Modern Power Station Practice

British Electricity International

Third Edition incorporating Modern Power System Practice

Modern Power Station Practice ELECTRICAL SYSTEMS AND EQUIPMENT Volume D

Pergamon Press

MODERN POWER STATION PRACTICEThird Edition(in 12 volumes)

incorporating Modern Power System Practice

Main Editorial PanelD. J. Littler, BSc, PhD, ARCS, CPhys, FInstP, CEng, F1EE (Chairman) +Professor E. J. Davies, DSc, PhD, CEng, F1EE F. Kirkby, BSc, CEng, MIMechE, AMIEE H. E. Johnson P. B. Myerscough, CEng, I-IMechE, FINucE W. Wright, MSc, ARCST, CEng, FIEE, FIMechE, FInstE, FB1M

Volume Consulting EditorProfessor E. J. Davies, DSc, PhD, CEng, FLEE

Volume Advisory EditorF. Beach, BSc(Eng), ACGI, DIC, CEng, F1EE, MIMechE

AuthorsChapter 1 A. E. Clegg, CEng, FIEE E. C. Adams, DipEE, AMIEE R. A. Colley, DipEE, CEng, MIEE J. E. Durrant, BSc(Hons), CDipAF, CEng, MIEE T. Lepojevic, DipING(Belgrade), AM1EE J. E. Simpson, BSc(Hons), CEng, MIEE P. J. Simpson, CEng, MIEE

Chapter 2 J. N. Dodd, CEng, MIEE F. J. W. Preece, BSc, MSc, CEng, MIEE G. T. Williams, DipEE, CEng, MIEE Chapter 3 M. J. Heathcote, BEng, CEng, MIEE

Chapter 4 L. T. Smith, BSciFlons), CEng, MIEE Chapter 5 D. F. Oldfield, CEng, MIEE Chapter 6 P F. Partridge, BSc, CEng, M1EE F. Beach, BSc(Eng), ACGI, DIC, CEng, FIEE, MIMechE B. R. Hill, BSc, CEng, MIEE C. W. Poole, BSc(Hons), DipMS, CEng, MIEE D. L. Threlfall, BSc, CEng, MIEE Chapter 7 B. Barker, CEng, MIEE Chapter 8 E. C. FitzGerald, CEng, M1EE F. Ashurst, KEE Chapter 9 C. H. Spear, BSclEngi, CEng, F1EE Chapter 10 M. Ballinger, MIEEIE Chapter 11 J. t3. Hadwick, BEng, MIEE W. Morgan, BSc, CEng, MIEE Chapter 12 B. R. Hill, BSc, CEng, MIEE

Series ProductionManaging Editor Production Editor Resources and Co-ordination P. M. Reynolds H. E. Johnson T. A. Dolling J. R. Jackson

MODERN POWER STATION PRACTICEThird Edition

Incorporating Modern Power System Practice

British Electricity International, London

Volume D

Electrical Systems and Equipment

PERGAMON PRESSOXFORD NEW YORK SEOUL . TOKYO

U.K. U.S.A. KOREA JAPAN

Pergamon Press plc, Headington Hill Hall. Oxford 0X3 OBW, England Pergamon Press, Inc., 395, Saw Mill River Road, Elmsford, New York 10523, U.S.A. Pergamon Press Korea, KPO Box 315, Seoul 110603, Korea Pergamon Press Japan, Tsunashima Building Annex, 3-20-12 Yushima, Bunkyo-ku, Tokyo 113, Japan Copyright 1992 British Electricity International Ltd

AN Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the copyright holder.First edition 1963 Second edition 1971 Third edition 1992Library of Congress Cataloging in Publication Data Modern power station practice: incorporating modern power system practice/British Electricity International. 3rd ed. p. cm. Includes index. 1. Electric power-plants. I. British Electricity International. TK1191,M49 1990 62.31'21 dc20 90-43748

British Library Cataloguing in Publication Data British Electricity International Modern power station practice.-3rd. ed. 1. Electric power-plants. Design and construction I. Title II. Central Electricity Generating Board 621.3121. ISBN 0-08-040510-X 112 Volume Set) ISBN 0-08-440514-2 (Volume DI

Printed in the Republic of Singapore by Singapore National Printers Ltd

ContentsVi Vii iX Xi

COLOUR PLATES

FOREWORD PREFACE CONTENTS OF ALL VOLUMES

Chapter

1

Electrical system design Electrical system analysis Transformers Generator main connections Switchgear and controlgear Cabling Motors Telecommunications Emergency supply equipment

I 84 193 287 325 427 623 649 748 799 868 948 987

Chapter 2 Chapter 3

Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8

Chapter 9

Chapter 10 Mechanical plant electrical services Chapter 11 Protection

Chapter 12 SynchronisingINDEX

V

Colour Plates(between pp 496 and 497)

FIG, 3.11

Large core being built (GEC Alsthom)

FIG. 3.12 Completed core, showing frame bolts (GEC Alsthom) Eic,, 3.58 Core and windings of single-phase CEGB generator transformer (GEC Alsthom) FIG. 3.60 800 MVA generator transformer bank at Drax power station (GEC Alsthom) Pc. 3.64 Cast-resin transformers for installation in 415 V switchgear (GEC Alsthom) FIG. 5.1/ Two poles (of a three-phase group) of a forced-air cooled generator circuit-breaker installed at Dinorwig pumped-storage power station (British Brown-Boveri Ltd)

FIG. 5.12 One pole of a forced-air cooled generator circuit-breaker, with side covers and the connection to generator busbar removed (British Brown-Boveri Ltd) FiG. 5.13 Three-phase water cooled generator circuit-breaker showing connection into the generator phase-isolated busbar system (British Brown-Boveri Ltd) Fic, 5.14 Generator circuit-breaker control panel (British Brown-Boveri Ltd) FIG. 5.15 Cooling water plant (British Brown-Boveri Ltd) FIG. 5.18 Air plant control panel (British Brown-Boveri Ltd) FIG. 5.29 Typical 3.3 kV switchboard of Reyrolle manufacture FIG. 5.48 3.3 kV switchboard of Reyrolle manufacture. The three left hand units are 'fused equipment Class SI4A' and the three right hand units are 'air circuit-breakers Class SA'. FIG. 5.49 Typical 415 V switchboard of GEC manufacture FIG. 5.51 Typical 415 V switchboard of Electra-Mechanical Manufacturing Co manufacture Fic., 5.69 Switchboard formation of control gear featuring vacuum interrupters in association with protection for 3.3 kV service (GEC Industrial Controls Ltd) HBC fuse

1-ro. 5.70 Example of control gear featuring vacuum interrupters in association with HBC fuse protection for 3.3 kV service, showing the demonstration of the circuit earthing switch (GEC Industrial Controls Ltd)

vi

ForewordG. A. W. Blackman, CBE, FEngChairman, Central Electricity Generating Board and Chairman, British Electricity International Ltd

FOR OVER THIRTY YEARS, since its formation in 1958, the Central Electricity Generating Board (CEGB) has been at the forefront of technological advances in the design, construction, operation, and maintenance of power plant and transmission systems. During this time capacity increased almost fivefold, involving the introduction of thermal and nuclear generating units of 500 MW and 660 MW, to supply one of the largest integrated power systems in the world. In fulfilling its statutory responsibility to ensure continuity of a safe and economic supply of electricity, the CEGB built up a powerful engineering and scientific capability, and accumulated a wealth of experience in the operation and maintenance of power plant and systems. With the privatisation of the CEGB this experience and capability is being carried forward by its four successor companies National Power, PowerGen, Nuclear Electric and National Grid. At the heart of the CEGB's success has been an awareness of the need to sustain and improve the skills and knowledge of its engineering and technical staff. This was achieved through formal and on-job training, aided by a series of textbooks covering the theory and practice for the whole range of technology to be found on a modern power station. A second edition of the series, known as Modern Power Station Practice, was produced in the early 1970s, and it was sold throughout the world to provide electricity undertakings, engineers and students with an account of the CEGB's practices and hard-won experience. The edition had substantial worldwide sales and achieved recognition as the authoritative reference work on power generation. A completely revised and enlarged (third) edition has now been produced which updates the relevant information in the earlier edition together with a comprehensive account of the solutions to the many engineering and environmental challenges encountered, and which puts on record the achievements of the CEGB during its lifetime as one of the world's leading public electricity utilities. In producing this third edition, the opportunity has been taken to restructure the information in the original eight volumes to provide a more logical and detailed exposition of the technical content. The series has also been extended to include three new volumes on 'Station Commissioning', `EHV Transmission' and 'System Operation'. Each of the eleven subject volumes had an Advisory Editor for the technical validation of the many contributions by individual authors, all of whom are recognised as authorities in their particular field of technology. All subject volumes carry their own index and a twelfth volume provides a consolidated index for the series overall. Particular attention has been paid to the production of draft material, with text refined through a number of technical and language editorial stages and complemented by a large number of high quality illustrations. The result is a high standard of presentation designed to appeal to a wide international readership. It is with much pleasure therefore that I introduce this new series, which has been attributed to British Electricity International on behalf of the CEGB and its successor companies. I have been closely associated with its production and have no doubt that it will be invaluable to engineers worldwide who are engaged in the design, construction, commissioning, operation and maintenance of modern power stations and systems.

March 1990 vii

PrefaceTne review of the original Modern Power Station Practice series carried out a few years ago revealed large gaps in its treatment of electro technology within power stations. Not unnaturally much of the content was also badly out of date. It was clear that a straightforward revision of the previous book would not suffice and that a totally new work was required. It was therefore with much enthusiasm that the team of authors of Volume D set out to write it since we all felt it was very timely to do so. The re-organisation of power station design and construction within the CEGB in 1971 created the Generation Development and Construction Division (GDCD). The Division set up an Electrical Branch which pulled together previously dispersed skills and experience in all aspects of power station electrical engineering covering power systems and plant as well as control, communications, data and instrumentation systems and equipment. Two of the volumes of MPSP, Volumes D and F, are largely based on the work done over many years by GDCD Electrical Branch. Volume D deals with the work of some 50 electrical design specialists in the power engineering field while Volume F covers a similar level of activity in the C and I field. One of the major tasks of the Electrical Branch has been to rationalise electrical system and plant design and development, and produce designs which meet operational needs in the most economic way and with the required level of reliability and performance. As the Head of the branch for many years I have felt privileged to edit Volume 1D. The twelve chapters describe in appropriate detail the design philosophies and techniques which have underlain the work of the Branch. They describe the solutions to the large number of design problems which have been identified and the plant which has been chosen and developed to equip electrical systems both within the different types of new power station which have been built as well as for replacement and modification tasks at existing stations. Since the formation of GDCD, CEGB projects have included most types of generating plant including AGR and PWR nuclear stations, fossil-fired stations (both oil and coal), gas turbine and diesel driven generators and a major hydro plant at Dinorwig. While most of the electrical equipment for these different sorts of power station is similar, the electrical system needs vary widely. The designs described in Volume D therefore deal with the requirements for all types of power station electrical plant and systems. Furthermore, while the rate of change of electrical power plant technology is not as fast as that in the light current area, there has nevertheless been considerable equipment development and an even greater change in design techniques and methodology. This is especially true in the analysis of electrical system design and performance and in some areas of plant development such as cable system design. Since the Electrical Branch has also been responsible for control, instrumentation, communication and data systems it has been possible to ensure co-ordinated complementary development of both heavy and light current systems. The many interfaces, e.g., cabling, power supplies, instrumentation, protection and metering have been engineered with a coherent systems approach. The light current technology in the control, instrumentation and data systems area is described in Volume F. The Advisory Editor for that volume, Mr M.W. Jervis, and I, as colleagues in Electrical Branch, have always striven to maintain a close co-operation on all aspects of electrical systems design. We hope that this integration of design effort will be apparent to the readers of Volumes D and F. I would like to record my most sincere thanks to my many colleagues who have produced Volume D and also Volume F. They have undertaken the work in parallel with their day to day responsibilities and have seen the task as an opportunity to put in writing a review of the results of their work for CEGB. I also wish to express my gratitude to Professorix

Preface John Davies and Mr. Peter Reynolds for the great help and support which they have given me in the preparation of the volume. I believe that Volume D stands as a record of many years of high quality electrical design activity and that it will remain relevant as an exposition of the science for a long time. F. BEACH Advisory Editor Volume D

Contents of All VolumesVolume A Station Planning and Design Power station siting and site layout Station design and layout Civil engineering and building works Boilers and Ancillary Plant Volume B Furnace design, gas side characteristics and combustion equipment Boiler,unit thermal and pressure parts design Ancillary plant and fittings Dust extraction, draught systems and flue gas desulphurisation Volume C Turbines, Generators and Associated Plant

The steam turbine Turbine plant systems Feedwater heating systems Condensers, pumps and cooling water systems Hydraulic turbines The generator

Electrical Systems and Equipment Volume D Electrical system design Electrical system analysis Transformers Generator main connections Switchgear and controlgear Cabling Motors Telecommunications Emergency supply equipment Mechanical plant electrical services Protection Synchronising

Volume E Chemistry and Metallurgy Chemistry Fuel and oil Corrosion: feed and boiler water Water treatment plant and cooling water systems Plant cleaning and inspection Metallurgy Introduction to metallurgy Materials behaviour Non-ferrous metals and alloys Non-metallic materials Materials selection

xi

Contents of All Volumes Welding processes Non-destructive testing Defect analysis and life assessment Environmental effects Volume F Control and Instrumentation Introduction Automatic control Automation, protection and interlocks and manual controls Boiler and turbine instrumentation and actuators Electrical instruments and metering Central control rooms On-line computer systems Control and instrumentation system considerations

Station Operation and Maintenance Volume G Introduction Power plant operation Performance and operation of generators The planning and management of work Power plant maintenance Safety Plant performance and performance monitoring

Volume H Station Commissioning Introduction Principles of commissioning Common equipment and station plant commissioning Boiler pre-steam to set commissioning Turbine-generator/feedheating systems pre-steam to set commissioning Unit commissioning and post-commissioning activities

Volume J Nuclear Power Generation Nuclear physics and basic technology Nuclear power station design Nuclear power station operation Nuclear safety

Volume K EHV Transmission Transmission planning and development Transmission network design Overhead line design Cable design Switching station design and equipment Transformer and reactor design Reactive compensation plant I-1 VDC transmission plant design Insulation co-ordination and surge protection Interference Power system protection and automatic switching Telecommunications for power system management Transmission operation and maintenance

xii

Contents of All Volumes Volume L System Operation System operation in England and Wales Operational planning demand and generation Operational planning power system Operational procedures philosophy, principles and outline contents Control in real time System control structure, facilities, supporting services and staffing Volume M Index Complete contents of all volumes Cumulative index

Evan John DaviesEmeritus Professor of Electrical and Electronic Engineering at Aston University in Birmingham, died on 14 April 1991. John was an engineer, an intellectual and a respected author in his own right. It was this rare combination of talents that he brought to Modern Power Station Practice as Consulting Editor of seven volumes and, in so doing, bequeathed a legacy from which practising and future engineers will continue to benefit for many years.

XV

CHAPTER 1

Electrical system design1 Introduction 2 System needs Station operating criteria 2.1 2.2 Grid system operation criteria 2.3 Plant and personnel safety needs 2.4 Nuclear hazard needs 3 System descriptions 3.1 Main generator and station systems 3.1.1 Main generators 3.1.2 Generator transformers 3.2 Electrical auxiliaries systems 3.2.1 Auxiliaries system transformers 3.2.2 Interconnection 3.2.3 Essential systems 3,2.4 Emergency generation 3.3 Types of stations 3.3.1 Fossil-fired power stations 3.3.2 Magnox nuclear power stations 3.3.3 AGR nuclear power stations 3,3,4 P M nuclear power stations 3.3.5 Hydro and miscellaneous 4 System performance 4.1 Station and unit start-up 4.1.1 Plant required 4.1.2 Synchronising to the grid 4.1.3 Synchronising unit to station 4.2 Shutdown and power trip 4.2,1 Controlled shutdown 4.2.2 Power trip 4.3 The effects of loss of grid supplies 4.4 Station plant outages and faults 5 System choice 5.1 Operational requirements 5.2 Reliability of main and standby plant 5.3 Economics 5.4 Plant limitations 5.4.1 Switchgear current rating 5.4.2 Switchgear short-circuit rating 5.4.3 Large electric motors 5.4.4 System performance calculations 5.5 Maintenance and safety 5.5.1 Operational 5.5.2 Maintenance 5.5.3 Other safety interlocking 5.5.4 Nuclear safety 5.6 Quality assurance 5.6,1 Design quality 5.6.2 Product quality 6 Uninterruptable power supply MPS) systems 6.1 Introduction 6.2 Earlier UPS and GlS schemes 6.2.1 Motor-generator (MG) set schemes 6.2.2 Static inverter schemes 6.3 Development of UPS systems 6.3.1 Littlebrook D power station schemes 6.3.2 Drax power station schemes 6.3,3 Heysham 1 power station 6.4 System configuration and method of operation 6.5 System considerations and components 6.5.1 Voltage regulation 6.5,2 UPS system loads 6.5.3 Step-down transformers 6.5.4 Standby and spares philosophy 6.6 UPS equipment specification 6.7 UPS equipment performance requirements 7 DC systems 7.1 Introduction 7.2 DC system duties 7,3 DC system design 7.3.1 250 V DC systems 7.3.2 220 V DC systems 7.3.3 110 V DC systems 7.3.4 48 V DC systems 7.3.5 250 V. 220 V and 110 V DC circuit earthing 7.4 DC system analysis 7.5 Battery chargers and batteries 8 Electrical system monitoring and interlocking schemes 8.1 Introduction 8.2 Operational interlocking, monitoring and indications 8.3 Relay systems 8.3.1 Switchgear auxiliary contacts 8,3.2 Application of interlock schemes 8.4 Computer-based systems 8.5 Maintenance interlocking equipment 8.5.1 Key exchange boxes 8.5.2 Scheme application 8.6 Other safety interlocking

Introduction The various electrical systems within a power station include those associated with the connection of the generating plant to the grid system and the very much larger number which are provided to distribute power supplies around the auxiliary plant within the station

boundaries. The total electrical systems therefore interface with the whole of the power station installation. The systems can be summarised as follows: Generator primary system and grid, typically 23.5 kV (660 MW) or 26 kV (900 MW) for units with grid voltages of 275 kV and 400 kV.1

Electrical system design Station board system from grid, typically II kV derived from 132 kV, 275 kV or 400 kV. Station and unit auxiliaries systems, typically at 11 kV, 3.3 kV and 415 V. Einereency pow er supplies systems, typically gasturbine, diesel-driven generators connected at 11 kV, 3.3 kV, 415 V. DC systems, typicall at 250 V, 220 V, 110 V, 48 V. Uninterruptable power supply systems (UPS), typically at 415 V single and 3 phase, 110 V single-phase. The security required of the electrical supplies is determined by the importance of the power station plant ot equipment. For example, auxiliaries associated with the main unit which if lost would immediately cause loss of unit output, clearly require more secure supplies than services such as sump pumps used occasionally. The nature of the supplies also requires careful consideration by way of voltage and frequency limits, susceptibility to transients caused by faults or switching operations and the consequences of short breaks in supplies. As a matter of course, most items of plant and equipment are specified and tested for compliance with known standards. This will include their electrical performance. If the need for new types of equipment is identified then performance limits should be defined at the outset of any development work, where standards cannot be quoted. The degree of security must also be taken into account since parts of nuclear power stations will warrant a much higher level than, say, a small hydro station. It is necessary then, to recognise from the outset the importance of each item of plant when determining the nature and degree of security of electrical supplies it requires. The sources for auxiliaries supplies range from the grid-derived AC supplies, through to batterybacked AC and DC supplies and the 'short break' supplies. The bulk of the electrical auxiliaries load is normally arranged to be taken from the grid-derived AC supplies. This will mean that the outline design of the electrical auxiliaries system can benefit from previous know ledge and experience when considering alternative supply arrangements to a certain level at an early planning stage of a project. The alternatives will include, for example, unit and station transformer schemes, generator soltage switchgear schemes and FIV switch isolator schemes. Detailed descriptions of t hese and other schemes are given later in this chapter. The timely and accurate design of electrical systems is always easiest if at the outset, and at appropriate stages of the project, full details of electrical loading, rating and duty information can be established from the plant specialists, particularly for the major items, e.g., reactor, boiler, turbine-generator and operational ancillary plant. One way of achieving this is by in2

Chapter 1 eluding standard electrical loading, rating and duty schedules in all the plant enquiry specifications, thereby committing tenderers to identify their design loads. It also assists in forming a comparison between competitive tenders and should be followed up with more accurate and detailed information at defined stages of the contract by the chosen contractors. By this means, the electrical system loadings can progressively be assembled and refined, enabling design ratings of transformers, switchgear, cables, etc., to be established for comparison of the various possible alternative electrical systems. The system designer would always present a recommended scheme by comparing the alternatives on a basis of first and lifetime costs and suitability for duty. Until this stage is reached, the electrical plant specialists cannot seriously begin to specify their requirements. It is possible, however, that the system designer has already taken account of the commercially available equipment, which will make the specifying of electrical components more straightforward. This chapter explains the approach and criteria used in determining the most suitable electrical systems for the various duties required at nuclear, fossil-fired and hydro power stations. There is a brief reference to other forms of generation, generally referred to as alternative sources of energy.

2 System needs 2.1 Station operating criteriaIn common with all other areas of design in power stations, the electrical system designers must have a clear definition of what operating criteria need to be achieved. In the case of the CEGB, station development particulars are formulated at the early planning stages which incorporate the Station Technical Particulars (STPs). The STPs contain the requirements for the main plant availability, operating flexibility and the control of units. In addition, they will include the technical requirements for the generator transformers, the plant auxiliaries supplies and the station protection arrangements. Various appendices will detail the specifications to be met and the finite limits to be achieved. From these, it will be apparent what minimum features need to be built into the electrical systems to achieve the station output while at the same time ensuring the safety of personnel and plant, a more onerous requirement on nuclear power stations. Further documentation is prepared for nuclear power stations to cover the safety aspects in the form of a Preliminary Safety Report. The interpretation of the STPs into electrical requirements becomes the designer's check list and generally will incorporate the following as typical: (a) Station rated output is required over a supply frequency range of 49.5-50.5 Hz, with pro-rata

System needs decrease in the range 49.5-47 Hz. Operation below 48.8 Hz will be very infrequent and for periods not longer than 15 minutes. (b) A fault, including a fire, in any section of any auxiliaries system shall not cause more than one main generator to trip under all normal operating conditions. in perspective, outside the range of 50.5 Hz, it is generally expected at the following estimated rates: Greater than 50.5 Hz, I incident per year. Greater than 52 Hz, 0.2 incidents per year. In addition to the frequency ranges above, the auxiliaries system will be required to accommodate the network voltage variations. The 400 kV supergrid system voltage will normally remain within the range 400 kV +5%. The maximum voltage which can arise is 440 kV, but this condition would not be permitted to last longer than 15 minutes. The 132 kV system voltage can vary between the limits of 132 kV 10 070. Internally generated switching or other transient overvoltages on the auxiliaries system were mentioned above, but added to these will be any transferred surges from the grid system. The amplitude of step changes of voltage on the 400 kV system are not expected to exceed +6%. The effects of total or partial loss of the grid connections to power stations vary depending on the type of station, the most significant effects being on nuclear power stations. It is essential to re-establish AC supplies within known timescales in these instances to maintain nuclear safety. This is described more fully in Section 2.4 of this chapter. In the case of conventional power stations, the safety of plant and personnel is normally taken care of by the DC systems if AC supplies are lost. Re-establishing the AC supplies does not usually require the same emphasis other than for returning the main generators to service.

(e) The plant auxiliaries systems shall remain stable for three-phase faults of duration up to 200 ms on 'close-up' sections of the supergrid and grid busbars and the adjacent system, over a specified range of operating conditions.(d) The plant auxiliaries supply arrangements shall be designed to meet all the operating flexibility requirements, e.g., two-shifting, part-loading and load rejection. (e) The plant auxiliaries system shall satisfactorily withstand any internally generated switching or other transient overvoltages. (1) The plant auxiliaries system shall accommodate the generator operating with a terminal voltage in the range of 95% to 105% of the rated value. In addition to the needs that the auxiliaries electrical system must meet as requirements of the STPs, the designer may incorporate system features to improve availability by supplementing those required by the STPs. For example, the incorporation of alternative supplies to selected switchboards could reduce outage ti me and consequently lost revenue from a main generator following an electrical fault; such a design feature will be subjected by the designer to economic justification. This and other 'additional requirements' will be explained in more detail in Section 4 of this chapter.

2.3 Plant and personnel safety needsIt will be appreciated that maximising the output from power stations must be achieved within recognised standards, codes of practice and rules to ensure the safety of the power station plant and personnel. In electrical system design terms, adequate safeguards must be incorporated to meet the statutory requirements of the Electricity Regulations and the Health and Safety at Work Act, relating these to safety rules. The CEGB Safety Rules set out the mandatory requirements for establishing the safety of persons at work. The electrical systems must build-in means of achieving operational and maintenance regimes to comply with all necessary safety requirements. Operationally, the major considerations are to ensure that the normal and abnormal duties and prospective fault capabilities for circuits and system configurations are not exceeded. The circuits must be rated for required voltage and for normal and fault currents calculated during the design, and in the case of the switchgear must be capable of making and breaking current during normal and fault operations. Interlocking or monitoring schemes need to be incorporated to ensure that ratings are not exceeded due to operator 3

2.2 Grid system operation criteriaWhile the power station has specified operating criteria, the grid system into which it generates also has such criteria defined for it. The significant ones are those associated with frequency, voltage and total or partial loss of the grid connections in the vicinity of the power station. The frequency ranges have been outlined in Section 2.1 of this chapter. In addition however, looking from the grid into the power station, it must be remembered that below 47 Hz the auxiliaries system may be protected by an automatic trip, although an excursion of this sort has a very low probability. There are also the onerous transient frequency excursions as a result of full-load rejection and possibly periods of steady high frequency up to 52 Hz, which the auxiliaries system will be expected to withstand without tripping for periods not exceeding 15 minutes. To put the likelihood of local frequency excursions

Electrical system design error. Descriptions of such schemes are contained later in this chapter. For maintenance of plant and equipment there is a CEGB mandatory requirement to isolate and earth all circuits at voltage levels of 3.3 kV and above before work can commence. At 415 V and below, proof of isolation must be established. Details of these features are described later in this chapter. Protection of the plant must be arranged to prevent damage without resulting in an increased loss of availability. For example, should a turbine-generator trip as a result of loss of AC supplies or trip and cause loss of AC supplies, the lubricating oil systems are maintained by means of DC motor-driven pumps. Other means of maintaining the safety of plant will be described later in this chapter when considering what safeguards need to be incorporated into the system.

Chapter 1 which each has been utilised is described later in Section 3 of this chapter.

3 System descriptionsThe generating units of each power station deliver their electrical output to the National Grid via connections at 400 kV or 275 kV, although at some older generating stations the generators are connected to the grid at 132 kV. As part of the design of new power stations, dependent on the network and capacity requirements of the transmission system in the area, consideration may be given to building a new 400 kV substation at locations where existing generating plant is connected at lower voltages, i.e., 275 kV or 132 kV. The present policy is to use SF6 insulated metalciad 400 kV switchgear, often mounted indoors, particularly on coastal or polluted sites. If extensions to existing substations is the economic method of connecting new generators, then 'open' busbar design would be employed using SF6 circuit-breakers. The stations require supplies to be available at all ti mes for supplying 'station' auxiliaries and depending on the system design, for providing a supply to the 'unit' auxiliaries for starting up and shutting down of the units as shown in Fig 1.1. In the cases where generators are connected to the grid via a generator voltage switch, the units are normally started and shutdown via the generator/unit transformer route, though a separate source for 'station' supplies would still be provided for the station auxiliaries and for standby to the unit transformer as illustrated in Fig 1.2. If available, this would normally be derived from a 132 kV source since, for the rating of 500 MVA and below, the transformers are well proven, economic and the switchgear is cheaper. If however, 132 kV is not available on the site, to create a 132 kV substation might require long cable routes or overhead lines and possibly provide additional intergrid reinforcement. This may be more costly than considering station transformers connected at 400 kV.

2.4 Nuclear hazard needsThe electrical systems provided at nuclear power stations must relate to the plant and equipment required to prevent the release ultimately of radioactivity to the atmosphere. Initially the favoured source of electrical supplies to these safety systems would be derived from the grid network. This network has finite li mits of its own, the voltage and frequency limits having been described in Section 2.2 of this chapter. However, when these limits are exceeded they can, particularly in the case of nuclear power stations, be regarded as being the equivalent to a total loss of grid supplies. The likelihood of this 'total loss' must be considered in relation to the time factors associated with maintaining nuclear safety. For example, in the case of the Sizewell B pressurised water reactor (PWR) station ti me bands of 0 to 2 hours, 2 to 12 hours and greater than 12 hours have been considered. The probability can be related to the required stage by stage availability of the plant needed to meet the safety case. Consideration of the needs of the safety related plant to the probabilities of losing grid supplies invariably leads to the provision of a supplementary AC source of supply by means of on-site generation. In most cases this has been provided by either gasturbine or diesel-driven generators. The choice between the two will depend on several factors including the rating and availability of the auxiliary generation required. For example, the CEGB have utilised gasturbine generators of 17.5 MW rating on earlier AGR nuclear power stations. For the later Heysham 2 AGR power station, diesel generators up to 8 MW rating have been installed. A significant factor regarding the Heysham 2 diesel generators was the need for a fast start-up/loading requirement. This influenced the generator parameters, e.g., a low subtransient reactance value was chosen to achieve fast start-up while still containing the prospective fault contribution to an acceptable level. The manner in4

3.1 Main generator and station systems3.1.1 Main generatorsGenerators of 660 MW (776 MVA) rating having a nominal output voltage of 23.5 kV. The output of the machine to the generator transformer is via phase isolated connections, naturally air cooled and rated at 20 000 A, either directly connected or switched by purpose built generator voltage switchgear, depending on the auxiliaries system design. Details of the generator main connections and generator voltage switchgear are given in Chapters 4 and 5 respectively. At present these are the largest generating sets installed in the UK. Designs are being developed for generators rated at 900 MW, in which case the gen-

System descriptionsm

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Fro. 1.1 Typical unit station system for a 660 MW generator

erator terminal voltage will probably increase to 26 kV. The present style of main generator connection arrangements will be capable of carrying the increased output current, although forced cooling by either air or water may be required. The limiting factor for naturally cooled connections would be accommodation of the greatly increased size, particularly within the generator terminal centres. This is fully described in Chapter 4.3.1.2 Generator transformers

Each generator is connected to the grid system via a generator transformer with the appropriate voltage ratio. The CEGB fit on-load tap changers to accommodate the grid voltage variations and the voltage operating range of the generator. It has established a 'registered design' of generator transformer for the 660 MW generating units rated at 800 MVA and made

up of 3 single-phase units. The intention of the registered design is to achieve a high level of reliability by avoiding all but essential change to proven systems in detail design, materials or components (see Chapter 3). The 800 MVA rating is based on taking the main generator 776 MVA rating plus the possible 44 MVA output from a gas-turbine generator, contributing from the 11 kV level via a unit transformer, less a minimum unit auxiliaries load of 20 MVA. For the generating units being considered at 900 MW (nominal) rating, a generator transformer rated at 1145 MVA, also in 3 single-phase tanks, is being developed, taking into account an overload capability from the main unit. As with the 800 MVA rating, on-load tapchangers will be fitted for the same reasons. There have been instances on nuclear power stations using generator voltage switchgear, where the on-load tapchanger has been arranged with an auto5

Electrical system design

Chapter 1

GRID

GRID

GENERATOR TRANSFORMER

GENERATOR VOLTAGE SWITCH

STATION TRANSFORMER UNIT TRANSFORMER INTERCONNECTOR

ti kV UNIT BOARD

11kV STATION BOARD

UNIT AUXILIARIES

STATION AUXILIARIES

CIRCUIT BREAKER CLOSED

(1: 1) CIRCUIT BREAKER NORMALLY OPEN

FIG. 1.2 Typical generator voltage switch system for a 660 MW generator

matic feature. This has been done to deal with the problem that arises when a generator trip results in the opening of' the generator voltage switch but retains the grid connection. Under these circumstances, the 11 kV switchboard voltage could fall to a level such that the direct on line starting of a boiler feed pump may not be achieved. The auto-tap facility raises the voltage in a timescale and to a level capable of achieving a pump start thus securing an initial boiler feed without relying on the emergency feed pumps. Should this scheme not achieve boiler throughput, the emergency pumps connected to the 3.3 kV system will still ensure reactor safety.

3.2 Electrical auxiliaries systemsThese systems provide the power for the station auxiliaries and are nowadays almost always designed on the unitised principle. In the past, in some cases, particularly early magnox and AGR stations, this principle was not strictly followed, the former consortia ho built the stations having used criteria different from present day practice. The general arrangements for electrical auxiliaries systems are described below, and in their design due comisance is taken of the limits and constraints imposed by the equipment commercially available which is or could be type approved for the particular application. In the unit principle, all auxiliaries associated with6

starting and running the unit at CMR output are connected to the unit electrical system. This must be designed such that one fault including a fire, does not lose the output of more than one generating unit. Similarly, plant which does not have an immediate effect on the running of the unit, will be connected to the station electrical system. It is required that a fault on this plant will not immediately propagate into the unit electrical system and affect unit output. To achieve this, the electrical and mechanical plant, switchgear and cabling is segregated between the units, and between the two halves of the station system, (normally known as 'station A' and 'station B'). Segregation is not normally provided between the unit and station systems. Standard voltage levels of 11 kV, 3.3 kV and 415 V have been selected to accommodate the very wide range of plant drives and equipment. In the case of the 11 kV unit system, a major constraint is the thermal current rating of the largest circuit-breaker commercially available and approved for use on CEGB systems. The present nominal rating is 3150 A which, when calculated in relation to the transformer standard BS171 requirements, relates to an incoming transformer rating of approximately 60 MVA. Therefore, if the unit load is in excess of this, two unit transformers are required. The fault interrupting capability of the switchgear also adds a constraint, which is discussed under 'system choice' in Section 5 of this chapter.

System descriptions Supplies to the unit board are derived from a unit transformer on the basis of one per switchboard, the primary of which is teed-off the generator voltage main connections system. All auxiliaries requiring electrical motor drives, whose combined operation is necessary to keep the unit venerating, are connected to the unit system. Large motors rated at 1500 kW and above are generally connected to the 11 kV system, e.g., electrical boiler feed pump , circulating water pumps, gas circulators at nuclear power stations and boiler fans at fossil-fired power staticns. In the case of nuclear power stations, particularly early AGRs (and some magnox), the 11 kV unit systems form part of the nuclear safety case. This is because the 11 kV provides a preferred source of supply to the essential system, and in some cases feeds essential plant directly, e.g., gas circulators. In this context, essential plant is that which is required following a reactor trip to shut down the reactor safely and remove post-trip decay heat. Back-up emergency generating facilities are provided at the appropriate voltage level should the grid connection fail. If the emergency drive is greater than 1500 kW and therefore requires connecting to the II kV system, emergency generation at II kV is provided. This is described in more detail later in this section. The 11 kV unit switchboard as well as supplying large motors also provides a feed to the 3.3 kV unit system via 11/3.3 kV oil-filled unit auxiliaries transformers located outdoors. For nuclear stations there is, in addition, an 'essential system' which normally derives supplies from 11/3.3 kV 'essential transformers'. If grid derived supplies are not available, the essential system is supplied by on-site generation (gas turbines or diesels). The thermal current limits applicable to unit transformer incoming circuit-breakers apply equally to the auxiliaries and essential transformers, which are limited by the largest approved circuit-breaker at 3.3 kV, rated at 2400 A, giving the largest practical size of transformer rating of around 14.5 MVA. Unit related auxiliaries in the range 150-1500 kW are connected at 3.3 kV, although motors outside this range may be connected for special cases. The 3.3 kV system also provides feeds to the 415 V unit system via 3.3/0.415 kV unit services transformers. These transformers are normally naturally air cooled ' AN' type and mounted in the switchboards. The 415 V system is distributed around the power station, with switchboards located in switchrooms as close to the load as possible. Motors up to approximately 150 kW are connected to this system although motors above this rating may be considered in special cases. To maintain a high availability of electrical supplies to auxiliaries, duplicate feeds are supplied to each switchboard. This may be achieved by two incoming supplies and a bus section switch, or by one incoming supply and a cabled interconnector to another switchboard which has its own incoming supply. For both these methods, each transformer feeding the switchboards must be rated to include the standby requirement of the other transformer. Those auxiliaries which are common to two or more units or are not necessary to maintain unit output, are connected to a 'station' electrical system, i.e., not specifically associated with any one unit. The 11 kV station system has several duties, and the rating chosen will reflect the duty it is called on to perform. It does however share the same constraints as the 11 kV unit system brought about by the circuitbreaker ratings. The station system may be required to provide a source of supply to large 11 kV 'unit' drives, directly in some system arrangements or in a standby mode in others to cater for a unit transformer outage. The station transformer rating must be chosen accordingly. The different duties expected of station transformers are outlined in Section 3.2.1 of this chapter. Feeds from the 11 kV station system are provided to a station 3.3 kV system via 11/3.3 kV station auxiliary transformers. Common station services, such as coal handling plant at fossil-fired power stations, would be supplied at this voltage level. The rating of motors at 3.3 kV would be on the same basis as the unit system. Station supplies at 415 V are derived from the 3.3 kV switchboards via 3.3/0.415 kV services transformers, usually of the 'AN' type, mounted in the switchboards and located in switchroorns as near to the load centre as layout permits. On some nuclear power stations while the station system is not required ultimately in the safe shutdown case, in many cases it may provide grid derived supplies to nuclear plant and relieve the demands on the essential system. For example, at Heysham 1 AGR, the 132 kV grid derived supplies can be made available from the station system to the main gas circulator motors via converters following a reactor trip. As such it would be required to be engineered with this duty in mind.

3.2.1 Auxiliaries system transformers

Unit transformersAs mentioned above, the supply to the II kV unit board is via a dedicated 23.5 kV/11 kV unit transformer, with a rating chosen to match the unit load, but limited to 60 MVA due to the largest approved rating of 11 kV circuit-breaker. Another consideration the designer must take into account is the choice of transformer impedance. A unit transformer has a typical impedance of approximately 15% on rating. When this value is used in the analysis of the station's electrical auxiliaries system, it may require alteration. For example, the electrical system regulation may be too high, making the starting of large 11 kV squirrel-cage induction motors direct-on-line (DOL) difficult. The maximum rating suitable for DOL starting at 11 kV 7

Electrical system design is about 11 MW. Also, unacceptable voltage conditions may be experienced at the lower voltage levels. In this case the impedance may need to be reduced. In con11-1 with this, too low an impedance may give rise to unacceptable fault levels on the 11 kV system, especially when unit and station supplies are paralleled during station start-up and shutdown procedures. The subject of parallel operation is discussed more fully Section 4 of this chapter. Solutions to this conflict are seldom easy and almost always cause complications and additional expenditure. The options are: Use assisted starting techniques, i.e., 'soft' starting, for the largest motors by utilising static or rotary converters. This may also be combined with woundrotor motors, rather than squirrel-cage.a

Chapter 1 The station transformers' duties may be summarised as follows: Supply the total 'station' load (due to outage of the other station transformer) as well as supplying the starting load of a unit. Supply its proportion of the station load and the CMR unit load when acting as replacement for a unit transformer. It should be noted that to accommodate the single fault criteria (that one fault should not lose all station supplies), a minimum of two station transformers will be required for the station. The above duties become more complex when more than two station transformers are used on multi-unit stations. However, the above principles remain the same. Similar to the principles outlined in the section on unit transformers, the impedance of the station transformer must be chosen to enable paralleling with the unit transformer, for start-up and shutdown and to allow the largest electric motor (normally the feed pump) to be started. As mentioned above, the use of higher rated sets may preclude paralleling and alternative methods may be required to achieve successful methods of changeover for start-up and shutdown supplies. It should be noted that the above criteria are a general guide, and each proposed electrical system is designed with the particular requirements of the station addressed specifically. More information is given in the following sections.

Use automatic fast transfer systems when switching between unit and station supplies to reduce the transfer time to one or two cycles. This permits break before make without allowing the speed of running motors to drop below recovery times. If make before break is ever regarded as an acceptable option, it would limit the time during which prospective fault levels exceed ratings.

Use generator voltage switchgear to provide start-up supplies via the generator and unit transformer. Use HV connected unit transformers, with HV disconnection of the generator/generator transformer combination. Increase the system voltage to say 15 kV thereby increasing the possible transformer rating. This is not being pursued in present designs, mainly because it would mean either creating a 15 kV system with a separate unit transformer for the very large drives only or raising the voltage for all the motors catered for at II kV, e.g., induced and forced draught fans, CW pumps. These problems have become more pronounced with he proposed introduction of larger generating sets, c.g., 900 MW, without steam-turbine-driven boiler feed pumps and relying on large full duty electric feed pumps in a 3 x 50c% configuration each rated at 13.5 MW. It should be noted that past practice has beLn to design systems whereby unit and station systems arc capable of being paralleled for start-up/shutdown and 'or standby duty without exceeding fault levels (sce Section 4 of this chapter). Slur on transformers The supply for the 11 kV station boards is via a 132 kV, 275 kV or 400 kV/11 kV station transformer, the rating of which is chosen to provide a starting facilit , for the unit, and standby capacity to the unit transformer in the case of its being unavailable, due to an outage. 8

3.2.2 InterconnectionTo enable flexibility of operation and to cater for planned or forced outages, interconnection between different switchboards at the same voltage levels is normally provided. These are usually cabled interconnections with circuit-breakers at each end. The associated circuit-breakers are arranged such that one is normally closed. This energises the cable permanently, so that any cable fault is detected and cleared by the protection immediately, rather than when the circuit is energised just prior to being required. Interconnection may be one of two distinct types: Where the two supplies may be paralleled, thereby giving continuity of supply. Where an alternative supply is required but the two sources may not be paralleled due to a paralleled fault level in excess of the switchgear certified rating. Where interconnection is provided between supplies which may be paralleled (as there is no fault level restriction) but may be out of phase and frequency, check synchronising facilities will be provided at the normally open circuit-breaker. Where interconnection

System descriptions would produce unacceptable fault levels at the switchboard, an indication or interlocking system is provided to ensure that the circuit-breaker is not closed. Indication and interlocking systems are discussed further in Section 8 of this chapter. 3.2.3 Essential systems All power tations require essential systems, but a fundamental difference exists between fossil and nuclear plant. Fossil plant only requires essential electrical systems to maintain unit output and to protect plant from damage following a loss of supply. The consideration of these systems only needs to examine economic and personnel safety issues, and the systems are designed to achieve these objectives. Nuclear plant requirements are much more onerous, due to the fission product decay heat which requires removal to avoid an unacceptable risk of a radiological hazard and expensive plant damage. Essential systems for all the power stations are based on additional on-site prime movers, either diesel generators or gas-turbines, together with batteries and chargers providing no-break supplies. Present designs also use uninterruptable power supplies (UPS) to provide instrumentation and power supply requirements which are battery-backed. These systems are also used in normal operation since they provide a stable voltage and frequency supply 'isolated' from the transients experienced by the main auxiliaries system. They are based on centralised schemes of static or rotary inverters, with a battery backing for a timescale in the region of 30 minutes to cater for loss of the battery charger or its AC supplies. For more details on UPS see Section 6 of this chapter. The DC system voltage levels are chosen for selected duties such as emergency drives and emergency lighting at 250 V, switchgear with the higher current closing solenoids at 220 V, protection, direct control and switchgear tripping at 110 V and telecommunications, remote control and indications at 48 V. The batteries are usually of the lead-acid Plante type. The DC systems are described later in Section 7 of this chapter, and the batteries and chargers in Chapter 9. 3.2.4 Emergency generation As mentioned previously, on-site generation is provided for emergency supplies to the auxiliaries system on all power stations. There are many differing types, dependent on the type of station and the needs which have to be met. Generators may be powered by gas turbines, or diesels and may be at voltages of 11 kV or 3.3 kV. On-site generation for large fossil-fired stations since the early 1960s has been provided by gas turbines at 11 kV, and has satisfied the following needs: To provide an independent supply to the auxiliaries of the main steam units in the event of unacceptably low frequency on the Grid system. Use as output plant capacity to meet system requirements. In this mode of operation the gas turbines will normally be used for 'Peak generation' purposes, and will also act as 'hot standby'. Ability to start-up a station without external Grid supply. To provide an independent supply in order to ensure the operation of essential drives, such as the main bearing lubricating oil, in the event of loss of normal supplies. This duty is, in effect, a back-up to the DC battery system. On-site generation for nuclear power stations assumes a more important role as it becomes part of the nuclear safety case. All plant required to safely shut down and cool the reactor is normally supplied from an essential system, which derives its preferred supply from the grid supply. Failure of the off-site connection requires the on-site generation to connect, usually automatically, to the essential system. The large quantities of decay heat in the reactor core/boiler system cause prolonged requirements for feedwater, steam dumping and reactor core cooling after the turbine-generator has been tripped.

3.3 Types of stationsThe CEGB have a wide variety of power stations from base load coal-fired and nuclear power stations to oil-fired, hydro, pumped-storage and gas turbine types, and gas-fired and wind power pilot installations. The bulk of the demand is of course met by the base load stations which this section will concentrate on. The present design policy to take the CEGB into the t wenty-first century is to have both large coal-fired stations and nuclear stations of the PWR design. Combined cycle gas-turbine (CCGT) installations are also a future possibility. The coal-fired stations will be at the 2 x 900 MW size and the first PWR will be at Sizewell B with a single reactor and 2 x 660 MW turbine-generator units. With the increasing concern for controlling the emissions from coal-fired stations, retrofitting of Flue Gas Desulphurisation (FGD) plant is taking place at selected existing coal-fired stations and included at the design stage for the new 2 x 900 MW designs. The additional loading imposed by FGD on the auxiliaries system is very significant, resulting in the designers assessing different schemes for meeting the various methods of providing FGD plant. FGD is an international problem being tackled in various ways, but initially the CEGB are employing the limestone/ gypsum method. The additional auxiliaries system load9

Electrical system design it-1g for this process at a 2 x 900 MW station is of the order of 45 MW for the entire plant. Considering now the auxiliaries systems for the various types of stations, this section describes the different aspects associated with each.

Chapter 1 justify providing facilities beyond what could be provided to meet only the STP requirements. Each 3.3 kV unit auxiliaries board is supplied by duplicate 8 MVA transformer feeders, each capable of supplying the 3.3 kV auxiliaries load and thereby providing standby to each other. The transformer i mpedance was chosen to enable both transformers to be in service at the same time. There was no need therefore to provide any unit/station interconnection at the 3.3 kV level. At the 415 V level, a sectionalised unit services/ station services switchboard was introduced, each section fed from its respective 3.3 kV auxiliaries board. This provides a better utilisation of transformer capacity at this level than having separate unit and station 415 V boards with duplicate supplies for each from the respective 3.3 kV unit or station auxiliaries system; thus reducing the cost, space and maintenance requirements. Transfer of loads from one transformer to its standby is carried out off-load since the prospective fault level at 415 V does not permit carrying this out on-load. For the fossil-fired stations, slightly different auxiliaries systems have evolved as the CEGB moved from the 500 MW unit period to the 660 MW units of the late 1960s. All systems used the unit/station principle. Most of the stations with 500 MW units had four units each with two station transformers, typically shown in Fig 1.5.

3.3.1 Fossil-fired power stationsThe majority of existing CEGB fossil-fired plant is fuelled by coal or residual oil with a small number capable of being fired by either. There are a few gas turbine stations with units of about 70 MW using distillate fuel, and Hams Hall C power station which is dual-fired, using coal or natural gas. The latter example using natural gas was a pilot conversion scheme to assess its feasibility. Basically for a given location and station output, the electrical auxiliaries system for a coal-fired or oil-fired station would differ only in respect of the loads associated with the fuel handling and combustion plant. For a coal-fired station, this plant consists of the unitised draught plant (induced draught, forced draught and primary air fans), coal mills and feeders and the precipitators together with the common services associated with coal handling, dust handling and ash disposal systems. For a 2000 MW coal-fired station of 4 x 500 MW units, operating at CMR, the auxiliaries load is typically 31 MVA per unit plus a station load of 20 MVA. The comparative figures for a similar sized oil-fired station are 20 MVA and 13 MVA respectively since the unit load will not have the PA fans, precipitators and coal mills and the station loads will not have the coal, ash and dust handling systems. The fuel oil system does not make the same load demands as the coal fuel systems. Take as an example, the electrical auxiliaries system provided for the 2000 MW (3 x 660 MW) Littlebrook D oil-fired station. The outline of the auxiliaries system is shown in Figs 1.3 and 1.4. An important consideration in the adoption of the most economic station supplies arrangement was the availability of an existing 132 kV substation on the site. One of the SIP requirements was for the output from the three gas turbines, for system reasons, to be available to the grid independent of the operation of the main units. Each gas turbine generator rating is 35 MW, which required three station transformers, since one transformer circuit (maximum rating 60 MVA) could not accommodate more than one gas turbine generator for thermal reasons nor could the auxiliaries system for prospective fault level reasons. The use of three station transformers however does lend itself to a simpler and more flexible system configuration than is possible with the more general two station transformers scheme. At the 11 kV level, the station/ station interconnections maximise the availability of the station transformers across all three units and their gas turbines. These interconnections are an example of how an auxiliaries system design can economically10

Drax power station was designed as a 6 x 660 MW unit station, with three units initially installed, followed in the early 1980s with the three remaining units. The station auxiliaries system catered for the six units from the outset by providing four station transformers. Despite the long time interval between the construction of the first and second halves of the station, there was great emphasis placed on replication wherever possible for the completion phase to ensure the operational and maintenance convenience of the station as a whole. The outline of the auxiliaries system for the six-unit station (to 11 kV level) is shown in Fig 1.6. The auxiliaries systems for the present 900 MW unit coal-fired station designs are being assessed as for past stations against their STP requirements and economics. The alternative systems considered include using generator voltage switchgear, which the CEGB first used at Hartlepool and Heysham AGR stations, but which to date has not been used at fossil-fired stations. All generator voltage switchgear used by the CEGB on their modern large units has been the 3 x singlephase airblast type, designed and manufactured by Brown Boveri. A description of the design, construction and performance of the types used by the CEGB is given in Chapter 5. It has not been a requirement at fossil-fired stations to make grid supplies available via the generator transformer, and generator voltage switchgear has not been economically justifiable compared with a unit/station transformer scheme.

System descriptions

4c30kV

415V


C4RCUIT BREAKER NORMALLY OPEN

FLO.

1.3 Littlebrook D electrical system

The introduction of FGD plant follows the CEGB policy decision to reduce the overall sulphur emission from its power stations. To achieve this, it is proposed in the first instance to retrofit FGD equipment to existing coal-fired stations starting with Drax. In addition, the CEGB will be providing FGD equipment on all their new coal-fired stations. For a 2000 MW station burning 2% sulphur content coal, the load consumption of the FGD plant using the limestone/ gypsum process is of the order of 53 MVA. When compared with a nominal station load of 51 MVA, this represents 104% additional auxiliary power required, which constitutes a significant increase in capital and through-life costs for the station. The electrical auxiliaries system currently proposed for a 2 x 900 MW subcritical coal-fired station design, which includes the FGD plant, is shown in Figs 1.7 and 1.8. It will

be seen that the FGD plant electrical supplies have been derived from the unit/station electrical systems. All voltage levels of 11 kV, 3.3 kV and 415 V are required to accommodate the loads, including large booster fans fed at II kV. Cabling system design is made more complex with this arrangement since the unit/station system is determined by the layout of the generator, station and unit transformers and major switchboards. These are located at the opposite end of the station to the FGD auxiliaries and plant. Alternatively, the FGD plant can be considered as a separate entity, giving rise to the provision of a dedicated FGD electrical auxiliaries system centred on a location adjacent to the FGD plant and with its own Grid connections. The comparison between the two approaches is mainly one of economics. For new11


Chapter 1

70

,

so"

7 4:

rata, ia 0 yd .,. .0 a.

77. 71711 1 1174 ,

11 0/..0:

13

Fic. 1.4 Littlebrook D electrical auxiliaries system

projects the most economic approach utilises the unit/ station electrical system, although the separate electrical system has clear benefits for retrofit FGD schemes.3.3.2 Magnox nuclear power stations

eyed differently for each station and differently from those which would be adopted today. However the fundamentals for reactor safety remain the same. They are: To ensure a main coolant flow over the reactor internals, so cooling the reactor core and fuel. To ensure a flow of feedwater to extract the heat developed in the reactor, and hence provide steam to power the main turbine-generators. To provide reactor auxiliaries and services, e.g., pressure vessel cooling water flow. To provide controls and indications for the above.

The CEGB has eight magnox reactor nuclear power stations. These stations were commissioned over a period spanning eleven years, from Bradwell in 1962, to Wylfa in 1973. The stations were built by different consortia as 'turnkey' contracts, and hence have many differences in terms of output and design. The design measures which ensure reactor safety, which is the most onerous requirement on system design, are achi12

4..7 .111aC.0

aal )a

System descriptions

.1

0.1 :10

C 2, So, 1 4V

1

00

'

244OC

1 2= ! i!E' L !

15.ss. Wre STATION ai.i 8., :SC 1:57 7 S: -

eo.00

-

ISO ti41 Pvul

uo ara

O

4;

3.3 1.1- ApOn

1 61.1 0.0 2

0 SD ARE FITTED WITH 3 3kV MOTOR SWITCHING DEVICES

FIG. 1.4

(cont'd) Littlebrook D electrical auxiliaries system

These requirements apply to a reactor whether it is operational, or in the initial period of shutdown, when fission product decay heating occurs, during the posttrip cooling period. AdeCluate post-trip cooling must be available for all credible faults and accidents that can be sustained by the reactor, and sufficient redundancy and diversity of mechanical plant and systems ensures this. Clearly, electrical equipment, where required as the power source for the mechanical plant and systems, must also be capable of meeting the redundancy and diversity requirements. Having satisfied the demands of reactor safety, the electrical system must also enable the station to be

operated at full or part output with the best possible efficiency and operational flexibility. To achieve all of the above objectives, the electrical system is structured into two parts: (a) The main electrical system. (b) The essential electrical system. The main electrical systems of all magnox stations are based on the unit and station system principle. This has already been described in Section 3 of this chapter. In the case of the rnagnox stations however, different voltage levels (e.g., 6.6 kV) and sometimes discrete systems for a particular purpose (e.g., gas circulator13

VI [Wit". 5 an III

,


Chapter 1

400kV

30 25MW GAS TURBINEMVA,

2 5',

8IkV 500M VA 30MVA

AS LNG No

30 0 TO UNIT No 2

2 1( 10 MVA 8%

,

5 0 1.105

3 30/

SCM VA

o LL:L m

c0

0}

4150

DEAERATOR PUMP B

01:10000 0 CI . ,_...,.._./ ,..____,_____, z IL 2-z < 1 `2 1 Lz. 1E 8 .0 L,CO

Z

415V

LJ1.1.1

EIL IL

E DIL 15 AIR HEATER DRIVE

IL IL

9ta cr a

ri p(7, 0

2

AIR HEATER OIL PUMP

IL

LU

0

IL

R'LE

co

EL ;IL

3ON MILLS MAIN AIR HEATER

MAIN AIR HEATER

ON MILLS

0' 50

415V

FIG. 1.5 Typical electrical system for 4 x 500 MW coal-fired units

supplies) are used. Although the details of the design are different in each case, the objectives are the same, viz safe and economic operation of the power station as a whole. The essential electrical system is an integral part of t he main electrical system, but is designed so that it14

can operate independently of it. The safety of the reactor is dependent on the essential electrical system as all items of plant necessary for post-trip cooling are connected to this system, e.g., emergency boiler feed pumps, pony motors for gas circulators, necessary auxiliaries, etc.

AIR PUMP a

System descriptions

8 88II3.3kv STATION SERVICES BOARDS COAL & ASH PLANT GENERAL SERVICES

I B 2B 0L ci; I So) I

iect

3 3kV

STANDBY SUPPLIES

TO No 3 UNIT

TO No 2 UNIT

TO No 4 UNIT

4l5V

STANDBY SUPPLIES

rtj CIRCUIT BREAKER CLOSE

CIRCUIT BREAKER NORMALLY OPEN

Flu. 1.5 (coed) Typical electrical system for 4 x 500 MW coal-fired units

The main electrical system provides the preferred supply to the essential electrical system, via unit or station auxiliary transformers. If there is a loss of grid causing loss of supplies to the essential electrical system, then on-site generation will provide supplies to the essential system only. Usually, gas turbine or

diesel generators are used for this purpose. There are certain items of plant which can tolerate a short interruption of supplies, and these will be connected to AC switchboards supplied by the on-site generation. The short break in supplies due to the starting period of the on-site generation is acceptable15


Chapter 1

GENERATORI '32 NIT

IGSNERATOR 2

3E1,EPACL.P

TRAT.S,': 2 5 % 1 E.-

UN'T BOARD 5

UNAT

BOARD

4

O BOARD

UNIT

3

UNFI BOARD 2

UNIT

BOARD

,

STATION TRANSEORMEP 'A

STATION TRANSFORMER 2A 4OF,e l.r

25

0

CW PUMP EmERGENCY BOILERFEET FLAPS

CV/ PUMP 8

0

38 0 29 EMERGENCY BOWES FEED PUMP

500TOLOWER COMPRESSORSAA

0

13

CIRC,u7 BREAKEq CLOSED

0SOOT8LOWER COMPRESSORS FIG. 1.6 Drax power station electrical system to 11 kV level

CIRCUIT BREAKER T.ORMAL,

from reactor safety considerations. Plant items which cannot tolerate any break in supplies (e.g., instrumentation, controls and indications) are connected to supplies derived from a DC system, i.e., batterybacked. Normally the DC supplies are provided from the essential system AC switchboards via rectifier units, the battery being maintained in a fully charged state. Following a loss of supplies to the essential system, the battery maintains supplies to the plant items connected to the no-break system during the short period of time while the on-site generation is starting up. When the on-site generation is fully available, the DC supplies are again provided from this source, and the battery is recharged to a fully charged state. The main and essential electrical systems thus provide supplies for both reactor and plant safety, and economic operation of the station when supplying power to the National Grid.3.3.3 AGR nuclear power stations

The choice and ratings for the auxiliaries systems for the AGR stations are similar to the fossil-fired stations 16

and provide supplies in the same way on a unit and station basis. Although the electrical auxiliaries system is a single integrated design, it has two major constituent parts namely the main electrical system and the essential electrical system. The most recent of the CEGB AGR stations is at Heysham 2 and its design has followed CEGB design philosophies evolved over the period since the first AGRs were designed. Heysham 2 as described represents the latest design aspects of the AGRs. The main electrical system function is primarily to operate the station in producing its output, while the essential system is to ensure that the CEGB meets the required safety criteria in supplying safety related reactor auxiliaries plant both following a reactor trip and in the general longer term. Heysham 2 (2 x 660 MW) is the largest auxiliaries power system for a two-unit station installed by the CEGB. Considering first the main electrical system, the auxiliaries power system for the two units, numbered 7 and 8, are illustrated in Figs 1.9 and 1.10. Unlike Heysham I, the grid connections have been made to both the 400 kV and 132 kV grid substations.

System descriptions

400kV 409 275 132AV

GE"LERATGR TRANS ,DPMEk 1" '1 9,1VA

STAT'ON TRANSFORMERS 20 60NIvA

GENERATOR TRANSFORmER

GENERATOR 909MW

60

MvA UNIT TRANSFORMERS

69 MVA

60MVA

2

25 UNIT TRANSFORMERS

UNIT BOARD 1A

STATION 0 BOARD IA UNIT BOARD'S

STATION BOARD'S

ir

$

116V D

OSTATION BOARD 2A O D

CI STATION BOARD 2B

0 UNIT BOARD 24

6

UNIT 804100 25

CIRCUIT BREAKER CLOSED CIRCUIT BREAKER NORMALLY OPENFIG. 1.7 Electrical system for 2 x 900 MW coal-fired units

This enabled the setting up of an electrical system which matched the reactor quadrant concept and provided higher integrity grid connections. Each reactor/ generator has four 11 kV switchboards; A, B, C and D. Station boards A and B deriving supplies from the 132 kV system and unit boards C and D from the 400 kV system. In this way, there is an 11 kV switchboard associated with each reactor quadrant and it is at this level that the four-trained electrical system starts and is continued to the lower voltages. The auxiliaries associated with each quadrant all derive supplies down through the various voltage levels from the same 11 kV source. The lower voltage levels are 3.3 kV and 415 V. The drives connected at each voltage level are: 415 V up to and including 150 kW. 3.3 kV up to and including 1500 kW. 11 kV above 1500 kW. The gas circulators (5220 kW), CW pumps (1700 kW) and emergency boiler feed pumps (10 500 kW) are therefore supplied at 11 kV. At the 3.3 kV level, two systems are established, the X system for cooling the reactor by forced circu-

lation and the Y system to feed water into the main boilers, after a trip. The 415 V system continues with X and Y systems. In addition, at 3.3 kV there are auxiliaries associated with the turbine-generator for which the electrical needs are similar to most 660 MW units, and a unit auxiliary and station auxiliary system has been created by deriving supplies from the appropriate 11 kV level (D and B). This follows through to the 415 V level and in addition provides supplies for the reactor services auxiliaries. Each generator output at 23.5 kV passes to the grid via a generator switch and a generator transformer to 400 kV. The generator also feeds its auxiliaries via t wo unit transformers (23.5/11 kV). Each unit transformer normally supplies one II kV board but it is rated (60 MVA) to be capable of supplying the normal loads of two 11 kV boards, via the 11 kV interconnectors. It can be seen that each unit board is interconnected to a station board, i.e., D to A and C to B. This makes possible either a 400 kV or 132 kV derived source. Likewise the two station transformer secondaries are each rated to supply the normal load of a unit/station board. The two station transformers are also interconnected at 11 kV. Again each secondary winding is rated17


Chapter 1

400kV

ild5MVA GENERATOR TRANSFORMER

1 , kV UNIT BOARD IA

GENERATOR I 900MW 60M VA 60M VA

Ikv UNIT BOARD 10

El 0SCE

El41)

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812 5MVA 12 5MVA

ELECTRIC FEED PUMPS

8

12 5MvA

12 5MVA

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OT O A RD

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3 3kV UNIT AuXIL1ARY BOARD B

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O 8 0 8 8 8

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a 5V FGD SERVICES BOARD B

415V PRECIPITATOR BOARD A

i

tt,

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415v FAN 4156 FAN SERVICES BOARD A SERVICES BOARD

415v MILL SERVICES BOARD 8

415V PRECIPITATOR BOARD B

0415V TURBINE BOARD A 415 BOILER BOARD A 475V BOILER BOARD B 415V TURBINE 0 BOARD a

FIG. 1.8 Electrical auxiliaries system for 2 x 900 MW units including FOD plant supplies

to supply the normal loading of two I I kV station boards. Two conditions need to be considered here; A station transformer can only act as standby to the other station transformer if its own reactor is shutdown. A station transformer can only act as standby to one unit transformer at any one time on the basis that generating with more than one normal 11 kV source unavailable is not permitted. This leads to the three-winding station transformer having two secondary windings rated at 60 MVA, whereas the primary is rated at only 90 MVA. The essential electrical system is an integral part of the main system and is centred on the 3.3 kV level. The diesel generators are the ultimate back-up for the provision of electrical supplies. They are connected at 3.3 kV since the critical safety auxiliaries are at this level and below.18

The decision to have a total of eight diesel generators for the two reactors was taken on cost grounds. The initial proposal had been for sixteen, which allowed one to be associated with each X and Y system but could not be economically justified. The restriction to eight diesel generators caused connection design problems both for operation and for cabling as each is connected and rated to supply the post-trip needs of corresponding X or Y systems of both reactors. This fixed the X diesel generator rating at 5.2 MW and the Y diesel generator rating at 6.735 MW. Y is the larger rating because the emergency feed pumps are much larger than any X system drive. The X system diesel generators, as well as supplying 3.3 kV and below, supply the main gas circulators via converters with variable frequency output, 1 Hz to 50 Hz, up to a voltage of 3000 V. The diesel generator supplies are regarded as short break supplies, i.e., loading of them cannot take place for approximately 26 s. There is however, a need for

System descriptions

32 , 275 , 400kV

120160.60FAVA

415v GENERAL SERVICES BOARDBOARDS

I SCE

STATIC CONVERTER EQUIPMENT


CIRCUIT BREAKER NORMALLY OPEN

FIG. 1.8 (coned) Electrical auxiliaries system for 2 x 900 MW units including FGD plant supplies

supplies to some loads which do not suffer a break, i.e., an uninterruptable power supply system (UPS). The UPS supplies at Heysham 2 are derived from battery-backed static inverters. The X system has a large UPS load requirement, including 3-phase drives. To maintain the 'trained' design concept, each X and each Y system has an appropriately rated UPS system; at 100 kVA, 3-phase 415 V output for each X system and 6.3 kVA single-phase 110 V output for each Y system., Additionally, each unit has unit and station UPS systems of 200 kVA, single-phase 415 V output for other than essential loads, e.g., unit guaranteed instruments and unit computer. As at all other stations, DC systems are provided both for normal usage and also for those situations when DC is absolutely essential, such as switchgear operation. For this reason all X and Y systems have discrete closing (220 V) and opening (110 V) batteries. In fact, the closing batteries are solely dedicated to that duty.

The unit and station DC system design needs to have in addition, a 250 V DC system for emergency lighting and turbine-generator emergency drives.3.3.4 PWR nuclear power stations The CEGB has embarked on a series of nuclear power

stations of the PWR type and have based the station design on the American SNUPPS system. The lead station is at Sizewell B in Suffolk, where there is an existing magnox station. The electrical auxiliaries system, however, accommodates a UK design evolved around a twin-generator/single-reactor system, whereas the SNUPPS design has a single generator. The electrical system chosen provided a grid connection at 400 kV for each generator and a similar 400 kV grid connection for each of the two station transformers. Although the grid connections are to a common substation, each connection can be electrically segregated from the other by means of isolators and circuit19

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Modern Power Station Practice

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