelay RLA which switches trip and ilarm contacts in the external circuits. operation is output of the to the input An instantaneous triP obtained by applying the bridge rectifier directlY voltage set bof tlie amplifier with a voltage 9el b-y R4 Hence--for hisher values of faultR4. Hence, for higher current, the inverse--time delay circuit is by-passed. / R 5 Time Setting R3 R7 *ve c3 Trip & Alarm l F " " Fig. 2.29 Electronic overcurrent relay circuit.
Circuit Protection 49 Both _the _magneticand electronic relays can be designed to give an almost instantaneous trip (typically less than 0.05 seconds or 50 ms) to clear a short- circuit fault. Thermal relays are commonlv fitted in mouldedcaseciicuit breakers(MiCns) and in miniature circuit-breakers (MCBs) to give 9 -'ll9ng time" thermal overcurrenttrip in addition to a magnetic action for an instantaneoustrip with a short-circuit fault. Overcurrent protection relays in large power circuits are generally driven by current transformers (CTs). The CT secondary usuallv has a 5 A or L A rating for full load iurrent in its primary winding. All overcurrent relays can be tested bv injecting calibrated iest currents into them to check their current trip levels and time delay settings. Primary injection is where a calibrated test current is fed through the normal load circuit. This requires a large current injection test set. The test set is 6ssentially a transformer and controller rather lik-e a welding set, i.e. it gives a low voltage - high current output. Small secondary injection currents (5-50 A) are fed current directly into the overcurrent relay usually via -a special test plug/socket wired into the ielay. Secondary injection does not prove the CT performance (as it is disionnected during the test) but is the usual method for testing an overcurrent relay. The- setting up of an overcurrent relay is obviously critical to its protective duty so is carried out in strict accordance with the manufacturer's instructions. Syqh setting up is done during new ship trials and at subsequentperiodic surveys. o Fuse Protection A fuse is the most common type of protection against a short-circuit fault in LV distribution circuits, motor circuits and for portable appliances. It is relatively simple, inexpensive and reliable. As re-wireable fuses tend to be less reliable than the cartridge type and are open to abuse (fitting the wrong stze of fuse wire), they are not recommended for marine practice. HRC (high rupturing c-apacity- e.g. 80 kA) cartridge-type fuse links are normally used. A tyfiiat construction is shown in Fig. 2.30, SilverFusibleElement Metal QuartzSand in CeramicBody Fig. 2.30 HRC fuse construction. A disadvantage of a fuse is its insensi- tivity to smallovercurrents. An HRC fuse wiII blow at currents as low as 25olo overload, but only after about 4 hours. - . Th" advantage-of a fuse is its very high speed of operation (a few milli- seconds) at high short-circuit fault current - faster than a circuit-breaker. Fuses are fitted in circuits to give protection againstshort-circuifs.Protection against relatively small overcurrents (e.g. due to shaft overloading on a motoi) is provided where necessary by an oaercurrentrelay (OCR). A starter overcurrent relav protects the motor against relatively tr"itt over- currents. The fuse links provide back-up protection for the supply cables and generators against a short-circuit fault. Motor fuses are typically rated at 2-3 times the motor full load current in order to withstand the large starting current surge (up to 6 times full load) of the motor. The motor manufacturer will specify the correct rating of fuse link for a particular motor rating. Hence a typical fuse designation for a motor circuit could be "32M63" which indicates
50 Electrical Distribution a continuous rating of 32 A but a rating of 63 A for the brief starting period. Important points to note concerning fuses are: o In the event of a fuse blowing, the cause of the fault must be located close the wrong circuit-breaker e.g. the breaker of a stoPPed and dead generator. If this circuit-breaker was flosed, the dead generator would be the equivalent of -a short-circuit fault on the bus-bars and cause a blackout. The undervoltage relay prevents the closure of the cirEuit-breike^rof the dead generator. QUESTTON A 3-phase short-circuit occurs on the main- bus-bars and the short-circuit trip of the running generator breaker fails to operate. Explain how the undervoltage relay provides a back-uPtriP. MainBus-bars Fig, 2.3L Under-voltage protection. The reference symbols used on an HRC fuse link are devised bY the particular manufacturer. They include the current rating, voltage, application (e.g. motor, transformer, diode, ge-neral use), physical size, and type of fixing arrangement. tr Undervoltage Protection An undervoltage(U/V) releasemechanism is fitted to all generator breakers and some main feeder circuit-breakers. Its main function is to trip the breaker when a severevoltage dip (around 50o/o) occurs. This is achieved by lifting the mechanical latch (which keeps the contacts closed) to allow the trip spring to function which oPens the breaker contacts. The U/V releaseon a generator circuit-breaker also prevents it being closed when the generator voltage is very low or absent. As shown in Fig. 2.31,,an undervoltage relay, which may be magnetic or elec- tronic, also provides back-up protection to short-circuitprotection. As an example, suppose during generator paralleling proCedures, an attempt was made to and repaired beforethe fuse link is replaced. The replacement fuse link must be of the correct current rating, grade and type. Usually this means the rephc6ment fuse iink is identical to th-eblown fuse link. Replace all three fuses in a 3-phase suppty even if only one is found bl<iwn after a fault. The others may be seriously weakened which makes them unreliable for future use. ANSWER The short-circuit reduces the voltage to zero which causes releaseto trip the breaker. bus-bar the U/V Undervoltage protection is also required for motor starters. The starter contactor normally provides this protectio_n as it drops ouf when the supply _voltage is losCor is drastically reduced. The starter circuit will not normally allow the motor
Electric Cables 51 to re-start when the voltage supply is restored except when special automatic re-starting facilities are provided. Undervoltage protection can be electro- magnetic or electronic. Checking and calibration of generator undervoltage relays can only be done accurately by calibrated aoltageinjection. A known variable voltage is directly applied to the undervoltage relay to check: o The voltage at which the relay pulls-in o The voltage at which the relay drops-out Generator U/V relays are usually slugged to allow a time-delay which prevents spurious tripping during transient voltage dips (typically 15"/") caused by large motor starting currents. 2.9. Electric Cables Ship wiring cables have to withstand a wide variety of environmental conditions, e.g. extremes of ambient temperature, humidity and salinity. Improved materials have led to ship wiring cables of a fairly standard design that are safe, durable and efficient under all conditions. The normal distribution voltage on ships is 440 V and cables for use at this voltage are designated 600i1000V, i.e. 600 V to earth or 1000 V between conductors. Higher voltage systems require cables with appropriate ratings, e.g. for a 3.3 kV 3-phase earthedneutral system the required cable rating is 1900/3300V. For 3-phase insulated systems the cable rating would be 3300/3300V. Cables are constructed of several basic parts: Conductors are of annealed stranded copper which may be circulr,r or shaped. Cableswith shaped conductors and cores are usually smaller and lighter than cableswith circular cores. Fig, 2.32 XLPE cable construction. Cable insulation has a thickness appropriate to the system voltage rating. Insulation materials are generally organic plastic compounds. Butyl rubber, which is tough and resilient, has good heat, ozone and moisture resistance. These excellent properties enable butyl rubber to replace natural rubber as an insulant. Even so, butyl rubber has now been largely supersededby ethylene propylene rubber (EPR) insulation. EPR has similar electrical and physical properties to roundcomoactedstranded conductors XLPEinsulation cold easy strip semi-conducting screenextrudedwith the insulation coppertape cores laid uo with polypropylenefillers extrudedPVC bedding steelwirearmour black PVC oversheath
52 Electrical Distribution butvl rubber but with better resistance to moisture and ozone. It should not, however, be exposedto oils and gteases. Cross-linked polyethylene (XLPE) as shown in Fig. 2.32, is also used as an insulant but has inferior mechanical and thermal properties when compared with EPR. Polyvinyl chloride (PVC) is not generally used for ships' cables, even [houeh it is very common ashore. PVC tend"s to soften and flow at high temperatures (melts at L50'C), and hardens and cracks at low temperatures ( - 8'C). Even at normal temperatures PVC tends to flow and become distorted under mechanical stress - for example necking occurs at cable glands causing the gland to lose its watertight properties. Multicore shipwiring cables have the cores identified by either colour, printed numerals on untaped cores or numbered tapes on taped cores. Extra mechanical protection is provided by armouring with basket-woven wire biaid of eithei galvanised steel or tinned phosphor bronze. The non-magnetic properties of phosphor bronze are pre- ierr'ed for singie-coiecables.A protective outer sheath-of CSP compound covers the wire braid. The wire braiding also acts as a screen to reduce interference (caused by magnetic fields) in adjacent communication and instrumentation circuits. QUESTION Will cable materials burn? QUESTION What is the purPose of the sheathon a cable? ANSWER The sheath of a cable Protects the insulation from damage and injury - it is not classed as an insulant. Sheath materials are required to be heat, oil and chemical resistant and flame retardant (HOFR). The sheath must also be tough and flexible. ANSWER Yes, all organic materials will eventually burn in i severe fire. Cable sheath materials commonly in use are organic plastic compounds that are classed as ilame retardant, i.e. will not sustain a fire. Most cable materials now achieve this property by developing chlorine gas and ^acid ftimes to smother the flame. PVC is notorious for its release of deadly acid fumes, but PCP and CSP do the same. EPR and XLPE do not. Some new materials do not produce acid _fumes when burning - an important feature for fire-fighting personnel. However, burning Cable-materials still tend to produce dense black smoke. Polychloroprene (PCP or neoPrene) is a cornmon sheath material but has been largely superseded by chlorosulphonated pol-yethyl-ene(CSP or hypalon). CSP- kOfn sheathing compound is well suited to shipboard conditions. It offers good resistance to cuts and abrasions, iesists weather and ozone, acid fumes and alkalis, and is flexible. . MIMS cables: Mineral Insulated, Metal Sheathedcables are very useful in high temperature, fire-risk-areas.These cableshave a mag- nesium oxide powder as insulation with a metal sheath - usually coPPer (MICC - Mineral Insulated, Copper Covered) which is further covered with PVC for weatherproofing where necessary_.A special termination is used with MIMS cibles to provide a moisture-proof seal for the hygroscopic insulation powder. For an MieC cable this is achieved by
Electric Cables 53 screw-onbrasspot PVC sleeve coppersheath. compound copperconductor cable sheath screwto pot Fig. 2.33 MICC cable termination. a compound filled brass pot screwed directly on to the copper sheath as shown in Fig. 2.33. The current ratin g oI a cable is the current the cable can carrv continuouslv without the conductor eiceeding 80'C with an ambient air temperature of 45"C (i.e. a 35oC rise). This rating must be reduced (de-rated)if the ambient exceeds 45"C, or when cables are bunched together or enclosed in a pipe or trunking which reduces the effective cooling. MICC cable current ratings are based upon a copper sheath temperature of 150 rc maximum. For all types of cable the size of conductors required for a particular installation is estimated from current rating tables issued by suppliers. These tables show current ratings for a range of cable types, conductor area and volt-dropiamp/metre. The volt drop in cables from the main switchboard to the appliance must not exceed 6"/" (in practice it is about 2"/o). The cables installed must comply with both the current rating and the volt-drop limitation. Cable volt drop only becomes a problem in very long cables. QUESTTON What is the purpose of a cable gland? ANSWER Cables are insulated, mechanically protected and watertight. They may be armoured and suitable for installation in a hazardous explosive area. A cable gland maintains these properties where the cable is terminated at an appliance, e.g. at a motor terminal box. The cable gland is screwed into the appliance terminal box. Nuts on the gland compress sealing rings to maintain watertight seals on the inner and outer sheaths and to clamp the armour braiding. The gland must be matched to the size and type of cable. A typical Ex-protected gland construction (which
54 Electrical Distribution C o b l e C l o m p Fig. 2.34 Exd cable gland. is more complicated than an equivalent industrial type) is shown in Fig. 2.34. In most cases earthing of the cable armouring is done by the cable gland. Where cables pass through watertight bulkheads and fire-stop barriers they must be specially glanded to maintain the integrity of such bulkheads. Conductor termination sockets can be solderedto the conductors but are more frequently cimped onto each wire by a compressiontool. Cablesocketsmust be securelvattached to the appliance terminal screw by nuts and shakeproofwashers A loose terminal will invariably become a source of localised overheating. Periodic main- tenance should always include checking the tightness of terminal connections. Small cables are terminated in terminal blocks of various designs. Cablesshould be periodically inspected and tested, ideally ^when chectinf their connected appliances. Cable insulation resistance should be measured and the value recorded. Cables in exposed and damp situations, E.g. for deck lighting, may develop a low insulation resistance. Usually this is a result of mechanical damage or a faulty gland permitting the ingress of water. Cablescan be dried out by injecting a heating current from a current injection set or a welding transformer as shown in Fig. 2.35. The procedure requires care not to overheat the cables which could cause further damage. The cable should be disconnected at both ends from equip- ment, and connected as shown. The injection cables must have good connections at each end. Current flow and cable temperature should be carefully monitored. When satisfactorv insulation values have been restored, a final check should be made with the cable at normal ambient temperature. The injected heating current must neoer exceed the rated current for the cable - it is advisable to use an ammeter
Electric Cables 55 Supplyfrom WeldingSet Short-circuit Link Two-coreCable Supplyfrom WeldingSet Fig. 2.35 Cable dry-out connections. and to start at the lowest available setting on the injection set. The voltage should be in the region of 30 to 55 V depending upon the current setting. The cable temperature can be measured with a contact thermometer secured to the cable or with an infra-red sensor and should not be allowed to exceed a temperature rise of 30'C. Temperature and insulation resistance should be measured and recorded every hour. When the insulation resistancebecomes steady the heating should be carried out for a further four hours before switching off. Final readings of at least 20 MO to earth and 100 MQ between cores should be expected. Mechanical damage to cables must be made good either by repairing the damage or replacing that section of cable. Unprotected metal armouring and insulation material are vulnerable to attack by moisture, chemicals and corrosive gases, while exposed live conductors are obviously dangerous. A temporary repair may be effected by preparing and binding the damaged section with a suitable adhesive plastic electrical insulating tape. Such a repair wtll not be acceptablein a hazardous zone on a tanker. Permanent cable repairs must be made as soon as possible.
5 / Chapter Three Generators and Main Circuit Breakers 3.0 Inhoduction 3.L AC Generator Operation 3.2 Generator Construction and Cooling 3.3 Excitation Methods 3.4 Automatic Voltage Regulation 3.5 Generators in Parallel 3.6 Emergency Generators 3.7 Generator Protection 3.8 Generator Maintenance 3.9 Main Switchboard 3.1,0 Main Circuit Breakers Page 57 57 51. 65 68 70 75 76 78 79 80 3.0. Introduction The electrical power demand aboard ship will vary according to the ship type (tanker, bulk carrier, ro-ro, container, fercy, cruise liner, offshore support etc.) and its day-to-day operational needs (at sea or in port). To meet the power demand, two or more main generators are used which are backed rp by an emergency generator and an emergency battery service. The construction, operation, protection and maintenance of generators is described together with a review of main circuitbreakersand the main switchboard. 3.L. AC Generator Operation Main generator power ratings range from, typically, 100 kW to 2 MW at 440 V, 60 Hz a.c. or 380 V, 50 Hz a.c. driven by diesel, steam turbine, gas turbine or propulsion shaft-driven prime movers. As the demand for increased electrical power installations arise (e.g. for specialist offshore vessels and cruise liners) it is necessary to generate at high voltage (HV) with voltages typically at 6.6 kV, 60 Hzbut 3.3 kV and Ll, kV are also used. An emergency generator, typically 20 kW to 200 kW at 440 V or 220 Y,
58 Generators and Main Circuit Breakers Fig. 3.1 Principle of generator operation. will be diesel driven and fitted with an automatic start facility. Battery supplies -from lead-acid or alkaline cells, usually rated at 24 V d.c., provide sufficieni power for the emergency alarm and communication systems together with some lighting and power essentialfor safety during a main power failure. As the vast majority of ships use alter- nating current (a.c.) generators (some times called alternators), the principles and operational features will cover this type only, and ignore the direct current (d.c.) type. The basic principle of an a.c. generator is very simple. Pairs of electromagnetic poles are driven (by the prime mover) past fixed coils of wire on the stator as shown in Fig. 3.1. An alternating electromotive f.orce (emfl which, ideally, has a sinusoidalwaveform, is inducedinto each stator phase winding. useful emf level (E) is called the mean square (rms) value and all equipment is rated in rms terms..A peak, or maximum, level is 1.4L4 (V2) times larger than the rms level. e.g. if E i s 4 4 0 V , t h e n E m e x : L . 4 L 4 x 4 4 0 : 622 ztolts. The size of emf generated depends on the strength of magnetic flux (@) and the rate at which this flux cuts the coils, so ai@where n is the rotational speed of the rotor poles in rev/s. The voltage available at the generator ter- minals is V: E - (l.Z) [phasorcalculationl where I is the load current flowing in the stator phase windings. An internal phase volt-drop of (l.Z) occurs due to the impedance Z of a phase winding which is made up from its resistance and reactance. The frequencyf (measured in Hertz) of the emf is the number of waveform cyclesper second.This obviously depends on the rotational speed and the number of poles,so Fi4 or f : (N/60).p where n: speedin rea/s,N : rea/min and p: pairs of poles.Related speeds and The root
AC Generator Operation Sg pole-pairs (p) for 60 Hz rea/min(N) for 50 Hz rea/min (N) 1 3600 3000 2 1800 1500 J 1200 1000 4 900 750 frequencies with F,"."yqber of pole_pairs are given in the table below: r The rated values of a machine always refer to line conditions 1us stuted or, rating plate). _. Angle @ is the phase angle between Vpp and lpa which'is deternii""J Uy tnu !yp"r_of electrical load on the generator (e.g. lighting, motors, galley efrurpment etc.). , C.ord i-slhe poToerfactor of the electrical rolg.and is typically about 0.g laeeine whrch means that the current *urrif6rrfi lags about 37. behind the voltage. QUESTION Il^" t9*"r,fu.l9r. metershownin Fig.3.2nas rts scale divided into /or.rrsegments- each calibrated 0-1.0.'What'i, th" significance of each segment? ANSWER An indication in the top half of the scale shows that the .ma;h.ihe is _gin;rating.The bottom half of the scale indicat8s 11"j .*, generator is motoring. noth top and bottom halves are fu-rther split 1119. tug8ing and leading po*", factor sections. These two basic relationships for emf a,nd frequency dictate how'to control tne voltage and frequency output of a generator. In practice ine speed is maintained praciically constani bv the generator's prime_mover whicn fixds ifre .o;^?n^t,,11!uency. The constant speed rnen ailows the size of generated'emf to,be directly controlled 6V tn" size of pole ilux (excitation) A- practical a.c. generator has three sets of coils, called fhase windings, located in slots in the stator surrou"nding the rotating magneticpoles. The emf induced rn each phase is 120" out of phase with tne other two .phas-es.Three_phase windings -are lab-elled as U_V_iry with colour coding of red, yellow and blue used on terminals and bus_bars.One end of each of the three-phase*i"ai"g, are joined together to form the neutral potnt or a star connection. The other ends of the phase windinss are connected to outgoing conductois called lines. The three output line. voltages (repre_ sented by V.) and the 3 Sitput tine currents (represented by l1) corirbine to create the three_phase electrical power output of: P : J3.Vr.Ir.cos6watts Fig. 3.2 Power factor meter. Vy is m3de up from two p In a star connection, arry line voltage :]-: -,1d" "gjlg- two pirase voltages, voltage where Vr: J3.Vpu.Th; Vs r;.?-i, due to the 120. diiplacem"rrt b"t*""r, phasevoltages.e.g. if Vr:iiO-i, thrn Vpu: 254 V.
60 Generators and Main Circuit Breakers A three-phase a.c. generator rated at 500 kW, M0 V at 0.83 lag will deliver a full load line current of: P- = - : - J5. vr.cos| This means that the phase windings, cable conductors and generator circuit breaker must be capable of carrying this full load current (FLC) continuously without exceeding their temperature limits. QUESTION If the above 500 kW generator circuit- breaker is protected by an over-current relay (OCR) setting oi \25"/o what will be the actual minimum tripping current level? ANSWER The full load line current the generator overcurrent at: 1.250/"x 790.5: 988A is 790.5 A so relay will trip 500,000 J 3 . 4 4 0 . 0 . 8 3 : 790.5 A The speed of an auxiliary diesel driven generitor (DG) is accurately manqgei 6y an electronic fuel governor which maintains an almost constant output frequency over its load range. A propulsion-shaft driven (9G) generaior- can be an efficient method ior extracting electric power from the ship's main engine as the Power is derived from lower cost fuel than that used for an auxiliary DG unit. The SG mav be fitted directly inJine with the slow speed propulsion shaft or, more commonly, be gear-driven uP to a higher speed. 440V 60Hz Shaft Generator three-phase bridge rectifier three-phase controlled inverter d . c . link P----* Ship's Load B - O frequency :"::,=d\-'L-xvn,--- 3 \-rt)rr""n;iifcontrol \-| | comPensatorl l Fig. 3.3 Shaft-driven generator control.
Generator Construction and Cooling 61 Also, by using a shaft generator as the main source of electric power during long sea passages,the DG units operate for short periods only with a reduced maintenance requirement. An apparent disadvantage of a shaft generator is that it has no direct frequency control as this is determined by the main engine which is set for the ship's full-away speed range (e.g. 70-100"/0). This means that the frequency must be separately regulated at the output of the shaft generator to maintain a constant 60 Hz to the ship's electric power consumers. Such a frequency regulator utilises an electrica.c.-d.c.-a.c.converter as shown in Fig. 3.3. At the three-phase rectifier stage the a.c. generator frequency is converted to a d.c. voltage. The three-phasecontrolled inverter converts the d.c. back to a ftxed output frequency by sequenced thyristor switching. A d.c. link inductor coil is interposed between the rectifier and in- verter to smooththe normal current flow and act as a current-limiter in the event of a short circuit fault. An inverter thyristor switch is turned- on by a positive current pulse to its gate when its anode is positive with respect to its cathode. The thyristor is onlv turned-off when its curreht is reduced tb (approximately) zero. This is a problem for the inverter thyristors when driving into the ship's inductive load (typically about 0.8 power factor lagging). In this case the current continues to flow in a thyristor after its voltage has gone through a zero point causing disruption of the inverter switching sequence. To overcome this problem it is necessary to have the thwistor current in-phase-with its voltage so that turn- off is automatically achieved (line commutation) at the end of each a.c. half-cycle. The addition of leading kVAr compensation to the power system to create an overall unity power factor solves the problem. Hence, the SG/ converter must only supply true power P (kW). At every instant the leading kVAr ( + Q) must exactly match the lagging kVAr ( - a) of the ship's load so the compensation must be automatically controlled. The practical solution is to include a synchronous motor, operating as a synchronous compensator, whose operating power factor is controlled by regulating its d.c. field current. Overall, the bus-bar voltage is fixed by the field flux in the shaft generator and the bus-bar frequency is regulated by the controlled inverter. 3.2. Generator Construction and Cooling o Construction The two main parts of any rotating a.c. machine are its stator and rotor. The fabricated steel stator frame supports the stator core and its three phase windings as shown in Fig. 3.4. The stator core is assembled from laminated steel with the windings housed in slots around the inner periphery of the cvlindrical core. Th"e stator coils are interconnected (in the end-winding regions) to form three separate phase windings with six ends.These phase ends are found in the stator terminal box as shown in Fig. 3.5. In some cases only three terminals are available in the terminal box. In this case,the neutral or star point connection is an internal part of the stator winding arrangement. The main outgoing cables connected to these terminals conduct the generator's electric power to its circuit-breaker at the main switchboard. The rotor of a main a.c. generator provides the field excitation from its electromagneticpoles. Two constructional forms of rotor are available as shown in Fig. 3.6.
62 Generators and Main Circuit Breakers Emergency Air InletPanel StatorCore (WithAir Ducts) Emergency AirOutlet Doors PilotExciter Main Exciter Diode Plate HeatExchanger Stator Endshield Air Flow Fanshield Fig. 3.4 Generator construction. TO GEN. C I R C U I T BREAKER Fig. 3.5 Generator terminal box. . salient pole type . cylindrical type The salient pole type poles bolted or keyed hub. Field excitation windings are fitted around each pole. This type of rotor is used with medium and slow shaft speeds (1800 rpm and below) and is the most common arrangement for marine generators. has projecting onto the shaft
Generator Construction and Cooling 63 Inter-poleConnector ExcitingWinding PoleShoe RotorBody ExcitingWinding in Slot SALIENTPOLE (4 - POLE) End Windings Fig. 3.6 Generator rotors, salient and cylindrical construction. Cylindrical type rotors are generally used with large power, high speed (1500-3600 rpm) steam/gas turbine drives. The excitation windings are wedged into axial slots around the steel rotor. Unwound sectionsof the rotor form the pole facesbetween the winding slots. The shaft bearings of large generators (and motors) are usually insulated to prevent stray currents from circulating through. Unbalanced(stray) end-winding magnetic flux induces an eml along the steel shaft. This will cause a current to circulate through the shaft, bearings and bedplate to produce arcing across the bearing surfaces and degradation of the oil layer. Under generator unbalanced fault conditions the bearing problem mav be severe. To prevent the flow of shaft current, one bearing (usually the non-drive end) is electricallv isolated from earth bv a thin layer of insulating material beneath the bearing pedestal. The pedestal holding-down bolts must alsobe insulated by suitable sleeving. In normal operation the effectiveness of the pedestal insulation can be checked by measuring its voltage to earth which may show as a few volts. The rotor poles are supplied with direct current (d.c.) from an exciter. If the exciter equipment is a conventional d.c. generator or is static (see section on excitation methods), the d.c. excitation current is fed into the field windings via carbon brushes on a pair of shaft- mounted slip-rings. To eliminate the maintenance problems associated with rotating contacts, a brushlessarrangement is usual for marine generators. All brush gear, commutators
64 Generators and Main Circuit Breakers Diode Flexible leads DIODEPLATE Fig. 3.7 Rotor diode plate, and slip rings are eliminated by using an a.c. exciter with its output being rectified by shaft-mounted silicon diodes as shown in Fig. 3.7. The diodes are connected as a three phase a.c./d.c. bridge circuit. The six diodes, mounted on the shaft, convert the a.c. exciter output to d.c. which is then fed directly into the main generator rotor field windings. Note, the a.c. exciter has its own d.c. field poles fitted on its stator while the rotor carries its three-phase a.c. exciter output windings. This construction layout is inverted compared with that of the main generator. o Cooling Power losses, typically 10"/" of the generator rating, cause internal heating in the windings and magnetic cores of both rotor and stator. This heat must be continuously transferred out of the generator io prevent excessive temperature rise causing breakdown of winding insulation. Forced air circulation in a closed circuit (to prevent ingress of dirt) via an air cooler is pressurised by a fan on the rotor shaft: Cooling air is forced through ventilation ducts in the stator core, between rotor poles and through the air gap (a few millimetres) between stator and rotor. Water cooling of the circulating air may also be used for generators with a large power rating. Temperature detectors (resistance type, thermistors or thermocouples) are used to monitor the temperature of stator windings, bearings and the cooling air/water of the generator. Single or grouped temperature alarms are activated at the main watchkeeping position. While the generator is stopped during standby or maintenance periods, low power electric heaters within the machine prevent internal condensation forming on the winding insulation. These heaters may be switched on manually or automaticailv from auxiliarv contacts on the generaior circuit-bre-aker.Heater power supplies are normally 220 V a.c. single-phase supplied from a distribution box local to the generator. QUESTTON The water cooling system on a large generator is out of service due to a faulty inlet valve. How will this affect the generator operation? ANSWER The generator can only be used to supply a much reduced electrical power output to keep the machine temperatures below their maximum permitted levels. External emergency doors in the generator's air cooling ducts may be opened in such cases. The penalty is that the normally closed air circuit of the generator is now open to the engine room atmosphere.
Excitation Methods 65 3.3. Excitation Methods The two f actors essential for the production of a generated emf in an a.c. generator are rotational speed (n) and magnetic flux (@). Field windings on the rotor create strong magnetic field poles when direct current is passed through them. Various methods have been devised to supply the correct d.c. field (excitation) current to produce the required a.c. output voltage from the stator terminals. The excitation must be continually regulated to maintain the generator output voltage as the load power demand fluctuates. Broadly, the excitation methods are either rotary or static. A rotary method utilises an a.c. or d.c. exciter which is shaft-mounted and rotates with the main generator rotor. Traditionally, rotary exciters were d.c. generators with stationary field poles, rotating armature, commutator and brushgear. Now the most common arrangement is to use a shaft mounted a.c. exciter. In some applications, a small additional rotary pilot exciter may be used to supply current to the main exciter field. A pilot exciter is a small permanentmagnet a.c. generator which is driven from the generator shaft. Its output voltage is generally at a high frequency (e.9. 1000Hz) but this is rectified to d.c. before being fed into the main exciter field. A "brushless" excitation scheme is shown in Fig. 3.8. The absence of brushes, brushgear and carbon dust improves reliabilitv and considerablv reduces generatof maintenance. Rectification of the a.c. exciter voltage is achieved by six shaft-mounted silicon diodes. The suppressionresistor connectedacrossthe main generator field protects the diodes against voltage surges arising from sudden changes in excitation current. QUESTTON What is likely to happen if one of rotating diodes fails and becomes: (a) an open circuit? and (b) a short-circuit? the GeneratorFrame RotatingElements Fig. 3.8 Brushlessexcitation scheme.
66 Generators and Main Circuit Breakers (a) ANSWER the remaining healthy diodes would continue to supply the main field. In manual (hand) control the total field current, and hence generator voltage, will be slightly reduced. Under AVR control, the exciter field current would be automatically boosted to maintain the correit generator voltage while the diode failure would probably be un- detected. The exciter will gradually overheat. a short-circuited diode is more serious as it leads to a short-circuited exciter. Rapid overheating of the exciter will occur. Although diode failures are rare, some generator field systems are fitted with an electronic detector relay to give an alarm and/or trip signal should such a fault occur. Usually, the detectormonitors the exciter field current whose size and shape are noticeably affected by a diode failure. Generators with rotary exciters, con- ventional or brushless, have a relatively sluggish response to sudden load changes. For example, it may take typically up to one second to correct a 15"/" aoltage dip caused by the start-up of a large pump motor. Inductance of main rotor field winding Inductance of exciter field winding Regulator (manual or automatic) resPonse (b) QUESTTON What factors govern the overall response of a generator to (transient) load changes? The transient voltage response of a generator can be improved by eliminat- ing the rotary exciter in favour of a static excitation method. In this arrangement, the generator field draws its d.c. current via a static excitation transformer/rectifier unit fed directly from the generator voltage and current output. This arrangement is known as compoundingas it is controlled by voltage (shunt effect) and current (series effect) feedback. Responsetimes as low as 0.1 second to correct a 15o/ovoltage dip are common with static excited compoundgenerators. This fast response is most desirable on general/bulk cargo ships where heavy and frequent load surges arise from deck cranes and winch gear. Such static excitation equipment may be located within the generator casing or inside the main switchboard. This type of generator has two shaft slip-rings and brushgear to connect the static excitation equipment to the rotor field winding. The basic scheme of a self-excited compoundedgeneratoris shown in Fig. 3.9 (single-phase operation is shown for simplicity). Note, compounded means that the excitation is derived from the generator output voltage and its current. On no-load, the generator excitation is provided by the PRI.1 winding of the exiitation transformer. On load, the generator current injects an additional excitation current via PRI.2 of the transformer to maintain a constant output voltage. If the excitation components are carefully designed, the generator voltage of a compounded generator can voltage s u d d e n ANSWER The main opposition to field current required generator output voltage changes in to correct are: the the
Excitation Methods 67 Stator AU Main Breaker i SlipRings : & Brushes ; i vI - r SEC. i A Fig. 3.9 Single-phase compound excitation circuit. Reactor Coils Capacitors islip Rings : & Brushes i Fig. 3.10 Three-phase compound excitation circuit.
68 Generators and Main Circuit Breakers be closely maintained at all loads without the use of an AVR or manual voltage trimmer. However, some generator manufacturers do include an AVR and a manual trimmer rheostat in such a compounded static excitation scheme. This addition may provide closer voltage regulation over the-load range and allo"w manual control of the generator voltage, e.g. for synchronising and kVAr load balancing between generators. A practical 3-phase static excitation scheme has additional components such as reactors and capacitors. The circuit in Fig. 3.10 has no AVR or manual trimmer regulator. A load current surge will automatically feed back an adjustment to the field excitation to correct the resulting voltage surge so quickly that the output voltage remains practically constant. Compound excitation systems require the static components to be designed to closely match its associatedgenerator. 3.4. Automatic Voltage Regulation Sudden load current surges (e.g. due to motor starting) on a generator cause a corresponding change in its output voltage. This is due to an internal voltage drop in the generator windings and the effect is usually called voltagedip. Similarly, load shedding will produce an oaervoltageat the bus-bars. An un- regulated or non-compounded generator excitation svstem would not be realistic on board ship due to the varying voltage caused by the fluctuating load demand. Automatic voltage regulation (AVR) equipment is necessaryto rapidly correct such voltage changes. See Fig. 3.11. An AVR will control the generator's voltage to +2.5"/o (or better) of its set value over the full load range. This is its steady-stntev oltage regulation.Transient voltage dip is usually limited to 15% for a specified sudden load change with recovery back to rated voltage within 1.5 seconds. In special cases where unusually large surges are expected (e.g. from heavy-duty cargo cranes) the generator/AvR performance limits may be extended. The AVR senses the generator output voltage and acts to alter the field current to maintain the voltage at its set value. A manual trimmer regulator may be fitted on the generator control panel to set the voltage level e.g. 440 V. More usually, the voltage trimmer potentiometer is voltage -*:fl{ \ unregulated (noAVR) time 100o/o+l- 25% Fig. 3.11 Generator/AVRvoltage response.
Automatic Voltage Regulation 69 a ,r- -,/-r, externalmanual',-7 - - i "trimmer"(if fitted) f - - - - { - . - Thyristor "set"voltage AVR UNITControl o t "... supplyfrom I rotaryor static I exciter d.c.field current 440 V, 60 Hz 3-pha.c. output Fig. 3.12 AVR block diagram. on the control card of the AVR so is not accessibleto an operator. The control circuit for a modern AVR consists of transformers, rectifiers, zener diodes, transistors and thyristors. These are mounted on one of more circuit cards fitted either within the switchboard or local to the generator. Although the_AVR control circuit design varies with the manufacturer scheme contains the following shown in Fig. 3.12. A thyristor is a fast-acting electronic switch controlled by a voltage signal at its gate terminal. This device rectifies and regulates the generator field current. Additional components and sub-circuits are included in the AVR to ensure: o Rapld response time with voltage stability o Fair current (and kVAr) sharing when generators are to be operated in parallel o Quick voltage build-up during generator run-up o Overvoltage/undervoltage alarm/trip protection The complete AVR circuit is fairly com- plex and includes a few pre-set variable resistors for the control of sensitivity, offset-error and stability (proportional, integral and differential- cohtrot;. These the basic elements The voltage sensing unit transforms d.own,rectifiesand smooths the generator output voltage. This produces a low voltage d.c. signal tha[ is proportional t-o the a.c. generator voltage. This actual d.c. signal is compared with a set d.c. value produced by ^ reference circuit of zener diodes and resistors. An ercor signal output from the comparator is then amplified and made stiitable for driving . the field circuit regulating thyristor(s).
70 Generators and Main Circuit Breakers are adjusted and set during generator trials to achieve an optimum and stable performance. It is recommended that you resist the temptation to fiddle with such pre-set controls unless fully competent with such a feedback control system. AVR running checks, as guided by the manufacturer, consist of a.c. and d.c. voltage measurements at installed test points. These are compared with values found acceptable during previous generator trials. The test voltmeter type and its range are usually specified for each test. Most ships will carry a spare AVR unit or spare cards which may be interchanged after a suspected failure. An AVR changeover should only be attempted when its generator is stopped and locked off. Checks at the test pbints on the new AVR excitation field current level and the manual regulator operation (if fitted) should be proven with the generator running on no-load before attempting to synchronise on to the bus-bars. When generators are load sharing in parallel, check for approximately equal current (or kVAr) sharing between the machines. This will indicate correct operation of their AVRs. QUESTION What precaution must be taken when testing the insulation of generator cables and wiring connected to an AVR unit? ANSWER Electroniccomponentssuch astransistors, integrated circuit chips, thyristors, etc. generator and its cables to earth and protect the electronic parts, either: Short-circuit all outgoing cable terminals during the IR test Remove electronic card(s) Disconnectall cablesat both ends and test separately 3.5. Generatorsin Parallel Main generator units (gas-turbine, steam turbine or diesel drives) have to be run in parallel to share a total load that exceedsthe capacity of a single machine. Changeover of main and standby generator units requires a brief parallel running period to achieve a smooth transition without blackout.For simplicity and security it is not normally possible to run a main generator in parallel with either the emergency generator or a shore supply. Circuit breaker interlocks are used to prevent such an arrangement. Essentially, parallel running is achieved in the two stagesof: . Synchronisingthen Load Shaing Both operations are, of course, usually carried out automaticallv but manual control is still in common use and is generally provided as a back-upto the auto control. The generator already on-the-barsis called the running machine and the generator to be brought into service is the incoming machine. are likely to be damaged during a high voltage (500 V) megger test. To test the To smoothly parallel the incomino generator, it must be synchronisedwith the live bus-bars.
Generators in Parallel 7'1. ANSWER At the instant of closing the breaker, the voltage phase difference causes a large circulating current between the machines which produces a large magnetic force to pull the generator voltages (and field poles) into synchronism. This means rapid acceleration of one rotor and decelerationof the other. The large forces may physically damage the generators and their prime-movers and the large circulating current may trip each generator breaker. Result? Blackout, danger and embarrassment! To achieve smoothmanual synchronising, the incomer must be brought up to speed to obtain approximately the same frequency as shown on the bus-bar frequency meter e.g. 60 Hz. Fig. 3.13 Two generatorsto be synchronised. QUESTTON What are the likely consequencesof attempting to close the incomer's circuit breaker when the generator voltages are not in svnchronism? R B incomer bus-bar volts synchroscope tncomer frequency incoming generator bus-bar frequency SYNC PANEL o l-sT[C-lI H= I |""i" Illl\ ll"r". I l(fi;t)llVll(.'iil)l i{t5_ffimsffi, l o l EIF Monitor t - ; l f f i l = = = l& d 1 " " " 1 Gen Alarms Selector o Switch Fig. 3. 14 Synchronising instruments.
72 Generators and Main Circuit Breakers The incoming generator voltage is set by its AVR or manually trimmed (if available) to be equal to the bus-bar voltage. Fine tuning of the speed can now be observed on the synchroscopeor syn- chronisinglamps.The incomer is adjusted so that the synchroscopeindicator rotates slowly clockwise (fast direction) at about 4 secondsper indicator revolution. The circuit-breaker should be closed as the indicator approachesthe 12 o'clock (in-phase) position. Breaker closing between S-to and S-past the 12 o'clock synchroscope position is satisfactory as long as the pointer rotation is fairly slow. o Lamps bright method (2 lamps) . Sequencemethod (3 lamps) In each case the lamps are connected between the incoming generator and the bus-bars. The sequencemethod, as shown in Fig. 3.15, is preferred as it displays a rotation of lamp brightness which indicates whether the incoming machine is running fast (clockwise) or slow (anti-clockwise). As with the synchroscope, the lamp sequence must appear to rotate slowly clockwise. Correct synchronisation occurs when the top or key lamp is dark and the two bottom lamps are equallybright. QUESTION What indication is available to show the optimum synchronised condition? ANSWER The incoming generator ammeter pointer will show very little kick when correctlv svnchronised.' A traditional pointer-type synchroscope is usually short-time rated (e.9. up to 20 minutes) to avoid overheatirg - do not forget to switch it off after a paralleling procedure. Modern synchroscope indicators use a circular set of LED's (light emitting diodes) which sequentially light up to show the phase difference between the generator voltages. As a back-up, or alternative, to the synchroscope a set of lamps may be used. The correct synchronised position may be shown by either of the following methods: o Lamps dark rrrethod (2 lamps) Fig. 3. L5 Synchronising with three lamps. QUESTTON How could vou monitor the correct instant for synchronising without the aid of a synchroscope or synchronising lamps? ANSWER Connect a voltmeter as shown in Fig. 3.16, (expect up to 500 V on a 440 V system) R Y B
Generators in Parallel 73 A check-synchronisingunit has an electronic circuit to monitor the voltage, phase angle and frequency of the incoming generator with respect to the bus-bars. Circuit breaker operation is initiated by the watchkeeper but the check-synchronising monitor only allows a permit-to-closesignal when all the synchronising conditions are within acceptablelimits. This method provides a useful safeguard against operator error but retains overall watchkeeper control for adjusting the voltage and frequency. Auto-synchronising of an incoming generator does everything an operator would do - senses and controls the voltage and frequency then initiates a circuit-breaker closesignal at the correct instant. The auto-synchronising equip- ment uses electronic circuits to monitor the size of voltage, frequency and phase angle difference, then acts to regulate them until they are equal to the existing bus-bar conditions. Usually, the check or auto synchroniser units are switched between a set of generators as and when required. When an incoming generator has been successfully synchronised the synchronising equipment should be switched off. Fig. 3. L6 Synchronising with a voltmeter. across one pole of the open incoming generator circuit breaker. This procedure is more easily (and safelv) performed at the synchroscope terminals behind the door of the synchronising panel at the front of the main switchboard. Check the circuit diagrams before such testing. Adjust the generator speed until the voltmeter oery slowly fluctuates from zero to maximurn. Close the breaker when the voltmeter indication passes through zero. Note, for this operation, an analogue (pointer and scale)meter is easier to follow than a digital type. b u s - b a r- - - - - - - : loadsharingand power factors Fig. 3.17 Generator load-sharing.
74 Generators and Main Circuit Breakers The total bus-bar load can now be shared between generators or totally transferred to the new machine. In parallel operation, a generator governor directly controls Power (kW) while its AVR trimmer or manual voltage regulator controls ReactiaeVolt Amps (kVAr) or poroerfactor. Manual kW load sharing is achieved by raising the governor setting of the incoming machine while lowering the setting on the running machine. The balanceof power sharing is dictated by the goaernor (speed)droop of each generator prime mover. Current (or kVAr) sharing is set by the aoltagedroop of each generator AVR. See Fig. 3.17. For equal load sharing of kW and kVAr, each machine must have similar droop characteristics which are typically 2-4o/o between no-load and full-load values. An overall balance of load sharing for kW and kVAr can be seen by comparing the power factor (cosg) meters of each generator. QUESTTON Two generators are load sharing in parallel: rb) tandr - Q' - 350 : 0.875 P2 400 s o 6 z : 4 1 . 2 " then,p.f2: cosrf2: cos41.2" :0.75 Lagging T o t a l P : 5 0 0 + 4 0 0 : r y andTotalQ : 375+ 350: 725kVAr (c) : 0 . 8 L a n dQ : 3 8 . 9 " so,oaerallloadp.f. : cos38.9" :0.78 Lagging Auto-load sharing equipment is yet again more black-boxelectronics. The circuits comparethe kW loading of each generator (via CTs and VTs) and any difference is used to provide an error signal to raise/lower the governor setting of each prime mover as necessary. Such equip- ment is usually trouble-free, requiring little maintenance other than an occa- sional visual inspection, cleaning and checking the tightness of connections. Manual load sharing is the obvious fallback if the auto-control equipment fails. QUESTTON Two generators are load sharing equally in parallel when a total loss of excitation occurs in No. 2 machine. What is the likely outcome? ANSWER Generator No. 2 will run as an induction generator drawing its excitation kVAr from No. 1. Both generator currents will rise rapidly with No. 1 becoming more lagging while No. 2 runs with a leading p.f. (indicated on cosS meter). A loss o 725 OaeraIItan@: = : -' P 9 0 0 Generator 1 delivers power factor lag, and Generator 2 delivers 350 kVAr lag. Calculate: (a) the kVAr loading of Generator 1 (b) the p.f. of Generator2 (c) the total bus-bar loading in kW, kVAr and power factor ANSWER ( a ) c o s $ 1 :0 . 8 s o 6 r : 3 6 . 9 " now (from PQSpoznertiangle): Q: P.tanbr: 500.tan36.9": 375kVAr 5 0 0 k W a t 0 . 8 400 kW and
Emergency Generators 75 of excitation trip (if fitted) or the over- current relay should trip No. 2 generator probably causing an overload on No. 1. Alternatively, No. 1 trips on overcurrent which deprives No. 2 of excitation and its breaker trips out on undervoltage. Result - total power failure ! 3.6. Emergency Generators The power rating of an emergency generator is determined by the size and role of the ship. On some small vessels a few kW will suffice for emergency lighting only. Larger and more com- plicated vessels, e.g. LPG carriers, passenger liners, etc., may require hundreds of kW for emergency lighting, re-starting of the main engine auxiliaries and to supply fire-fighting pumps. The construction and operation of an emergency generator is similar to that of a main generator. Excitation supplies, either static or rotary, will usually be governed by an automatic voltage regulator. In some cases where a static compounded exciter provides a reason- ably constant generator voltage, the AVR may be omitted. Generally, the emergency generator output voltage is at the same level as that of the main generators, €.8. 440 V, 60 Hz,3-phase a.c.. In an HV/LV system e.g. 6.6 kVl440 V, the emergency generator will usually operate at 440 V and the emergency switchboard will be interconnected with the Engine Room M0 V switchboard in normal operation. However, smaller emergency generator sets may deliver power at 220 V 3-phase a.c. or even single-phasea.c. for lighting and essential navigation aids only. An emergency generator is connected to its own emergency switch- board and they are located together in a compartment above the water-line, e.g. on the boat deck. In normal operation the emergency board is supplied from the main board by u cable called the interconnector. It is not normally possible to syn- chronise the emergency and main generators. Special interlocks in the control circuits of the circuit-breakers, at each end of the interconnector,prevent parallel running. Starting of the emergency generator prime mover is generally automatic.The run-up is initiated by an electrical relay which monitors the normal voltage supply (e.g. aaOQ. Falling mains frequency or voltage causes the start-up relay to operate the engine starting equipment. The prime mover may be electrically cranked from its own 24 V battery ind starter motor or air started from its own air reservoir fitted local to the generator engine. A manual start-up may be initiated by push buttons in the main control room and in the emergency generator room. Small generator prime movers can usually be manually cranked with a starting handle. Correct functioning of the auto-start equipment is obviously vital to the production of emergency power. Weekly testing of the emergency generator should include simulation of the loss of normal power. The start-up equipment may provide a push button to interrupt the normal voltage supply to the control panel which then triggers the start-up sequence. Loss of main power supply can easily be simulated by pulling a fuse in the auto start panel which supplies the under-voltage or under-frequency relay. Emergency generators should be regularly checked and run up to speed for short test runs to comply with safety regulations. These no-load running checks should, when practicable, be supplemented occasionally by an actual
76 Generators and Main Circuit Breakers load test. This requires the disconnec- tion of normal mains power from the emergency board while the emergency generator is loaded up to near its rated value. Only a proper load test will prove the performance of the emergency generator, its prime-mover, and the circuit-breaker operation. 3.7. Generator Protection Apart from direct temperature measure- ment of the stator windings and the internal air, the protection of a generator is largely based on the sensing of current and voltage from CTs and VTs. The number and type of protective relay functions increases with the generator kVA rating and voltage level. Protective relays are electromagnetic (traditional) or electronic (increasingly more common) which are mounted on the generator front panel of the main switchhoard. Some protective functions may be grouped together within a single relay case. Settings for level and time-delay must be periodically checked by injecting currents and/or voltages directly into the relay (usually via a special multi-pole socket adjacent to the relay and internally wired to it). Also see Chapter Two for general circuit protection methods. Some typical relay types employed for generator protection are outlined in Fig. 3.18. tr ocrT The Over Current Inverse Time relay function monitors general balanced overloading and has current/time settings determined by the overall protective discrimination scheme. N E R (HV system) > l D | F F .I Generator Circuit Breaker FaultTrips Fig. 3.18 Generator protection scheme.
Generator Protection 77 Typical setting ranges for current (I) and time (t) are: I>:0.7-2.In, (In : normalor rated generator current) and t: 1-10s o oc(rNsr.) "Instantaneous" trip to protect against extremely high overcurrent caused by a short-circuit fault. Typical setting ranges are: I > : 2-10.In, and t: 0.1-1s D NPS A Negative Phase Sequence relay deter- mines the amount of unbalancein the stator currents which is an indirect measure of the generator stator and rotor temperature. A relatively small degree of unbalance causesa significantly increased temperature rise so the NPS current setting is low at around 0.2.In. D DIFF This is a differential measurement of current at each end of a stator phase winding. This comparison of current is to detect an internal fault in the stator windings which may be caused by partially short-circuited coil turns and/or earth faults. Current settings for this very serious fault are very low e.g. about 0.1.In. t r E L An Earth Leakage relay (sometimes called Zero Phase Sequence)detects an earth fault current returning back through the earthed neutral connection. In a ship's HV generator system the earth fault current is limited by a high impedance NER (neutral earthing resistor) or earthing transformer so the pick-up current setting is very low, e.g. 1-5A with a time delay of 0.1-0.5 s. D UViOV Under Voltage and Over Voltage func- tions are monitored by these relays with settings of around 0.8.Un and 1.2.Un respectively (Un : rated voltage) with time delays of about 2s. An overvoltage function may not be required in many protection schemes. D UF/OF Under and Over Frequency settings are typically 58 Hz and 62 Hz for a 60 Hz system. t r L O This is the master Lock Out or tripi hand-reset relay responsible for tripping the generator circuit breaker. Its action is instantaneous when triggered by ^ protective relay. It can also be used to trip the generator prime-mover and initiate generator field suppression together with the signalling of an alarm. A R P Generators intended to operate in parallel must have reaersepoToerprotection (RP). A reverse power relay monitors the direction of power flowing between the generator and the load. If a prime-mover failure occurred the generator would act as a motor. The reverse power relay detects this fault and acts to trip the generator circuit-breaker. The pick-up power level setting and time-delay setting are adjustable and are pre-set to suit the prime-mover. If the prime mover is a turbine, very little power is absorbed when motoring and a reverse-powerpick-up setting of 2-3o/o is usual. If the prime mover is a diesel then a setting range of 5-15% is usually adopted. A time delay range of about 0.5-3 s is usual. The RP relay operation is easily checked during a generator changeover. The outgoing generator is gradually throttled down so that it motors causing the reverse power relay to trip its generator circuit-breaker.
78 Generators and Main Circuit Breakers 3.8. Generator Maintenance Regular inspection and the correct maintenance of generators and their associated control gear is essential to prevent failure and inefficient operation. CAUTION; Always ensure that the generator prime-mover is shut down and locked off before you begin any main- tenance. Also ensure that the generator circuit breaker is locked off, auto-start circuits are disabled and electric heaters are switched off and isolated. All wiring to the generator should be inspected for damage or frayed insulation and tightness of terminal connections. Particularly check for signs of oil and water contamination of cable insulation within terminal boxes. Check that the cooling air intake and exhaust openings are not blocked and are free of dirt and dust. Inspect and clean the generator rotor and stator windings by removing dust with a dry lint-free cloth. Low pressure, dry compressed air may be used to dislodge heavier dirt but be careful not to drive the dirt deeperinto the windings. An industrial type vacuum cleaner is very effective for removing dirt from the windings. Use a rubber or plastic coated nozzle on the vacuum cleaner tube to prevent abrasive damage to the sensitive winding insulation. Oil on the surface of winding insulation will reduce the insulation resistance and shorten its life. The oily deposits can be removed by washing the windings with special degreasant liquids. Minor abrasions to winding insulation can be repaired, after cleaning, by the application of a suitable air-drying varnish. Rotor sliprings must be checked for uniform (even) wear and that the carbon brushes have free movement in their boxes. Correct brush pressure can be checked using a pull-type spring balance and compared with the manufacturer's instructions. A pull of around 1-1.5 kg is usual. If the brushes become too short (below about 2 cm) the reduced spring pressure will cause sparking at the slipring contact. Replace brushes with the correct type and bed them to the curvature of the slip rings. This can be done by placing a thin strip of glass paper (not emery paper) over the slip ring with its cutting surface under the carbon brush. Pull the glasspaper around the slip ring until the brush surface has the same contour as the ring. The last few passes of the glass paper should be made in the same direction as the normal rotor direction. Remove all traces of carbon dust with a vacuum cleaner. Generator excitation transformers, AVR components and rotating diodes must be kept free of dirt, oil and dampness. A special contact greaseis used between the diode connectionsto prevent electro- lytic action occurring between dissimilar metals. Check such contactsfor tightness but do not disturb them unnecessarily. Measure the insulation of the stator and rotor windings to earth and between stator phases (assuming that the neutral point is available for disconnection at the terminal box). Rememberto disconnect or short-circuit any electronic circuit components which are likely to be damaged by u high voltage insulation test. Consult the wiring diagrams and the manufacturer's instructions before testing. Record the IR values and note the prevailing temperature and humidity. Compare with previous test results. A minimum IR value is usuallv taken to be 1 MO but a lower value may be acceptable to a surveyor based on 1 kO/volt, e.g. 450 kO or 0.45 MO for a 450 V generator. However, it is the historical trend of the machine IR values which will give a better picture of the insulation condition. Generators with very low IR values (less than 0.5 MO) should be given a
Main Switchboard 79 thorough cleaning then dried out. If the IR has recovered to a reasonable value which has become steady during the drying period, its windings should be covered with high-quality air-drying insulating varnish. Should the IR value remain low during a dry-out, the machine insulation needs to be completely re- impregnated or rewound (generally by a specialistcontractor). After maintenance, no-load running checks should precede synchronising and loading. On load, particularly check for excess temperature rise and load- sharing stability when running in parallel. Finally, if a generator is to be left idle or a long time, make sure that its windings are suitably heated to prevent internal condensation forming on its insulation. As with all electrical equip- ment - dirt, overheating and dampness are the enemy! 3.9. Main Switchboard The central section of the main switch- board is used for the control of the main generators.The switchgear cubicles on either side of the generator panels are used for essential services and flanking these are the grouped motor starter panels. Handles for opening the doors on switchboard cubicles are usuallv linked (or interlocked) to an isolating- switch. This ensuresthat supplies to components in the cubicle are switched off before the door can be opened. Fused isolators are isolating switches that incorporate fuses. The action of opening the switch isolates the fuses so that they can be replaced safely. Fused isolators can also be interlocked with the cubicle door handle. Motor starters frequently incorporate this arrangement. One type of interlocked fused isolator can be completely withdrawn and removed to ensure complete safety when carrying out maintenance on equipment. Maintenanceon fused isolatorsconsists of periodically checking the operating mechanism. Contacts must be inspected for damage and lightly greased with an electrical lubricant. The interlock mechanism (if fitted) should also be examined for correct and safe operation. A typical layout of switchboard is shown a ship's main in Fig. 3.19. 440V MainSwitchboardOutline n uutlo Ut]U! I F F A - - - - - l l l l l l l l - - - - l STARTERS SHORE TO & FEEDERSSUPPYEMERG PORT SWBD GEN1 BUS-TIE GEN2 (MAINBREAKERSBEHIND) TO SHORE 2 2 O V S U P P Y SECT. STBD STARTERS & FEEDERS 220V SECTION FEEDERS Fig. 3.19 Main switchboard layout.
80 Generators and Main Circuit Breakers A separate section switches the three phase 220 V a.c. low power and lighting services. Check vour own switchboard and particularly note the controls and instruments on the generator panels; the link to the emergency switchboard; steering gear supplies (duplicated); other essential services to the engine-room; navigation equipment supplies and section board feeders. Note the alarms and insulation resistance (earth fault) monitors on both the 440 V and the 220 V sections. The 4J:01220V lighting transformers may be located inside the main switchboard or, more likely, will be separately mounted nearby. The main generator supply cables are connected directly to their respective circuit-breakers.Short copper bars from each generator circuit breaker connect it to the three bus-bars which run through the length of the switchboard. The bus-bars-mavbe seen if the rear doors of the switchboard cubicle are opened, but they may be in a special enclosed bus- bar duct acting as an internal fire barrier. Take care when opening doors on switchboards, live parts are exPosed- you are in danger. The ship's electrical diagrams will include drawings of the front, and perhaps the rear, of the main switchboard showing the as-fittedequipment. The electrical distribution diagrams will follow the physical arrangement of the main switchboard lavout. You should studv the electrical circuit and layout diagrams for your ship to identify, locate and appreciate the role of each key component in the scheme. Efficient fault-finding on a distribution network can only be achieved by a thorough understahding of the scheme and its normal operation. Switchboard instruments and controls for particular functions are grouped together. For example, the generator synchronis- ing panel has all the instruments, relays and switches necessary for generator paralleling. Each generator panel has all the instruments, relays, switches, controls and status lamps- necessary for control of the generators. The instruments on panels of outgoing circuits are usuallv limited to an ammeter, status 1amp6,function switches (e.g. manual/offlauto) and push buttons. Low power control and instrument wiring is of relatively small cross- sectioh, with multicoloured plastic insulation which is clearly identified against the larger main power cables. The instrumentation and control wiring is supplied from fuses which are located behina the appropriate panel. Green and yellow striped earth wiring from instruments and panel doors etc., is connected to a common coPper earth- bonding bar running the length of the switchboard at its rear. This earth bar is electrically bonded to the ship's steel hull. 3.L0. Main Circuit Breakers LV generator circuit-breakers and other large distribution circuit-breakers (600-6000 A) on board ship are traditionally of the air break type called ACB (air circuit breaker).This means that the circuit-breaker contacts separate in air. An ACB outline is shown in Fig. 3.20. High voltage (HV) installations e.g. ,at 6.6 kV and L1 kV generally use the vacuum interrupter type or gas-filled (sulphur hexafluoride - SF6) breakers. Outlines shown in Fig. 3.21. In a vacuum interrupter the contacts only need to be separated by a few miliimetres as the insulation level of a vacuum is extremely high. The quality of the vacuum in the sealed interrupter
Main Circuit Breakers 81 tio; i'.c"i*---- -- __i InternalStructure . Motorchargingtype Storedenergytype 1 Overcurrenttrip device(Solid-state) 2 Arc chutes 3 lsolatingcontactsof controlcircuits 4 Line-sidemain circuitterminals 5 Draw-outmouldedbase 6 Load-sidemain circuitterminals 7 lsolatingcontactsof main circuits 8 Fixed arcing contacts 9 Movingarcingcontacts 10 Fixedmain contacts 11 Movingmain contacts 12 Closingmechanism 13 Tripbar 14 Inst.trip devices(for marineuse) 15 CT (for overcurrenttrip device) 16 Chargingmotor 17 Closinglatch release 18 Closingspring 19 Charginghandle 20 Quick-close/Slow-closeselectorlever 21 Auxiliaryswitches Fig. 3.20 Circuit breaker components. contacts in vacuum chamber metal bellows fixed contact bolt ceramic insulator movrng contact boltceramtc insulator Vacuumlnterrupter(onephase) SF6 Interrupter(onephase) Fig. 3.21 Vacuum and SF6 interrupter units.
82 Generators and Main Circuit Breakers ConnectedPosition Test Position lsolatedPosition RemovePosition Fig. 3.22 Circuit breaker positions. chamber is checked by applying a short duration HV pulse (e.g. 10 kV for a 6.6 kV breaker) acrossthe open contacts. In the gas breaker the contacts separatein a special interrupter chamber containing SF6 gas typically at 500 kPa (5 bar) at 20oC. The operating mechanism for vacuum and SF6 breakers is similar to that employed for an ACB. Fig. 3.22 shows how each main circuit breaker is mounted on guide rails inside a main switchboard cubicle from which it must be withdrawn and isolated from the bus-barsfor maintenanceand testing. The breaker and its guide rails are usually mounted in a special cassette bolted into the switchboard cubicle and electrically connected to the bus-bars. If repair *ork demands that the breaker is to be completely removed from its cassette then usually a special hoist or fork-lift is required for large, heavy-duty units. The action of withdrawing the circuit breaker causes a safetv shutter to cover the live bus-bar contacis at the rear of its cubicle. The mechanical linkage in a circuit- breaker is quite complex and should not be interfered with exceptfor maintenance and lubrication as specified by the manufacturer. The main fixed and moving contacts are of copper (sometimes of special arc-resistant alloy or silver tipped) and usually silver-alloy coated. Main contacts shouli not be siraped or filed. If the main contacts suffer severe burning they will probably require realignment as specified by the manufacturer.
Main Circuit Breakers 83 Arcing contacts normally suffer burning and may be dressed by a smooth file as recommended bv the manufacturer. Carborundum and- emery should not be used - the hard particies can embed themselves in the soft contacts and cause future trouble. The arc chutes or arc splitter boxes confine and control the inevitable arc to rapidly accelerateits extinction. These must be removed and inspected for broken parts and erosion of the splitter plates. Various types of circuit breaker closing mechanism may be fitted: o Independent Manual Spring The spring charge is directly applied by manual depressionof the closing handle. The last few centimetresof handle move- ment releases the spring to close the breaker. Closing speed is independent of the operator. WARNING: Circuit breakers store energy in their springs for: o Store-charge mechanisms in the closing springs. o Contact and kick-off springs. Extreme care must be exercised when handling circuit breakers with the closing springs charged, or when the circuit breaker is in the ON position. Isolated circuit-breakers racked out for maintenance should be left with the closing springs dischargedand in the OFF position. Circuit-breakers are held in the closed or ON position by u mechanical latch. The breaker is tripped by releasing this latch allowing the kick-off springs and contact pressure to force the contacts oPen. Tripping can be initiated: o Manually - a push button with mechanicallinkage trips the latch. Undervoltage trip coil or relay (trips when de-energised). Overcurrent/short-circuit trip device or relay (trips when energised). Solenoid trip coil - when energised by 1 remote push-button or relay (such as an electronic overcurrent relay). Mechanical interlocks are fitted to main circuit breakers to prevent racking-out if still in the ON position. Care must be taken not to exert undueforce rt the breaker will not move, otherwise damage may be caused to the interlocks and other mechanical parts. Electrical interlock switches are connected into circuit-breaker control circuits to prevent incorrect sequence -operation,,e.9.-when a shore-supply breaker is closed onto a switchboard-.- The ship's generator breakers are usually interlocked OFF to prevent parallel running of a ship's generator and the shore supply. o Motor Driven Stored Charge Spring (most common type for marine applications) Closing springs are charged by a motor-gearbox unit. Spring recharging is automatic following closure of the breaker which is initiated by a push- button. This may be a direct mechlnical release of the charged spring, or more usually, it will be releasedelectricallyvia a solenoid latch. a This Manual Wound Stored ChargeSpring is similar to above method but with manually charged closing springs. o Solenoid The breaker is closed by a d.c. solenoid energisedfrom the generator or bus-bars via a transformer/rectifier unit, contactor, push button and, sometimes, a timing relay.
85 Chapter Four Motors and Starters 4.0 4.1. 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 Introduction Motor Construction Enclosures and Ratings Induction Motor Operation Control Equipment Direct-on-Line Starting Reduced Voltage Starting Speed Control Motor Protection Single Phase Motors Maintenance Page 85 85 87 90 92 93 95 100 105 712 114 4.0. Introduction The drive power for compressors,pumps and fans aboard ship comes from electric motors. By far the most common type of motor is the 3-phase a.c. cage-rotor induction motor. It is popular because it is simple, tough and requires very little attention. Another advantage is that starting and stopping these motors can be done with simple and reliable direct-on-line contactor starters. Three phase induction motors are usually supplied at 440 Y, 60 Hz, but 3.3 kV and 5.5 kV, 60 Hz are sometimes used for very large drives such as bow thrusters, cargo pumps, air compressors and gas compressors. Special types of motor can also be found on board ships. DC commutator motors are sometimes used for driving deck machinery where speed control is important. Single-phase a.c. motors are used in low power drives such as galley equipment and domestic tools. High power synchronous a.c. motors are frequently used for electric propulsion drives, see Chapter Eight. This Chapter will deal principally with the three-phasea.c. cage rotor induction motor, together with its control and protection. Additionally, the more common types of motor speed control methods are outlined, followed by maintenance procedures for motors and starters.
86 Motors and Starters 4.1. Motor Construction The induction motor has two main components, the statorand the rotor. The stator carries three separate insulated phase windings which are spaced I20" (electrical)apart and lying in slots cut into a laminated steel magnetic core. This type of stator winding is similar to the construction used for an a.c. generator. The ends of the stator windings are ter- minated in the stator terminal box where they are connectedto the incoming cable from the three-phasea.c. power supply. The rotor consists of copper or aluminium conductor bars which are connected together at their ends by short-circuiting rings to form a cage winding. The conductor bars are set in a laminated steel magnetic core. The essential reliability of the induction motor comes from having this type of simple, robust rotor which usually has no insulation on the conductor -bars and does not have any troublesome rotary contacts like brushes, commutator or sliprings. The diagram in Fig. 4.'l-. and the following component list identifies the main items used in the construction of a typical totally enclosed, fan ventilated (TEFV) induction motor. Fig. 4. L Induction motor components.
Enclosures and Ratings 87 No. Component N o . Component No Component 1 endshield, driving end 11 stator frame 21 terminal box cover 2 grease nipple 12 eyebolt 22 terminal box gasket J grease relief screw 13 inside cap 23 terminal board 4 end securing bolt 14 ball bearing 24 terminal box 5 anti-bump nuts 15 circlip 25 terminal box gasket 6 ball bearing, drive end 16 endshield 26 raceway plate 7 false bearing shoulder 17 inside cap screws 27 raceway gasket 8 flume 18 fan 28 "D" flantge 9 cage rotor 19 fan cover 29 "C" face flange 1.0 drain plug 20 lubricator extension pipe 4.2. Enclosuresand Ratings D Motor Enclosures Enclosure protection for electrical equipment is defined in terms of its opposition to the ingress of solid particles and liquids. The enclosure protection is defined by the Ingress Protection (lP) Code where a two-figure number is used to indicate the degree of protection against the ingress of solids and liquids as shown below. Drip-proof open ventilated motors are used where the risk of liquids leaking from overhead pipes and valves may be a problem. Air is drawn into the machine by an internal fan to provide cooling. The ventilation ducts are fitted with mesh screens to prevent any objects from entering the motor and causing damage. These screens must always be kept clean and free from dust otherwise the motor will overheat due to inadequate ventilation. When a greater degree of protection is required the enclosure is made Totally EnclosedFan Ventilated (TEFY) and jet- proof. No external at is allowed inside the motor. To improve heat transfer the motor casing is finned surface area, and airflow is achieved by means of and cowl arrangement. to increase the across the fins an external fan Fig. 4.2 TEFV motor enclosure. Motors located outside on weatherdecks have deck watertight enclosuresbut the external fan is omitted because of the possibility of ice formation. Deck watertight motors (IP56) have sealed bearings and a watertight terminal box. They can be completely immersed in shallow water for short periods. Sealing washers are fitted under all screws and a coat of special corrosion resisting paint is generally applied to all external and internal surfaces.
88 Motors and Starters " 1st numeral Degree of Protection of persons against contact with live or moving parts inside the enclosure and protection of equipment against ingress of solid bodies 2nd numeral Degree of Protection againstingressof liquids 0 No protection of persons against contact with live or moving parts inside the enclosure. No protection of equipment against ingress of solid foreign bodies. 0 No protection 1 Protection against accidental or inadvertent contact with live or moving parts inside the enclosure by a large surface of the human body, for example, a hand but not protection against deliberate accessto such parts. Protection against ingress of large solid foreign bodies. 1 Protection against drops of condensed water: Drops of condensed water falling on the enclosure shall have no harmful effect. 2 Protection against contact with live or moving parts inside the enclosure by fingers. Protection against ingress of medium size solid foreign bodies. 2 Protection against drops of liquid: Drops of falling liquid shall have no harmful effect when the enclosure is tilted at any angle up to 15 " from the vertical. 3 Protection against contact with live or moving parts inside the enclosure by tools, wires or such objects of thickness greater than 2.5 mm. Protection against ingress of small solid foreign bodies. 3 Protection against rain: Water falling in rain at an angle up to 50" with respect to the vertical shall have no harmful effect. 4 Protection against contact with live or moving parts inside the enclosure by tools, wires or such objects of thickness greater than 1 mm. Protection against Ingress of small solid foreign bodies. 4 Protection against splashing: Liquid splashed from any direction shall have no harmful effect. 5 Complete protection against contact with live or moving parts inside the enclosure. Protection against harmful deposits of dust. The ingress of dust is not totally prevented, but dust cannot enter in an amount sufficient to interfere with satisfactory operation of the equipment enclosed. 5 Protection again water-jets: Water projected by a nozzle from any direction under stated conditions shall have no harmful effect. 6 Complete protection against contact with live or moving parts inside the enclosure. Protection against ingress of dust. 6 Protection against conditions on ships'decks (deck watertight equipment): Water from heavy seas shall not enter the enclosure under prescribed conditions. Note that the higher the numeralof the l.st and 2nd characteristic. the greaterdegreeof protectionthe enclosureoffers: e.g. lelproof lP55 meetsall the lessonerousdegrees suchas 1P22,1P23,lP34 and 1P54. 7 Protection against immersion in water: It must not be possible for water to enter the enclosure under stated conditions of pressure and time. 8 Protection against indefinite immersion in water under specified pressure. It must not be possible for water to enter the enclosure.
Enclosures and Ratings 89 Insulation Class Maximum Temp. ("C) Typical Materials A 105 Cotton, natural silk, synthetic silk, presspan E 120 Wire enamels with a base of polyvinyl acetyl, epoxy or polyamide resins B 130 Mica products, wire enamels with a base of polyterephthalate, laminated glass-fibre materials F 155 Mica products, glass fibre, wire enamels with a base of imide-polyester and esterimide H 180 Mica products, glass fibre, wire enamels with a base of pure polyimide Deck motors for tankers must have a flameproof (Exd) enclosure if they are within 3m (4.5m for some ships) of an oil tank outlet. D Motor Ratings The motor converts electrical energy taken from the electric power supply into rotational mechanical energy at the motor shaft. Power lossesoccur during the energy conversion which results in the production of heat in the motor. These losses increase when the load on the motor increases because the motor takes more current from the supply. The life of the insulating materials used on motor windings depends on the temperature at which it is operated. Insulating materials are selected for marine practice based on an ambient temperature of 45oC. An adequate life-span for the insulation is based on the assumption that the maximum temperature limit is not exceeded. Motor nameplate definitions: o RatedFuIl Load Current (FLC) This is the maximum value of current that the motor can continuouslv take from the supply without exceeding the temperature limit for the insulating materials used. o Rated Voltage The motor has been designed to operate successfullv when connected to this value of supply voltage. If the rated voltage is not applied, overheating, stalling and burn-out can result. o Rated Frequency The motor speed is directly affected by the supply frequency; so are the motor losses. If the motor is operated at other than rated frequency overheating can occur. o PowerRating This is the shaft power output of the motor when it is connected to rated voltage and frequency when drawing its rated current from the supply. o Rated Speed This is the full load speed of the motor when connected to rated voltage and frequency. . lP Number Indicates the degree of protection given by the motor enclosure. The motor rating details are shown on the motor nameplate as in the example in Fig. 4.3.
90 Motors and Starters 3 PhaseInductionMotor 180-306Drive End 180-306N/DriveEnd Ser.No. 2238J 420 Fig. 4.3 Motor rating label. Standard three-phase a.c. induction motors are manufactured in about 60 frame sizeswith power ratings from about 0.37 kW to 500 kW. A sample selection of output power ratings and their average full load current (FLC) for A-pole, 440 V motors are listed below: 4.3. Induction Motor Operation When the 3-phase a.c. supply voltages are connected to the three stator phase windings, the resulting phase currents produce a multi-pole magnetic flux (@). This flux is physically rotatedaround the stator core by the switched sequenceof the R-Y-B currents at a speed called synchronousspeed (n,). The value of synchronous speed depends on how many magnetic pole-pairs (p) fixed by the stator winding arrangement and by the frequency (/) of the voltage supply connected to the stator winding. f.60 o r N . : L r e a / m i n p QUESTION What is the synchronous speed of a 6-pole motor supplied at 60 Hz? ANSWER 20 rev/s or 1200revimin The stator rotating magnetic flux cuts through the rotor conductors to induce an alternating emf into them. Since the rotor conductors are connected together at the ends, the induced emf's set up rotor currents. The rotor currents also produce a magnetic flux which interacts with the stator rotating flux which produces a torque (T) on the rotor conductor bars as shown in Fig. 4.4. n, : f rea/s p kw 0.5: 1 . 5 4 . 0 11 22 37 55 / J 100 200 500 A 1..4 J . I 7 . 9 20.1 39 64 90 125 162 32\ 780 twistingforce ( t o r q u e = F x r a d i u s on rotorbars statorcoreand phasewindings ri))!/.,1*, statorflux 'rotating statorflux Fig. 4.4Induction motor action.
Induction Motor Operaton 91 Rotor torque size is determined as: TaQJp.cos@where @ is the stator flux, Ia is the rotor cunent and @is the angle between @ and Ip. The rotor reactance varies with the rate of cutting flux which depends on the rotor speed. Hence cos@ (power factor) will vary during motor start-up as it acceleratesup to its rated speed. lf. cosf is ignored (for simplicity) then the shaft torque is approximately given by: TeV2 (as Q a V and /p c @). The directionof the rotor torque causes the rotor to rotate in the same direction as the rotating magnetic field. QUESTTON How is the rotor direction reversed? ANSWER Simply by swapping over any two supply line connections at the stator terminal box. This reversesthe direction of the rotating magnetic field. An induction motor cannot run normallv at synchronous speed. This is becaus-e the rotor conductors would then be stationary with respect to the rotating magnetic field. No emf would be induced in the rotor and there would be no rotor current and no torque developed. Even when the motor is on no-load the rotor speed has to be slightly less than the synchronous speed fl.5so that current can be induced into the rotor conductors to produce the torque to overcome the mechanical rotational losses of friction and windage. Slip speed is the differencebetween the synchronous speed (nr) of the rotating magneticflux and actual rotor speed (np). Slip is usually expressedas a percentage of the synchronous speed: ': l*#l'roov' QUESTTON If a 6-pole motor is supplied at 60 Hz and runs with a slip of 57o, what is the actual rotor speed? ANSWER The synchronous speed is 1200rpm, and the rotor slips by 5% of 1200, i.e. by 60 rpm so the rotor runs at 1140rpm. If the load torque on the motor shaft is increased, the rotor will tend to slow down (increasing the slip) which allows the rotor conductors to cut the flux at an increased rate. This causes more current to flow in the rotor which is matched by more stator supply current to meet the increased shaft torque demand. The motor will now run at this new, slightly reduced, speed. The fall of motor speed between no-load and full-load is very small (between 1.o/oand 5%) so induction motors are considered to be almostconstant speed machines. The characteristicin Fig. 4.5 shows the variation of torque with slip for a standard cage-typeinduction motor. Also shown is a typical load characteristic which indicates the torque necessaryto drive the load at different speeds. At start-up the motor develops more torque than is necessaryto turn the load so the motor and load accelerate. The speed increasesuntil, at the intersection oT the two characteristics, the torque developed by the motor is the same as the torque required by the load at that speed. The motor and load will then run at this steady speed as the torque supplied exactly matches the demand.
92 Motors and Starters 2 . 5 max.torquepoint (stallingor pull-out) Example: A4 pole,60 Hz motorhas a statorflux speed of: n" = f/p = 6012= 30 rev/s or 1800rpm. On full loadwith a typicalslip of 4%, the actual rotor speed will be 96% of n" which is 1728 rpm. On lightloadthe slip is typically1% so the rotor speed rises to 1782 rpm. Hence,over the load rangethe shaftspeed is almost constant. 2 . 0 1 . 5 1 . 0 lfullload I running I point a t 3 - 5 % slip 0.25 0 . 7 5 0 . 5 0 . 5 0 . 7 5 0.25 . 1 . 0p . u .s p e e d Fig. 4.5 Motor torque/speedcurve and shaft loading. 4.4. Control Equipment When an induction motor is connected directly to its three-phase a.c. supply voltage, a very large stator current of 5-8 x full-load current (FLC) is taken. This is due to the maximum rate of flux cutting (s : 100%) in the rotor, creating large inducedrotor currents. The corresponding supply can match the supply voltage to the start-up and load conditions. Such a controller aims to maintain the operating power factor as high as possible to minimise supply current and power losses.Note, this type of.aoltagecontroller does not control shaft speed (which is controlled by frequency). Most induction motors are Direct-on-Line (DOL) switch-started because such starters are inexpensive and simple to operate and maintain. The high starting current surge will not cause serious heating damage to the motor unless the motor is repeatedly started and stopped in a short time period. When very large motors are started DOL they cause a significant disturbance of voltage (aoltagedip) on the supply lines due to the large starting current surge. This voltage disturbance may result in the malfunction of other electrical equipment connected to the supply e.g. lighting dip and flickering effects. To limit the starting current some large induction motors are started at reduced voltage and then have the full supply factor at start-up is very low, about 0.2 lagging, which rises 0.5 lagging on no-load then 0.85 lagging on full-load. This starting surge current reduces as the motor acceleratesup to its running speed. Operating on light loads at low power factor is inefficient as the supply current is relatively high causing significant I2R resistive (copper) losses.The only way to improve the power factor of the motor on light loads is to reduce the supply voltage. This can be achieved with an electronic voltage controller called a soft-starter and/or energy manager which Power typically to about to about
Direct-on-Line Starting 93 Fasteningpanel Fixedsectionof electromagnet Phase shift nng Coil Blockcontaining the fixedand movingcontacts Fig. 4.6 Contactor construction. voltage reconnected when they have acceleratedclose to their rated sireeds. Reducedyoltage starting is used for large motors driving loads like cargo pumps and bow thrusters. Two methods of reduced voltage starting by switching are called star-delta starting and autotransformerstarting but an electrortic "soft" starting option is also used. Contactors,as shown in Fig. 4.6, perform the switching action in startersto-connect and disconnect the power supply to the motor. and/or remote stop/start push buttons. If the current goes above the rated current for the motor, its contactor will be tripped out automatically by an overcurrent relay (OCR) to disconnect the motor from the supply (see motor protection). 4.5. Direct-on-Line Starting In the example circuit shown inFig. 4.7, the induction motor is directlv sutitched onto the three-phase a.c. po#er supply lines. This is ^a very sirirple stariin! arrangement which is used for the majority of induction motor drives. l The contactor is an_electromagnetically operated3-pole switch initiated from locdl
94 Motors and Starters ${i. DOL STARTER t- powenI t crRc,gll_f stop& reset @ E - - - T - - - I stop \ r r- F ocRtrip ocR t O i t rrremote lcontrol I station control transformer r u n n r n g l a m p Q 1 : 6 Fig. 4.7 DOL starter circuit. The switching sequence for this starter circuit is as follows: power circuit operation control circuit operation manual closing of fused-isolator Q1 closing of line contactor KML KM1 contactor "holds-in" KM1 contactor drops out, motor stops control circuit voltage available (e.9. 110V from control transformer) press start button "I" (local or remote) auxiliary contact on KM1 "latches" contactor remote indicator lamp "on" press stop button "O" (local or remote) on overload the OCR trips out the stop button OCR must be manually reset (after thermal time delay)
Reduced Voltage Starting 95 Further circuit additions can be made for remote control (e.g. by liquid level switch) and motor reversing -(with an extra contactor). DOL switching demands a short duration (a few seconds) but large starting current, typically 5 x FLC fixed by the motor impedance. This is generally acceptable to the supply generator as long as the corresponding boltage dip is not greater than 10-15o/o within the run-up period. For large motor drives this starting surge will cause an un- acceptable uoltage dip at the supply bus-bars with likely malfunctions^of other consumers e.g.-lighting flicker and possible drop-out of supply contactors. The voltage dip is further compounded as all the other connected motors com- pensate by demanding an increased current to maintain their original power output. If prolonged, this sudden current loading may cause supply line and generator protection to trip. Hence large motors (e.g. bow and stern thrusters) require_a more complicated starting method to limit the size of startinf current and so protect the generatoi supply and other consumers. This means applying a reduced voltage at start-up. 4.6. ReducedVoltage Starting During the run-up period the size of motor starting current can be limited by applying a reduced supply voltage or inserting some additional circuit impedance. The most common arrange- ment is to apply reduced voltage which is sub-divided into the methods of star-delta switching, auto-transformer starting and "soft" starting. tr Star-Delta Starting If a motor is direct-on-line started with the stator winding star connected, it will only take one-thirdof the starting current that it would take if the windings were delta connected. The starting iurrent of a motor which is designed t6 run delta connected can be reduced in this way. Star-deltastarters for small motors rnav be operated by a manual changeover switch. For large power motors, thephase windings are automatically switched us-rng contactors controlled by u timing relay as shown in Fig. 4.8. A choice oT time delay relays are available whose action is governed by thermal, pneumatic, mechanical or electronic control devices. The switching sequencefor this starter circuit is as follows: power circuit operation control circuit operation manual closing of fused-isolator Q1 closing of contactor KM1: star connection closing of KM2: motor supply opening of KM1: star connection opens closing of KM3: delta connection KM2 & KM3 contactors drops out, motor stops control circuit voltage available (e.g. 110V from control transformer) press start button 52 to close KM1 KM1 closesKM2 "hold-in" of KM1 - KM2 by KM2 auxiliary opening of KM1 by KM2 auxiliary closing of KM3 by KM1 auxiliary Stop by 51 button or OCR trip F1 Note: KM2 has a pair of auxiliary contacts with a time delay action (typically 40 ms) between the opening of the N/C and the closing of the N/o contacti
96 Motors and Starters l:, u STAR-DELTA STARTER Fig. 4.8 Star-deltastarter circuit. QUESTION Whv is the time delav necessarybetween the'KM2 auxiliary contacts? ANSWER To provide an electricalinterlock between contactors KM1 and KM3. This is to prevent a full short-circuit fault across the supply lines during the changeover from star to delta. supply voltage (V) and the impedance of the motor phase windings (Zpn). Compare the starting current when star connected to the starting current when delta connectedas in Fig. 4.9. Fig. 4.9 Star-deltaconnections.At the instant of starting when the supply has just been switched on and the motor has not yet started to rotate, there is no mechanical output from the motor. The onlv factors which determine the current ta(en bv the motor are the "/*., ratio of: Ir(Y) : Ir(a) _ 1 36V
Reduced Voltage Starting 97 This shows that the starting current of a delta connected motor can be reduced to one thfud if the motor is star connected for starting. The shaft torque is also reduced to one-third which reduces the shaft acceleration and increases the run-up time for the drive but this is not usually a problem. When an induction motor is running on load it is converting electrical energy input to mechanical energy output. The input current is now determined by the load on the motor shaft. An induction motor will run at the same speed when it is star connected as when it is delta connected because the flux speed is the same in both cases being set by the supply frequency. This means that the power output from the motor is the same when the motor is star connected as when the motor is delta connected, so the power inputs and line currents must be the same when running in either connection. If the motor is designed to run in delta but is run as star connected, and on full load, then each stator phase winding will be carrying an oaercurrentof ",13xrated phase current. This is because phase and line currents are equal in a star connection. This will cause overheating and eventual burnout unless tripped by the overcurrent relay. Remember that the motor copper losses are produced by the 12& heating effect so the motor will run (V3)2: 3 times hotter if left to run in the star connection when designed for deltarunning. This malfunction may occur if the control timing sequence is not completed or the star contactor remains closed while a mechanical interlock prevents the deltacontactor from closing. For correct overcurrent protection, the overcurrent relays must be fitted in the phaseconnections and not in the line connections. Check the position of the overcurrent devices in the previous schematic diagram, Fig. 4.8, for an automatic star-deltastarter. tl Autotransformer Starting Starting a large motor with a long run-up period will demand a very high current surge from the supply generator for a few seconds. This causes a severe voltage dip which affects every load on the system. Reducedvoltage starting will limit the starting surge current. One way to reduce the initial voltage supplied to the motor is to step it down using a transformer. Then, when the motor has acceleratedup to almost full speed, the reduced voltage is replaced by the full mains voltage. The transformer used in this starter is not the usual type with separate primary and secondary windings. It is an autotransformerwhich uses only one winding for both input and output. This arrangementis cheaper, smaller and lighter than an equivalent double-wound transformer and it is onlv in operation during the short starting period. For induction motor starting, the autotransformer is a 3-phase unit, and, becauseof expense, this method is only used with large motor drives, e.g. electric cargo PumPs. Fig. 4.10 shows the supply voltage is connected across the complete winding and the motor is connected to the reduced voltage tapping. A number of tappings are usually available on the transformer winding, giving voltage outputs ranging from about 50% to 80% of the mains supply voltage. e.g. a 60"/" tap on an autotransformer supplied at 440 V would provide a voltage output of. 60"/" of M0 : 264 Y. The autotransformer usuallv has a few tapping points to give i set of reduced voltages (e.g. 40"/", 50o/oand 65"/") which help to match the motor current demand to the supply capability. As with the star-delta starter, the auto- transformer mav use what is called an open-transitioniwitching sequence or a closed-transition switching sequence
98 Motors and Starters tx % . V r l to motorI t * z100% tl v , lsupply 1*.1 l'""l x%"v' 1 -phase unit tapping star point tapping points x = Nr/ N., Fig. 4. L0 Autotransformer connections. between the start and run conditions. In the former, the reduced voltage is supplied to the motor at start then disconnectedand the full supply voltage rapidly reconnectedto the motor. The problem with open-transitionis that a very large surge current can flow after the transition from reduced to full voltage. QUESTION What causes the large current surge in gpen transition starters when going from the start to the run condition?- ry11ing induction motor the magnetic field does not immediately colhpse. The motor begins to slow down but-still generates an emf. When reconnected in open transition, the supply voltage and motor emf are not necessarily in- phase (the condition is similar to synchronising a generator onto the bus-bais). An addi-- tional current surge is therefore likely at the changeover stage, causing furiher voltage dip and so affect other cohsumers. Closed transition starters overcome this because the motor is never actuallv disconnected from the supply during th"e starting cycle. Most autotransformer startersused the closedtransition method. ANSWER A typical circuit closedtransition starter All motors generate a back emf against circuit is shown in Fig. 4.11. llj t"pp.ty voltage when they are running. The switching sequence for this starter when the supply is removed from a circuit is as fjllowi: power circuit operation control circuit operation manual closing of fused-isolator QL closing KM1: star connection of transformer closing of KM2: motor supply via transformer opening of KML: star connection opens closing of KM3: direct supply to motor (Note the mechanical interlock of KM1-KM3) KM3 contactors drops out, motor stop control circuit voltage available (e.g. 110V from control transformer) press start button 52 to close KML interlocking of KM3 by KM1 closing of KA1 by KM1 closing of KM2 by KA1 hold-in of KM2 opening of KM1 by KA1 (after time delay) closing of KM3 by KM1 interlocking of KM1 by KM3 hold-in of KM3 opening of KM2 by KA1 Stop by 51 button or OCR trip F1
Reduced Voltage Starting 99 f r AUTOTRANSFORMER STARTER Q 1 F 1 2 4 Fig. 4.1L Autotransformer starter circuit. 3 phasea.c. suppty,e.g. 4 4 0 V , 6 0H z a.c. voltagecontrolby delayed switchingon both half-cycles motor voltage." l jV nsrng rms voltage Fig. 4.12 "Soft" starter block diagram. tr "Soft" Starting This method of supplying a gradually increasing a.c. voltale grlinf start_uir generally refers to an efficienl electronic switching technique. A basic method shown in Fig. 4.I2, is to use back-to-back connected thuristors or triacs in the supply lines whiih are"gated" to delay "lurn-on,, within each a.c. half-cycle. This delayed switching
100 Motors and Starters applies a reduced average a.c. voltage to the motor. The applied motor voltage is gradually ramped up by the starter sciftware program until the full voltage level is reached. To achieve maximum efficiency, the electronic switching circuit can now be bypassed for normal running. A "soft" starter may be further adapted to become a voltage controller over the motgl operating load range. In this type of efficient "energy mnnager" application, the controller monitors the motor power factor which is a measure of the motor loading. On light load and full voltage, the power factor is low so the controller reduces the motor voltage which reduces current while improving power factor and efficiency. Note, this type of "soft- start/energy manager" is not a speed controller. To electrically change the speed of an induction motor it is necessaryto vary the applied frequency. Motor speedcontrol methods are outlined in a later section. The DOL starter is simple and cheap but causes a large starting surge. Star- delta starting reduces the surge but is somewhat more complex, requiring three contactors and a timer. The autotransformer method can be arranged to match the motor surge current and run-up period to meet the supply limitations by a suitable choice of voltage tapping. This starter is considerablymore expensive than the other two starter types. 4.7. Speed Control The standard cage-rotor a.c. induction motor operates as an almost constant speed drive over its load range. This feature is satisfactorv for most of the ship's auxiliary services supplying power to ventilation fans and circulating pumps. Variable speed control is necessary for cranes, winches, windlass, capstans, forced-draught fans etc. Ship's electric propulsion with electronic speed control mav use d.c. motors or a.c. induction moiors for low/medium power applica- tions. Large power electric propulsion, e.g. for a passenger cruise ship, will use a.c. svnchronous motors - see Chapter Eight. Two main forms of speed change/control are available: o Pole-changing for induction motors to give two or more fixed speeds, e.g. 2-speed forced-draught fans and 3-speedwinches o Continuously variable speed control, e.g. smooth control of deck cranes, winches and electric ship propulsion using variable frequency Fixed set speeds can be obtained from a cage-rotor induction motor by using a dual wound stator winding, each winding being designed to create a different number of magnetic poles. QUESTTON Estimate and compare the likely starting current surges for a motor that takes 200 A on full load when started: (a) DOL (b) Star-Delta (c) Autotransformer with a 50% tapping. ANSWER (a) When starting DOL the initial surge current is about 5 x FLC, i.e. 1000A. (b) A star-deltastarter reduces the initial starting surge to one-third of the equivalent DOL value, i.e. to about 330 A in this case. (c) The autotransformer method reduces the initial starting surge to (r)2.Ipe1 where r: tapping point. In this examplex: 0.5, so the surge current level is 0.52.1000: 250 A. -
Speed control L01 u 1 ,v 1 ,w 1 u 1 .v 1 .w 1 u3.v3.w3 doublestar (highspeed) Fig. 4.13 Star-doublestar connections. QUESTION A dual-wound induction motor is arranged to create 6 pole and 10 pole stator magnetic fields. Estimate the rated speeds assuming that the rotor slips by 57o and the power supply is at a frequency of 60 Hz. to give medium speed (8-pole) and high speed (4-pole) outputs. Speed control and drive direction are achieved by u set of switching and reversing contactors operated from the winch control pedestal. Remember that to reverse the rotation of an induction motor it is necessary to switch over two of the supply lines to the stator winding. An alternative method giving two fixed speeds in a 2:1 ratio from a cage-rotor induction motor is to use a single stator winding which has centre-tap connections available on each phase. This method uses a starter with a set of contactors to switch the phase windings into either single-star(low speed) or double-star(high speed). The supply lines to the stator windings are shown in Fig. 4.13. Note that two of the supply lines are interchanged in the double-starconnection - this is to maintain the same direction of rotation as in the low speedconnection. A continuously variable speed range of motor control involves more complication and expense than that required tb obtain a couple of set speeds. Various methods are available which include: ANSWER From f : nr . p we get nr: f/p [raheren, is the synchronousspeedof the rotating magneticfluxl so, at high speed (6 poles, p : 3), nr: 60/3 : 20 rea/sor 1200rpm but rotor runs at np: 95Y".20: 19 rea/sor 1L40rpm and, at low speed (10 pole, p :5), nr: 60/5: 12 rea/sor 720 rpm but rotor runs at nn: 95o/o.12: 11.4 rea/sor 684 rpm A 3-speed pole-changewinch motor can be arranged by having two cage rotors mounted on the same drive shaft. One stator winding (usually 2[-pole) gives a low speed while the other is dual wound
702 Motors and Starters o o Electro-hydraulic drive. Wound-rotor resistance control of induction motors- Ward-Leonard d.c. motor drive. o Variable-frequency induction or synchronous motor control. The electro-hydraulic drive, often used for deck crane control, has a relatively simple electricalsection.This is a constant single-speed induction motor supplied from a DOL or star-delta starter. The motor runs continuouslv to maintain oil pressure to the variable-speedhydraulic motors. A crude form of speed control is provided by the wound rotor induction motor. The rotor has a 3-phase winding (similar to its stator winding) which is connected to 3 sliprings mounted on the shaft as shown in Fig. 4.'1,4.An external 3-phase resistor bank is connected to brushes on the rotor sliprings. A set of contactors or a slide wiper (for small motors) varies the amount of resistance added to the rotor circuit. Increasingthe value of external resistance decreasesthe rotor speed. Generally, the starters of wound-rotor motors are interlocked to allow start-up only when Laminatedrotor Brushes Windings External resistancebanks maximum rotor resistance is in circuit. This has the benefits of reducing the starting current surge while providing a high starting torque. The wound-rotor arrangement is more expensive than an equivalent cage-rotor machine. It requires more maintenance on account of the sliprings and the external resistor bank which may require special cooling facilities. Where continuously variable speed has to be combined with high torque, smooth acceleration, including inching control and regenerative braking, it is necessary to consider the merits of a d.c. motor drive. Speed and torque control of a d.c. motor is basically simple requiring the variation of armature voltage and field current. The problem is: where does the necessaryd.c. power supply come from on a ship with an a.c. electrical system? A traditional method for lifts, cranes and winches is found in the Ward-Leonard drive as shown in Fig. 4.15. Here a constant speed induction motor drives a d.c. generator which in turn supplies one or more d.c. motors. The generator output voltage is controlled by adjusting its small excitation current via the speed regulator. The d.c. motor speedis directly controlled by the generator voltage. Fig. 4. 14 Wound rotor construction.
Speed Control 103 A ^ A ^ u . u . generator motor Fig. 4.15 Ward Leonard speed control method. I -H- field rectifier (+') I L ( NI T a . c .- d . c . thyristor controller f+) ( ' - 3-phasea.c. supply speed control T 0."\-7 load shaft t motor Fig. 4.16 Electroniccontrol for a d.c. motor. Obviously the motor-generator (M-G) set requires space and maintenance. An alternative is to replace the rotary M-G set with a static^electronic thizristor controller which is supplied with constant a.c. voltage but delivers a variable d.c. output voltage to the drive motor as shown in Fig. 4.16. Although the Ward-Leonard scheme provides an excellent power drive, practical commutators are limited to
104 Motors and Starters t r t t r n trtr bridge recifier I -l(roar I controlled inverter d . c .l i n k capacitor fixed 3-phasea.c. supply r variablefrequency | 3-phasea.c. supply r to motor e . g . 440 V 60 Hz i".s. \-/ i 0.5Hz- 120Hz Fig. 4. L7 ElectronicVSD controller. about 750 V d.c. maximum which also limits the upper power range. The commutators on the d.c. machines also demand an increased maintenance requirement. To eliminate these problems means returning to the simplicity of the cage-rotor induction motor. However, the only way to achieve a continuously variable speed output by electrical control is to vary the supply frequencyto the motor. A static electronic transistor or thyristor (high power) controller can be used to generate such a variable frequency output to directly control the speedof the motor as in the example diagram in Fig. 4.17. In an electronic variable speed drive (VSD), the fixed a.c. input is rectified and smoothed by a capacitor to a steady d.c. link voltage (about 600 V d.c. from a 440 V rms a.c. supply). The d.c. voltage is then choppedinto aariable-width,but constant level, voltage pulses in the computer controlled inverter section using IGBTs (insulated gate bipolar transistors). This process is called pulse width modulationor PWM. See Fig. 4.18. By varying the pulse widths and polarity of the d.c. voltage it is possible to generate an aaeragedsinusoidala.c. output over a wide range of frequencies. Due to the smoothing effect of the motor inductance, the motor currentsappear to be approximately sinusoidal in shape. By directing the currents in sequence into the three stator winding a reversible rotating magnetic field is produced at a frequency set by the PWM modulator. Accurate control of shaft torque, acce- leration time and braking are a few of the many operational parameters that can be programmed into the VSD, usuallv via a hand-held unit. The VSD can b-e closelv tuned to the connected motor drive to achieve optimum control and protection features for the overall drive. Speed regulation against load changes is very good and can be made very precise by the addition of feedback from a shaft speed encoder. VSDs, being digitally controlled/ can be easily networked to other computer devices e.g. programmable logic con- trollers (PLCs) for the overall control of a complex process. A disadvantage of choppinglarge currents with such a drive creates harmonicaoltnges
Motor Protection L05 I --1(rcer I PWM inverter PWM principle (one-phaseonly shown) Fig. 4.18 PWM control method. back into the power supply network. A harmonic voltage waveform is a distorted sinusoidal waveshape. The analysis (not covered here) of a distorted waveshape reveals a set of sinusoidal harmonic voltages super- imposed upon the base (or fundamental) frequency. Harmonic frequencies are integer (whole number) multiples of the fundamental frequency. In an a.c. system, even numbered harmonics are conveniently self-cancelling as are multi- ples of three in a 3-phase network. This leaves harmonic numbered frequencies of.5, 7, 1'1,,13, 17, 19 etc. Fortunately, the higher the harmonic number the lower is the amplitudeof the harmonic voltage. For a 60 Hz fundamental (L't harmonic), a 5th harmonic would be at a frequency of 300 Hz and a 7th harmonic would be at 420 Hz. The amplitude of a 5th harmonic may be up to about 20o/o of the fundamental while the 7th will be down to about 14o/oand so on. Such harmonic voltage disturbances caused by current switching can interfere with other equipment connected to the power system. - e.g. progressive insulation breakdown due to high voltage spikes, flickering of the lighting, malfunction of low current devices such as electronic computers and instru- mentation/control circuits. Minimising harmonic disturbance involves good circuit design and the fitting of harmonicfilters adjacent to the VSD drive. A harmonic filter is a com- bination of inductance and capacitance units tuned to absorb the unwanted frequencies. Be guided by the manufacturers' installation notes regarding the need for filters, acceptablecable rating and length, earthing and bonding etc. before fitting such a drive. Very large drives use thyristor converters and synchronousmotors, e.g. for ship's electric propulsion as outlined in Chapter Eight. 4.8. Motor Protection The circuits in Fig. 4.19 show typical motor control circuits on LV and HV supplies. In the HV motor protection scheme above, the back-up fuses are the tigger type. This type of fuse releasesa trigger actuated by u spring held in tension until the element melts. When released,
106 Motors and Starters HV e . g .6 . 6 k V isolating VT 6600/110V fused isolator contactor OCR with single-phasing (differential) protection W 3i:x$ fi'irr;;,dLE[?,"ilil; :t + : ' . d T r i n . ' l o c k o u t II I I I I contactor\..''lP. . I r-olt iri")i"."t ) | l< " '. r."i"y ;:5'iln.1, ; : LV motorwith thermalOCR ;";1J Y ararms : I * r focrrlc r s l i t . . . > lo r r r l e.q.5oo/5AI I EF I- | combinedmotorrelaywith: I ouercurrentinversetime I differential(single-phasing) at\ earthraurt I I HV motorwith \ 3 rV /combined \ , / p r o t e c t i o nr e l a y Fig. 4.19 LV and HV motor protection scheme. the trigger may be used to indicate a blown fuse or to trip a circuit breaker or contactor. Trigger fuses are an additional protection against a single-phasing fault so that the motor is definitely tripped out when a single fuse blows. Protecting an electric motor basically involves preventing the motor from getting too hot. Remember, every L0'C above the maximum recommended temperature of the insulation can reduce its working life by half. Obviously, the best way to protect a motor against overheating is to directly monitor the temperature of the motor windings. If the temperature exceeds the maximum set value for the motor insulation its contactor is tripped to stop the rnotor and allow it to cool down. Three main types of direct temperature sensorscan be used. These are: o a o Thermocouple Resistancetemperature device (RTD) Thermistor Tlne thermistorsensoris probably the most common as its thermal characteristic more closelv matches that of a motor than the other types. Thermistors are small pellets of semiconductor material
Motor Protection 107 which are embedded into the insulation of all three motor stator windings during manufacture. When a thermistor gets hot its resistance changes dramatically. They are connected so that if the motor temperature gets too high the starter contactor will be tripped by an electronic protection relay to stop the motor. Direct thermistor protection is usually only fitted to large motors, e.g. bow thrusters, FD fans, air conditioning compressors,etc. Most motors are protected by monitoring the temperature indirectly by measuring the current flowing in the supply lines. This method uses electronic, thermal or electromagnetictime-delayed overcurrent relays (OCRs) in the motor starter. The system is designed so that if the motor takes too much current because it is mechanically overloaded, the OCR will trip out the contactor coil, after a pre-set time delay, beforesevere overheating can occur. The largest oaercurrentpossible is the current taken when the motor has stalled. This, of course, is the starting current of the motor which will be about five times the full load current. The contactor is capable of tripping this stalled current quickly and safely. lf a short-circuit occurs in the motor, the starter, or the supply cable, then a huge fault current will flow. If the trippingtime(t) A ' 1 6 a c) E 4 2 1_, _ t _ inverse/time graphshave log/logscalesa o C o o o a 10 currentx FLC overloads- trippedby OCR - faults- trippedby fuses Fig. 4.20 Motor protection curves.
108 Motors and Starters contactor tries to open under short-circuit conditions, serious arcing will occur at its contacts such that it mav fail to interrupt the fault current. The prolonged short-circuit current will cause serious damage to the motor, starter and cable with the attendant risk of an electrical fire. To prevent this, a set of fuses or a circuit breaker is fitted upstream of the contactor which will trip out almost instantaneously thereby protecting the contactor during a short-circuitf.auIt. It is important that the tripping characteristics,as shown in Fig. 4.20, of the OCR and fuses/circuit breaker are co-ordinated so that the contactor trips on thermal oaercurrentwhile the fuses/circuit breaker interrupt short- circuit fault currents. This contactor + fuse arrangement is usually called back-upprotection. QUESTION At what value of current should the OCR be set? ANSWER To protect a modern CMR (Continuous Maximum Rating) motor the thermal OCR should be set at the full-load current (FLC) rating of the motor. This will ensure that tripping will not occur within 2 hours at '1.05o/o FLC. At 120"/" FLC tripping will occur within 2 hours. capacity to allow for short duration DOL starting currents without blowing. Consequently they do not protect against normal overloads but do protect the motor and supply system against a short-circuit fault. Fuses designed for motor circuit back-up protection have a restricted continuous current rating (called "M" tating) as compared with their fusing characteristic. Hence a typical fuse designation for motor circuits could be "32M63" which indicates a continuous rating of 32 A but a rating of 63 A for the starting period. QUESTTON A motor is protected by a thermal OCR and back-up fuses. Can the motor exceed its rated temperature without being tripped by the protection? ANSWER Yes! Although overheating is usually indicated by the current drawn by the motor rising above its rated value, a number of other situations can contribute to motor overheating. For example: very high ambient temperature; inadequate ventilation; a star-delta starter stuck in the star con- nection; stopping and starting too often; worn or dry shaft bearings. The motor windings can only protected against these conditions using direct thermalprotection. b e by It must be emphasised that the motor fuses are not chosen for their rated current but for then inaersecurrent/time (I/t) characteristic. This means that the current rating of fuses used to protect a motor does not appear to have any direct relationship to the FLC rating of the motor. Fuses used for back-up protection for motor circuits have a special time/current characteristic. They are generally carrying steady currents well below their rated There are three types of overcurrent relay (OCR) used for motor protection: Electronic Thermal Electromagnetic Electronic OCIT (overcurrent inverse time) relays have largely superseded
Motor Protection 109 A I r 8 . ^ l 0 ) l 3 t 4 . s l E r ^ 1 Z t 4 t l- a - I I I I o t 2 0 E I $ ro l $ r r . 1 0 I I r 5 I I I I I C U R R E N T SETTING 3 4 5 6 ( x F L C ) I Fig. 4.21, Electronicovercurrent relay and I/t curves. electromagnetic types as they have no moving parts (except for their output trip relay) and their very reliable tripping characteristicscan be closely matched to the motor circuit. Such relais are robust, smaller and lighter than the equivalent electromagnetictype. A block diagram of such an electronic OCR is shown in Fig. 4.21.. The block diagram of the electronic OCIT relav shows that the current and time settings can be adjusted over a limited range to match the motor FLC and run-up time. A self-test of the OCR performance can usually be applied with a fixed setting of, typically, 6 x FLC and the tripping-time can be measured and compared against the manufacturers current/time characteristics. Although electromagnetic devices with time delays can give adequate protection against large, sustained overloads to motors which are operated well below their maximum output and temperature, they have been found to be inadequate for continuous maximum rated (CMR) motors. Most LV motors are protected by less expensive thermal OCRs. Inverse-time thermal OCRs usuallv work with bi-metal strips as shown in Fig. 4.22. The strips are heated bv the motor current and bend depending on the temperature. If the motor takes an overload current, the strips operate a normally-closed (NC) contact which trips out the line contactor to stop the motor. The minimum tripping current of such a device can be adjusted over a small range. This adjustment alters the distance the strips have to bend before operating the trip contact. For larger motors, the heaters do not carry the full motor current. They are supplied from current transformers (CTs) which proportionally step-down the motor current so that smaller heater components may be used.
110 Motors and Starters Hot IndirectlyHeated Fig. 4.22 Bi-metallic overcurrent action action. fAtt f \,, causingshaft vibration unbalanced statorcurrents Fig. 4.23 Single-phasingfault. To operate correctly, induction motors must be connected to a three phase a.c. supply. Once started they may continue to run even if one of the three supply lines becomes disconnected. This is called single-phasingand can result in motor burn-out. Single-phasing,as shown in Fig. 4.23, is usuallv caused when one of the three back-up fuses blows or if one of the contactor contacts is open-circuited. The effect of single-phasing is to increase the current in the two remaining lines and cause the motor to become very noisy due to the uneven torque produced in the rotor. An increase in line current due to single-phasing will be detected by the
Motor Protection 111 protective OCR. The three thermal elements of an OCR are arranged in such a way that unequalheatingbf the bi-metal strips causes a differentialbi-metal strips causes a diffe Healthy conditior, (balanced) Single-phasingt'ault condition (unbalanced) o/oof rated FLC o/" of rated FLC lp and I6 Ia and I3 I 6 60 102 62 131 70 130 79 161 100 243 129 185 levels of motor phasing fault as a single- 4.23. loading shown during in Fig. of the movement which operates the OCR switch contacts to trip out the motor contactor. - For large HV machines a separate device, called a negatiaephase sequence (NPS) relay, is used to measure the amount of unbalancein the motor currents. For star connected motor windings the phase and line currents are equal io the line connected OCR is correctly sensing the winding current. If the overcurrent setting is exceededduring a single-phase fault the motor will be tripped off. The situation is not so simple with a delta connected motor. Normally the line current divides phasorally beiween two phases of the motor windings. The phase current is just over half the line current as l p H : + : 0 . 5 7 7 1 r . V 3 When one of the lines becomes open- circuited a balancedthree phase condition no longer exists. Now the-setsof line and phage currents are no longer balanced. The table below shows typical values of line and phase currents at various Particularly note that the current in winding C is considerably higher than that in the other two windings. Look at the condition where the motor is at 50% of full load when single-phasing occurs: the line currents are 102"/" of the full-load value but the current in winding C is 131% of its full-load value. The 102o/oline current will probably not activate a line connected OCR and the motor remains connected. However, the local overheating in winding C of the motor will quickly result in dhmage. Motors can he protected against this conditio_n by using a differential type relay which trips out with unbalaiied currents. In fact, most modern thermal OCRs for motors have this protection against single-phasing incorporated as a normal feature. A differential action is shown in Fig. 4.24. bimetal 3 bimetalscold position 3 bimetalshot (balanced) 2 bimetalshot, lcold (differential) Fig' 4'24 Bi-metallic single-phasing protection (differential action).
L12 Motors and Starters If single-phasing occurs when in operation on light load, the motor keeps on running unless the protection trips the contactor. If the motor is stopped, it will not restart. When the contactor is closed, the motor will take a large starting current but develop no rotating torque. The OCR is set to allow the starting current to flow long enough for the motor, under normal conditions, to run up to speed. With no ventilation on the stationary motor, this time delav will result in iapid and sever" olr"i- heating. Worse still, if the operator makes several attempts to restart the motor, it will burn out. If a motor fails to start after two attempts, you must investigate the cause. Undervoltage protection is necessary in a distribution system that supplies motors. If there is a total voltage loss or black-out, all the motors must be disconnected from the supply. This is to prevent all the motors restarting together which would result in a huge current surge, tripping out the generator again. Motors must be restarted in a controlled sequenceafter a supply failure. Undervoltage (UV) protection for LV motors is simply provided by the spring- loaded motor contactor because it will drop out when the supply voltage is lost. For large HV motor the UV protection function will be covered bv a relav separate from the OCR funciion or it may be part of a special motor relay which incorporates all of the necessary protection functions. When the supply voltage becomes available, the motor will not restart until its contactor coil is energised. This will usually require the operator to press the stop/resetbutton before initiating the start sequence. For essential loads, the restart may be performed automatically by a sequenie restart system. This svstem ensures that essbntial servicei are restarted automatically on restoration of supply following a blackout. Timer relays in the starters of essential motor circuits are set to initiate start-up in a controlled sequence. 4.9. Single Phase Motors Low power motors for power tools, domestic equipment, refrigerators, vacuum cleanersetc are typically supplied at 220 V a.c. 50160Hz. Common types are: . Split-phase induction motor . Capacitor start/run induction motor o Shaded pole induction motor . a.c. commutator motor . Split-phase induction motor: A single phase induction motor has a cagerotor similar to that used in a three phase type. A single stator winding produces a pulsatingmagnetic field when energised with single-phasea.c. current. This field cannot exert a rotating force on the cage rotor. One method used to produce a rotational force is to employ two stator windings fitted 90o to each other with both connected across the same supply. This is the split-phase motor. To get the effect of a shifting magnetic field (and hence induce a rotating force into the rotor), one winding is electrically phase-shiftedby adding capacitance in series with one of the windings. o Capacitor start/run induction motor: When the motor has started to run, the additional phase winding circuit may be disconnected and the rotor will continue to be pulsed around by the magnetic flux. This is called a capacitor start motor which is onlv useful for driving a very light load.
Single Phase Motors 113 M 7 - , A \ /6t 19,\ /f ^ \\ caqef g \ t t @ l \b /- J/ roror + centrifugal----{_l switch stator w i n d i n g s S C start C run Fig. 4.25 Capacitor-startmotor circuit. For starting and running, two capacitors are used in circuit as shown in Fig. 4.25. During the starting period the two paralleled capacitorscreate a large phase angle to the "S" winding current. As the rotor runs up to speed a switch cuts out one of the capacitors. The switch may be a centrifugal type on the rotor shaft or a current-operated time-delay relay in the motor terminal box. This typd of motor gives good starting and running torque with reasonable power factor. Most split phase motors are arranged for a 4-pole stator winding so at 50 Hz its synchronous (flux) speed will be 25 rev/s or L500 rpm. As with all induction motors, the rotor wrTl slip causing the shaft speed to be about 24revls or 14/:0rpm on no-load. On-load, a single-phase induction motor will run with greater slip and operate with less efficiency, than a three phase version. o Shaded pole induction motor: This is a low torque machine useful for low power drives such as small cooling fans in ovens and electronic equipment. Fig. 4.26 shows how the face of each salient stator pole is partially split with one side carrying a thick copper wire called a shadingring. The pulsating a.c. flux divides into each half of the pole but is time delayed in the part with the s h a d i n g n n g Fig. 4.26 Shaded-polemotor construction. sshading ring. This is due to an induced current in the ring which opposes flux change in the shadedpart. To the rotor, this delay appears as a flux shift across the overall pole face which drags the rotor with it bv the normal induction motor action. Obviously, the developed torque is small and the machine is not very efficient but is an inexpensive drive for very low power applications. As with all induction motors the shaft base speed is fixed by the supply frequency, so at 50 Hz the maximum speed is 3000 rpm and shaft loading will cause the rotor to slip below this value. . a.c. commutator motor: This is basicallv a d.c. series motor construction dedigned to operate very effectively on an a.c. voltage supply. See Fig. 4.27. Fig. 4.27 Commutator motor construction.
I 114 Motors and Starters The shaft torque produced is given by TxQJ where @ is the flux produced by the series connected stator winding and I is the armature (and supply) current in the rotor. As @ is produced by the same current the torque is essentially Tq12 which makes this single phase a.c. motor more powerful than the induction types. At 220 V a.c. the shaft speed on light load is typically 12,000 to 18,000 rpm and is easily controlled by an additional series resistanceor an electronic voltage regulator. The speed falls rapidly with increasedload torque. This type of motor is mainly used intermittently in equipment's rated up to a few hundreds of watts. Typical examples would include power drills, sanders, jig-saws, food-mixers and vacuum cleaners. The commutator and brush contacts will cause some sparking in normal operation which can causeradio/television interference so a high frequency voltage suppressor is usually fitted to this type of motor. 4.L0. Maintenance QUESTION What is the most common cause of induction motor failure? ANSWER Failure of stator insulation due to dampness is a major problem with marine motors. Open ventilated motors are most at risk, particularly when they are not used for long periods. Anti-condensation heaters should be regularly checked to see that they are actually working and keeping the motor drv. For all motors, cleanliness is next to godliness. A regular cleaning routine is required to remove harmful deposits of dust, dirt, greaseand oil from both inside and outside the motor. The cleaning of the external surface is especially important for totally enclosed motors which run continuouslv. The heat generated in these motois is removed through the external surface. A thick laver of dust will reduce the heat dissipation and result in very high temperatures. Internal dust and dirt in open ventilated motors must be regularly removed by blowing or extraction and ventilation screensand ducts clearedout. If motors are to be blown out, the air used must be absolutelv drv and the pressure should not be more than'1..75 bar. If the pressure is higher than this it forces the dust into the winding insulation rather than removing it. When blowing out a motor remember to cover up other machines in the area to protect them from flying dust. Suction cleaning is better than blowing out. QUESTTON How often should a motor be cleaned? The maintenance requirements for cage-rotor induction motors are very simple: . Keep insulation resistance high and contact resistancelow Lubricate correctlv and maintain a uniform air gap Ensure both the interior and exterior are always clean and dry Provided these requirements are met, an induction motor should give trouble- free service during its long life.
Maintenance LL5 ANSWER Basically this will be determined by the local conditions and the type of ventilation. Onlv the external surfaces of totally enclosed motors will require regular cleaning. But both the outside and inside of open ventilated motors will require routine attention. The inside of a totally enclosedmotor can be cleaned if the mbtor has been dismantled for bearing replacement. Motors in areas where considerableamounts of air-borne dust are expected, hatch-cover motors are an example, will obviously require more frequent cleaning. Contamination by oil and grease from motor bearings is often a cause of insulation failure. The insulation should be cleaned by brushing or spraying with one of the many proprietary brands of cleaning fluid which are available. Badly contaminated motors may require total immersion of the stator windings in cleaning fluid. Broken or missing bearing covers must be repaired or replaced to prevent grease escaping. When a motor has been dismantled for cleaning and overhaul it should be thoroughly inspected. In this way, faults can be detected beforethey evolve into a major breakdown. . Stator Look at the stator windings for damaged insulation causedby carelessreplacement of the rotor into the stator. Discoloured insulation is an indication that the winding has been overheated. The cause of overheating must be found and corrected before allowing the motor back into service. Carefullv examine the stator core for signs of rubbing with the rotor, usually caused by u worn bearing. Even slight rubbing of the rotor against the stator will generateenough heat to destroy the stator insulation. Replace the bearings before putting the motor back into service. Laminated steel core plates which have been badly scored may cause a local hot spot to be generated when the motor is running. This is because the Fe (iron) losses will increase in the damaged area. After the motor has been put back into service with new bearings, check the motor running temperature. After a short period of service dismantle the motor and check for discolouration at the core damage which will indicate local heating. If you suspect core hot spots then the motor core will need to be dismantled for the laminations to be cleaned and re-insulated - definitela a shore job. The insulation resistance reading is the best indication as to the presence of moisture in the motor windings. Break- downs due to insulation failure usuallv result in an earth fault, short-circuitei turns in a phase or phase-to-phasefaults. QUESTTON How do vou check resistance- between induction motor? the insulation phases on an ANSWER Larger motors are usually six-terminal, which means that all six ends of the stator windings are brought out to the terminal block. Links between the terminals are used to star or deltaconnect the motor. Disconnect the supply leads and remove the links. Test between phases with an insulation resistance tester as shown in Fig. 4.28.
116 Motors and Starters view. The best policy is to renew the bearings as part of a planned maintenance programme. If this is not possiblebecause of cost or a shortage of replacements, then bearings should be removed, cleaned and inspected for signs of damage before a decision to refit or renew is taken. Before opening up a bearing, make sure that the complete area around the housing is clean and dry. Manufacturers recommend that bearings should be removed from the shaft as seldom as possible, but cleaning and inspection is best done with the bearing off the shaft. If the correct size of wedges or pullers is used, then removal should not cause any damage. Bearings should be cleaned by immersion in a solvent such as clean white spirit or clean paraffin, then thoroughly dried in a jet of clean, dry compressed air. Bearings should not be spun by the air jet because skidding can damage the rolling elements and racewavs. Once dry, the bearing must be lightly oiled. Any traces of metal particles, such as brass, indicate cage wear and the bearing must be replaced. If there is no evidence of metal particles, carefully examine the raceways and rolling ele- ments for signs of wear or damage. Hold the inner race in one hand and slowlv turn the outer race. Any sticking or unevenness in the rotation requires a re-wash of the bearing and rotation in the cleaning fluid. If the sticking persists the bearing must be rejected. Similarly, bearings with visible signs of corrosion, overheating or damage, and those with a noticeable degree of roughness in rotation should also be replaced. When fitting a bearing to a shaft, first clean the shaft and apply a thin film of light oil. Set the bearing square on the shaft and, with a tubular drfit (pipe), force the bearing against the shaft shoulder. The drift should bear on the inner race as close to the shaft as possible. Large bearings can be heated for 10-15 minutes in clean mineral oil Fig. 4.28 Motor IR test. A problem can arise on small, three- terminal motors where the star or delta connection is made inside the motor. Only one end of each winding is available at the terminal block. Phase-to-phase insulation resistance cannot be checked. If a three terminal motor is to be rewound, ask the repairer to convert it to a six terminal arrangement. o Bearings Induction motors are fitted with ball andior roller bearings. These bearings are robust and reliable and should give very little trouble provided they are properly fitted, kept absolutely clean and lubricated correctly. Many engineers argue that if a bearing seems to be operating correctly it should not be tampered with. Portable vibration detection results, sampled periodically and analysedcan be a very useful way to recognisethe onset of a bearing failure. Bearing temperature, e.g. using embedded detectors or with portable Infra Red (IR) spot checks, is another indicator the general health of a shaft bearing. Otherwise, it is not easy to predict (with any degree of certainty) the un- expired life of bearings that have already run for some time. Also, inspection may not show damage to raceways and rolling elements in areas hidden from
Maintenance 717 up to 80'C to facilitate fitting. Lubricate the bearings with the correct type and quantity of grease as recommended by the manufacturer. Fill the bearing about one third to one half full with grease. Overgreasing causeschurning and friction which results in heating, oxidation of the grease and possible leakage through the seals. On account of the high ambient temperature and excessive vibration which many marine motors endure, greaselife cdn be short and fresh grease should be applied at regular intervals. Unless the bearing housing has a vent hole to allow excess grease to escape, it will be necessary to clean out the bearing housing bef6re charging it with fresh grease. Because of the vibration on ships, bearings can be damaged when the motor is not running. The shafts of stationarymotors should be periodically rotated a quarter turn to minimise vibration damage to the bearings. o Rotor As you will have gathered, maintenance of cage-rotor induction motors tends to mainly involve the stator windings and bearings. Cage-rotorsrequire little or no specialcare in normal service. Inspect for signs of damage and overheating in the cagewinding and its laminated steelcore. Make sure that all core ventilating ducts are clean and clear. If an internal fan is fitted it must be in good condition if it is to provide adequate cooling. QUESTION A cage-rotor induction motor has been flooded with sea water and its insulation resistance is down to zero MO. What is the procedure for putting the motor back into service? ANSWER The main problem is to restore the insulation resistanceof the stator winding to a high value. This is achieved in three stages: o Cleaning . Drying o Re-varnishing Salt contamination can be removed bv washing with clean, fresh water. Any grease or oil on the windings has to be removed using a degreasant liquid such as Armaclean. Dry the stator windings with low power electric heaters or lamps with plenty of ventilation to allow the damp- ness to escape. Alternatively, the windings can be heated by current injection from a welding set or from a special injection transformer. Be sure to keep the injected current level weII below the motor's full load rating. With the windings clean and dry, and if the IR test remains high over a few hours, apply a couple of coats of good quality air-drying insulating varnish. The motor starter and other control equipment should be regularly inspected to check and maintain the following items: ,, Enclosure: Check for accumulations of dirt and rust. Any corroded parts must be cleaned and repainted. Examine the starter fixing bolts and its earth bonding connection - particularly where high vibration is present, e.g. in the steering flat and the forecastle. r Contactors and relays: Check for any signs of overheating and loose connections. Remove anv dust and grease from insulating components to prevent voltage breakdown by surface tracking. Ensure that the magnet armature of contactors moves freely. Remove any dirt or rust from magn-et faces which may prevent correct closing.
118 Motors and Starters z Contacts: Examine for excessivepitting and rough- ness due to burning. Copper contacts may be smoothed using a fine file. Copper oxide, which acts as a high resistance, can be removed using glass- paper. Do not file silver alloy contacts or remove silver oxide as it acts as a good conductor. A thin smear of electrical contact lubrication helps to prolong the life of all contacts. When contacts have to be replaced, always replace both fixed and moving contacts m Pars. Check contact spring pressure and compare adjacent contact sets for equal pressure.Examinepower and control fuse contacts for signs of overheati.g - lubricate the contact blades on fuse- holders. ,z Connections: Examine all power and control con- nections for tightness and signs of overheating. Check flexible leads for fraying and brittleness. tz Overcurrent relays: Check for proper motor FLC). settings (relate to grease and corrosion of movement (not electronic type of Inspect for dirt, and for freedom possible with an ocR). A thorough OCR performancetest can only be carried out by calibrated current injection. r Control operation: Observe the sequence of operation during a normal start-up, control and shut-down of the motor. Particularly look for excessivecontact sparking (only possible with open-type contactors). Remember to check the operation of emergency stop and auto-restart functions.
119 Chapter Five Ancillary Electrical Services 5.0 Introduction 5.1, Ships'Lighting 5.2 IncandescentLamps 5.3 Discharge Lamps 5.4 Voltage Effects on Lighting 5.5 Navigation and Signal Lights 5.5 Emergency Lighting 5.7 Maintenance of Lighting Fittings 5.8 Refrigeration and Air Conditioning 5.9 Galley and Laundry 5.1.0 Cathodic Protection 5.LL Battery Supplies Page 119 779 120 r22 127 128 130 737 132 137 140 143 5.0. Introduction To ensure a safe working environment, together with off duty comfort in the accommodation quarters on board your ship, a considerable proportion of the generated electric power is absorbed in the ancillary sensices. Lighting of the ship's deck areas, engine room and accommodation to meet specified levels of illumination is provided by various light fittings (Iuminaires)designed to work safely in their particular locations. The hotel services for food storage, preparation and cooking, together with accommodation air-conditioning and laundry services, are essential for the general maintenance of the mariner. This chapter will examine ships' lighting and refrigeration/air conditioning together with galley and laundry services. Additionally, hull protection by the impressed current cathodic protection method and battery supplies are also included. 5.L. Ships' Lighting Historically, the original application of electricity in ships was for lighting. Oil lanterns were a definite fire risk and the ship's lamp trimmer had great
120 Ancillary Electrical Services difficulty in maintaining his navigation lights in stormy weather. To meet the safety and comfort levels of illumination required throughout your ship a wide range of lighting fittings (Iuminaires)are used. The power ratings of the lamps used will vary from a few watts for alarm indicator lamps to a few kW for deck floodlights and searchlights (e.9. a Suez Canal Projector Light). The amount of light falling on a particular area can be checked with a luminance meter which is calibrated in lux (lx). One lux is the illumination of one lumen/sq. metre (lm/m2) where a lumen is the unit of luminousflux. For example, container loading requires a minimum illumination level of 50 lx while a main engine control room may be illuminated to a level of 500 lx. The minimum illumination standards for crew spaces in UK registered ships are specified in "The Merchant Shipping (Crew Accommodation) Regulations". The luminous efficiency of a light fitting is defined as the ratio of lumens/ watt. This efficiencv reduces in time mainly becausethe limp deteriorates as the lumens emitted gradually get less while the watts input remains constant. Dirt on the lamp reflector and lamp-glass will also reduce its luminous efficiencv. Group replacement of lamps is often considered by shipping companies to be more economic and convenient than individual replacement following lamp failure. Cleaning of the fittings can also be carried out during group lamp replacement so maintaining a high luminous efficiency. Lamp end caps are many and various but the most common types are screlo and bayoneffittings. The old names, €.9. Goliath Edison Screw (GES) and Bayonet Cup (BC) are now re-designated to indicate the cap type and its dimensions. A selection of old names and the current codes are listed below. The first letter of the current code indicates the cap type (Edison or Bayonet). The first number indicates the nominal outer diameter of the cap barrel or screw in millimetres (rntrr). The next number gives the overall length and the final number (if listed) is the diameter of the flange. Broadly, the luminaires employ one of two general lamp types classified as: o lncandescent . Discharge 5.2. IncandescentLamps The most common lamp used for general lighting is the simple filament type as shown in Fig. 5.1. A current is passed through the thin tungsten wire filament which raises its temperature to around 3000'C when it becomes incandescent (it glows). The glass bulb is filled with an inert gas such as nitrogen or argon which helps to reduce filament evaporation to allow an operating life expectancy of about 1000 hours. Lamp power ratings are available from 15 W to 1000 W. Old Code Description Current Code ES Edison Screw 827t27 GES Goliath Edison Screw E40t45 SES Small Edison Screw E14123x15 MES Miniature Edison Screw E10/13 LES Lilliput Edison Screw E5/9 BC BayonetCap 8.22125x26 SBC Small Bayonet Cap 815124x17 MCC Miniature Centre Contact BA 9s/14
Incandescent Lamps 121 arc quenching foam filling supportwires wire filament high internal gas pressure Fig. 5.L Incandescentlamp construction. The ordinary filament lamp is called a GLS (general lighting service) lamp. One variation of the basic lamp design has a special coiled-coilfilament which increases the life expectancy of low power (up to 150 watt) lamps and are referred to as double-lifelamps. Specially reinforced construction lamps (called rough-seruice)have a tough filament for use in areas where shock and vibration are expected - this type is useful with portable handlamps. Other variations include: clear glass bulb, inside frosted glass bulb (pearl) to reduce glare, tubular construction, internal reflector lamps, decorativelamps (e.9. candle shape) and heating lamps. Typical lamp power ratings and average light outputs (for a 240 V supply): QUESTTON Estimate the luminous efficiencv of 100 W single-coil and coiled-coil lamps. ANSWER Efficiency : Output lumensiinput watts : 1'1.601100: 1'1..6lm/W and 12.6 lm/W A popular variation of the incandescent lamp is the tungsten-halogentype. This lamp construction has a gas-filled quartz tube or bulb which also includes a halogen vapour such as iodine or bromine. When the filament is heated, evaporated tungsten particles combine with the halogen vapour to form a tungsten-halide. At the high filament temperature, the tungsten vapour re- forms onto the filament. This regenerative process continues repeatedly creating a self-cleaning action on the inner surface of the glass tube or bulb. In an ordinary GLS lamp the tungsten evaporation from the filament causes an internal blackening of the glass bulb which is eliminated in the tungsten-halogenlamp. Two basic lamp forms for the tungsten- halogen design are the linear double-ended lamp (K class) and the single-endedlamp (M class)as shown in Fig. 5.2. Fig. 5.2 K & M lamp construction. Linear tungsten-halogen lamps must be used in the horizontal position otherwise the halogen vapour will concentrate at its lower end which results in rapid blackening of the tube and a reduced single-coil type double-coil type Power(W) Light (lm) Power(W) Light (Im) t5 150 25 200 40 325 40 390 60 5 / 5 60 665 / 3 885 100 1160 100 1250 150 1960 150 2075 200 2720 For high vibration areas, single coil lamps are preferred as they are more robust than the double-coil type 300 4300 500 7700 750 12,400 1000 17,3W M type (single-ended)
122 Ancillary Electrical Services operating life. Both the linear and bulb type are particularly useful for display, floodlighting and spotlighting. Examples of tungsten-halogen lamp details: o Linear double-ended (240V rated and life expectancyof 2000 hours): . Single-ended: Tungsten-halogen lamps must be care- fully handled when being fitted. If the outside surface of the quartz tube or bulb is touched with dirty or greasy hands, premature failure can occur due to fine surface cracks in the glass. Handle the tube by its ends only, or use a paper sleeve over the lamp during fitting. If accidentally handled, the lamp glass may be cleaned with a spirit solvent, carbon tetrachloride or trichlorethvlene. 5.3. Discharge Lamps The light output from a discharge lamp is generated by the flow of current in an electric arc between two electrodes through a gas and metal vapour inside a sealed glass bulb or tube. The most common metal vapours employed in discharge lamps are: f Mercury (as used in a fluorescent tube) and B Sodium Low and high-pressuretypes of mercury and sodium lamps are available. A suitable voltage applied between the electrodesof a dischargelamp causes an arc discharge through the gas. This ionisation of the gas either creates visible light directly or by secondary emission from a phosphor coating on the inside wall of the lamp glass. The discharge lamp current must be carefully controlled to maintain the desired iignt output and some form of current limiting ballast is required. This ballast is often an iron-cored inductor (choke coil) but special transformers and electronic regulator ballast circuits are also used. The ballast must match tlrre lamp (e.9. a 20 W fluorescent tube must have a matching 20 W ballast unit) to ensure correct lamp operation for high luminous efficiency and long life. D Mercury Fluorescent These are manufactured as low pressure and high pressure lamp types. . Low-pressuremercury fluorescent Wpe The most obvious example of this type is the popular fluorescent tube as shown in Fig. 5.3. It is classified as an MCF lamp ( M : m e r c u r y , C : l o w p r e s s u r e ,F - fluorescent coating). Type Power (W) Light output (lm) K9 300 5000 K1 500 9500 K 2 & K 3 750 15000 K4 1000 21000 K5 1500 33000 K 6 & K 8 2000 44000 TyPe Voltage (v) Power (!v) Life (hours) Light outpul (lm) }J.t29 6 10 100 210 M30 6 20 100 420 M34 6 20 2000 350 M28 12 100 2000 2150 M32 12 50 2000 900 M35 12 20 250 450 M36 za 250 2000 s750 M38 240 300 2000 5000 M40 AO 300 2000 8500
Discharge Lamps 123 glasstubewithinner coating bi-pincap mercury bi-pincap cathode argonand mercuryvapour cathode andshield andshield Fig. 5.3 Fluorescentlamp construction. chokecoil (ballast) Fig. 5.4 Fluorescentlamp circuit and glow starter. The inside surface of the glass tube is coated with a fluorescent phosphor which emits white light. Variations of phosphor material create different light colours of which the most common are called: o Warm White Colour 29 o Warm White de luxe Colour 32 o Daylight Colour 33 o White Colour 35 Fluorescent tubes are available in lengths from 150 mm to 2400 mm with power ratings from 4 W to I25 W. The tube ends are usually fitted with bi-pin lamp caps or miniature bi-pin for the small tubes. Typical luminous efficiency for a fluorescent tube is about 70 lm/W with an average operating life of 5000 hours. To stike a fluorescent tube, its gas filling (usually argon or krypton) must be ionised by a voltage between its cathodes that is slightly higher than that required to maintain the normal discharge. Two common methods are used to strike the tube: o the switch-start circuit o the transformer quick-start circuit. The circuit in Fig. 5.4 shows a typical switch-start circuit. The starting action is initiated by a glow type starter switch which is connected between opposite ends of the tube. When the supply voltage is applied to the circuit, the full mains voltage appears across the starter switch. A glow
124 Ancillary Electrical Services - chokecoil (ballast) Fig. 5.5 Transformer-startfluorescentlamp circuit. discharge occurs between the starter contactswhich quickly heat up, bend and touch each other. This allows current to flow through the lamp cathodes which will cause the tube ends to heat up and glow before the tube actually strikes. The tube strikes when the starter switch re-opens as it cools down during its closed (non-glow) period. When the starter switch opens it interrupts an inductive coil (choke) circuit which produces a surge voltage acrossthe tube which then strikes. The tube is now full-araay and the reduced arc voltage across it is not sufficient to re-start the glow discharge in the starter so its contacts remain open. In fact, in the normal running condiiion, the starter switch can be unplugged from its bayonet cap base and the lamp will function normally. The tube will not, of course, re-strike after switch off without the starter action- Two or three stikes may be required to get the lamp running normally as the starter contacts may open before the cathodes are sufficiently heated. Such cold-striking reduces the lamp life by erosion of the cathode material which causes irregular lamp flashing. In normal operation as a series reactance the choke coil acts to limit the lamp discharge current. Severe blackening at the tube ends is a sure sign that its useful life is finished. Each time the starter switch closes, a large current surges through the choke coil which increases its temperature. Excessivelamp flickering must, therefore, be swiftly corrected by lamp or starter replacement. Most choke coils are potted in a thermosetting polyester compound within a steel case. While an earth fault is unlikely to occur within a choke, an open-circuit is possible and can be simply checkedusing an ohmmeter. No repair of a choke-coii is feasible so it must be completely replaced with an identically rated unit. Similarly, glow starters should only be replaced with an equivalent which matches the size of tube it is to be used with. An example of a transformer quick-start circuit is shown in Fig. 5.5. The lamp discharge begins as soon as the cathodes reach their operating temperature. A capacitive effect between the cathodes and the earthed metalwork of the fitting ionises the gas and the tube stikes very quickly. Most tubes have a conducting path through the phosphor coating ot, alternatively, a special metal earth strip running between the end caps which assists the starting process. The trans- former ballast gives an immediate start but some difficultv can occur with low ambient temperatures, low supply voltage
Discharge Lamps 125 and poor earthing. Many other variations of quick-start circuits using transformers and resonant effects are used. Capacitorsare used with dischargetubes for: Power factor correction (PFC) Radio interference suppression (RIS). The PFC capacitor is used to raise the supplypower factor to around 0.9 lagging. Without this capacitor the power factor may be as low as 0.2 lagging due to the high choke-coil inductance to cause the supply current to be 4 to 5 times larger than normal. For a \25 W tube a PFC value of about 7.2 pF is typical. Radio interference from discharge tubes is caused by the ionisation process of the discharge through the tube. This is suppressed by a capacitor fitted across the tube ends. In glow-switch circuits, the RIS capacitor is actually fitted within the starter. Typical RIS capacitor values are around 0.0005pF. Starter switches with an electronic time-delay can be used to eliminate flicker at iwitch on. An electronicballastcircuit can be used to improve luminaire efficiency and supply power factor by increasing the lamp frequency to around 30 kHz. QUESTION What would happen if in a glow-switch failed the RIS capacitor to a short-circuit? a a ANSWER The tube would not strike but would glow at its ends while the choke may overheat and eventuallv fail. A similar result would occur if tlie bi-metal strips of the starter welded together. . High-pressuremercury fluorescent type A typical high-pressure lamp and its circuit is shown in Fig. 5.6. This type of lamp is coded as MBF ( M : M e r c u r y , B : h i g h p r e s s u r e ,F : fluorescent coating). An additional suffix to lamp codes may be lU or lV meaning that the lamp is designed for fitting in a Uniaersal or Vertical position respectively, e.g. MBF/U. The high-pressure mercury lamp comes in sizes ranging from 50 to 1000 W and is fitted with Edison screw (ES) or Goliath Edison screw (GES) lamp caps. Its luminous efficiency is in the range of 40-60 lmiW with an average life of secondary electrode quartz discharge tube seriesresistor inertgas filling of argon & mercuryvapour internallycoated outer jacketMBF lamp circuit Fig. 5.6 HP mercury lamp and circuit.
126 Ancillary Electrical Services around 7500 hours. The lamp takes several minutes to reach full brightness. It will not immediatelv re-strike when rapidly switched off then back on because the vapour pressure prevents this happening. Re-striking will occur when the discharge tube has sufficiently cooled down. Lamp gas ionisation is obtained between a secondary electrode fitted close to one of the main electrodes which warms up the tube and an arc strikes between the main electrodes.Mercury, which was condensedon the tube will now vaporise and the main arc passesthrough it. The secondary electrode ceasesoperation as the lamp pressure builds up. tr Sodium Vapour These, like the mercury lamps, come in low-pressure and high pressure versions. o Low-pressure sodium vapour type A low-pressure sodium lamp is coded as SOX (SO : sodium vapour, X - standard single-ended lamp of integral construction). A typical lamp shape and its circuit is shown in Fig. 5.7. The lamp has a U-shaped arc tube containing metallic sodium and an inert gas such as neon. Common lamp power ratings are 35 W, 55 W, 90 W, 135 W and 180 W with luminous efficiencies in the range of '1.20-175lm/W with an average operating life of 6000 hours. The low-pressure sodium lamp needs a high voltage (480-650V) which requires a special transformer ballast or electronic ballast circuit. When first ignited the SOX lamp gives a red glow as the discharge is initially through the neon gas. As the lamp warms up the sodium begins to evaporate to take over the dischargefrom the neon causing the lamp colour to change from red to yellow. The time taken to reach full brightness is between 6-15 minutes. . High-pressure sodium vapour type A typical lamp and its circuit is shown in Fig. 5.8. The basic lamp type is coded as SON (SO : Sodium vapour, N : high pressure), but two other variations are labelled as SON-T (a tubular clear glass type) and SON-TD (a tubular double- ended clear quartz type). The SON lamp gives a wide spread of illumination with a golden-white light. Lamp starting is achieved by a high voltage pulse from an electronic igniter circuit which ceases to function once the main arc has been struck. SOX lampcircuit "U" arc tubewith dimpleformation for sodiumretention Fig. 5.7 SOX lamp and circuit.
Voltage Effects on Lighting 127 sinteredaluminium isothermalouterjacket with oxide white internal supportrods and conductors SON lamp circuit Fig. 5.8 SON lamp and circuit. A start-up delay of about 5 minutes is required for the lamp to reach full brightness but will uiually re-strike within 1 minute of extinction from the hot condition. , PON -F-p. pgy_e1 ratings range between 70 and 1000W with corresponE_ i1S t"*i1ous efficiencies being beiween 80 and 120 lm/W. 5.4. Voltage Effectson Lighting Naturally, all lamps are designed to produce their rated luminous" output at their rated voltage-. An overvolt'age on an incandescent lamp produces a b.righter and whiter ligh't b'ecause the filament temperature i-s increased. Its operating .lif_e.is, however, drastically reduced. A 5"/" increase over its ratei voltage will reduce the lamp life by 50"/o. Co,nversely,a- supply voltage reduction wrll rncreasethe operating life of a GLS lamp but it produces a iuller, reddish light. Lamps- rated at 240 V are often ".t:1^i13 :llp" lighting system operating at 220 V. This under-runniTgshoujd *orE than double the lamp life." Similar effects on lieht output and operating life apply to-discharge lamps but if the supply-voltage is d"rastical'ly reduced (below 50%) the arc discharge ceases and will not re-strike until tfre voltage is raised to nearly its normal value. A fluorescent tube will begin to flicker noticeablyas the voltage is re"cluced below its rated-value. The normal sinusoidal a.c. voltage wave- form causesdischargelamps to eitinguish at the end of every half iycle, i.e. every 10 ms at 50 Hz or-every6.7 ms at 60 Hi. Altho-ugh this rapid light fluctuation is not detectable by the human eye, it can cause a stroboscopiceffect whereby rotating shafts in the vicinity of discharg'e t9mp9 may appear stationary or rotating .r.lorylywhich could be i dangerous illusion to operators. QUESTION Give 3 methods to alleviate a stroboscobic problem. ANSWER o Use a combinationof incandescentand discharge lighting in the same area. o Use twin discharge lamp fittines with each lamp wired"as a fead-Iag"circuit, r-e. -the _lamp currents are phase displaced-.so -that they go thiough zero at different times,-hence t[e ov.erall.tig\t output is never fully extinguished.
128 Ancillary Electrical Services o Where a 3-phase supply is available, connect adjacent dischargeluminaires to different phases(Red, Yellow, Blue) so the light in a given area is never extinguished. 5.5. Navigation and Signal Lights The number, position and visible range of navigation lights aboard ships are prescribed by the International Maritime Organization (IMO) in their "International Regulations for Preventing Collisions at Sea". In the UK, the National Authority for maintaining marine safety standardi is the MCA (Maritime and Coastguard Authority). By far the most common arrangement is to have five specially designed navigation running lights referred to as Foremast,Mainmast (or Aftmast), Port, Starboardand Stern See Fig. 5.9. Two anchorlights, fitted forward and aft, may also be switched from the Navigation Light Panel on the bridge. The side lights are red for Port and green f.or Starboard while the other lights are uthite. For vessels length more than 50 metres, the masthead light(s) must be visible from a range of six nautical miles and the other navigation lights from three nautical miles. To achieve such visibility, special incandescent filament lamps are used each with a typical power rating of 65 W but 60 W and 40 W ratings are also permitted in some cases. Due to the essential safety requirement for navigation lights it is common practice to have two fittings at each position, or two lamps and lampholders within a special dual fitting. Each light is separately supplied, switched, fused and monitored from a NaaigationLight Panelin the wheelhouse. The electric power is provided usually at 220 V a.c. with a main supply fed from the essential services section of the main switchboard. AFT STERN S I D E LrGHT(S) FWD ANCHOR FORE MASTHEAD ANCHOR Fig. 5.9 Ship navigation lights arrangement.
Navigation and Signal Lights 129 ALARMAND -------L /l\ BUzzER INDICATION ' --r SUPPLIES r?\ INDICATOR KY L.,Vtr,K SWtTcHf-1il To orHER l____;PNAV.LrGHr L_*J CTRCUtTS NAV.16ATHON, Ll€HTr'PAN,€L EMERGENC MAINLIGHT FOREMAST AUXILIARY POWER SUPPLIES Fig. 5.10 Navigation light panel. An alternatiaeor standbypower supply is fed from the emergency switchboard. A changeover switch on the Navigation Light Panel selects the main or standby power supply. The Navigation Light Panel has indicator lamps and an audible alarm to warn of any lamp or lamp-circuit failure. Each lamp circuit has an alarm relay which monitors the lamp current. The relay may be electromagnetic or electronic. A basic double navigation light scheme with alternative power supplies is shown in Fig. 5.10. Various signal lights with redr gr€€n, white and blue colours are arranged on the signal mast as shown in Fig. 5.11. These lights are switched to give particular combinations to signal states relating to various international and national regulations. Pilotage requirements, health, dangerous cargo conditions, etc., are signalled with these lights. White Morse-Code flashing lights may also be fitted on the signal mast. EXAMPLEOF SIGNALLIGHTS (XMASTREE) Fig. 5.1L Signal lights arrangement. The NUC (Nof signalled using Under Command)state is two all-round red lights
130 Ancillary Electrical Services vertically mounted at least 2 m apart. Such important lights are fed from the 24 Y d.c. emergency supply but some ships may also have an additional NUC light-pair fed from the 220 V a.c. emergency power supply. 5.6. Emergency Lighting Depending on the ship's classification, e.g. ferry, ro-ro, gas carrier, etc., and tonnage the Safety of Life at Sea(SOLAS) Convention prescribesminimum require- ments for emergencylighting throughout the vessel. Emergency light fittings are specially identified, often with a red disc, to indicate their function. Most of the emergency lighting is continually powered from the ship's emergency switchboard at 220 Y a.c. A few emergency lights may be supplied from the ship's 24 V d.c. battery, e.g. at the radio-telephone position in the wheelhouse, the main machinery spaces and the steering flat. Some shipping companies now fit special battery-supported lighting fittings along main escaperoutes in the engine room, accommodation and at the lifeboat positions on deck. Generally, such emergency lights in the accommodation are arranged to produce light immediately on mains failure. Boat station emergency lights are switched on when required. Inside the fitting a maintenance-freebattery, usually nickel-cadmium, is continually trickle- chargedfrom the normal main-s supply powersupply and batteryunit mainssupply 24V 220V LED modules LLLsystem outline extruded to house metalchannel LEDmodules Fig. 5.12 Low location lighting (LLL).
Maintenance of Lighting Fittings 13L via a transformer/rectifier circuit. The battery is then available to supply the lamp via a d.c. to a.c. inverter when the main power is absent. Usually the battery will only function for a few hours. This power supply arrangement is called an uniterruptiblepoTnersupply or UPS. Such battery supported light fittings can be simply tested by switching off the normal mains power supply or, in some cases, by u test switch on the actual fitting. Periodic inspection and testing of all emergency lights is an essential require- ment on all ships. A visible, illuminated escape route reduces uncertainty and assists orderly evacuation. Passenger ships carrying more than 36 passengers are required by IMO resolution A752(18) to be fitted with Lora LocationLighting (LLL) to identify escape routes where normal emergency lighting is less effective due to the presence of smoke. An LLL system must function for at least 50 minutes after activation and it should indicate a line along the corridors of an escaperoute. The installation of LLL should be on at least one side of the corridor, either on the bulkhead within 300 mm of the deck or on the deck within 150 mm of the bulkhead. In corridors more than 2m wide, it should be installed on both sides. The LLL light sources may be low power LED's, incandescent lamps or a photoluminescent material containing a chemical that stores energy when illuminated by visible light. Of these sources,the LED and incandescentlamp are the most effective. For hazardous areas such as car decks on a ferry, an intrinsically safe (Exia) version can be installed. Fig. 5.12 indicates the main components of an LLL system where the LED's are wired onto a printed circuit board within a clear polycarbonate rectangular tube with connectors at each end. A similar 5.7. Maintenance of Lighting Fittings The performance of electric lamps will deteriorate with time. Eventually thev fail and the lamps must be replaced. Simple lamp replacement becomes the most obvious maintenance task. When a luminaire fails to light-up when switched on, it is natural to suspect lamp failure. If this does not solve the problem, checks on the lamp control equipment and power supply must follow. An incandescentlamp may be checked (out of circuit) for low-ohm continuity using a multimeter. If the lamp appears intact then the fault must lie in the supply or its connections. Voltage and continuity checks of the supply, fuse/ MCB and ballast circuit must be applied. Remember that a single earth fault on an insulated two-wire lighting supply will not blow a fuse. However, a similar earth fault on an earthed supply system (as used for a 110V transformer supply to deck sockets for portable tools and handlamps) will blow a fuse. QUESTTON Why will a single earth fault blow a fuse in an earthedsupply system? ANSWER Fig. 5. L3 Earth fault effect in an system. 2201110V transformer(55 V to earth) .--llF - - ( ' l l l l ) ( t earth fault ilr, - - __ __ -ElF_p_"I[_ - _- -' arrangement is available using low power incandescent lamps. earthed supply
132 Ancillary Electrical Services The single E/F completesa low resistance path back to the neutral or centrepoint of the supply with a resulting large fault current to rupture the fuse. A maintenance check list: ,z Remember that it is good practice to replace both tuses after clearing a fault which has ruptured only one of them. ,z Cleaning of the lamp glass and reflectors is essential for safety and necessary to maintain the luminous efficiency of the luminaire. r Particular care should be paid to the maintenance of the watertight integrity of exposed luminaires (e.9. for navigation, signal and deck lighting) at their flanged joints and cable gland entry. Similarly, a regular inspection of all portable handlamps and portable cargo light fittings, together with their flexible cables and supply plugs, should be undertaken.When replacing a lamp, ensure that the circuit is dead and isolated whrle removing the old lamp and inserting the new one. The glass bulb or tube of an old and corroded fitting may break loose from its end-cap while attempting to remove the lamp. If the supply is still connected, it is relatively easy to cause an accidental short- circuit during the removal process and the correspondingarc flash may cause blindness, burns and fire. Always replace a lamp with the correct size, voltage and power rating for the fitting it is housed in. Overheating and fire can easily result by using a higher powered incandescent lamp than the fitting was designed for. Check the lampholder wire connections behind the lampholder for signs of overheating (hard, brittle insulation on the wires) and replace if necessary. Take care when disposing of lamps, particularly discharge tubes, which should be broken (outdoors) into a container (e.g. a strong plastic bag) to avoid handling the debris. Remember that in a fluorescent lamp circuit the capacitor may remain charged for a while after switch off unless fitted with a dischargeresistor. Play safe, discharge the capacitor with a screwdriver blade before touching its terminals. Maintenance of flameproof(Exd) lighting equipment (e.9. in tanker pumprooms) is covered in Chapter Six. 5.8. Refrigeration and Air Conditioning The basic electric power and control elements for refrigeration and air- conditioning are outlined below: . Refrigeration The safe storage of food necessitates that it is maintained at low temperatures which requires the process of refrigera- tion. For bulk foods one large industrial refrigeration plant will serve separate cold rooms for the storage of meat, fruit and vegetables, dairy products, etc. Smaller domestic sized refrigerators are used to meet the daily catering needs in the galley, pantries, duty messrooms and in cabins. The refrigeration process is also utilised in deep-freezers,water- chillers and air-conditioning plant. Large scale cargo space refrigeration is also necessaryfor the transportation of foods and certain liquid chemicals and gases. Whatever the size or role of the ship's refrigerators, the basic principle is
Refrigeration and Air Conditioning 133 discharge(hotgas)line solenoidvalve suctionline evaporatorin cold room Fig. 5.14 Refrigeratorscheme and main components. common to them all. Each will have an evaporator (cooling unit), a refrigerant compressor and a condenser. The refrigerant is generally Freon-12 (CCI2F2) or Freon-22, but ammonia is also used in large systems. Freon re- frigerants in general use are colourless and almost odourless, while also being non-toxic, non-corrosive and non- flammable. However, when exposed to an open flame, a highly toxic phosgene gas is produced. Additional components to the basic refrigerant cycle may include filter-driers, heat exchangers, accumulators and pre-coolers. Also required are the operating and protective controls such as thermostats, relays, defrost controls and overcurrent trips. Above the domestic sized refrigerator, the compressor motor will invariably be a 3-phase type driving a reciprocating compressor. The domestic version will usually be a single-phase motor driving a rotary compressor. The basic refrigerant circuit of a direct (or primary) expansion system used for the cooling of meat and vegetablerooms is outlined in Fig. 5.14. Each cold room is fitted with a thermostat which operates a solenoid valve between set temperature limits. The quantity of refrigerant flowing in the system is regulated by the expansion valve. This valve is controlled by a liquid phial connected by u capillary tube attached to the vapour return pipe at the outlet of the evaporator. When the room temperature falls to the pre-set level, the thermostat de-energisesthe solenoid valve to stop
134 Ancillary Electrical Services common control relay V a . c . R rotor Fig. 5.15 Basic compressormotor control. circulation of the refrigerant. The resulting pressure drop in the compressor suction line will operate a low-pressure cut-out valve and stop the compressor. The rooms or compartments are cooled by natural air circulation through the evaporator coils or by forced-air from a fan blowing across a bank of cooling tubes. In a domestic refrigerator the cooling effort is controlled by using a control thermostat to switch the compressor on or off. The hermetically sealed compressor motor is the split-phase type having two separate windings - start and run as shown in Fig. 5.15. The motor is acceleratedby connecting both start and run phase windings to the supply. When the motor reaches about 80% of its rated speed, the start winding is tripped out of circuit. For compressor drives, this switch is usually in the form of a current-operated relay which is fitted adjacent to the comPressor. QUESTION If the motor terminal markings are unknown, how could you identifv the start, run and common terminal connections? ANSWER The start winding (being short-time rated) has a higher resistance than the run winding so a resistancecheck should identifv the terminals as follows: Using a multimeter on the low resistance range, find the two terminals that have the highest resistancebetween them. These are the start and run terminals. The remaining terminal must be the "common". Connect one lead of the meter onto the common terminal and touch the other meter lead onto the other terminals in turn and note the readings. The highest reading indicates the start winding terminal. The other remaining terminal is the run cor:rrtection. Typically, the run winding is 1.5-6 Q and the start winding is 6-22 O.
Refrigeration and Air Conditioning 135 door switchF V a . c . cabinet ti I heat via I ocR- - - . i - - - - - + capilrary i _rL thermostati | - l Fig. 5.16 Domestic refrigerator electric circuit. The main temperature control device in the refrigerator is the thermostat which sensesthe evaporator temperature via a capillary- tube_ The sef temperature is adjusted by a control kriob which tensions the control spring against the pressureof the bellows. For motor protection a bimetallic overcurrent relay (OCR) trip is included as part of the contiol reiay ifo"jria" tn" compressor. The motor supply current either passes directly throueli i bi-metal strip or disc or the bi-melal is heated indirectly from a small resistanceheater alongside it. A motor overcurrent will cause the bi-metal to deflect and cause a snap-action switch to open. -Fig. .5.16 shows the cbmplete circuit of..3 simple domestic refrigerator (i.e. wrthout timers, automatic defrost or air-circulation fans). Wh"l the evaporator temperature rises, the thermostat switch clbses allowing current to flow through the motor run qinding and the reliy solenoid coil. I nls current is .initially ligh causing The motor will now begin to accelerate from standstill causing ils run winding current to reduce to a level where thE start-relay will drop off. The motor will now run continuously on the run_phase only. When the evaporator reach^esits set temperature the thermostat resets and the motor is switched off. The most common way to achieve automatic defrosting of the evaporator is to use a time-switch to cut out the refrigeration circuit and initiate a defrost heater circuit. The timer may be a small motor with a cam driven -changeover switch or an electronic timer witfr relav changeovercontacts. J A bimetallic defrost thermostat controls the defrost heater in or below the eva- porator. Most defrost thermostatsclose at 20oF + 5o and open at 55o + 5o. Defrost periods may vary from 15 to 45 minutes with up to four defrost cyclesin 24 hours depending .on the fridgeTfreezerdesign. , Some refrigerators lnd freezers may have electric heaters fitted for varioui duties such as a dewpoint heater (to prevent sweating on the cabinet in the treezer area) and a compartment divider panel or stile heater (to prevent sweating on the panel). the solenoid to clos-eth6 rel{, switch-t8Lr.L ovrLrrvru r(J LruDe [Irg rglav swl allow current into the start windintg.
136 Ancillary Electrical Services Additionallv there may be condenser and evaporaiot fans which are driven by single-phaseshaded-poletype motors. o Air Conditioning Air conditioning is a process which heats, cools, cleans and circulates air together with the control of its moisture content. The air must be delivered to a room with a definite temperature and specified relative humidity. For summerduty, the usual method is to cool the incoming air to a temperature below the deuspoint to allow condensation to occur until the mixture has the desired specific humidity then heating the air to the required delivery temperature and relative humidity. In winter, the incoming air may have to be heated and have water added to achieve the correct inlet conditions. In most plants the bulk of the mixture is re-circulating air with fresh air intake forming about one third of the total required. The amount of make-up air is a statutory requirement which is typically between 17 mslhr and 28 m3/hr. The electrical aspects of accommodation air conditioning (A/C) comprises the power equipment of motors and starters for the compressor(s),fans and sea-water cooling pumps. Associatedcontrol equip- ment will include electric solenoid valves, high and low-pressure and temperature switches together with safety cut-outs for overcurrent, loss of refrigerant, low compressor oil pressure, etc. The usual air-conditioning system used for the accommodation spaces of cargo ships is the central single-duct type, shown in Fig. 5.17.ln its simplestform a single compressor serves the whole accommodation. fresh f fli,,f + _=__> I rooms -.->re-circ f + A I R C O N D I T I O N I N G PLANTOUTLINE starterboard compressor Fig. 5.17 Air-conditioning scheme and main components.
Gallev and Laundrv 137 The compressor is generally a multi- cylinder reciprocating type with a power rating in the range of 50-200 kW, although rotary-vane or screw-action compressors may also be encountered. Large passenger vessels may have a total power requirement of more than 5 MW for the AC compressor drives to maintain air delivery to the hotel and staff accommodation areas. Capacity control of the reciprocating compressor is by automatic unloading of cylinders by -valve control usin-"g seivo oil Pressure. The compr€ssorr air fan and sea water pump are driven by simple fixed speed, 3-phase a.c. induction motors each with its own starter and supplied from a distribution board fitted in the air-conditioning plant room. Routine electrical maintenance and fault finding on the motors and starters will involve cleaning,checkingof connections, IR (megger)/continuity tests and running tests as described in Chapter Four. Inspection of connections and correct operation of any electric heaters must _alsobe performed. Such heaters may be used for heating the compressor crankcase oil and f-or separating the refrigerant (Freon R12 or R22) from the oil in an oil reservoir. Regular inspection and testing of control and safety thermostats and pressurestats should be carried out in accordance with the manufacturer's instructions. In particular the compressor's low oil pressure alarm and trip circuit should be tested periodically for correct operation. 5.9. Galley and Laundry The following section outlines the basic power and control units utilised for galley and laundry services: o Galley The electrical power in a galley is largely absorbed in producing heat. Ovens, deep fryer pans, water boilers and the hotplates on the galley range all employ resistive heating elements which are usuallv controlled bv bimetallic thermostats. Other miscellaneous elec- trical galley equipment may include oven air circulating and range exhaust fans, meat slicers, food mixers and grinders, dishwashers, potato peelers and garbage disposal units. Most of this equipment will utilise small electric motors together with the necessary control switches, safety interlocks and indicator lamps. Becauseof the large power requirement for food preparation and cooking, the major galley items are supplied from the 3-phase a.c. M0 V system. Smaller galleys may be supplied from the low- voltage 220 V a.c. system. The electrical equipment has to work safely in the usual galley atmosphere of high humidity and high temperature. Catering staff have been known to wash down ovens and ranges with an enthusiasm that demonstrates a scant regard for Ohm's Law! All in all, the galley electrics work in a tough area so be prepared for faults caused by the environmental hazards of grease, dust and dampness. Heating elements are usually formed from Nichrome-wire insulated with a magnesium-oxide (MgO) powder within an inconel tube which forms the outer sheath. Power ratings vary from 1 kW to about 4 kW and some elements are arranged to be switched to give varying levels of heat. The simplest arrangement is obtained using a 3-heat, 4-position switch to control 2 elements within a single hot- plate on a single-phasesupply. The two resistance elements are interconnected by the switch to give a choice of OFF, LOW, MEDIUM and HIGH settings as shown in Fig. 5.18.
138 Ancillary Electrical Services and hot-plates. Be careful not to megger test low-voltage electronic components during maintenance and fault finding. Check the manufacturer's instructions and drawings before fault chasing around electrical control circuits. The most likely fault in a heating element is a simple open-circuit. Earth faults within the element or on the wires supplying it are also probable. Loose wire connections cause localised overheating with the wire burning away to leave an open-circuit at least, but the possibility of a short-circuit or earth fault also arises. The connecting wires lying close to heating elements should be covered with high-temperature silicone or fibre glass sleeving or with ceramic beads. QUESTION What would be the continuitv resistance of a healthy 2kW,220V heatingelement? ANSWER The element current is: Fig. 5.18 Three-heatswitching circuit. For larger heating power control using M}V 3-phasea.c., three heatingelements can be interconnected into star and delta configurations. Closer control of heating elements is obtained by using simmerstafswitches and electronic switching. The simmerstat switch type shown in Fig. 5.19, houses a bi-metallic switch which cltcles the heating element on and off at a rate determinedby the switch setting. Average hot-plate temperature is fixed by the ratio of time that the element is on to the time it is off. Circuit current heats the bi-metal which operates the control switch. Fig. 5.19 Simmerstat controller circuit. Oven simmerstat controls have a similar switching action but a temperature sensing capillary tube, located in the oven, deflects a diaphragm or bellows which activates the switch. Electronic switching devices such as transistors, thyristors and triacs may also be used for temperature control of bvens , P 2 0 0 0 w v 2 2 0 v a n d P : I 2 . R s o , :2:1 4 D R : ' : : lt : 24.2 Q :9 . 1 2 When measured cold, the resistance value may be lower than the calculated value. High power ovens and hotplate ranges are often supplied from the 3-phase, 440 V a.c. supply. Thermostats control the on-off heating cycle. A simple oven circuit is shown in Fig. 5.20 as an example of contactor control. Many and varied circuits occur in practice and the manufacturer's drawings must be checked in a particular case. Microwave ovens provide rapid defrosting and cooking of foods.
Galley and Laundry 139 LP2 ! 1 2 3 4 ul 6 7 elements 1 A ; € (top) ll' "rl;"Jnt'il fl, positions5ll U 1 4 2 U o o 1 A Fig. 5.20 Basic oven temperature control. The microwaves are produced by a specialvalve called a magnetronoperating at around 4000 V with a frequ-ency o-f 2450 MHz. Specialised knowledg-e is required Jor the repair of this type of oven and internal Tault finding -is not recommendedwithout the manuficturer's guidance. Inspection and maintenance of galley equipment is most important. The main objective is to keep the electrical parts clean and free of water, oil, dust and grease. Pay particular attention to all connection points in high current heating circuits where loose connections cause overheating and future problems. For operator safety, all enclosure metalwork must be earthed and regular checks of earthing straps must be given priority. Insulation resistance (IR) tests on heating elements, when cold, may reveal surprisingly low values (10-100 kh) even with new elements. This is becausethe element insulation (magnesium-oxide powder) is somewhat hygroscopic (ab- sorbs moisture). The insulation reiistance value of a healthy heating element should _rise rapidly after being operated for a few minutes. Obviously, if the IR value of an element remiins low when hot it is defective and must be replaced. o Laundry Washing machines, spin dryers and tumble dryers utilise heat and mechanical rotation during their laundry processes. The sequence of events is controlled by timers which are often simple electric timer motors driving cam-operated switches. Alternatively, electronlc timers with relay switching or solid state electronic switching using thyristors or triacsmay be employed. Small washing machines operating on a single-phase supply have motors which are usually the split-phase type of the capacitor-start,capacitor-runvariety.
140 Ancillary Electrical Services Larger washing machines operate from the 3-phasea.c. power supply with a 3-phaseinduction motor drive. Control items in a washing machine include water level switches, temperature switches (bi-metallic) and solenoid valves in the inlet and outlet water lines. Lid and door switches interrupt the main power supply if operated after the washing sequencehas begun. Spin dryers have a safety door interlock that prevents it being opened while the drum is still revolving. Tumble dryers often only have one motor with a double- ended shaft for drum and blower fan drives. Lint and fluff collects on the motor and wiring which causes no trouble while it remains drv and in small quantities. Periodic removal of the fluff will help prevent faults arising where dampness may combine with the fluff to cause conductive tracking between live conductors and to earth. Small single-phase motors are sometimes protected by a thermal cut-out attached to the stator end windings. 5.L0. Cathodic Protection The outer surface of a ship's hull is subjected to electro-chemical attack by corrosive currents that flow between areas of the hull which are at slightly different electric notentials. Dissimilar metlls, variations in struc- tural and chemical uniformity in hull plates and welding, differences in paint thickness and quality, water temperature, salinity and aeration all combine to cause ireas of the hull to become either anodic(positive) or cathodic(negative). Fig. 5.21 shows that in the hull, electrons flow from anode to cathode leaving positively charged iron ions at the anodic area. At the cathode the effect of the arrival of electrons is to produce negatively charged hydroxyl ions (OH) by electrolysis of the sea water. These negative ions flow through the sea to the anodic area where thev combine Fig. 5.21, Cathodic protective action.
Cathodic Protection 141 with the positive iron ions to form ferrous hydroxide Fe(OH)2. This ferrous hydroxide is further oxidised bv dissolved oxyg-ento form ferric hydroxide Fe(OH)3 which is rust. Thus the anodic area is gradually corroded away whilst no corro_ sion takes place at the iathodic area. This naturally corrosive action can be overcome if the complete hull is made cathodic, i.e. electrohs are allowed to arrive at the hull surface and produce ,negativehydroxyl ions but no eiectrons leave the hull to produce positive iron ions. This is achiev-edby fittins insulated lead or platinised titanium inodes to the hull and _applying a positive d.c. potential to them with reipect to the hull. The.negatively charged hydroxyl ions (OH) now pass to the insulated lead a-nodes causing the lead surface to change to lead peroxide pbo2. The potentia[ is of such J value that it just overcomes the original corrosion current and gives rise to an impressed protection current which flows-in the complete circuit. The value of protection current must be critically conlrolled to just prevent corrosion, is beyond this value the increase in the rate bf release of hydroxyl i-o1s will cause sponginess and flaking of the anti-fouling paint. Initially the electrolytic action will form lead peroxide (PbO2) on the surface of the anodes and when this skin is formed the action reduces. The anodes take on a rich brown appearance(positive lead-acid battery plate) and in service are expected to last 7-10 years. The correct value of protection current can be determined bv reference elec- trodes. These are either of zinc or silver attached to the hull, but insulated from it, below the waterline. The voltage measured between the hull and re-ferenceelectrodes of an unprotected ship with sea water as an electrolvte is: c Zinc electrode (450mV negatioeto hull) o Silver electrode (600mV positiaeto hull) When satisfactorily ptot"cted, the pro- tection current will make the hull 200^mV more negative, i.e. a zinc reference electrodewill register 250 mV negative to hull and silver 800 mV positive" to hull as shown in Fig. 5.22.-The reference electrode voltage may, therefore, be used to monitor the protection, but more important, is used as the signal source to automatically regulate the value of protection current. Cathodic protection systems fitted in thry.r c.onsistof a number of anodes (lead or platinised titanium) fitted to the hull at selectedplaces below the waterline, and control. equipment which automatically regulates the anode current to thi) required value. Direct current is supplied to the anodes, after transformatio^n-and rectification, from the ship's M0 V 60 Hz 3-phase a.c. distributio^nsystem. The control_equipment comprises reference electrodes,an amplifier assemblyand one or more transformer rectifier units. _ neoative l l l, r r l l l_ positive , - l l l t lr r t l l r I n IJ fL_ t-l U 2 5 0 m V U 800mV Fig. 5.22 Protection voltages.
142 Ancillary Electrical Services I I I I I I I I I I I ; I L d.c. power supply + . " h a n d / : eleGlronlC auto I protective anodesregulator I rectifier to reference anodes protective anodes basic circuitarrangementsfor cathodicprotection "set" fi,r hand value f reg. ; I I Fig. 5.23 Ship anodes and impressed current control system. The anode current control is usuallv regulated by electronic thyristor con- trollers and the diagram in Fig. 5.23 outlines a typical scheme. The control equipment automatically monitors the size of anode current required which will vary with the ship's speed, water temperature and salinity, condition of paint work etc. Typical anode current densities range from 10 mAim2 to 40 mA/m2 for the protection of painted surfaces and 100 to 150 mA/m2 fbr bare steel surfaces. The total impressed current for a hull in good condition may be as low as 20 A. Maximum controller outputs may be up to about 600 A at 8 V. Cathodic protection does not appear to deter molluscular growth on the ships hull, so a top coat of anti-foul (poisonous) paint is still necessary. Typical referenceand main anode outlines are shown in Fig. 5.24. Monitoring facilities in the cathodic protection control cabinet may provide measurementsof: o Reference electrode voltage (hull potential) o Amplifier output voltage o Total anode current o Individual anode current Measurementsshould be regularly logged together with the ship operating con- ditions, e.g. location, draught, water
Battery Supplies 143 fibre glass cover smallholes to silver-chloride lead-alloystripsmountedin a glassfibreinsulatingmoulding mainanode o . , , o o Fig. 5.24 Reference and main anode construction outlines. temperature, etc. Changesin underwater hull area, speed, water temperature/ salinity and paint condition will all cause the anode currents to vary. The hull potential should, however, remain cons_tantin a properly regulated system. Although the reference elecirodes and the monitoring facilities give a reasonable day to day check they are gnly_ measuring in the vicinity oi the fitted electrodes. When the ship is moored singly or stgppel at sea, voltage readings iah be taken between a portable silver or zinc test electrode and the ship's hull. This portable electrode is lowered 2-3 metres below the water surface and as close as possible to the hull at specified positions around the ship. Check the manufacturer's instructions regarding -the storage and setting up of the portable electrode. Some fiave to be immersed in a plastic bucket of sea water for about 4 hours before the hull test. With the cathodic protection switched on and working noimally, the voltage measured between hull ind a silver/silver chloride portable electrode should be 750-850 mV using a high resistance multimeter (e.g. analogue or digital type); the electrode being p-ositive with respect to hull. When dry docked, ensure that the main anodes and reference electrodes are covered with paper tape to prevent paint contamination. To ensure that the rudder, propeller screw and stabiliserfins receive the same degree of cathodic protection as the hull it is necessary to electricallv earth-bond these items to the hull. The iudder stock may be bo-nded by a wire braid linking the top of the stock to the deckhead directly above it. Carbon brushes rubbing on the rotating main propulsion shaft effectively bond the shaft to the hull. A periodic inspection of such earthing is worthwhile as the brushes wear awav and may occasionally stick in their brush holders. 5.LL. Battery Supplies A.-properly maintained storage battery will instantly supply electric power wheir required. This feature makes a battery the key element in the provision of essential and emergency power supplies on board ships. Essential routine power supplies, e.g. for radio equipment,-telephone -exchang6, fire detection, general alarm circuits etc., are often supplied from two sets of batteries worked on a regular chargei discharge cycle. Emergency battery supplies, e.g. for emergency, generator start-up and emergency lighting, are used in a standby
IM Ancillary Electrical Services role to give power when the main supply fails. Ships' batteries are usually rated at a nominal voltage of 24 V d.c. In some casesa battery system of 110 Y or 220 Y d.c. may be used where a large amount of emergency lighting and power is vital or where a battery is the only source of emergency power. The two main types of rechargeable battery cell are: o Lead-acid o Alkaline The nominal cell voltages of each type are2Y for lead-acidand'1,.2Vfor alkaline. Hence, twelve lead-acid cells or twenty alkaline cells must be connected in serie-s to produce a nominal 24 V. More cells may be connected in parallel to increase the battery capacity which is rated in Ampere-hours (Ah). The battery capacity is usually rated in terms of its discharge at the l-0 hour rate. A 350 Ah battery would be expected to provide 35 A for 10 hours. However, the battery will generally have a lower capacity at a shorter dischargerate. The manufacturer's discharge curves must be checked for such details. After a 10 hour discharge a lead-acid cell voltage will have fallen to ap- proximately 1,.73V. The equivalent figure for an alkaline cell is 1,.14V. Battery installations for both types of battery are similar in that the battery room -should be well ventilated, clean and dry. Both types generate hydrogen gas during charging so smoking and naked flames must be prohibited in the vicinity of the batteries. Steelwork and decks adjacent to lead-acid batteries should be covered with acid-resisting paint and alkali resisting paint used near Ni-cad cells. Acid cells must never be placed near alkaline cells otherwise rapid electrolytic corrosion to metalwork and damage to both batteries is certain For similar reasons, neaer use lead-acid battery maintenance gear (e.g. hydrometer, topping up bottles, etc.) on an alkaline installation or vice-versa. Battery maintenance includes keeping the cell tops clean and dry, checking the tightness of terminal nuts and applying a smear of petroleum jelly to such connections to prevent corrosion. Be most careful when handling the battery electrolyte (e.9. when using a hydrometer to check its specific gravity). Use protective rubber gloves and eye goggles when handling electrolyte. Insulated spanners should be available for use on cell connections to prevent accidental short-circuiting of battery terminals. Such a short-circuit acrossthe terminals of just one cell of a battery will cause a blinding flash with the probability of the cell being seriously damaged. QUESTION An alkaline cell has an electrolvte of potassium hydroxide while a lead-acid cell uses sulphuric acid. Both are diluted with distilled water. What first aid treatment would vou apply should you be splashed with eiiher electrolyte? ANSWER In both casesrapidly wash eyes and skin with plenty of fresh water. The electrolyte of alkaline cells causesskin burns which should be treated with boracic powder and the eyes washed out with a solution of boracic power - one teaspoonful to a pint of water. Sulphuric acid splashescan be washed with a saline solution - two teaspoonfuls of household salt to one pint of water. For both types of battery first aid equipment should be in the battery compartment.
BatterySupplies 145 NegativeTerminal PositiveTerminal PositivePlate NegativePlate Separator Fig. 5.25 Lead-acidcell construction. The diagram in Fig. 5.25 outlines the principal features of a lead-acidcell. The state of charge held by a lead-acid battery is best indicated by a test on the el-ectrolyte specific gravity (SG) by using a hydrometeras shown in Fig. 5.26. A fully charged lead-acid cell has an SG of about 1..27-1..285(often written as'1.270-1285) which falls to about 1.1 (or 1100) when fully discharged. The cell voltage also falls during discharge and its value can also be used as an indication of the state of charge. A lead-acid battery may be safely discharged until the cell voltage drops to approximately 1,.73V (measuredwhile delivering load current). float in dilute acid liquid within glass syringe high float = high SG low float = low SG rubbertubeto withdrawliquid from battery Fig. 5.26 Hydrometer testing.
1,46 Ancillarv Electrical Services PositiveTerminal NegativeTerminal FillerCap Gas ReleaseValve PositivePlate NegativePlate PositiveActive Material NegativeActive Material InsulatingRods EdgeInsulator SteelContainer FinelyPerforated PocketEnvelopes Fig. 5.27 Alkaline cell construction. The open-circuit (no-load) battery voltage readings can be misleading as a high value does not necessarilyindicate that the cells are in a healthy charged state. Note, the SG values quoted above for lead-acid cells are based on an ambient temperature of 15'C. Corrections to the SG value at any other ambient tempera- ture are as follows: o Add 0.007 to reading for each 10oC above 15'C o Subtract0.007 from reading for each 10"C below 15"C. e.g. a hydrometer reading taken during an ambient temperature of 25"C is'1,.27. The equivalent SG value at LSoC is 1,.27+ 0.007: 1..277(or 1277) Fig. 5.27 outlines the principal features of an alkaline cell. The state of charge of an alkalinebattery cell cannof be determined from its SG value. The electrolyte density does not change during chirge/dischirge cycles but gradually falls during the lifetime of the battery. New alkaline cells have an SG of around 1190. When this reduces to
Battery Supplies 147 about 1145 (which may take 5-10 years depending on the duty cycle) the electrolyte must be completely renewed or the battery replaced. Discharge of alkaline cells should be discontinued when the cell voltage has fallen to about 1.1 V. Battery charging equipment uses a transformer/rectifier arrangement to supply the required d.c. voltage to the cells. The size of voltage depends on the battery type (lead-acid or alkaline) and the mode of charging, e.g.charge/ discharge cycle, boost charge, trickle or float charge. Check the manufacturer's instructions for details of the required charging voltages. Do not allow electrolyte temperatures to exceed about 45'C during charging. A lead-acid cell will gas freely when fully charged but an alkaline cell gases throughouf the charging period. The only indication of a fully charged alkaline cell is when its voltage remains at a steady maximum value of about 1 . 6 - 1 . 8V . Generally, alkaline cells are more robust, mechanically and electrically, than lead- acid cells. Nickel cadmium cells will hold their charge for long periods without recharging so are ideal for standby duties. Also they operate well with a float-chargeto provide a reliable emergency supply when the main power fails. For all rechargeable batteries (other than the sealed type) it is essential to replace lost water (causedduring gassing and by normal evaporation) with the addition of distilled water to the correct level above the plates. Exposure of the cell plates to air will rapidly reduce the life of the battery. On all ships and offshore platforms there are particular essential services which are vital during a complete loss of main power. Such services include switchgear operation, navigation lights, foghorns, fire and gas detection, internal communications, some radio communica- tions, alarm systems. To avoid the loss of essential services they are supported by an uninterruptiblepower supply or UPS. These can be for battery supported d.c. supplies or A.c. supplies both of which can be configured as continuous UPS or standbyUPS. Fig. 5.28 shows an a.c. supported UPS arrangement: continuousUPSsystem standbyUPSsystem a . c a . c Fig. 5.28 UPS systems.
148 Ancillary Electrical Services 44OV main switchboard T 440v emergencyboard ./ ./ ./ 3-phasetransformer bridgerectifier blockingdiodes blockingdiodes 1 1 0V or 24V d . c . battery No.1 1 1 0V o r 2 4 V d . c . battery N o . 2 d.c. services __=_ I _:_ Fig. 5.29 UPS d.c. battery charger. The arrangement shown in Fig. 5.29 is typical of.a continuousUPS d.c. supported supply system. The essential d.c. services are normally supplied from the 440 V main power system through charger 1 which con- tinuously trickle charges its battery. During a loss of main power, battery 1 maintains a transitional supply while the emergency generator restores power to the emergency board and hence to charger 2. Either battery is available for a few hours if both main and emergency generators are unavailable. Some critical emergency lights have an internal battery supported UPS within the luminaire where its battery charge is continuously maintained during non- emergency conditions.
r49 Chapter Six Special Electrical Practice for Oil, Gas and Chemical Tankers 6.0 Introduction 6.1. Tanker Classification 6.2 Hazardous Zones 6.3 Electrical lgnition of Gas 5.4 Apparatus Gas Groups 6.5 Temperature Class 6.6 Types of Explosion Protection 6.7 Exd Flameproof Enclosure 5.8 Exi Intrinsic Safety 6.9 Exe Increased Safety 6.1,0 Exn Non-Sparking 6.11, Exp PressurisedEnclosure 6.\2 Exs Special Protection 6.13 Certification and Identification 6.14 Electrical Testing in Hazardous Areas 6.15 Maintenance of Ex-protectedApparatus Page 149 L50 153 1,53 1,54 155 L55 L56 1"58 150 161, 161. 162 163 164 164 6.0. Introduction Ships and offshore installations that transport, process and store bulk quantities of oil, gas and liquid chemicals are subject to special codes of practice regarding their electrical installations. Statutory authorities and classification societies generally base their recom- mendations on Publication 92 of the International ElectrotechnicalCommission (rEC). The object of all such guidance is to prevent the hazards of fire and explosion occurring on board these tank ships. Spacesin tankers where explosive gas-air mixtures may be expected to be present are called dangerousor hazardous.AII other areas being regarded as safe. The best way to avoid explosions caused by electrical equipment is simply not to install such equipment in the
150 Special Electrical Practice for Oil, Gas and Chemical Tankers hazardous areas. However, special electrical equipment is permitteii and this chapter will provide a guide to the range and maintenance of such explosion (Ex) protected equipment. D Type D Tankers for the carriage in bulk of other flammable liquid cargoes. This includes those cargoes which are potentially more dangerous than those conveyed by Type A and Type C tankers, and those products which exhibit chemical instabilitv. D Type A Tankers Dangerous * Cargo 5.1. Tanker Classification Shore practice f.or hazardous areas is to divide the areas into three zones (0,1,2) which recognises the degree of hazard by indicating the likelihood of an explosive gas-air mixture being present. This practice is not used on tankers. However, electrical equipment is manu- factured on the basis of such zones. On tankers, areas are designated as either dangerousor normally-safespaces. A dangerous space is defined as an area where flammable gas-air mixtures would normally be expected to occur. The degreeof hazard or danger presented by ^ dangerous space is determined, initially, by the nature of the flammable cargo of the tanker. On this basis, four types of tanker are recognised as: tr TypeA Oil tankers intended for the carriage in bulk of non-boiling oil cargoes having a flash point (closed test) of 60'C oi less. These include crude oil carriers, gasoline carriers, etc. O TypeB Oil tankers intended for carriage in bulk of non-boiling oil cargoes having a flash point (closed test) in excessof 60'C. These include tankers for carrying bituminous or asphalt products, oi foi carrying diesel or fuel oils. D Type C Gas carriers intended for the carriage * * tanks. width of the vessel plus 3 m * * * * Compartments for cargo hoses. * Enclosed or semi-enclosed having a direct opening into the spacesor areasmentioned and aft on open deck, up to of 2.4 m above the deck. sPaces any of above. the full forward a height in bulk of liquefied petroleum gas (LPG) only be Electrical equipment and cables should onlv be located in daneerous sDacesdangerous spaces * Cofferdams adjoining cargo tanks. Cargo pump rooms. Enclosed or semi-enclosed spaces immediately above cargo tanks, or having bulkheads above and in line with the cargo tank bulkheads. Enclosed or semi-enclosed spaces immediately above cargo pumprooms, or above vertical cofferdams adjoining cargo tanks, unless separated by u gastight deck and suitably mechani- cally ventilated. Spaces, other than cofferdams, adjoining and below the top of the cargo tanks, e.g. trunks, passageways and holds. Areas on open deck, or semi-enclosed spaceson open deck, within at least 3 m of any cargo oil tank outlet or gas or vapour outlet. * Areas on open deck over all cargo tanks, including all ballast tanks within the cargo tank block and to or liquefied natural gas (LNG). when it is absolutely necessary.
Tanker Classification 151 Only intrinsically safe (Exi) electrical equipment is allowed inside cargo tanks. Electric motors are not perniitted in cargo PumP rooms. . Fl.ameproof(Exd) or pressurised(Exp) luminaires may be used in pumprooms. The switches and fuses for the luminaires must be located in a normally safe space outside the pumproom. Ai least 1wo independent circuits must be provided for the lighting. If maintenanceis carried out on the luminaires of one circuit this circuit must be de-energised while the other circuit provides su-fficient light for the work to be safely completed. - or semi-enclosed spaces immediately above a cargo tank, above a cargo pump room and in compartments foi storing cargo hoses. The switches and fusei must be located in a normally safe area and must switch both lines of the circuit (i.e. double pole switching). D Type B Tankers Dangerous spaces are not defined for vesselsof this type, but it is recommended that care be exercised so that potential sources of ignition are reduced is far as possible. Also, the following practices should be followed: * Use intrinsically safe (Exi) for any monitoring or instrumentation equip- ment which is in direct contact wiih oil in the cargo tanks or in the oil circuits. * Cargo pump motors should be inueasedsafety (Exe) type if they are located in the cargo pump room. * All portable electrical equipment used in the cargo tanks must be suitably explosionprotected(Ex). D Type C Tankers (Gas Caniers) Dangerous Spaces: * A space in the cargo area which is not equipped with approved arrange- ments to ensure that its atmosphere is at all times maintained in a safe condition. * An enclosed space outside the cargo area through which any piping terminates, unless approved arrange- ments are installed to prevent any escape of product vapour into th-e atmosphere of that space. * A cargo containment system with cargo piping: (a) a hold space where cargo is carried in a cargo containment !/stem requiring a secondary barrier; QUESTTON Tanker pumprooms require two separate lighting circuits. How Can the circriits be arranged so that the luminaires can onlv be opened rp when the correct circuit has been isolated? ANSWER Many tankers use the following arrangement: The luminaires on one of the circuits have bolts with a different size or type of head to those on the luminair'es slPplied by- the other circuit. Typically, these would be two different types bf triangular bolt head The keys to remove the bolts are actually the operating handles of the circuit isolators. A key can onlv be removed from its trapped positiotr or, the switch after the ^iircui? has been isolated. This key can only open up those luminaires connected to the circuit which has been isolated. Cable runs are permitted through most ctang.erous- spaces except cargo tanks, provrcted they are continuously moni_ tored for earth leakage. . Flameprool(Exd) or pressurised(Exp) luminaires are permitied in enclosdd
152 Special Electrical Practice for Oil, Gas and Chemical Tankers (b) a hold space where cargo is carried in a cargo containment system not requiring a secondary barrier. * A space separatedfrom a hold space described in (a) above by u single gastight steel boundary. * A cargo pump room and cargo compressorroom. * A zone on open deck, or semi- enclosed space on open deck, within 3 m of any cargo tank outlet, gas or vapour outlet, cargo piped flange, cargo valve or of entrances and ventilation openings to a cargo pumproom and cargo compressor rooms. * The open deck over the cargo area and 3 m forward and aft of the cargo area on open deck up to a height of 2.4 m above the weather deck. * A zone within 2.4 m of the outer surf ace of a cargo containment system where such surfaceis exposed to the weather. * An enclosed or semi-enclosedspace in which pipes containing products are located. * A compartment for cargo hoses. * An enclosed or semi-enclosedspace having a direct opening into any dangerous space or area. The recommendations for the use of electricalequipment in dangerous spaces are the same for this type of vessel as they are for Type A tankers. There are, however, two important additional recommendationsfor gas carriers: * Cargo Pump Motor Submerged cargo pump motors and their cables are permitted in cargo tanks subject to the atmosphere of the tank being controlled to prevent presence of a gas-air mixture when the motors are energised. * Gas Compressor Motors These motors are allowed, under certain circumstances, to be sited in the same space as the compressors. In these instances the motors are required to be pressurised (Exp) with air, inert gas or water. Alternatively, an increasedsafety (Exe) motor within a flameprool(Exd) enclosure may be used and marked overall as Exe d. D Type D Chemical Carriers The products carried in these vessels may produce explosive gas-air mixtures and can also be intenselv corrosive. In cases like this elecirical equipment must not only be explosion protected but also designed to withstand corrosion. These products are categorised as follows in order to give guidance on the electrical equipment which would be suitable. Types of chemical product: * Products which have similar proper- ties to those carried by vessel types A, B and C. The recommendations given for those vesselswould apply. * Products which are considered to be more hazardous than those above. The extent of dangerous areas is increasedfrom 3 m to 4.5 m. * Products which are susceptible to chemical instabilitv which creates flammable gases. -Special arrange- ments would be required for this type of product. * Products which will damage any electrical equipment with which they come into contact. Materials and enclosures must resist the corrosive effect of these products.
Electrical Ignition of Gas 153 Hazardous areas ashore are classified into zoneswhich indicate the probability of an explosive gas-air mixture being present and, therefore, the likelihood of an explosion occurring. * Zone 0 In which an explosive gas-air mixture is continuously present, or present for long periods. * Zone 1 In which an explosive gas-air mixture is likely to occur in normal operation. * Zone 2 In which an explosive gas-air mixture is not likely to occur in normal operation and, if it occurs, will exist for only a short time. 6.2. Hazatdous Zones An area which is not classified Zone 0,'L or 2 is assumed to be a non-hamrdous or safe area. Examples of this zoning applied to ships could be: * Zone 0 Interior spacesof oil cargo tanks, pipes, pumps, etc. * Zone 1 Enclosed or semi-enclosedspaceson the deck of a tanker, the boiler firing area on a gas carrier using methane boil-off as a fuel and battery rooms. * Zone 2 Open spaces on the deck of a tanker. The cargo pump rooms of tankers are, at present, consideredas falling somewhere between Zone 0 and Zone 1. 5.3. Electrical Ignition of Gas In practice, three essential components must be present to start a fire or causean explosion: * A flammable gas or vapour (hazard) * Air or oxygen to support combustion (oxidiser) * Something to start the explosion (source of ignition) When all three of these components are brought together ignition can take place, often with devastating results. I S O U R C EO F I G N I T I O NI Fig. 6.1 Fire triangle. The occurrence of a fire or ignition depends on the probability of the simultaneous occurrence of all three components shown in the fire triangle diagram shown in Fig. 6.1. Gases, when concentrated above the Louter FlammableLimit (LFL), can be ignited by heat generated from various electricalsourcese.g.: * Arcing between switch contacts.
154 Special Electrical Practice for Oil, Gas and Chemical Tankers * Arcing between a live conductor and earth. * An internal arcing fault within an electrical enclosure. * Overheating causing hot spots. * An electrostatic spark discharge between charged bodies or between a charged body and earth. * Chemical action. * Lightning strikes. As might be expected,the flammabilityof a gas/air mixture is dependent upon the ratio of. gas to air. A ratio of 100o/ogasl air concentration will not burn and, as can be expected, 0% will also not burn. Furthermore, each gas is quite different and the flammabilityrangedepends on the gas type as shown in the table below: The above table shows that hydrogen has a very low ignition enerry, but a very high ignition temperature.Acetylene, however has a low ignition energy and low ignition temperature (beware, it is very easy to ignite). Methane with its very high ignition temperature and high ignition energy can prove quite difficult to ignite. (Natural gas igniters are di{ficult to design!) On the other hand, the amount of energy released(in MJ/m3) by a given volume of gas does vary, methane containing three times as much energy as Hydrogen and Butane eleven times as much (see table below). A medium size camping gas cylinder of butane can contain more than enough energy to destroy an average-sizedhousehold garage. The terms used to describe these limits are called: LFL, lowerflammablelimit, and UFL, upper flammablelimit (previously called LEL and UEL, lower and upper explosioelimits). This is still not all that must taken into consideration; there is also the amount of minimum ignition enerry required to ignite the gas, and the temperatureat which the gas automaticallyignites. Some examples are shown in the table below: 6.4. Apparatus Gas Groups The flammable gases in which explosion protected electrical equipment may have to operate are grouped according to the amount of electrical energy,in the form of an arc, which is needed to ignite the gas. Gases associated with the mining industry are fire-classedas GROUP I, all other industrial gases are classed as GROUP II which are listed in three sub-groups according to their ease of ignition. It should be noted that equipment certified for use in group IIC may also be used for IIA and IIB. Equipment certified for IIB mav be used for IIA. Equipment certifi-edfor IIA may be used with no other group. Gas Flammable Limits Lower o/" I Higher % Acetylene 1 . 5 100 Hydrogen 4 75.6 Methane 5 15 Butane 1 . 5 8 . 5 Gas Net Calorific Value M|/m3 Acetylene 51 Butane 112.4 Hydrogen 10.2 Methane 34 Gas Auto- Ignition Tenperature ("c) Minimum ignition energy (mI) Acetylene 305 0.02 Butane 365 0.25 Hydrogen 560 0.02 Methane (firedamp) 595 0.29
Types of Explosion Protection 155 Gas or Vapour Methane (Firedamp) Ammonia Industrial methane Blast furnace gas Carbon monoxide Propane Butane Pentane Hexane Heptane iso Octane Decane Benzene Xylene Cyclohexane Acetone Ethyl methyl ketone Methvl acetate Ethyl acetate n-Propyl acetate Butyl acetate Amyl acetate Chloroethylene Methanol Ethanol iso Butanol n-Butanol Amyl alcohol Ethyl nitrite Buta-1, 3 diene Ethylene Diethylether Ethylene oxide Town gas The gas grouping can affect the design and construction of some types of explosionprotectedequipment (Exd and Exi) 6.5. Temperature Class This defines the maximum surfacetemp- erature of the components in the electrical equipment under normal and fault conditions. This maximum surface temperature must not exceed the gas ignition temperature. The temperature class is stated with reference to a maximum ambient temperature of 40"C, should any other reference temperature be adopted, regulations require that this temperature be shown on the equipment. It is important to note that the apparatus gas grouping and temperature class are not related. For instance, hydrogen requires very little spark energy to ignite, but the surface temperature necessary for ignition is very high (560'C). The following table relates the tem- perature class to the maximum surface temperature under fault conditions. Temperature Class Maximum surface temperature T1 450"c T2 300"c T3 200"c T4 135"C T5 100"c T6 9 5 "c For example, an electric motor may have a maximum surfacetemperature of 120'C and would be classedas T4. Temperature Classifications and Apparatus Groups for all Group II gases can be found in BS 5345 Part L. 6.6. Types of Explosion Protection There are a number of different constructional techniques employed in preventing electrical equipment causing explosions in hazardous areas. Some techniques, such as flameproolenclosures, have long been established but others, such as intinsic safety and increasedsafety,
156 Special Electrical Practice for Oil, Gas and Chemical Tankers Fig. 6.2 Ex identification marks. are the result of developments in material and electrical/electronic circuit design. U K T e s t H o u s e 6.7. Exd Flameproof Enclosure Type 'd' protection, code EExd, uses a flameproof enclosure to contain the electrical apparatus. The internal apparatus may include parts which arc and surfaces which become hot. Gas may be inside the enclosure so it must fulfil three conditions: The enclosure must be strong enough to withstand an internal explosion without suffering damage. The enclosure must prevent the flame and hot gasesfrom being transmitted to the external flammable atmosphere. The external surface temperature of the enclosure must remain below the ignition temperature of the surrounding gas under all operating conditions. The transmission of flame and hot gases from a flameproof enclosure is prevented because all joints, such as flanges, spigots, shafts and bearings are closely machined to achieve a small gap which is less than a defined maximum. The pressure of an internal explosion is then released through the small g"p between machined faces which cools the gas sufficiently to prevent it from igniting any external flammable atmosphere. The maximum permitted gap depends upon three factors: o The type of gas with which the apparatus is safe for use. This is indicated by ApparatusGroup. o The width of the joint (L). o The volume of the enclosure (V). It has been internationally agreed that explosion protected equipment be identi- fied by the symbol "Ex" followed by u letter indicating the type of protection employed. The following table lists the types ot protection: Some equipment may use more than one of these types of protection in its construction. In this case, the primary type of protection is quoted first. For example, an increasedsafety motor with a flameprool terminal box would be marked Exe d. Equipment may also be marked with a prefix "E" which denotes compliance with European Standards e.g. EExed. Symbol Type of Protection Exd flameproof enclosure Exi intrinsic safetv Exe increased safety Exn non-sparking Exq powder filled (not applicableto ships) Exo oil immersed (not applicableto ships) E*P pressurisation Exs special protection
Exd Flameproof Enclosure 157 FlangeJoint SpigotJoint ScrewedJoint Fig. 6.3 Exd flamepaths. These factors are illustrated in Fig. 6.3 designed for use in apparatus gas for a flanged enclosure: group IIC. The ignition t-emperature of QUESTTON A ship's battery room is fitted with a flameproof luminaire marked Exd IIC T4. Is this luminaire certified for use in the battery room? 560'C and the tenperature of the luminaire is T4. This it's surface temperature will 135", so the temperature classificatio is satisfactorv. The cable entrv into an Exd enclosure must also be maintain ed flameprooby using a certified Exd gland. This type of gland, shown in Fig. 6.4, has a compound filling which forms a barrier between the indiaidualconductorsandpreaentsentry of explosiae products f rom the enclosureentering the cable. hydrogen is classification means that not exceed ANSWER Yes. The hazard is hydrogen gas batteries which requires from the apparatus Fig. 6.4 Exd cable gland.
158 Special Electrical Practice for Oil, Gas and Chemical Tankers 5.8. Exi Intrinsic Safety These are circuits in which no spark nor any thermal effect produced uhder prescribed test conditions (which include normal operation and specified fault conditions) is capable of causing igni- tion of a given explosive atmosphere. Generally, this means limiting the circuit conditions to less than 30 V and 50 mA. Naturally, this restricts the use of Exi protection to low power instrumentation, alarm and communication circuits. The design of the circuit will depend on the type of gas present (gasgrouping). In the UK, two grades of intrinsic safety are recognisedbased on the safety factor of the equipment involved: * Exia the highest categorybased on a safety factor of 1.5 with two faults on the circuit. * Exib based on a safety factor of 1.5 with one f.ault on the circuit. In addition to apparatusin the hazardous area being rated as intrinsically safe, an electrical safetv barrier mav also be fitted to the circuit. The purpose of such a barrier is to limit ooltages and currents in the hazardous area when faults occur on the circuit. A separate barrier is required for each Exi circuit and thev must be fitted outside the hazardous arei. See Fig. 6.5. A safety (or zener) barrier comprises: ,z A fuse to limit the maximum current through the shunt (zener) diodes. ,z A set of resistors to limit the maximum current into the hazardous area. r A set of shunt connectedzener diodes to limit the maximum voltage appear- ing on the circuit within the hazardous area. hazardouslocation terminals internallayoutof barrier Fig. 6.5 Exi barrier construction.
Exi Intrinsic Safatv 159 All components are sealed into a compact package with clearly marked terminals at each end of the barrier. The circuit in Fig. 6.6 shows a single- channel zener barrier. It illustrates the preventive action in the event of a high voltage being accidentally applied to the non-hazardous terminals. The zener diode characteristic shows that when connected with reverse bias it has an approximately constant voltage across it irrespective of the size of current flow. In normal operation the instrumentation circuit has a supply voltage lower than the U" voltage rating of the zener diodes so no current flows through them. IA/hen an accidental high voltage appears at the input to the barrier, the diodes conduct to clamp their voltages to their U" rating. This then limits the maximum F u s e 3 H a z A r e a T e r m i n a l s 1 S a f e A r e a T e r m i n a l s 2 Z e n e rD i o d e C h a r a c t e r i s t i c 3 H a z A r e a T e r m i n a l s 1 E x a m p l e : o v e r v o l t a g e d u e t o a n e x t e r n a lf a u l t l 2 v o l t a g e Fig. 5.5 Exi barrier operation.
160 Special Electrical Practice for Oil, Gas and Chemical Tankers S c r e e n s A r m o u r E a r t h S t r u c t u r a lE a r t h s Fig. 6.7 Exi cable terminations. voltage appearing on the hazardous area wiring. While the zeners are conducting, the current level is designed to blow the fuse which now isolates the circuit to maintain safetv in the hazardous area. In the eveni of a short-circuit on the hazardous area wiring or equipment, the inJine resistors within the barrier will limit the size of fault current while the fuse blows. Two or three zener-resistor combinations are used within a barrier to provide back-up voltage anchorswhile the fuse is blowing. After clearing a fault, the complete zener barrier must be replacedwith an identical unit. No alterations to the original is allowed - remember this is a certifiedEx safety device. Cables for intrinsicallv safe circuits aboard ships should be separatedfuom power cables and the crossing over of such cables should be at 90". This is to minimise electromagnetic interference from the power cables affecting the intrinsicallv safe circuits. The metallic cable screensof intrinsically safe ckcuits should be earthed at the power supply end only to prevent circulating currents within the sheath. See Fig. 6.7. Power and intrinsicallv safe cable runs should be separatelyidentified. i.e. by labels or by using cableswith a distinctive colour (typically blue for Exi). 6.9. Exe Increased Safety Increased safety equipment is based primarily on the elimination of open sparking as at relay and switch contacts or on the commutators or slip-rings of motors and generators, and on the close control of surface temperatures. Also, the construction of the equipment is to a very high standard to prevent faults developing. Extra insulation is used, creepage distances between bare
Exp Pressurised Enclosure 16L C r e e p a g e C l e a r a n c e D i s t a n c ea c r o s ss u r f a c e o f i n s u l a t i n gm a t e r i a l D i s t a n c ei n a i r Fig. 6.8 Creepage and clearance distances. terminals are made longer and special enclosures to protect against damage due to entrv of moisture and mechanical damage are also specified. See Fig. 6.8. The enclosure is made to withstand impact and to prevent ingress of solids and liquids. Applications include cage-rotor induc- tion motors, luminaires and connection boxes. Special Exe cable glands, metal or plastic, are used with Exe apparatus. 6.L0. Exn Non-Sparking Similar to Exe, the designation Exn applies to equipment which has no arcing contacts or hot surfaces which could cause ignition. The Exn requirements are less stringent than for Exe, and designs are very close to that of normal electrical apparatus. The main consideration is extra care to ensure locking of terminal connections to avoid any risk of electric sparking or flashover. 6.11,.Exp PressurisedEnclosure Clean, dry air or an inert gas is supplied to the equipment slightly above atmospheric pressure to prevent entry of the external flammable gas. This method is sometimes used for motors, instrumentation enclosures and lighting. The diagrams in Fig. 6.9 show that the internal pressure may be maintained by leakage compensation or by continuous circulation. A pressurisation system requires a purge flow before the internal electrical equipment is permitted to operate. Also, the pressurised enclosure must be fitted with alarm and trip
162 Special Electrical Practice for Oil, Gas and Chemical Tankers S a f e A r e a P r o t e c t i v e G a s l n l e t L e a k a o e H a z P r o t e c t i J e Area 4.i ,{t l l U o f G a s 4lU e /- + + O u t l e t Valve ( S hu t ) S p a r k A r r e s t er \_ <- ( O P e n - f o r + P u r g e ) P r o t e c t i v e G a s O u t l e t n v 0 '.& S a f e A r e a H a z A r e a + €= t f f C h o k e t o r e s t r i c t e x i t o f '---2' Protective E=> Gas S p a r k A r r e s t e r <- e + c h o k e P r o t e c t i v e + c r e a t e s G a s O u l e t overpressu re i n t h e e n c l o s u r e n s y s f , e m )n '.il,..tl''{l P r e s s u r i s e d E n c l o s u r e ( w i t h l e a k a g e c o m p € n s a t i o n ) P r e s s u r i s e d E n c l o s u r e c i r c u l a t i o n Fig. 6.9 Exp (pressurised)enclosurearrangements. signalling for a reduction of pressure which in turn will switch-off the enclosed electricalcircuits. 6.12. Exs Special Protection This includes precautionstaken to prevent explosions which are not specifically covered by the previous designations. The table below shows the type of protection which is allowed in the three hazardous zones: Zone Type Of Protection 0 Exia Exs (specially certified for use in Zone 0) 1 Any type of protection suitable for Zone 0 and: Exd Exib ExP Exe Exs 2 Any type of protection suitable for Zone O or L and: Exn, Exo, Exq
Certification and Identification 163 6.13. Certification and Identification When a manufacturer produces an item of explosion protected equipment, it must be tested and inspected to ensure that it complies with the required standards relating to that type of protection. In the UK this work is carried out by BASEEFA (British Approvals Service for Electrical Equipment in Flammable Atmospheres) and SCS (SIRA Certification Service). BASEEFA/SCS issue a certificate for each explosion protected device they test. The BASEEFA certificate number is shown on the equipment name plate. Some other national certification authorities: USA: There is no national certifying body in the USA. Two separate insurance based organisations carry out tests on equipment and issue approvals (listings) of apparatus and equipment acceptable to their published standards. The organisations are: FM Factorv Mutual Research Corporation. UL Underwriters Laboratories. South Africa: South African Bureau of SABS Standards. The Canadian Standards Association is the national body responsible for certify- ing equipment for hazardous areas. CSA have an affangement with BSI to allow U.K. equip- ment which has a BASEEFA approval to be certified to CSA Standards in the U.K. The Standards Association of Australia is responsible for certification in Australia. SAA has an arrangement with BSI to help U.K. organisations to obtain SAA Approval. PTB are the testing and certi- fication authority, certifying either to CENELEC Standards or to VDE Standards. (VDE is the German equivalent of BSr). Danmarks Eleclriske Material- Kontrol. Denmark also recognises the CENELEC Standards. Det Norske Veritas: an approvals body similar to Lloyds. Canada: CSA Australia: SAA Germany: PTB Denmark: DEMKO Norway: DNV The following example gives a reminder of the meaning of the Ex identification marks on a rating label for an item of explosion-protectedapparatus: Example: EExia IIC T4 No. BASEEFA Ex 78229X E Ex ta nc T4 No. to European Standard EN50 020 Explosion Protection Type of protection Apparatus Group Temperature Class Certifying Authority and Certificate Number
164 special Electrical Practice for oil, Gas and Chemical Tankers QUESTTON Explain the meaning of the Ex label listed above ANSWER Intrinsically safe (Exi) to the highest safety factor (a) which is suitable for installation in Zone 0. Apparatus group (IIC) is suitable for hydrogen. Temp- erature class (T4) allows a maximuh surface temperature of 135'C. The certifying authority is the UK test house BASEEFA. 5.14. Electrical Testing in Hazardous Areas All electrical apparatus and associated circuits are required to be tested periodically in accordancewith a definite testing routine with recorded test results. Insulation resistance,earth loop resistance and earth continuity resistanie tests are required to be made, the last two in relation to the setting or rating of the protective devices associated with the apparatus and its circuitry. It is important that insulation resistance tests are NOT made in such a way that the safety devices and insulation- used in intrinsically safe apparatus and circuits are damaged by excesstest voltages. No apparatus should be opened in a danger area until it has been made dead and effective measures (e.g. locking-off the isolating switch) have been tiken to prevent- its being made live again inadvertently. Where, for the purpose of electrical testing, it is necessary to restore the power supply before the apparatus is reassembled,tests should be made using a suitable gas detector and continueii during the operation to ensure that the combustible does not approach the explosive limit. Unless the hazardous area can be made gas-freeor otherwise safe, or the electrical equipment is removed from the area, then insulation resistance testing should be carried out using a 500 V d.c] t_esterof certified intrinsicilly safe (Exi) design. The.testing and maintenance of flameproof or intinsically safe equipment shoulit be entrusted only to competent persons who have reieived ins^truction' in the special techniques involved. The body material of instruments and tools_qe_quiredfor maintenance purposes should be designed so that they will not make a hot spark when dropped. The energy output of all intinsically safe instruments should be so small that thdv do not produce hot sparks. An insulation tester has a drooping characteristic to prevent -high currents and may be intrinsically safe when applied to circuits of small iriductance or ciairacitancebut a risk may arise when sucki energy-storing properties of a circuit have an aiirreciablE value. Where such instruments are used the test leads should be firmly connected throughout and on compl6tion of the test they should not be detached until the circuit has been discharged through the testing instrument (leavi the testrer for one minute after test is finished). 5.L5. Maintenance of Ex-protected Apparatus The previous sections covering zoning, gas grouping, temperature classification and the various types of protection methods show that the design bf electrical
Maintenance of Ex-protected Apparatus 165 equipment for hazardous areas is very special. Maintenance of such not, in any way, cause be less safe than in its state. This most important point means that the maintenance must be carried out by a competent person. Temporary lash-ups, refitting with wrong sized components (e.9. lamps), failing to employ the correct number of cover bolts etc., is absolutely forbidden. apparatus must its operation to original certified t/ Bolts Make sure that there are no missing bolts. This is particularly important on flameproof luminaires because a missing bolt will invalidate the certification. Replacementbolts must be of equivalent strength as originals (usually high tensile steel). t/ Mountings Ensure all mountings are secure. Corro- sion and vibration are severe on ships and can cause premature failure. ,/ Flamepaths Examine the flamepath for signs of corrosion or pitting. If the flamepath needs cleaning, this should be done with a non-metallic scraper and/or a suitable non-corrosive cleaning fluid. ,/ Cement Examine the cement used around lamp- glass assembliesboth inside and outside. If the cement is eroded, softened or damaged in any way, advice should be sought from the manufacturer regarding repair. If deterioration of the cement has occurred, a complete new lampglass assembly should be fitted. ,/ Lampglass Check lampglass; if cracked or broken a complete new lampglassassemblyshould be fitted. Clean the lampglass. When re-assembling an Exd enclosureyou must ensure that the following points are covered: '/ Lightly grease all flamepaths and threaded components with an ap- proved form of non-setting silicone grease.Care must be taken to ensure that blind tapped holes are free from accumulated dirt or excessive grease Fig. 6.10 Exd (flameproof) motor construction. The inspection and maintenance of Exd (flameprooflenclosures for luminaires, switches, junction boxes, push-buttons, etc., requires meticulous care. The following example gives a guide to the inspection and maintenance points as applied to a flameproofluminaire: t/ Corrosion This will reduce the enclosure strength. To ascertain the extent of corrosion, remove dirt, loose paint and surface corrosion with a wire brush. If only the paintwork is deteriorating, the enclosure should be repainted to prevent further
166 Special Electrical Practice for Oil, Gas and Chemical Tankers which can prevent the correct closure o{ flamepaths, or cause damage to the tapped components. Fit new lamp of the correct rating. Ensure bolts are not over-tightened as this can distort flamepaths, cause excessive stress on lampglasses or distort weather proofing gaskets, if fitted, allowing the ingress of liquids and dusts. Check the luminaire is installed in accordance with the requirements of the installation, particularly the classification of th^e area if - it is hazardous and that the correct rating of lamp is fitted. ,/ Remove any build-up of dust on the luminaire, this can cause overheat- ing as well as acting as a corrosive agent. Before attempting any maintenance work on Exd equipment check for any particular inspection and overhaul instructions given by the manufacturer.
167 Chapter Seven Electrical Survey Requirements 7.0 Introduction 7.'t solAs 7.2 ClassificationSocieties 7.3 Main Electrical Survey Items 7.4 Generators and Governors 7.5 Circuit Breakers 7.6 Switchboards and Fittings 7.7 Cables 7.8 Insulation Resistance 7.9 Motors and Starters 7.10 Emergency Power and Associated 7.17 Parts of Steering Gear 7.\2 Navigation Light Indicators 7,13 UMS Operation 7.14 Tankers Equipment Page 767 767 L58 169 169 171. 172 173 174 175 176 177 178 179 180 Introduction The electrical equipment aboard ship is inspected and tested.during the complete engine survey which occurs every four years. Such a survey is prescribed under the Rules and Regulations for the Classification of the Ship. The electrical survey guidance given in this chapter is based on the periodical Survey regulations of Lloyds Register of Shipping, London. Other classification societies have their own rules which, although similar to Lloyds, should be consulted prior to an electrical survey. 7.1..SOLAS The International Maritime Organization (IMO), which met for the first time in 1959, is a specialised agency of the United Nations devoted to maritime affairs. Its main interests can be summed
L68 Electrical Survey Requirements up in the phrase "safershippingand cleaner oceLns". Of all the international conventions dealing with maritime safety, the most important is the International Convention for the Safety of Life at Sea, better known as SOLAS, which covers a wide range of measures designed to improve the safety of shipping. The Convention oldest of its kind: adopted in 19'J.4 of the Titanic with 1,500lives. is also one of the the first version was following the sinking the loss of more than Since then there have been four more versions of SOLAS. The present version was adopted in 1974 and entered into force in 1980. The Convention in its consolidated edition dated 1997 has eleven chapters. Electrical regulations are part of Chapter II-1 which outlines the require- ments for Ship construction- sub-diaision and stability, machinery and electrical installations. This Chapter has five Parts as follows: + Part A General + Part B Sub-dioisionand stability + Part C Machineryinstallations + Part D Electicalinstallations + Part E Additionalrequirementsfor peiodicallyunattended machineryspaces The electrical installations (Part D) is sub-divided into regulations as: + Regulation40 General + Regulation41 Main sourceof electrical pozoerandlightingsystems + Regulation42 Emergencysourceof electicalpowerin passengerships + Regulation42-1 Supplementaryeffiergency Iightingfor ro-ropassenger ships + Regulation43 Emergencysourceof electricalpoToerin cargo ships M 45 + + Regulation Regulation Starting arrangementsfor emergencygeneratorsets Precautionsagainstshock, fire and other hamrds of electical oigin 7.2. Classification Societies Some of the main Classification Societies for ships are: o American Bureau of Shipping, New York. o Bureau Veritas, Paris. o Germanischer Lloyd, Hamburg. o Nippon Kaiji Kyokai, Tokyo. o Det Norske Veritas, Oslo. o Registro Italiano Navale, Genoa Electrical equipment and services aboard ship must also meet the minimum standards specified by various national and international organisations. For British registered ships in particular, it is necessaryto comply, with: o Regulations for the Electrical and Electronic Equipment of Ships - Institution of Electrical Engineers (IEE). In conjunction with the British Standards Institute (BSI) these Regulations are being combined with the Recommendationsfor the Electrical and Electronic Equipment of Offshore Installations. o The Merchant Shipping Rules - Maritime and Coastguard Agency (MCA) o Safety of Life at Sea (SOLAS) - IMO Convention British Standards (BS) International Electrotechnical Commission (IEC). a o
Generators and Governors 169 Generators and Governors Circuit-Breakers Switchboards and Fittings Emergency Power Equipment Navigation Light Indicators The standards specified by the above organisatio,ns are met when the ship is designed, built, approved and classified. It is for the shipowner and the operating staff to maintain the vessel and its electrical installation to the require- ments of the Classification Societv throughout the ship's lifetime. Th" periodical electrical survey is, therefore, to check that the instaliation is maintained to the Rules of the ClassificationSociety. 7.3. Main Electrical Survey Items The following survey items apply general to all ships: ln For UMS operation, a survey of the associated alarms, controls and fire detection is required. For tankers/gas carriers and other ships transporting flammable cargo, an adcli- tional survey of all electrical equipment in hazardous areas is carried out iiuring each docking survey and annual survey. This means that hazardous area electric-al equipment is surveyed every year. 7.4. Generators and Governors The surveyor will require that main and emergency generators are clean, respond correctly to controls and load changes, and show stable operation when required to run in parallel with other generators. Generator windings on stator and rotor must be free of dust, oil and moisture. See Fig. 7.'1..A visual check will be made for any obvious deterioration, abrasion or cracking of the insulation around the end winding coils on the stator. An insulation test to earth and between stator phase windings (if the neutral point can be disconnectedat the terminal box) should be carried out while the machine is still hot after running on load. See Fig. 7.2.
170 Electrical Survey Requirements Emergency Air InletPanel StatorCore (WithAir Ducts) PilotExciter Heat Exchanger Fanshield Fig. 7.1 Generator construction. Insulation resistance (IR) test. U1 V1 W1 @ @ @ @ @ @ u2 v2 w2 F i g . 7 . 2
Circuit Breakers 171 QUESTION Would an IR test result of 0.5 MO to earth be acceptable for a 440 V main generator? ANSWER Although a minimum of 1.5 MO is generally specified for new equipment, Lloyds rules suggest that 0.1 MO is acceptable in special cases. However, most surveyors would insist on at least 1 kO/volt, i.e. M0 kf), say, 0.5 MO as a reasonable minimum value for a M0 V generator. For HV equipment the usual recommended minimum IR level is (kV + 1) MO. e.g. for a 6.6 kV motor, the acceptable minimum IR would be 7.6 i0/.{O. Remember to disconnect all AVR equip- ment, instrument connections and generator heater supplies when testing for IR. The rotor circuits must also be tested for insulation value, taking care to short out the rotating shaft diodes of a brushlessexcitation system as the diodes usually have a low- PIV (Peak Inverse Voltage) rating. Special attention to the contact surface of any commutator or slip-rings is required. The contact surfaces must be smooth and concentric without any signs of pitting or deep grooves. Carboir brushes must be of adequate length, maintained at the correct spring pressure and properly contoured onto its rotating commutator or slip-ring. Be sure to remove any excess carbon dust in the vicinity of the brush gear and around rotor coils. Generator running tests, on load, should confirm the proper operation of governor and AVR controls with correct voltage, frequency and current values indicated on the generator control panel. Governor droop and its response to sudden load changes must be within the declared specification for the prime mover/ generator combination. Stability of load sharing of kW and kVAr (or load current/power factor) between two or more generators running in parallel must be demonstrated. 7.5, Circuit Breakers A visual examination of circuit-breakers (e.g. as shown in Fig. 7.3) in main, emergency and section boards will usually precede operational tests. The surveyor will particularly check the condition of main, arcing and auxiliary contacts for signs of wear, misalignment and overheating. A similar inspection of fixed and moving isolator contacts at the rear of a circuit-breakerwill be made. Arc chutes must be clean, free of arc debris and correctly aligned. All internal wiring should be in good condition and its end connections must be tight. All mechanical linkages will be checked for any signs of wear or stress. Fig. 7.3 Air-break circuit breaker outlines. Tests on a circuit-breaker will include close and trip operations while in its isolatedposition (i.e. not connected in circuit). The racking mechanism for
172 Electrical Survey Requirements moving the breaker from the service to the isolatedposition must be demonstrated to be free moving and the fixed main terminals must be seen to be shuttered off when the breaker is withdrawn. Emergency hand charging (if fitted) of the closing spring will be tested. Correct operation of the mechanical indicators to show whether the breaker is open, closedor isolated,is required. The underaoltagerelease mechanism and oaercurrenttrip settings for level and time delay may have to be demonstrated to the surveyor's satisfaction. An overcurrent trip for a generator breaker is typically set for 130% of full load current (FLC) with a typical time delay of 3 s, but this has to suit the thermal capacity of the generator and be co-ordinated with the overall protection scheme for the power system. Although the overcurrent and time delay settings on the breaker can be seen to be correctly adjusted to the desired values, only a proper currentinjectiontest will prove these settings against the manufacturer's I/t characteristics.In this test the circuit-breaker is isolated from the bus-bar and a set of calibrated currents from a current injection set are fed directly through the closed circuit-breaker (primary injection) or (more usually) through the overcurrent relay (secondaryinjection).This is generally a specialisttask for an outside contractor. Circuit-breaker time delav mechanisms with oil dash pots must have the pots filled to the correct level with clean oil of a type recommended by the manufacturer. 7.6. Switchboards and Fittings An obvious survey requirement for any switchboard (as in Fig. 7.4), section board or distribution board is that they are clean. This includes all internal surfaces as well as the external panel surfaces, instrument faces and control switches. A thorough cleaning job on the inside of the main switchboard can only be safely carried out when the boird is completely dead (all generators stopped and prime movers locked-offl. All the main bus-bar and auxiliarv connections throughout the boardi should be checked for tightness during the deadperiod of a major internal clean up. Overheating signs at a connection junction are probably due to a loose joint. Direct heat testing on load with an infra-red thermal camera is now a very useful technique for locating hot-spots. 440 V MainSwitchboardOutline N U U U ( ] "tr?! E O = I :,.:.,::i,: l . l l . , ' . 1" ' J t , .I . , . . . . ' ,, : . , 1 L : : : : , : , : : . . : : . . . 1 li ..: ii-4...1 N o t r o o N o E o o STARTERS SHORE TO & FEEDERS SUPPYEMERG PORT SWBD GEN 1 BUS-TIE GEN2 (MAINBREAKERSBEHIND) TO 220V SECT. S H O R E SUPPY STBD STARTERS & FEEDERS 220V SECTION FEEDERS Fig. 7.4 Main switchboard arrangement.
Cables 173 Bus-bar supports will be examined for surface tracking and damage to the insulation material. All internal wiring within the switchboard panels must be securely fixed. Cable entries at the bottom of the switchboard should be sealed with a non-flammable material to exclude dirt and act as a fire stop. The main switchboatd earth bar must be securely bonded to both the frame of the board and, of course, to the ship's hull. One secondary terminal of each current transformer (Cf; and the metal cases of instruments and relavs should be wired to the main earth bar. Hinged panel doors should be bonded with-an earth strap to the main switchboard trame. QUESTTON What is the reason for earthing one end of the secondary winding of a CT? ANSWER Should the insulation between primary and secondarybreak down, the seiondary circuit can be raised to full primary voltage; e.g. M0 V above earth which could damage the secondary insulation with a serious risk to personnel. By earthing one end of the CT, the circuit is anchoredto zero volts. As a bonus, the earth connection will allow such a fault to be detected on the earth fault monitor. Feeder isolator and fuse holder contacts must be checked for anv mechanical wear or damage due to overheating or arcing at the contacts. A slight smeal of a proprietary electrical contact lubricant on such moving contacts is usually recommended. Operational tests on a main switchboard under this heading will focus on the synchronising controls and generator protection relays such as reverse power and preferential load shedding trips. Typical reverse power trip settings may range between 5-150/" of the generator power rating, with a time delay of 0.5-2.5 s for a diesel drive. Equivalent settings for a turbo-generator may be 2-5"/o and 5 s. Such time delay settings must allow for the operating practice on the ship. For example, cargo winches and cranes may, at times, feed power back into network. Under light load such regenerative feedback a generator to trip on reverse time delay was set too short. the supply conditions may cause power if its 7.7. Cables Apart from an IR (megger)test on cable run (e.g. along the flying a maln bridge Fig. 7.5 Cable construction outlines.
174 Electrical Survey Requirements of a tanker) the survey of cables and their installation is largely based on a close visual examination. Inspection would search for any external damage of a cable'E outer sheath and wire or basket weave armouring (if fitted). The cable must, of course, be adequately supported along horizontal and verticil runs by suitable cable clips or ties. Where cable-runs along an open deck have expansion loops, these must be examined for abrasion and wear. Where cables pass though fire check bulkheads they must be correctly glanded or pass through stopper boxes which prevent the passage of fire between compartments. Probably the most common ship-board cable insulations used are EPR (ethylene propylene rubber) or butyl rubber which is sheathed with either PCP (poly- chloroprene) or CSP (chlorosulphonated polyethelene). mechanical damage. In normal operation a flexible cable may be repeatedly dragged and chafed so reducing its safety. If in doubt replace flexible cables. A copper strap or flexible earthing braid/wire is used to bond the steel frame of all electrical motors and other equipment to the ship's hull. QUESTTON \A/hy is such an earth bond required? ANSWER Without an earth strap, a loose internal wire may touch the frame causing it to become liae at mains voltage with obvious danger to operators. The earth strap electrically anchorsthe frame to the ship's hull (zero volts) to eliminate the shock hazard to personnel. 7.8. Insulation Resistance The surveyor will require a list which shows the results of recent insulation tests on all main 440 V and 220 V circuits. Such a list should also indicate the test date's, weather conditions (hot, humid, etc.) together with any comments relevant to the test conditions (e.g. machine hot or cold). An example of an IR log and its graphical trend for a motor is shown in Fig. 7.6. For essential items such as generators and main motors, the surveyor will be more interested in the IR trend, so a set of past results showing the insulation history of such machines may be requested. QUESTION What are the functions of EPR and PCP or CSP? or butyl ANSWER EPR or butyl rubber are good electrical insulators but are not mechanicallv stro,ng o_rresistant to oil. This is why a sheath of PCP or CSP (which is stronger and has greater oil and fire resistance) is fitted around the inner insulation. Where EPR/butyl cable terminations mav be subjected to oil vapour it is usual t6 tape or sleeve the cable ends to prevent deterioration of the insulation. Check that such taping is secure. Flexible cables to light fittings, power tools, etc., should be inspected for
Motors and Starters 175 AC Gompressor Motor No. 1 rR(MO) ER cold (dry-dock) Fig.7.6 IR log and trend. 7.9. Motors and Starters After checking through the IR test results list, a surveyor may ask to witness a repeat test on selected motors. A visual examination of a motor frame and terminal box will reveal any damaged or missing parts. General neglect will be suspected if the motor is covered with dirt, oil or rust. Totally enclosed fan ventilated (TEFV) induction motors require little attention as their windings are protected against the external atmosphere. The surveyor will be more likelv to concentrate on motors with drip proof, weatherproofand deck-watertightenclosures. It may be necessary to open up such motors to check for ingress of oil and water which could damage insulation and cause internal corrosion. Specialmachinessuch as d.c. commutator or a.c. slip-ring types used, for example, on an electric windlass, must have their rotary contacts and brush gear checked. Cargo cranes and winches are not strictly part of a survey as they are not considered essential to the safety of the ship. Fig. 7.7 Motor construction. A running test on a motor will reveal any vibration problems, undue noise and
176 Electrical Survey Requirements worn out bearings. OnJoad, the motor running current (shown on the ammeter at the starter) should be checked against the value indicated on the motor rating plate. With starters and associated control gear such as remote stop/start buttons, regulating resistors etc., an inspection will check mainly for badly burned and misaligned contacts. The general condition of starter equipment will also be examined. This would include an inspection for loose connections, worn pigtails on moving contacts, badly carbonised arc-chutes and signs of overheating on coils, transformers and resistors.Dust and weather-proof sealing features on a starter must be in place and in a serviceable condition. Functional checks will test the normal operation of the starter from its local, remote and emergency control (if applicable)positions. Signal status lamps showing the motoristarter condition, e.g. running, off, tr@ed, etc., must be demonstrated as working correctly. Overcurrent trip settings should be compared with the motor full-load current (FLC) rating. Motor starter back-up fuse size and type may be checked against the ship's/manufacturer's drawings and the motor rating. 7.10. Emergency Power and AssociatedEquipment This section surveys the operation of the emergency generator andior battery power equipment (inspection of the emergency generator itself is covered under the heading of Generators and Governors). The emergency generator must be started, manually or automatically, while the initiation sequence and operation of starting equipment is observed. Electrical supplies taken from the emergency switchboard should be checked as receiving their rated voltage, current and frequency when powered from the emergency generator. Emergency lighting, fire pump and other emergency electrical equipment must be functioning correctly-.Electrical interlock- ing arrangements between main and emergency switchboard must be checked. Auto-start initiation relays, whether voltage or frequency operited, will be examined and tested. The ship's emergency battery installa- tion and its charging rectifier will be examined. In particular the battery environment must be dty and well ventilated. The battery tops must be clean with terminal posts and connections appearing free from corrosion. A typical lead-acidcell outline is shown in Fig. 7.8. Grease all connections with petroleum jelly. Battery electrolyte should be at its proper level and have the correct value of specific gravity (SG) as checked on a hydrometer. Safetynotices and personnel safety clothes (gloves, apron and goggles) should be available adjacent to the batteries. The ventilation arrangements for the battery locker will be checked. Battery charging equipment should be given the normal checks for dirt, over- heating, loose connections and correct functioning of indicators, instruments and alarms.
Parts of Steering Gear 177 NegativeTerminal Vent/ FillerCap PositiveTerminal PositivePlate NegativePlate Separator Fig. 7.8 Lead-acidcell outline. 7.11..Parts of Steering Gear Fig.7.9 indicateshow an electrohydraulic steering gear system can be envisaged from the surveyor's viewpoint as being in three parts: o Power unit o Steering control o Indications and alarms The power unit comprises duplicate electric motors and starters supplied from either side of the main switchboard. jq 'TEERTNG <|} coNrRoL INDICATIONS and ALARMS PARTSOF STEERINGGEAR Fig. 7.9 Main steering-gearcomponents.
178 Electrical Survey Requirements On many ships one of the steering gear motors will be supplied via the emergencyswitchboard as recommended by the SOLAS requirements for certain vessel types, e.g. passenger ships and ferries. The motors, starters and any changeover supply switch units will b-e inspected under the same criteria out- lined earlier in the section on Motors and Starters. Rudder control from the bridge position may be via an hydraulic telemotor or via an electric controller or both. Main and alternative electric supplies, including any changeover facilities for the electric control from the steering wheel and for the auto pilot, must be tested The steering gear and its control must be functionally tested for its response. This is generally specified to be that the rudder must be swung from 32o port to 32" starboard in 28 seconds. Nbte, a fully loaded response can only be obtained when the ship is loaded and under way at sea. Steering gear status indications must be operating correctly in the steering flat, main control room and on the bridge. The rudder position indicators on the bridge may be checked during the functional testing of the steering gear. The bridge indication should be compared with the direct mechanical indicator on the rudder stock in the steering flat. Motor overcurrent alarms can be initiated by simulating the action of the overcurrent relay. Remember that a steering gear mbtor does not have overcurrent trip protection; the only main circuit protection being from the back-up fuses which are essential for short-circuit protection. Hydraulic fluid low level alarms, if fitted, must be checked for correct initiation by the oil level sensors. 7.12. Navigation Light Indicators Essentialll, the surveyor will expect to prove that the navigation light indicator operates correctly and gives the ap- propriate alarms. A broken wire or lamp can be simulated by pulling the appropriate fuse. The power supply for the navigation lights must be duplicated (usually the alternative supply is obtained from the emergency switchboard) and the changeover facilities must be checked. See Fig. 7.10. ALARMAND ___---R /J BUZZER |ND|CAT|oN '.\-l SUPPLIES ________{?\ |ND|CATOR KY CHANGE OVER SWITCH NAVIGATION LIGHTPANEL MAINLIGHT EMERGENCY POWER S U P P L I E S AUXILIARY Fig. 7.10 Navigation light indicator panel.
UMS Operation 179 Although the actual light fittings for navigation are part of the Safety Equipment Survey, the electrical survey will naturallv include a check on the supply cables to the lights. 7.13. UMS Operation If your ship is classified for Unattended Machinery Space (UMS) operation, the electrical survev will be extended to include all the -alarms, fire detection, controls and fail-safe features of such an installation. All alarms associated with the main engine, auxiliary machines, lubrication and cooling are to be tested for correct operation. Testing of the electricalcircuits from the various sensors is relatively straightforward. This can be achievei by operating the sensor switch by hand or by simulating the switch action under the expected alarm condition. To prove that the overall sensor (pressurestat, flow switch, level switch, temperature switch, etc.) is functioning correctly is obviously more involved. Often, specialist contractors may be called upon to service and cahbrate the sensdrs and alarm annunciators. Particular attention will be paid to the main engine and auxiliary generators in respect of their alarms for lubrication and cooling. Initiation and action of automatic shut-down features will be tested. Essential drives for lubrication, cooling and fuel supply are duplicated and arranged so that one pump-can be selected on a duty/standby basis. Loss of pressure at the duty pump should automatically start up the standby unit. Automatic start-up of the emergency generator must be demonstrated. The initiation of the undervoltage or under-frequency relay can usually be accomplishedby pulling the fuses in the detection unit. The emergency generator should then run up to speed and supply voltage to the emergency switchboard. UMS requirements demand that a standby main generator starts auto- matically on loss of the duty generator. The standby generator is to start and close onto the dead bus-bars within 45 seconds. This is followed by automatic se- quential re-starting of essential auxiliaries for lubrication, cooling, fuel and steering. The correct functioning of the system will be tested. The duplicate bilge level alarms together with automatic bilge pumping must be proven to the surveyor's satisfaction. The main and standby electric power supplies to the overall alarm monitoring system must be inspected and tested. The standby power arrangement usually includes battery back-up. It will be necessaryto insp-ectthe general condition of the battery and its trickle-charger. Tests are made on the UMS alarm system to verify: r that alarms displayed on the main console in the engine control room are relayed to the smaller group alarm panel on the bridge; r that the duty engineer call system is operating in the accommodation areas, i.e. in the cabin of the selected duty engineer and in the duty mess and lounges; r that the selected duty engineer is allowed 2-3 minutes to respond to a machinery alarm. If the engineer has not reached the control room and accepted the alarm within this time, a deadman alarrn should be sounded generally in the alleyway adjacent to the engineers' accommodation. A complete inspection and test of the fire detection apparatus must be performed. All smoke, heat and flame sensors must function correctly to initiate the appropriate audible arid visual alarms
180 Electrical Survey Requirements on the bridge, in the main control room and in the accommodation. Hand operated fire-alarm switches of the break-glasstype must also be examined and tested to be in proper working order. Fig. 7.11 Break-glassswitch. 7.14. Tankers Electrical equipment in the hazardous areas of oil/gas carriers and other ships carryring potentially dangerous cargo will be surveyed during the normal engine survey (every four years) and during docking and annual surveys. Consequently, the haznrdous area electricalequipmentis effectively surveyed every yeat. The most common form of hazardous area electrical equipment is the flameproof enclosure type (marked Exd on the equipment certification label). This type of enclosure will be found on light fittings, motors, starters, push-buttons and alarm bells within the hazardous zones. The flameproof enclosure will be inspected for surface cleanliness (which affects the surface temperature), corrosion and secure mountings. On lighting fittings the cement that bonds the lamp glass to its frame must be closely inspected for cracks or indentations. All bolts must be in place, evenly torqued-up and of the correct type. The edges of flamepath flanged joints must not be painted over or impeded in any way. Exposed flameproof equipment on deck must be adjudged weatherproof with the correct (approved) gaskets or "O" rings in place. An Exd fitting may be opened up to check the condition of its flamepath surfaces for corrosion, pitting or scratch marks as shown in Fig. 7.L2. Main engine controls must function correctly and will be tested from the bridge position, local position (engine control room) and at the emergency position alongside the engine. The operational features of the electrical equipment for main engine control and indication will be best demonstrated during a full engine test during an engine survey. Such electrical equipment and connections associated with engine control will be examined as usual for wear and tear, insulation level, cleanliness,loose connections and overheating.
Tankers 181 FlangeJoint SpigotJoint ScrewedJoint Fig. 7.12 Exd flamepaths. The Ex Certification label and equipment rating label must not be painted over. Remember that no alterations to the Exd equipment are allowed without per- mission from the Certification Authoritv. This applies also to the lamp size aid its rating for a particular light fitting - it must have the correct lamp fitted. rooms have pressurised (marked Exp on the areasmust be intrinsically safe(marked Exi on the Certification label). In most cases/ zener barriers, as shown in Fig. 7.13, are connected in line with intrinsically s#e circuits and are fitted in a safe area just outside the hazardous area. The surveyor cannot easily test zener barriers in situ as this would involve special equipment and it is generally accepted that such protection equipment will function correctlv when circuit fault conditions arise. Thii is no different to accepting that a fuse will blow when a short-circuit occurs. However, the surveyor will visually inspect the zener barrier installation The barriers must have secure connections and be properly bolted to an earth strap, which in turn, must be solidly bonded to the ship's hull. Some pump light fittings Certification label). Here it is necessary to confirm that the fittings are purged and pressuisedbef.orethe light is allowed to be switched on. Similarly the lights should automaticallv be switched off rt the air pressure drofs below its set value. Electrical instrumentation and communi- cation equipment used in hazardous F u s e 3 H a z A r e a T e r m i n a l s 1 S a f e A r e a T e r m i n a l s 2 Fig. 7.1,3 Exi barrier circuit.
183 Chapter Eight Electric Propulsion and High Voltage Practice 8.0 Introduction 8.1 Elechic Propulsion Scheme 8.2 Power Supply Network 8.3 Review of Motor Operation 8.4 Controlled Rectification and Inversion 8.5 Converter Types 8.5 Propulsion System Operation 8.7 Harmonics 8.8 Propulsion Auxiliaries and Protection 8.9 High Voltage on Ships 8.10 High Voltage Safety 8.1,L High Voltage Equipment Testing Page 183 784 189 19't 194 196 20\ 204 208 217 273 216 8.0. Introduction The earliest electric propulsion for ships was demonstrated in Russia in 1832 with a d.c. motor powered from a battery. In 1886 an electrically propelled vesselcalled the Voltacrossedthe English Channel. By 1888 the improvements to batteries and motors led to the first commercial applications in passenger launcheson the River Thamesin London. As with road transport, electric river boats were soon eclipsed by the arrival of the internal combustion engine. Electric propulsion for many new ships is now re-established as the popular choice where the motor thrust is governed by electronic switching under computer control. The high power required for electric propulsion usually demands a high voltage (HV) power plant with its associatedsafety and testing procedures.
L&l Electric Propulsion and High Voltage Practice 6 i E @ @ 6 t 6 E i E @ E A E E E F F A q & A B q A S A q E @ 6 s @ g E q l g t @ @ @ G t @ @ @ @ E l E I E I E @ E g I G E & I @ E E I E I E I E @ B " t , a q c a . l ' a . a u l u . c r 6 a q Q @ & d r a 6 r g a t r a q @ @ a a a 6 8 e q * . E E r * a ' E E q e q s g o n € e e \ ;ee-";;;;;;;*=*=*m=r#"'=*""" EaErqE _Erg6E|E|a6eE D E D . } l t B ' i } F D . } ! & s } D D ! D . ' F s s ' ' E r D . s } D E s . r f f i ercrElorrbBlsllE 'ffi'HrttnEEsathffiDlrtsrhwm Fig. 8. L Passengercruise ship with electric propulsion. 8.1-.Electric Propulsion Scheme services has obvious advantages, but this would have to be a fixed voltage and frequency system to satisfy the requirements of the ship service loads. The provision of high power variable speed drives from a fixed voltage and frequency supply has always presented problems. Also, when the required propulsion power was beyond the capacity of a single d.c. motor there was the complication of multiple motors per shaft. Developments in high power static converter equipment have-presented a very convenient means of providing oariablespeeda.c. and d.c. drives at the largest ratings likely to be required in a marine propulsion system. The electric propulsion of ships requires electric motors to drive the propellers and generator sets to supply the electric power. It may seem rather illogical to use electric generators, switchgear and motors between the prime-movers (e.9. diesel engines) and propeller when a gearbox or length of shaft could be all that is required. There are obviously sound reasons why, for some installations, it is possible to justify the complication of -electric propulsion and some of the reasons advanced are: o Flexibility of layout o Load diversity between ship service load and propulsion Electric propulsion of ships has a long but somewhat chequered history. There have been periods when it has enjoyed popularity, with a significant number of installations being undertaken, whilst at other times it has been virtually ignored as a drive system. Passenger ships have always been the largest commercial vessels with electric propulsion and, by their nature, the most glamorous. This should not, however, obscure the fact that a very wide variety of vessels have been, ani are, built with electric propulsion. Early large passengervesselsemployed the turboelectric system which involves the use of variable speed, and therefore aariablefrequency, turbo-generator sets for the supply of electric power to the propulsion motors directly coupled to the propeller shafts. Hence, the generator/motor system was acting as a speed reducing transmission system. Electric power for auxiliary ship services required the use of separate constant frequency generator sets. A system that has generating sets which can be used to provide power to both the propulsion system and ship
Electric Propulsion Scheme L85 a o a Economical part-load running Ease of control Low noise and vibration characteristics Flexibility of layout The advantage of an electric transmission is that the prime-movers, and their generators, are not constrained to have any particular relationship with the load as a cable run is a very versatile transmission medium. In a ship pro- pulsion system it is possible to mount the diesel engines, gas turbines etc., in locations best suited for them and their associatedservices, so they can be ECR HV mimic -l I tr r:-r n l engineconsole propulsion computer localcontrol position propulsion computer P E M l 6.6 kV (HV) switchboard 440V engineroom switchboard DECK4 DECK3 DECK2 Engine Control Room - - t converterl I transformer@ "onu"rt"r[l]l "onu"rt"'.lLlltj transformerCf) o.c.O coilsO propulsion electric motors transformer converter d'.,O a7r\| 6qilsrl \)L/ transformer diesel- gen sets Fig. 8.2 Propulsion plant layout.
186 Electric Propulsion and High Voltage Practice remote from the propeller shaft. Diesel generator sets in containers located on the vessel main deck have been used to provide propulsion power and some other vesselshave had a 10 MW generator for ship propulsion duty mounted in a block at the stern of the vessel above the ro-ro deck. An example of an electric propulsion plant layout (for a large cruise ship) is shown in Fig. 8.2. Another example of the flexibility provided by an electric propulsion system is in a semi-submersible, with the generators on the main deck and the propulsion motors in the pontoons at the bottom of the support legs. Load diversity Certain types of vessels have a re- quirement for substantial amounts of electric power for ship services when the demands of the propulsion system are low. Tankers are one instance of this situation and anv vessel with a substantial cargo discharging load also qualifies. Passenger vessels have a substantial electrical load which, although relatively constant, does involve a significant size of generator plant. There are advantages in having a single central power generation facility which can service the propulsion and all other ship loads as required. Economical part-load running Again this is a concept that is best achieved when there is a central power generation system feeding propulsion and ship services, with passenger vessels being a good example. It is likely that a typical installation would have between 4-8 diesel generator sets and with parallel operation of all the sets it becomes very easy to match the available generating capacity to the load demand. In a four engine installation for example, increasing the number of sets in operation from two that are fully loaded to three partially loaded will result in the three sets operating at a 67"/" load factor which is not ideal but also not a serious operating condition. It is not necessary to operate generating sets at partJoad to provide the spare capacity to be able to cater for the sudden loss of a set, becausepropulsion load reduction rl:.av be available instant- aneously, and in- most vessels a short time reduction in propulsion power does not constitute a hazard. The propulsion regulator will con- tinuously monitor the present generator capability and any generator overload will immediately result in controlled power limitation to the propulsion motors. During manoeuvring, propulsion power requirements are below system capacity and failure of one generator is not likely to present a hazardous situation. Ease of control The widespread use of controllable pitch propellers (cpp) has meant that the control facilities that were so readily available with electric drives are no longer able to command the same premium. Electric drives are capable of the most exacting demands with regard to dynamic performance which, in general, exceed by a very wide margin anything that is required of a ship propulsion system. Low noise An electric motor is able to provide a drive with verv low vibration character- istics and this is of importance in warships, oceanographic survey vessels and cruise ships where, for different reasons,a low noise signatureis required. With warships and survey vessels it is noise into the water which is the critical factor whilst with cruise ships it is structure borne noise and vibration to the passenger spaces that has to be minimised. An overview of practical electric drive options is shown in Fig. 8.3.
Electric Propulsion Scheme 187 driving a.c. 0-20Hz synchronous Fig. 8.3 Electric propulsion options. For very high power, the most favoured option is to use a pair of high efficiency, high voltage a.c. synchronousmotorswith fixed pitch propellers (FPP) driven at variable speed by frequencycontrol from electronic converters. A few installations have the combination of controllable pitch propellers (CPP) and a variable speed motor. Low/medium power pro- pulsion (1-5 MW) may be delivered bv a.c. induction motors with variable fiequency converters or by d.c. motors with variable voltage converters. The prime-movers are conventionally constant speed diesel engines driving a.c. generators to give a fixed output frequency. Gas turbine driven prime- movers for the generators are likely to challenge the diesel option in the future. Conventionally, the propeller drive shaft is directly- driven- from the pro- pulsion electric motor (PEM) from inside the ship. From experience obtained from smaller external drives, notably from ice-breakers, some very large propulsion motors are being fitted within rotating pods mounted outside of the ships hull. These are generally referred to as azipods,as shown in Fig. 8.4, as the whole pod unit can be rotated through 360o to apply the thrust in any horizontal direction, i.e. in azimuth. This means that a conventional steering plate and stern side-thrustersare not required. Ship manoeuvrability is significantly enhanced by using azipods and the external propulsion unit releases some internal space for more cargo/passengers while further reducing hull vibration. Gradual progress in the science and application of superconductivity suggests that future generators and motors could be super-cooled to extremely low temp- eratures to cause electrical resistance to become zero. In this condition, the electrical power losses (I'R) are also zero so it is possible to drive extremely large currents ( > 100,000A) through very thin wire coils to create an exceptionally large magnetic field. The combination of a large current and a large magnetic field will produce a very large electromagnetic
188 Electric Propulsion and High Voltage Practice hydraulic steering unit slipringunit for power and control ventilation and cooling installation block azipodunit fixedpitch propeller (FPP) variablespeed/direction synchronousmotor Fig. 8.4 Azipod drive unit. Fig. 8.5 Linear electric propulsion.
Power Supply Network L89 force as F ocO.I. One way of applying such a direct force into the water for ship propulsion (a long-term ongoing experiment in Japan) is outlined in Fig. 8.5. A large d.c. current is driven between metal plates mounted in a open tube below the hull. The conductor for this current is the sea water. Coils of wire at a superconducting temperature (e.g - 269"C cooled by helium) are fitted around the propulsion tube to create a magnetic field 90' to the current flow. The combination of current and magnetic field produces a direct mechanical force on the conductor (water) to create a linear thrust without the need for a rotating propeller. By dividing port and starboard thrust tubes into short sections along the hull, the size and location of thrust can be distributed so that conventional steering and side thrusters are not required. This is a very interesting experiment into the direct application of electromagnetic force for ship propulsion. 8.2. Power Supply Network As the demand for electrical power increaseson ships (particularly passenger ferries, cruise liners, and specialist off- shore vessels and platforms) the supply current rating becomes too high at 440V. To reduce the size of both steadv state and fault current levels, it i; necessaryto increasethe system voltage at high power ratings. Note: In marine practice, 1000 V are considered LV HV (high voltage) is any are 3.3 kV or 6.6 kV but 11 kV is used on some offshore platforms and specialist oil/gas production ships e.g on some FPSO (floating production, storage and offloading) vessels. By generating electrical power at 6.6 kV instead of 440 V the distribution and switching of power above about 6 MW becomesmore manageable. e.g. A three phase 6 MW ships load on a M0 V system supplied by 3 x 2 NNV, 0.8 pf diesel-generatorunits requires the switchboard fault leaelto be about 90 kA and each generator circuit breaker and system cabling has to handle a full-load current (FLC) of: I :2,000,000 w/v3.440. 0.8 : 3300A The same system al 6.6 kV requires the HV switchboard and cables to be rated for a fault level of about 9 kA with generator circuit breakers rated only for an FLC of 220 A. The component parts of an HV supply system are now standard equipment with HV diesel generator sets feeding an HV main switchboard. Large power consumers such as thrusters, propulsion motors, air-conditioning (A/C) com- pressors and HV transformers are fed directly from the HV switchboard. An economical HV svstem must be simple to operate, reasonably priced and require a minimum of maintenance over the life of the ship. Experience shows that a 9 MW svstem at 6.6 kV would be about 20% more expensive for installation costs. The principal parts of a ships electrical system operated at HV would be the main generators, HV switchboard, F{V cables, HV transformers and HV motors. voltages below (low voltage). voltage above An example of a high voltage power LV. Typical marine HV system voltages system is shown in Fig. 8.6.
190 Electric Propulsion and High Voltage Practice ffi/;\ [\V MAIN GENERATORS /e\ v) 440 V 60 Hz EMERGENCYSWBD. A l.l V ) 2 2 O V6 0 H z E C R S U B 6.6 kV 60 Hz MAIN SWBD l / \ rI l l t [ r l I I I I I I I I I t l I 440 V 60 Hz ECR SWBD. Fig. 8.6 HV power system. In the example shown the HV generators form a central power station for all of the ship's electrical services. On a large passenger ship with electric propulsion, each generator may be rated at about 10 MW or more and producing 6.6 kV, 60 Hz three-phasea.c. voltages. The principal consumers are the two synchronous a.c. propulsion electric motors (PEMs) which may each demand 12 MW or more in the full away con- dition. EachPEM has two stator windings supplied separately from the main HV switchboard via transformers and frequency converters. In an emergency a PEM may therefore be operated as a half-motorwith a reduced power output. A few large induction motors are supplied at 6.6 kV from the main board with the circuit breaker acting as a direct-on-line (DOL) starting switch. These motors are: o Two forward thrusters and one aft thruster, and o Three air conditioning compressors Other main feeders supply the 440 V engine room sub-station (ER sub) switch- board via step-down transformers. An interconnector cable links the ER sub to the emergency switchboard. Other 440 V sub-stations (accommodation, galley etc.) around the ship are supplied from the ER sub. Some installations may feed the ships sub stations directly with HV and step-down to M0 V locally.
Review of Motor Operation L91 The PEM drives in this example are synchronous motors which require a controlled low voltage excitation supply current to magnetise the rotor poles. This supply is obtained from the HV switchboard via a step-down transformer but an alternative arrangement would be to obtain the excitation supply from the 440 V ER sub switchboard. a.c. versions mav be the induction or synchronou.smodeis. The following is a brief review of the basic action and control possibilities for the various types. o d.c. motors The d.c. motor drive is still used where very high torque and/or precise speed control is acquired. Traction drives such as electric trains, submarines and offshore drilling rigs use d.c. motors. The torque is governed by: T c O.Ia and the speed is due to: n a V/O where O is the magnetic field flux and Ia is the armature current. See Fig. 8.7. As the armature current and field flux can be independently controlled, the d.c. motor is able to provide very useful torque/speed characteristicsfor power drives. The major drawback of a d.c. motor is that the necessary switching of the armature current is achieved bv a mechanical"commutator" on the rotaiing shaft. Apart from the maintenance required for the commutator and its carbon brushes, the applied voltage for the armature is limited to about 750 V d.c. Many regional "Metro" train systems run at 1500V d.c. where two d.c. motors are connectedin series acrossthe supply voltage. QUESTTON Assuming 100"/" efficiency, calculate the FLC then estimate the DOL starting current for a three phase, 100 kW; 0.9 p.f. induction motbr supplied at: (a) M0 Y; (b) 6.6 kv ANSWER (a) 145.8A;729 A (b) 9.7 A; 49 A (assuminglps;: 5 x lgc) Review of Motor Operation Electric motors may be of the for ship propulsion duty d.c. or a.c. type. The statorcorewith projecting fieldpoles DC MOTOR ACTION armature with conductors, commutator and brushes d.c.motorcircuits F i g . 8 . 7 d . c .
192 Electric Propulsion and High Voltage Practice statorcoreand 3-phasea.c. phase windings input currents INDUCTION MOTOR force(F) L 1 \ r l r lI I / twistingforce ( t o r q u e = F x r a d i u s ) on rotor barsL2 'rotating statorflux Fig. 8.8 Induction motor action. . a.c. motors * induction type The most common motor drive is a three phase a.c. induction motor with a cage-rotorbecauseit is extremely.robust as-there are no electrical connections to the rotor. See Fig. 8.8. Three time-displaced supply currents to the three stalor windings produce a rotating magnetic field which induces currenti into the cage winding on the rotor. The interaction of stator flux O and rotor current Ip produces a torque on the shaft from TaO.In.cost' where @ is the phase angle between O and In. To be able to induce currents into the rotor, its running speed must be slightly lower than that of the stator rotating field. This difference is called the slip speed and ranges between about L-S"/o'overthe load range for a standard induction motor. The speed n, (synchrono-usspeed) of the rotating flux produced by the stator is fixed U"y the'number 5f wlnding pole-pairs "p" and the supply frequency "f" asi n, : f/P (rev/s). An example: for a motor designed for 4-poles (P : 2) to run on a 50 Hz suPPlY with a full-load slip of 4o/o, the speed of the rotating flux is n": $Ql/ : 25 rev/s (1500 r&imin; but the actual rotor speed will be 1n:96o/o of 25:24 rev/s or 1440 rev/min. \A/hile the cage-type induction motor is simple and low-c-os.tit has some-practical disddvantages. When supplied with a fixed voltage and frequency the motor runs at an almost constant speed and has a high starting current of typically 6 times its full load value. If the motor in the above examPle is designed for M0 V with a full load rated output of 100 kW with an efficiency of.90"/"and a power factor of 0.8 lagging, its full load supply current will be found from the three- phase power formula: P : V 3 . V r . I r . c o s d So, the electricPowerinPut is 100/90%: L11.1kW and l7: Q11.1x 103)h/3.440.0.8: 1.82.2A then the initial starting current surgeis about 911-A! ,.! synchronoustype This is a three phase motor that produces a magnetic field rotating at a speed of n,: f1p (rev/s) just like the induction motor type. The rotor has a set of magnetic poles with d.c. excitation which locks in syn- chronism with the stator rotating flux-
Review of Motor Operation 193 statorcoreand 3-phase input currents L 1 a . c . phasewindings force (F) SYNCHRONOUS MOTOR L2 ) ^ " ' " ' /rotating fbrce{F) statorflux twistingforce ( t o r q u e = F x r a d i u s ) rotorpoles lockedin sync. with statorflux Fig. 8.9 Synchronousmotor action. This means that the shaft is always running at the synchronous speed det by the supply frequency. See Fig. 8.9. To start the motor from standstill can be a problem - it is either: . Pulsedforward at a very low frequency with the rotor poles excited, or o Dragged up to slip speed as an induction motor with an embedded cage rotor then locked into syn- chronism by energising the d.c. rotor field. For normal running, the operating power factor of a synchronous motor c-an be lagging or leading as this is determined by the size of the d.c. excitation field current. o Basic speed control of motors Many industrial installations can benefit from direct and smooth speed control of a drive which is moving the process material (water, compressed air, oil, conveyor belts, lifts etc.). Smooth, controlled acceleration and deceleration also reduces shock loading in the system. For a d.c. motor on a fixed voltage supply, this is easily achieved by using resistance in the armature or field circuits to control the armature current or field flux (or both). The disadvantage is the overall loss of efficiency due to the power losses in the external control resistance(s). For an a.c. induction motor or synchronous motor on a fixed voltage and frequency supply, resistance control would only affect the size of operating current but the speed is constant due to the fixed supply frequency. This can only be overcome by changing the frequencyof the stator supply currents. To prevent overheating (by over-fluxing) of the motor while frequency changing, the supply voltage must be changed ih direct proportion. o Advanced speed control Computer controlled variable speed drives (VSDs) are now applied to d.c. and a.c. motor types of all sizes. The most popular application is for induction motors for the main industrial power range but synchronous motors are used in large installations e.g. marine electric propulsion. The a.c. motor drives produce a variable frequency output by fast voltage switching from a transistor or thyristor converter which may be ac-dc-ac (PWM and synchroconverter) or ac-ac (cycloconverter). These drives use a mathematical model of the motor and the computer controls the converter output to precisely match the set
194 Electric Propulsion and High Voltage Practice inputs for speed, torque, acceleration, deceleration,power limits etc. Such drives mav even be tuned to create optimum cohditions for run-up/ down, braking and energy savings against the connected shaft load. o Problems arising The fast switching (or chopping) of the voltagesto VSDs *ilI produce a distorted waveform which includes high frequency harmoniccomponents whose frequencies are exact multiples of the fundamental (basefrequency) value. For example a 7th harmonic of a 60 Hz fundamental will be at 420 Hz. Such harmonics create additional heating in equipment and possible interference (often called radio frequency interference or RFI). Practical solutions to a harmonic problem include good initial system design, filtering and suppression. See later section on harmonics. 8.4. Controlled Rectification and Inversion voltage and frequency. This is generally at M0 V and 60 Hz but for high power demands it is likelv to be 6.6 kV and 60 Hz. Speed control for a propulsion motor requires variable voltage for a d.c. drive and variable frequency * voltage for an a.c. drive. The set bus-bar a.c. voltage must be converted by controlled recti- fication (a.c.*d.c.) and/or controlled inversion (d.c.-*a.c.) to match the propulsion motor type. A basic rectifier uses semiconductor diodes which can onlv conduct current in the direction of anode (A) to cathode (K) and this is automatic when A is more positive than K. The diode turns-off automatically when its current falls to zero. Hence, in a single-phase a.c. circuit a single diode will conduct only on every other half-cycle and this is called half-taaoe rectification. Other single-phase circuits using a bi-phase arrangement with two diodes and a centre-tapped transformer will create full-waae rectification Similarly, four diodes in a bridge formation will also produce a full-wave d.c. voltage output. An equivalent three phase bridge requires six diodes for full-wave operation. A diode, having only two terminals, cannot control the size of the d.c. output from the rectifier. The generated three power supply on a phase a.c. electrical ship has a fixed THYRISTOR CURRENTCONTROL a.c.- d.c. "puck" K thyristor construction for largecurrents Half-waverectifiedd.c by delayedswitching . < - - } delayangle:c[ Fig. 8.10 Single-phasecontrolled rectification.
Controlled Rectification and Inversion 195 (current T smoothing) ^DC Fig. 8.11 Three-phase controlled rectifier bridge circuit. o controlled rectification process For controlled rectification it is necessary to use a set of ihree-terminal devicei such as thyristors (for high currents) or transistors (for low - medium currents). A basic a.c.*d.c. control circuit using a thyristor switch is shown in Fig. 8.10. Compared with a diode, a thyristor has an extra (control) terminal called the gate (G). The thyristor will only conduct when the anode is positive with respect to the cathode and a brief trigger voltage pulse is applied between gate and cathode (gate must be more positive than cathode). Gate voltage pulses are provided by ^ separate electronic circuit and the pulse timing decides the switch-on point for the main (load) current. The load current is therefore rectifiedto d.c. (by diode action) and controlledby delayed switch- irg. In this circuit an inductor coil (choke) smoothsthe d.c. Ioadcurrenf even though the d.c. voltage is severelychopped by the thyristor switching action. An alternative to the choke coil is to use a capacitor acrossthe rectifier output which smoothsthe d.c. aoltage. Full wave controlled rectification from a three-phase a.c. supply is achieved in a bridge Circuit with six thyristors a shown in Fig. 8.11. For a M0 V (r.m.s.) a.c..line voltage the peak voltage is 440x"J2:622 Y. The equivalent maximum d.c. aaeragevoltage output is taken to be about 600 V as it has a six-pulse ,ipple effect due to the three-phaseinput waveform. o controlled inversion process A d.c. voltage can be inverted (switched) repeatedly from positive to negative to form an alternating (a.c.) voltage by using a set of thyristor (or transistor) switches. A controlled three-phase thyristor bridge inverter is shown in Fig. 8.12. The inverter bridge circuit arrangement is exactlv the same as that for the rectifier. Here, the d.c. voltage is sequentially switched onto the three output lines. The rate of switching determines the output frequency.For a.c. motor control, the line currents are directed into (and out of) the windings to produce a rotating stator flux wave which interacts with the rotor to produce torque.
196 Electric Propulsion and High Voltage Practice Fig. 8. 12 Three-phase inverter circuit and a.c. synchronous motor. 8.5. Converter Types The processes of controlled rectification and inversion are used in conaertersthat are designed to match the drive motor. The principal types of motor control converters are: . a.c.-*d.c. (controlledrectifier for d.c. motors) a.c.*d.c.+a.c. (PWM for induction motors) a.c.*d.c. +a.c. (synchroconverter synchronous motors) o a-.c.+a.c. (cycloconverterfor syn- chronous motors) These are examined below. o a.c.-'d.c. converter This is a three phase a.c. controlled rectification circuit for a d.c. motor drive. Two converters of different power ratings are generally used for the separate control of the armature current (Ia) and for the field current magnetic flux (O). have a fixed field which produces the Some systems may current which means that the field supply only requires an uncontrolled diode bridge as shown in Fig. 8.13. Motor torque is determined from T c O.Ie and the speed is controlled from N c Ve/O. Shaft rotation can be achieved by reversing either the field current or the armature current direction. Ship applications for such a drive would include cable-laying, offshore drilling,i . diving .and supply, ocean survey and suDmarrnes. o a.c.-'d.c.->a.c.PWM converter This type of converter is used for induction motor drives and uses transis- tors as the switching devices. Unlike thyristors, a transistor can be turned on and olf by a control signal and at a high switching rate (e.9. at 20 kHz in a PWM converter).See Fig. 8.14. The input rectifier stage is not controlled so is simpler and cheaper but the converter will not be able to allow power from the motor load to be