Transmission and Distribution Electrical Engineering

Transmission and Distribution Electrical Engineering

This Page Intentionally Left Blank

Transmission and Distribution Electrical Engineering Second edition Dr C. R. Bayliss CEng FIEE

Newnes An imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 200 Wheeler Road, Burlington, MA 01803 First published 1996 Second edition 1999 Reprinted 2001, 2002, 2003 Copyright © 1996, 1999, C. R. Bayliss. All rights reserved The right of C.R. Bayliss to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone: (;44) 1865 843830, fax: (;44) 1865 853333, e-mail: permissions/elsevier.co.uk. You may also complete your request on-line via the Elsevier homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’ British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 7506 4059 6

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Contents

About the author List of Contributors Preface 1 System Studies 1.1 Introduction 1.2 Load flow 1.2.1 Purpose 1.2.2 Sample study 1.3 System stability 1.3.1 Introduction 1.3.2 Analytical aspects 1.3.3 Steady state stability 1.3.4 Transient stability 1.3.5 Dynamic stability 1.3.6 Effect of induction motors 1.3.7 Data requirements and interpretation of transient stability studies 1.3.8 Case studies 1.4 Short circuit analysis 1.4.1 Purpose 1.4.2 Sample study 2 Drawings and Diagrams 2.1 Introduction 2.2 Block diagrams 2.3 Schematic diagrams 2.3.1 Method of representation 2.3.2 Main circuits

xx xxi xxiii 1 1 1 1 2 8 8 10 14 17 28 28 29 34 42 42 42 50 50 50 51 51 55 v

vi Contents

2.3.3 Control, signalling and monitoring circuits 2.4 Manufacturers’ drawings 2.4.1 Combined wiring/cabling diagrams 2.4.2 British practice 2.4.3 European practice 2.4.4 Other systems 2.5 Computer aided design (CAD) 2.6 Case study 2.7 Graphical symbols Appendix A—Relay identification—numerical codes Appendix B—Comparison between German, British, US/Canadian and international symbols B1 General circuit elements B2 Operating mechanisms B3 Switchgear 3 Substation Layouts 3.1 Introduction 3.2 Substation design considerations 3.2.1 Security of supply 3.2.2 Extendibility 3.2.3 Maintainability 3.2.4 Operational flexibility 3.2.5 Protection arrangements 3.2.6 Short circuit limitations 3.2.7 Land area 3.2.8 Cost 3.3 Alternative layouts 3.3.1 Single busbar 3.3.2 Transformer feeder 3.3.3 Mesh 3.3.4 Ring 3.3.5 Double busbar 3.3.6 1 Circuit breaker  3.4 Space requirements 3.4.1 Introduction 3.4.2 Safety clearances 3.4.3 Phase-phase and phase-earth clearances 4 Substation Auxiliary Power Supplies 4.1 Introduction 4.2 DC supplies 4.2.1 Battery/battery charger configurations 4.2.2 Battery charger components

55 55 55 61 63 68 68 69 70 72 82 83 86 89 92 92 92 92 93 93 94 94 94 94 95 95 95 97 101 103 104 106 106 106 108 109 115 115 115 115 118

Contents vii

4.2.3 Installation requirements 4.2.4 Typical enquiry data—DC switchboard 4.3 Batteries 4.3.1 Introduction 4.3.2 Battery capacity 4.3.3 Characteristics of batteries 4.3.4 Battery sizing calculations 4.3.5 Typical enquiry data 4.4 AC supplies 4.4.1 Power sources 4.4.2 LVAC switchboard fault level 4.4.3 Auxiliary transformer LV connections 4.4.4 Allowance for future extension 4.4.5 Typical enquiry data 4.4.6 Earthing transformer selection 4.4.7 Uninterruptible power supplies 5 Current and Voltage Transformers 5.1 Introduction 5.2 Current transformers 5.2.1 Introduction 5.2.2 Protection CT classifications 5.2.3 Metering CTs 5.2.4 Design and construction considerations 5.2.5 Terminal markings 5.2.6 Specifications 5.3 Voltage transformers 5.3.1 Introduction 5.3.2 Electromagnetic VTs 5.3.3 Capacitor VTs 5.3.4 Specifications 6 Insulators 6.1 Introduction 6.2 Insulator materials 6.2.1 Polymeric and resin materials 6.2.2 Glass and porcelain 6.3 Insulator types 6.3.1 Post insulators 6.3.2 Cap and pin insulators 6.3.3 Long rod 6.4 Pollution control 6.4.1 Environment/creepage distances 6.4.2 Remedial measures

121 124 125 125 125 128 129 132 134 134 134 134 136 138 139 143 147 147 147 147 147 151 152 154 155 155 155 155 156 157 160 160 160 160 161 161 161 165 166 167 167 169

viii Contents

6.4.3 Calculation of specific creepage path 6.5 Insulator specification 6.5.1 Standards 6.5.2 Design characteristics 6.6 Tests 6.6.1 Sample and routine tests 6.6.2 Technical particulars 7 Substation Building Services 7.1 Introduction 7.2 Lighting 7.2.1 Terminology 7.2.2 Internal lighting 7.2.3 External lighting 7.2.4 Control 7.3 Distribution characterization 7.4 Heating, ventilation and air conditioning 7.4.1 Air circulation 7.4.2 Air conditioning 7.4.3 Heating 7.5 Fire detection and suppression 7.5.1 Introduction 7.5.2 Fire extinguishers 7.5.3 Access, first aid and safety 7.5.4 Fire detection 7.5.5 Fire suppression 7.5.6 Cables, control panels and power supplies 8 Earthing and Bonding 8.1 Introduction 8.2 Design criteria 8.2.1 Time/current zones of effects of AC currents on persons 8.2.2 Touch and step voltages 8.2.3 Comparison of touch and step potential design criteria 8.3 Substation earthing calculation methodology 8.3.1 Boundary conditions 8.3.2 Earthing materials 8.3.3 Earthing impedance and earthing voltage 8.3.4 Hazard voltage tolerable limits 8.4 Computer generated results 8.4.1 Introduction 8.4.2 Case study References

170 171 171 175 176 176 177 179 179 179 179 185 187 195 196 199 199 201 206 206 206 207 207 208 211 212 214 214 215 215 215 217 220 220 222 225 226 228 228 231 232

Contents ix

9 Insulation Co-ordination 9.1 Introduction 9.2 System voltages 9.2.1 Power frequency voltages 9.2.2 Overvoltages 9.3 Clearances 9.3.1 Air 9.3.2 SF  9.4 Procedures for co-ordination 9.4.1 Statistical approach 9.4.2 Non-statistical approach 9.5 Surge protection 9.5.1 Rod or spark gaps 9.5.2 Surge arresters References 10 Relay Protection 10.1 Introduction 10.2 System configurations 10.2.1 Faults 10.2.2 Unearthed systems 10.2.3 Impedance earthed systems 10.2.4 Solidly earthed systems 10.2.5 Network arrangements 10.3 Power system protection principles 10.3.1 Discrimination by time 10.3.2 Discrimination by current magnitude 10.3.3 Discrimination by time and fault direction 10.3.4 Unit protection 10.3.5 Signalling channel assistance 10.4 Current relays 10.4.1 Introduction 10.4.2 Inverse definite minimum time lag (IDMTL) relays 10.4.3 Alternative characteristic curves 10.4.4 Plotting relay curves on log/log graph paper 10.4.5 Current relay application examples 10.5 Differential protection schemes 10.5.1 Biased differential protection 10.5.2 High impedance protection 10.5.3 Transformer protection application examples 10.5.4 Pilot wire unit protection 10.5.5 Busbar protection 10.6 Distance relays 10.6.1 Introduction

234 234 234 234 235 245 245 246 247 247 248 248 248 250 265 266 266 267 267 267 267 268 268 271 271 272 272 272 273 274 274 274 277 277 278 289 289 292 293 297 300 303 303

x Contents

10.6.2 Basic principles 10.6.3 Relay characteristics 10.6.4 Zones of protection 10.6.5 Switched relays 10.6.6 Typical overhead transmission line protection schemes 10.7 Auxiliary relays 10.7.1 Tripping and auxiliary 10.7.2 AC auxiliary relays 10.7.3 Timers 10.7.4 Undervoltage 10.7.5 Underfrequency 10.8 Computer assisted grading exercise 10.8.1 Basic input data 10.8.2 Network fault levels 10.8.3 CT ratios and protection devices 10.8.4 Relay settings 10.9 Practical distribution network case study 10.9.1 Introduction 10.9.2 Main substation protection 10.9.3 Traction system protection 10.9.4 21 kV distribution system and protection philosophy 10.9.5 21 kV pilot wire unit protection 10.9.6 21 kV system backup protection 10.9.7 Use of earth fault indicators 10.9.8 Summary

304 305 309 311 312 316 316 321 321 321 322 325 325 325 326 326 326 326 328 328 331 332 333 335 335

11 Fuses and Miniature Circuit Breakers

336

11.1 Introduction 11.2 Fuses 11.2.1 Types and standards 11.2.2 Definitions and terminology 11.2.3 HRC fuses 11.2.4 High voltage fuses 11.2.5 Cartridge fuse construction 11.3 Fuse operation 11.3.1 High speed operation 11.3.2 Discrimination 11.3.3 Cable protection 11.3.4 Motor protection 11.3.5 Semiconductor protection 11.4 Miniature circuit breakers 11.4.1 Operation 11.4.2 Standards 11.4.3 Application

336 336 336 339 339 344 349 350 350 351 354 355 357 359 359 360 361

Contents xi

References 12 Cables 12.1 Introduction 12.2 Codes and standards 12.3 Types of cables and materials 12.3.1 General design criteria 12.3.2 Cable construction 12.3.3 Submarine cables 12.3.4 Terminations 12.4 Cable sizing 12.4.1 Introduction 12.4.2 Cables laid in air 12.4.3 Cables laid direct in ground 12.4.4 Cables laid in ducts 12.4.5 Earthing and bonding 12.4.6 Short circuit ratings 12.4.7 Calculation examples 12.5 Calculation of losses in cables 12.5.1 Dielectric losses 12.5.2 Screen or sheath losses 12.6 Fire properties of cables 12.6.1 Toxic and corrosive gases 12.6.2 Smoke emission 12.6.3 Oxygen index and temperature index 12.6.4 Flame retardance/flammability 12.6.5 Fire resistance 12.6.6 Mechanical properties 12.7 Control and communication cables 12.7.1 Low voltage and multicore control cables 12.7.2 Telephone cables 12.7.3 Fibre optic cables 12.8 Cable management systems 12.8.1 Standard cable laying arrangements 12.8.2 Computer aided cable installation systems 12.8.3 Interface definition References 13 Switchgear 13.1 Introduction 13.2 Terminology and standards 13.3 Switching 13.3.1 Basic principles 13.3.2 Special switching cases

367 368 368 368 371 371 371 382 382 383 383 383 385 386 387 390 392 403 403 403 404 404 405 405 406 406 407 407 407 408 410 416 416 419 425 428 429 429 429 431 431 443

xii Contents

13.3.3 Switches and disconnectors 13.3.4 Contactors 13.4 Arc quenching media 13.4.1 Introduction 13.4.2 Sulphur hexafluoride (SF )  13.4.3 Vacuum 13.4.4 Oil 13.4.5 Air 13.5 Operating mechanisms 13.5.1 Closing and opening 13.5.2 Interlocking 13.5.3 Integral earthing 13.6 Equipment specifications 13.6.1 12 kV metal-clad indoor switchboard example 13.6.2 Open terminal 145 kV switchgear examples 13.6.3 Distribution system switchgear example 13.6.4 Distribution ring main unit 14 Power Transformers 14.1 Introduction 14.2 Standards and principles 14.2.1 Basic transformer action 14.2.2 Transformer equivalent circuit 14.2.3 Voltage and current distribution 14.2.4 Transformer impedance representation 14.2.5 Tap changers 14.2.6 Useful standards 14.3 Voltage, impedance and power rating 14.3.1 General 14.3.2 Voltage drop 14.3.3 Impedance 14.3.4 Voltage ratio and tappings — general 14.3.5 Voltage ratio with off-circuit tappings 14.3.6 Voltage ratio and on-load tappings 14.3.7 Basic insulation levels (BIL) 14.3.8 Vector groups and neutral earthing 14.3.9 Calculation example to determine impedance and tap range 14.4 Thermal design 14.4.1 General 14.4.2 Temperature rise 14.4.3 Loss of life expectancy with temperature 14.4.4 Ambient temperature 14.4.5 Solar heating

446 447 453 453 454 460 461 463 465 465 469 471 471 471 475 481 485 490 490 490 490 492 494 494 497 507 508 508 509 509 510 510 511 511 511 514 522 522 523 524 525 526

Contents xiii

14.4.6 Transformer cooling classifications 14.4.7 Selection of cooling classification 14.4.8 Change of cooling classification in the field 14.4.9 Capitalization of losses 14.5 Constructional aspects 14.5.1 Cores 14.5.2 Windings 14.5.3 Tanks and enclosures 14.5.4 Cooling plant 14.5.5 Low fire risk types 14.5.6 Neutral earthing transformers 14.5.7 Reactors 14.6 Accessories 14.6.1 General 14.6.2 Buchholz relay 14.6.3 Sudden pressure relay and gas analyser relay 14.6.4 Pressure relief devices 14.6.5 Temperature monitoring 14.6.6 Breathers 14.6.7 Miscellaneous 14.6.8 Transformer ordering details References 15 Substation and Overhead Line Foundations 15.1 Introduction 15.2 Soil investigations 15.3 Foundation types 15.4 Foundation design 15.5 Site works 15.5.1 Setting out 15.5.2 Excavation 15.5.3 Piling 15.5.4 Earthworks 15.5.5 Concrete 15.5.6 Steelwork fixings 16 Overhead Line Routing 16.1 Introduction 16.2 Routing objectives 16.3 Preliminary routing 16.3.1 Survey equipment requirements 16.3.2 Aerial survey 16.3.3 Ground survey 16.3.4 Ground soil conditions

526 529 530 531 532 532 533 535 537 538 540 541 543 543 543 544 544 544 545 545 547 553 555 555 555 556 565 565 565 565 566 567 568 573 575 575 575 577 577 577 577 577

xiv Contents

16.3.5 Wayleave, access and terrain 16.3.6 Optimization 16.4 Detailed line survey and profile 16.4.1 Accuracy requirements 16.4.2 Profile requirements 16.4.3 Computer aided techniques 17 Structures, Towers and Poles 17.1 Introduction 17.2 Environmental conditions 17.2.1 Typical parameters 17.2.2 Effect on tower or support design 17.2.3 Conductor loads 17.2.4 Substation gantry combined loading example 17.3 Structure design 17.3.1 Lattice steel tower design considerations 17.3.2 Tower testing 17.4 Pole and tower types 17.4.1 Pole structures 17.4.2 Tower structures References 18 Overhead Line Conductor and Technical Specifications 18.1 Introduction 18.2 Environmental conditions 18.3 Conductor selection 18.3.1 General 18.3.2 Types of conductor 18.3.3 Aerial bundled conductor 18.3.4 Conductor breaking strengths 18.3.5 Bi-metal connectors 18.3.6 Corrosion 18.4 Calculated electrical ratings 18.4.1 Heat balance equation 18.4.2 Power carrying capacity 18.4.3 Corona discharge 18.4.4 Overhead line calculation example 18.5 Design spans, clearances and loadings 18.5.1 Design spans 18.5.2 Conductor and earth wire spacing and clearances 18.5.3 Broken wire conditions 18.5.4 Conductor tests/inspections 18.6 Overhead line fittings 18.6.1 Fittings related to aerodynamic phenomena

577 579 581 581 582 584 586 586 587 587 588 592 598 599 599 611 611 611 613 618 619 619 619 620 620 621 624 625 626 626 628 628 629 632 636 639 639 650 661 661 661 661

Contents xv

18.6.2 Suspension clamps 18.6.3 Sag adjusters 18.6.4 Miscellaneous fittings 18.7 Overhead line impedance 18.7.1 Inductive reactance 18.7.2 Capacitive reactance 18.7.3 Resistance 18.8 Substation busbar selection—case study 18.8.1 Introduction 18.8.2 Conductor diameter/current carrying capacity 18.8.3 Conductor selection of weight basis 18.8.4 Conductor short circuit current capability 18.8.5 Conductor support arrangements References 19 Testing and Commissioning 19.1 Introduction 19.2 Quality assurance 19.2.1 Introduction 19.2.2 Inspection release notice 19.2.3 Partial acceptance testing 19.2.4 System acceptance testing 19.2.5 Documentation and record systems 19.3 Works inspections and testing 19.3.1 Objectives 19.3.2 Specifications and responsibilities 19.3.3 Type tests 19.3.4 Routine tests 19.4 Site inspection and testing 19.4.1 Pre-commissioning and testing 19.4.2 Maintenance inspection 19.4.3 On-line inspection and testing 19.5 Testing and commissioning methods 19.5.1 Switchgear 19.5.2 Transformers 19.5.3 Cables 19.5.4 Protection Appendix A Commissioning test procedure requirements Appendix B Drawings, diagrams and manuals 20 Electromagnetic Compatibility 20.1 Introduction 20.2 Standards 20.3 Testing

665 665 665 665 665 667 668 668 668 668 669 672 673 677 680 680 680 680 682 682 682 683 685 685 685 685 686 686 686 687 687 691 691 701 704 707 723 724 726 726 726 727

xvi Contents

20.3.1 Magnetic field radiated emission measurements 20.3.2 Electric field radiated emission measurements 20.3.3 Conducted emission measurements 20.3.4 Immunity testing 20.4 Screening 20.4.1 The use of screen wire 20.4.2 The use of screen boxes and Faraday enclosures 20.4.3 The use of screen floors in rooms 20.5 Typical useful formulae 20.5.1 Decibel reference levels 20.5.2 Field strength calculations 20.5.3 Mutual inductance between two long parallel pairs of wires 20.5.4 Attenuation factors 20.6 Case studies 20.6.1 Screening power cables 20.6.2 Measurement of field strengths References 21 System Control and Data Acquisition 21.1 Introduction 21.2 Programmable logic controllers (PLCs) 21.2.1 Functions 21.2.2 PLC selection 21.2.3 Application example 21.3 Power line carrier communication links 21.3.1 Introduction 21.3.2 Power line carrier communication principles 21.4 Supervisory control and data acquisition (SCADA) 21.4.1 Introduction 21.4.2 Typical characteristics 21.4.3 Design issues 21.4.4 Example (Channel Tunnel) 21.5 Software management 21.5.1 Software—a special case 21.5.2 Software life cycle 21.5.3 Software implementation practice 21.5.4 Software project management References 22 Project Management 22.1 Introduction 22.2 Project evaluation 22.2.1 Introduction

728 730 732 732 734 734 734 738 739 740 740 741 741 742 742 745 747 748 748 748 748 750 753 758 758 761 766 766 767 769 770 772 773 774 778 780 783 784 784 784 784

Contents xvii

22.2.2 Financial assessment 22.2.3 Economic assessment 22.3 Financing 22.3.1 Responsibilities for funding 22.3.2 Cash flow 22.3.3 Sources of finance 22.3.4 Export credit agencies 22.3.5 Funding risk reduction 22.4 Project phases 22.4.1 The project life cycle 22.4.2 Cash flow 22.4.3 Bonds 22.4.4 Advance payments and retentions 22.4.5 Insurances 22.4.6 Project closeout 22.5 Terms and conditions of contract 22.5.1 Time, cost and quality 22.5.2 Basic types of contract 22.5.3 Standard terms and conditions of contract 22.5.4 Key clauses 22.6 Tendering 22.6.1 Choosing the contractor 22.6.2 Estimating 22.6.3 Tender evaluation 22.7 Model forms of contract—exercise Appendix A Project definition/questionnaire Appendix B Bidding checklist 23 Distribution Planning 23.1 Introduction 23.2 Definitions 23.2.1 Demand or average demand 23.2.2 Maximum demand (MD) 23.2.3 Demand factor 23.2.4 Utilization factor (UF) 23.2.5 Load factor (LDF) 23.2.6 Diversity factor (DF) 23.2.7 Coincident factor (CF) 23.2.8 Load diversity 23.2.9 Loss factor (LSF) 23.2.10 Load duration 23.2.11 Loss equivalent hours 23.2.12 Peak responsibility factor (PRF) 23.3 Load forecasting

785 792 796 796 796 797 798 798 800 800 803 804 806 806 806 807 807 808 810 812 815 815 816 817 819 821 845 849 849 851 851 851 852 852 853 853 855 855 856 860 861 862 863

xviii Contents

23.3.1 Users of load forecasts 23.3.2 The preparation of load forecasts 23.3.2 The micro load forecast 23.3.4 The macro load forecast 23.3.5 Nature of the load forecast 23.4 System parameters 23.4.1 Distribution feeder arrangements 23.4.2 Voltage drop calculations 23.4.3 Positive sequence resistance 23.4.4 Inductive reactance 23.4.5 Economic loading of distribution feeders and transformers 23.4.6 System losses 23.5 System reliability 23.5.1 Introduction 23.5.2 Reliability functions 23.5.3 Predictability analysis 23.6 Drawings and materials take off

875 876 877 877 880 882 886

24 Harmonics in Power Systems

888

24.1 Introduction 24.2 The nature of harmonics 24.2.1 Introduction 24.2.2 Three phase harmonics 24.3 The generation of harmonics 24.3.1 Transformers 24.3.2 Converters 24.3.3 The thyristor bridge 24.3.4 AC railway traction systems 24.3.5 Static VAr compensators and balancers 24.4 The effects of harmonics 24.4.1 Heating effects of harmonics 24.4.2 Overvoltages 24.4.3 Resonances 24.4.4 Interference 24.5 The limitation of harmonics 24.5.1 Harmonic filters 24.5.2 Capacitor detuning 24.6 Ferroresonance and subharmonics 24.6.1 Introduction 24.6.2 A physical description of ferroresonance 24.6.3 Subharmonics 24.7 References

863 864 865 867 868 870 870 871 873 874

888 888 888 889 889 890 890 892 894 894 894 894 896 897 898 900 900 902 903 903 905 906 906

Contents xix

25 Fundamentals 25.1 Introduction 25.2 Symbols and nomenclature 25.2.1 Symbols 25.2.2 Units and conversion tables 25.3 Alternating quantities 25.4 Vector representation 25.5 Vector algebra 25.5.1 The j operator 25.5.2 Exponential vector format 25.5.3 Polar co-ordinate vector format 25.5.4 Algebraic operations on vectors 25.5.5 The h operator 25.6 Sequence components 25.6.1 Theoretical background 25.6.2 Calculation methodology and approximations 25.6.3 Interpretation 25.7 Network fault analysis 25.7.1 Introduction 25.7.2 Fundamental formulae 25.7.3 Simplified network reduction example 25.8 Design optimization 25.8.1 Introduction 25.8.2 Technical problems 25.8.3 Loss reduction 25.8.4 Communication link gain or attenuation 25.8.5 Reactive compensation 25.8.6 Power factor correction calculation procedures References Index

908 908 908 909 914 917 922 922 924 925 925 925 925 926 926 927 928 932 932 933 938 944 944 945 949 958 958 962 966 969

About the author

Colin Bayliss gained a first class honurs degree in Electrical and Electronic Engineering at Nottingham University and went on to receive a PhD in Materials Science. He has worked on major power projects both at home in the UK and throughout the world with client, contractor and consultancy organizations. He was appointed Engineering Director by the Channel Tunnel main contractors (Transmache Link — TML) during the last two years of that project’s construction, having been involved previously in the earlier design stages. He is currently Planning, Performance and Engineering Director of the United Kingdom Atomic Energy Authority (UKAEA).

xx

Contributors

The preparation of a book covering such a wide range of topics would not have been possible without contributions and advice from major manufacturers, electrical supply utilities, contractors, academics and consulting engineers. Indeed, encouragement for the preparation of this book has come from the Institution of Electrical Engineers (IEE) Transmission and Distribution Professional Group, P7, under the chairmanship of David Rigden (Hawker Siddeley Switchgear) and John Lewis (Scottish Power). The names of the major contributors are listed below. D. Auckland A. Baker R. H. Barnes P. Bennett N. Bird K. Blackmore L. Blake S. A. Bleazard D. Boulu D. Brady D. Brown J. Finn H. Grant G. Harris M. R. Hill P. Hindle I. Johnston

Professor of Electrical Engineering, University of Manchester Principal Geotechnical Engineer, Balfour Beatty Projects and Engineering Associate Director and Principal Systems Analyst, Engineering and Power Development Consultants (EPDC) Director, Centre for Software Engineering Director, Balfour Beatty Cruickshank Ltd Senior Engineer, Interference Technology International Yorkshire Electricity Group Reyrolle Limited, Tyne and Wear Principal Engineer, Tractabel, Belgium General Manager, Optimal Software Ltd Director, BICC, Wrexham Principal Engineer, Reyrolle Projects (formerly Power Systems Project Manager, TML) Deputy Chief Design Engineer, Parsons Peebles Transformers, Edinburgh Livingstone Hire Marketing Director, Bowthorpe EMP Ltd Principal Engineer, GEC-Alsthom T&D Protection & Control Senior Software Engineer, Centre for Software Engineering xxi

xxii Contributors

C. Lau F. J. Liptrot G. Little I. E. Massey T. Mennell E. Meyer A. Monro R. Monk

Senior Data Transmission and Control Engineer, TML Technical Director, Allied Insulators Balfour Kilpatrick, Hackbridge, London Senior Civil Engineer, Balfour Beatty Projects and Engineering Head of Engineering, EMMCO, Merlin Gerin Control Engineer, Technip, Paris Design Engineer, Peebles Power Transformers, Edinburgh Senior Applications Engineer, GEC-Alsthom T&D Protection & Control D. Moore Principal Engineer, National Grid Company (formerly Ewbank Preece Consulting Engineers) P. G. Newbery Technical Director, Cooper Bussmann (formerly Hawker Fusegear) G. Orawski Consultant Engineer, Balfour Beatty Power S. D. Pugh Senior SCADA Engineer, Centre for Software Development D. Rigden Director, Hawker Siddeley Switchgear A. Smith Design Draughtsman, EPDC M. Swinscale Principal Technical Engineer, Furze, Nottingham M. Tearall Senior Building Services Engineer, Wimpey Major Projects M. Teliani Senior Systems Engineer, Engineering and Power Development Consultants Ltd (EPDC) A. Thomas Senior Communications Engineer, Ewbank Preece Consulting Engineers

Preface

This book covers the major topics likely to be encountered by the transmission and distribution power systems engineer engaged upon international project works. Each chapter is self-contained and gives a useful practical introduction to each topic covered. The book is intended for graduate or technician level engineers and bridges the gap between learned university theoretical textbooks and detailed single topic references. It therefore provides a practical grounding in a wide range of transmission and distribution subjects. The aim of the book is to assist the project engineer in correctly specifying equipment and systems for his particular application. In this way manufacturers and contractors should receive clear and unambiguous transmission and distribution equipment or project enquiries for work and enable competitive and comparative tenders to be received. Of particular interest are the chapters on project, system and software management since these subjects are of increasing importance to power systems engineers. In particular the book should help the reader to understand the reasoning behind the different specifications and methods used by different electrical supply utilities and organizations throughout the world to achieve their specific transmission and distribution power system requirements. The second edition includes updates and corrections, together with the addition of two extra major chapters covering distribution planning and power system harmonics. C. R. Bayliss

xxiii

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1

System Studies

1.1 INTRODUCTION This chapter describes the three main areas of transmission and distribution network analysis; namely load flow, system stability and short circuit analysis. Such system studies necessitate a thorough understanding of network parameters and generating plant characteristics for the correct input of system data and interpretation of results. A background to generator characteristics is therefore included in Section 1.3. The analysis work, for all but the simplest schemes, is carried out using tried and proven computer programs. The application of these computer methods and the specific principles involved are described by the examination of some small distribution schemes in sufficient detail to be applicable for use with a wide range of commercially available computer software. The more general theoretical principles involved in load flow and fault analysis data collection are explained in Chapter 25.

1.2 LOAD FLOW 1.2.1 Purpose A load flow analysis allows identification of real and reactive power flows, voltage profiles, power factor and any overloads in the network. Once the network parameters have been entered into the computer database the analysis allows the engineer to investigate the performance of the network under a variety of outage conditions. The effect of system losses and power factor correction, the need for any system reinforcement and confirmation of economic transmission can then follow.

1

2 System Studies

1.2.2 Sample study 1.2.2.1 Network single line diagram Figure 1.1 shows a simple five busbar 6 kV generation and 33 kV distribution network for study. Table 1.1 details the busbar and branch system input data associated with the network. Input parameters are given here in a per unit (pu) format on a 100 MVA base. Different programs may accept input data in different formats, for example % impedance, ohmic notation, etc. Please refer to Chapter 25, for the derivation of system impedance data in different formats from manufacturers’ literature. The network here is kept small in order to allow the first-time user to become rapidly familiar with the procedures for load flows. Larger networks involve a repetition of these procedures.

1.2.2.2 Busbar input database The busbars are first set up in the program by name and number and in some cases by zone. Bus parameters are then entered according to type. A ‘slack bus’ is a busbar where the generation values, P(real power in MW) and Q (reactive power in MVAr), are unknown. Therefore busbar AO in the example is entered as a slack bus with a base voltage of 6.0 kV, a generator terminal voltage of 6.3 kV (1.05 pu) and a phase angle of 0.0 degrees (a default value). All load values on busbar AO are taken as zero (again a default value) due to unknown load distribution and system losses. A ‘P,Q generator bus’ is one where P and Q are specified to have definite values. If, for example, P is made equal to zero we have defined the constant Q mode of operation for a synchronous generator. Parameters for busbar BO in the example may be specified with base voltage 6.0 kV, desired voltage 6.3 kV and default values for phase angle (0.0 degrees), load power (0.0 MW), load reactive power (0.0 MVAr), shunt reactance (0.0 MVAr) and shunt capacitance (0.0 pu). Alternatively, most programs accept generator busbar data by specifying real generator power and voltage. The program may ask for reactive power limits to be specified instead of voltage since in a largely reactive power network you cannot ‘fix’ both voltage and reactive power — something has to ‘give way’ under heavy load conditions. Therefore busbar BO may be specified with generator power 9.0 MW, maximum and minimum reactive power as 4.3 MVAr and transient or subtransient reactance in per unit values. These reactance values are not used in the actual load flow but are entered in anticipation of the need for subsequent fault studies. For the calculation of oil circuit breaker breaking currents or for electromechanical protection relay operating currents it is more usual to take the generator transient reactance values. This is because the subtransient reactance effects will generally disappear within the first few cycles and before the circuit breaker or

System Studies 3

Figure 1.1 Load flow sample study single line diagram

Table 1.1

Load flow sample study busbar and branch input data Bus data Voltage

Bus Name Slack AO A BO B C

Bus Bus Number Type

pu

Angle

MW

MVAR MW

Shunt L or C MVAR pu

1 2 3 4 5

1.05 1.0 1.0 1.0 1.0

0.0 0.0 0.0 0.0 0.0

0 0 9 0 0

0 0 4.3 0 0

0 0 0 0 9

1 2 3 4 5

Gen

Load

0 0 0 0 25

0 0 0 0 0

Branch data

Bus

Bus

Rpu 100MVA base

1 2 2 3 4

2 4 5 4 5

— 0.8 0.2 — 0.08

Circ

Xpu 100MVA base

Bpu 100MVA base

Tap ratio

0.5 1.73 1.15 0.8 0.46

— — — — —

1.02 0.0 0.0 1.02 0.0

4 System Studies

protection has operated. Theoretically, when calculating maximum circuit breaker making currents subtransient generator reactance values should be used. Likewise for modern, fast (say 2 cycle) circuit breakers, generator breakers and with solid state fast-relay protection where accuracy may be important, it is worth checking the effect of entering subtransient reactances into the database. In reality, the difference between transient and subtransient reactance values will be small compared to other system parameters (transformers, cables, etc.) for all but faults close up to the generator terminals. A ‘load bus’ has floating values for its voltage and phase angle. Busbar A in the example has a base voltage of 33 kV entered and an unknown actual value which will depend upon the load flow conditions.

1.2.2.3 Branch input data base Branch data is next added for the network plant (transformers, cables, overhead lines, etc.) between the already specified busbars. Therefore from busbar A to busbar B the 30 km, 33 kV overhead line data is entered with resistance 0.8 pu, reactance 1.73 pu and susceptance 0.0 pu (unknown in this example and 0.0 entered as a default value). Similarly for a transformer branch such as from busbar AO to A data is entered as resistance 0.0 pu, reactance 0.5 pu (10% on 20 MVA base rating : 50% on 100 MVA base or 0.5 pu), susceptance 0.0 pu (unknown but very small compared to inductive reactance), load limit 20 MVA, from bus AO voltage 6 kV to bus A voltage 33.66 kV (1.02 pu taking into account transformer < 5% taps). Tap ranges and short-term overloads can be entered in more detail depending upon the exact program being used.

1.2.2.4 Saving data When working at the computer it is always best regularly to save your files both during data-base compilation as well as at the end of the procedure when you are satisfied that all the data has been entered correctly. Save data onto the hard disk and make floppy disk backups for safe keeping. Figure 1.2 gives a typical computer printout for the bus and branch data files associated with this example.

1.2.2.5 Solutions Different programs use a variety of different mathematical methods to solve the load flow equations associated with the network. Some programs ask the user to specify what method they wish to use from a menu of choices (Newton—Raphson, Gauss—Seidel, Fast decoupled with adjustments, etc.). A

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Figure 1.2 Load flow sample study busbar and branch computer input data files

6 System Studies

Figure 1.3 Load flow sample study base case busbar and branch report

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Figure 1.4 Load flow sample study. Base case load flow results superimposed upon single line diagram.

full understanding of these numerical methods is beyond the scope of this book. It is worth noting, however, that these methods start with an initial approximation and then follow a series of iterations or steps in order to eliminate the unknowns and ‘home in’ on the solutions. The procedure may converge satisfactorily in which case the computer continues to iterate until the difference between successive iterates is sufficiently small. Alternatively, the procedure may not converge or may only converge extremely slowly. In these cases it is necessary to re-examine the input data or alter the iteration in some way or, if desired, stop the iteration altogether. The accuracy of the solution and the ability to control round-off errors will depend, in part, upon the way in which the numbers are handled in the computer. For accurate floating-point arithmetic, where the numbers are represented with a fixed number of significant figures, a microcomputer with separate maths coprocessor integrated circuit or a central processing unit (CPU) with in-built maths coprocessor (for example the Intel 80486DX integrated circuit) will be required. It is a most important principle in numerical work that all sources of error (round-off, mistakes, nature of

8 System Studies

formulae used, approximate physical input data) must be constantly borne in mind if the ‘junk in equals junk out’ syndrome is to be avoided. Some customers ask their engineering consultants or contractors to prove their software by a Quality Assurance Audit which assesses the performance of one software package with another for a single trial network. Figure 1.3 gives typical busbar and branch reports resulting from a load flow computation. It is normal to present such results by superimposing them in the correct positions on the single line diagram as shown in Fig. 1.4. Such a pictorial representation may be achieved directly with the more sophisticated system analysis programs. The network single line diagram is prepared using a computer graphics program (Autocad, Autosketch, GDS, etc.) and the load flow results transferred using data exchange files into data blocks on the diagram.

1.2.2.6 Further studies The network already analysed may be modified as required, changing loads, generation, adding lines or branches (reinforcement) or removing lines (simulating outages). Consider, for example, removing or switching off either of the overhead line branches running from busbars A to C or from B to C. Non-convergence of the load flow numerical analysis occurs because of a collapse of voltage at busbar C. If, however, some reactive compensation is added at busbar C — for example a 33 kV, 6 MVAr (0.06 pu) capacitor bank — not only is the normal load flow improved, but the outage of line BC can be sustained. An example of a computer generated single-line diagram describing this situation is given in Fig. 1.5. This is an example of the beauty of computer aided system analysis. Once the network is set up in the database the engineer can investigate the performance of the network under a variety of conditions. Refer to Chapter 25 ‘Fundamentals’, Section 8.5 regarding Reactive Compensation principles.

1.3 SYSTEM STABILITY 1.3.1 Introduction The problem of stability in a network concerns energy balance and the ability to generate sufficient restoring forces to counter system disturbances. Minor disturbances to the system result in a mutual interchange of power between the machines in the system acting to keep them in step with each other and to maintain a single universal frequency. A state of equilibrium is retained between the total mechanical power/energy input and the electrical power/energy output by natural adjustment of system voltage levels and the common system frequency. There are three regimes of stability.

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Figure 1.5 Load flow sample study. Computer generated results superimposed on single-line diagram-reactive compensation added.

Steady state stability describes the ability of the system to remain in synchronism during minor disturbances or slowly developing system changes such as a gradual increase in load as the 24-hour maximum demand is approached. Transient stability is concerned with system behaviour following an abrupt change in loading conditions as could occur as a result of a fault, the sudden loss of generation or an interconnecting line, or the sudden connection of additional load. The duration of the transient period is in the order of a second. System behaviour in this interval is crucial in the design of power systems. Dynamic stability is a term used to describe the behaviour of the system in the interval between transient behaviour and the steady state region. For example, dynamic stability studies could include the behaviour of turbine governors, steam/fuel flows, load shedding and the recovery of motor loads, etc. The response of induction motors to system disturbances and motor starting is also thought of as a stability problem. It does not relate specifically to the ability of the system to remain in synchronism. This description is divided into two parts: the first deals with the analytical nature of synchronous machine behaviour and the different types of stability; the second deals with the more practical aspects of data collection and interpretation of transient stability study results with case studies to illustrate

10 System Studies

the main points and issues. The complexity of such analysis demands the use of mini- or microcomputing techniques and considerable data collection.

1.3.2 Analytical aspects 1.3.2.1 Vector diagrams and load angle Figure 1.6a shows the synchronous generator most simply represented on a per phase basis by an internally generated voltage (E) and an internal reactance (X). The internal voltage arises from the induction in the stator by the rotating magnetic flux of the rotor. The magnitude of this voltage is determined by the excitation of the field winding. The reactance is the synchronous reactance of the machine for steady state representation and the transient and subtransient reactance for the representation of rapid changes in operating conditions. The generator terminals are assumed to be connected to an ‘infinite’ busbar which has the properties of constant voltage and frequency with infinite inertia such that it can absorb any output supplied by the generator. In practice, such an infinite busbar is never obtained. However, in a highly interconnected system with several generators the system voltage and frequency are relatively insensitive to changes in the operating conditions of one machine. The generator is synchronised to the infinite busbar and the bus voltage (U) is unaffected by any changes in the generator parameters (E) and (X). The vector diagrams associated with this generator arrangement supplying current (I) with a lagging power factor (cos ) are shown in Figs 1.6b to 1.6e for low electrical output, high electrical output, high excitation operation and low excitation operation respectively. The electrical power output is UI cos per phase. The angle
between the voltage vectors E and U is the load angle of the machine. The load angle has a physical significance determined by the electrical and mechanical characteristics of the generator and its prime mover. A stroboscope tuned to the supply frequency of the infinite busbar would show the machine rotor to appear stationary. A change in electrical loading conditions such as that from Figs 1.6b to 1.6c would be seen as a shift of the rotor to a new position. For a generator the load angle corresponds to a shift in relative rotor position in the direction in which the prime mover is driving the machine. The increased electrical output of the generator from Figs 1.6b to 1.6c is more correctly seen as a consequence of an increased mechanical output of the prime mover. Initially this acts to accelerate the rotor and thus to increase the load angle. A new state of equilibrium is then reached where electrical power output matches prime mover input to the generator. Figures 1.6d and 1.6e show the effect of changing the field excitation of the generator rotor at constant electrical power output and also with no change in electrical power output from the Fig. 1.6b condition — that is UI cos is unchanged. An increase in (E) in Fig. 1.6d results in a larger current (I) but a more lagging power factor. Similarly, in Fig. 1.6e the reduction in (E) results in

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Figure 1.6 Vector diagrams and load angle

a change in power factor towards the leading quadrant. The principle effect of a variation in generator internal voltage is therefore to change the power factor of the machine with the larger values of (E) resulting in lagging power factors and the smaller values for (E) tending towards leading power factors. A secondary effect, which is important in stability studies, is also the change in load angle. The increased value of (E) shown in Fig. 1.6d (high excitation operation) has a smaller load angle compared to Fig. 1.6e (low excitation

12 System Studies

operation) for the same electrical power. Figures 1.6f and 1.6g show approximately zero lag and lead power factor operation where there is no electrical power output and the load angle is zero.

1.3.2.2 The power/load angle characteristic Figure 1.6b represents the vector diagram for a low electrical power output: P : UI cos (per phase) also for the vector triangles it is true that: E sin
: IX cos

substitute for I: P:

U cos ; E sin
UE sin
: X X cos

The electrical power output is therefore directly proportional to the generator internal voltage (E) and the system voltage (U) but inversely proportional to the machine reactance (X). With (U), (E) and (X) held constant the power output is only a function of the load angle
. Figure 1.7 shows a family of curves for power output vs load angle representing this. As a prime mover power increases a load angle of 90 degrees is eventually reached. Beyond this point further increases in mechanical input power cause the electrical power output to decrease. The surplus input power acts to further accelerate the machine and it is said to become unstable. The almost inevitable consequence is that synchronism with the remainder of the system is lost. Fast-acting modern automatic regulators (AVRs) can now actually enable a machine to operate at a load angle greater than 90 degrees. If the AVR can increase (E) faster than the load angle (
): dE d
 dt dt then stability can be maintained up to a theoretical maximum of about 130 degrees. This loss of synchronism is serious because the synchronous machine may enter phases of alternatively acting as a generator and then as a motor. Power surges in and out of the machine, which could be several times the machine rating, would place huge electrical and mechanical stresses on the machine. Generator overcurrent relay protection will eventually detect out-of-synchronism conditions and isolate the generator from the system. Before this happens other parts of the network may also trip out due to the power surging and the whole system may collapse. The object of system stability studies is therefore to ensure appropriate design and operational measures are taken in order to

System Studies 13

Figure 1.7 Power/load angle relationship

retain synchronism for all likely modes of system operation, disturbances and outages.

1.3.2.3 The synchronous motor Operation of the synchronous motor may be envisaged in a similar way to the synchronous generator described in Section 1.3.2.2 above. In this case, however, the power flow is into the machine and, relative to the generator, the motor load angle is negative. An increase in load angle is in the opposite direction to shaft rotation and results in greater electrical power consumption. A leading power factor corresponds to high excitation and a lagging power factor low excitation.

1.3.2.4 Practical machines In reality practical machine characteristics depart from the behaviour of the simple representations described above. However, in most cases the effects are

14 System Studies

small and they do not invalidate the main principles. The principle differences are due to saturation, saliency and stator resistance. Saturation describes the non-linear behaviour of magnetic fluxes in iron and air paths produced by currents in the machine stator and rotor windings. Saturation effects vary with machine loading. Saliency describes the effect of the differing sizes of air gap around the circumference of the rotor. This is important with salient pole rotors and the effect varies the apparent internal reactance of the machine depending upon the relative position of rotor and stator. Saliency tends to make the machine ‘stiffer’. That is, for a given load the load angle is smaller with a salient pole machine than would be the case with a cylindrical rotor machine. Salient pole machines are in this respect inherently more stable. The effect of stator resistance is to produce some internal power dissipation in the machine itself. Obviously the electrical power output is less than the mechanical power input and the difference is greatest at high stator currents.

1.3.3 Steady state stability 1.3.3.1 Pull out power Steady state stability deals with the ability of a system to perform satisfactorily under constant load or gradual load-changing conditions. In the single machine case shown in Fig. 1.7 the maximum electrical power output from the generator occurs when the load angle is 90 degrees. The value of peak power or ‘pull out power’ is given as: EU (from Section 1.3.2.2) P : +6 X With (U) fixed by the infinite bus and (X) a fixed parameter for a given machine, the pull out power is a direct function of (E). Figure 1.7 shows a family of generator power/load angle curves for different values of (E). For a generator operating at an output power P , the ability to accommodate an  increase in loading is seen to be greater for operation at high values of (E) — increased field excitation. From Section 1.3.2 and Figs 1.6d and 1.6e, operation at high values of (E) corresponds with supplying a lagging power factor and low values of (E) with a leading power factor. A generator operating at a leading power factor is therefore generally closer to its steady state stability limit than one operating at a lagging power factor. The value of (X), used in the expression for pull out power for an ideal machine, would be the synchronous reactance. In a practical machine the saturation of the iron paths modifies the assumption of a constant value of synchronous reactance for all loading conditions. The effect of saturation is to give a higher pull out power in practice than would be expected from a

System Studies 15

Figure 1.8 Typical generator operation chart

calculation using synchronous reactance. Additionally, in practical machines saliency and stator resistance, as explained in Section 1.3.2.4, would modify the expression for pull out power. Saliency tends to increase pull out power and reduces to slightly below 90 degrees the load angle at which pull out power occurs. Stator resistance slightly reduces both the value of pull out power and the load angle at which it occurs.

1.3.3.2 Generator operating chart An example of the effect of maximum stable power output of a generator is given in the generator operating chart of Fig. 1.8. This is basically derived as an extension of the vector diagrams of Fig. 1.6 where the value of internal voltage (E) and load angle
is plotted for any loading condition of MW or MVAr. In the operating chart, the circles represent constant values of (E) and load angle is shown for an assumed operating point. The operating points for which the load angle is 90 degrees are shown as the theoretical stability limit. Operation

16 System Studies

in the area beyond the theoretical stability limit corresponds with load angles in excess of 90 degrees and is not permissible. The theoretical stability limit is one of the boundaries within which the operating point must lie. Other boundaries are formed by: 1. The maximum allowable stator current, shown on the chart as an arc of maximum MVA loading. 2. The maximum allowable field excitation current shown on the chart as an arc at the corresponding maximum internal voltage (E). 3. A vertical line of maximum power may exist and this represents the power limit of the prime mover. Whichever of the above limitations applies first describes the boundaries of the different areas of operation of the generator. In a practical situation, operation at any point along the theoretical stability limit line would be most undesirable. At a load angle of 90 degrees, the generator cannot respond to a demand for more power output without becoming unstable. A practical stability limit is usually constructed on the operating chart such that, for operation at any point on this line, an increased power output of up to a certain percentage of rated power can always be accommodated without stability being lost. The practical stability limit in Fig. 1.8 is shown for a power increase of 10% of rated power output. The dotted line beyond the theoretical stability limit with a load angle
 90 degrees shows the stabilizing effect of the AVR.

1.3.3.3 Automatic voltage regulators (AVRs) The AVR generally operates to maintain a constant generator terminal voltage for all conditions of electrical output. This is achieved in practice by varying the excitation of the machine, and thus (E), in response to any terminal voltage variations. In the simple system of one generator supplying an infinite busbar, the terminal voltage is held constant by the infinite bus. In this case changes in excitation produce changes in the reactive power MVAr loading of the machine. In more practical systems the generator terminal voltage is at least to some degree affected by the output of the machine. An increase in electrical load would reduce the terminal voltage and the corrective action of the AVR would boost the internal voltage (E). Referring to the generator operating chart of Fig. 1.8, an increase in power output from the initial point A would result in a new operating point B on the circle of constant internal voltage (E) in the absence of any manual or automatic adjustment of (E). Such an increase in power output takes the operating point nearer to the stability limit. If, at the same time as the power increase, there is a corresponding increase in (E) due to AVR action the new operating point would be at C. The operation of the AVR is therefore to hold

System Studies 17

the operating point well away from the stability limit and the AVR can be regarded as acting to preserve steady state stability.

1.3.3.4 Steady state stability in industrial plants From Section 1.3.3.3 it can be seen that the steady state stability limits for generators are approached when they supply capacitive loads. Since industrial plants normally operate at lagging power factors the problem of steady state stability is unlikely to occur. Where power factor compensation is used or where synchronous motors are involved the possibility of a leading power factor condition is relevant and must be examined. Consider the Channel Tunnel 21 kV distribution scheme shown in Fig. 1.9. This consists of long 50 km lengths of 21 kV XLPE cable stretching under the Channel between England and France. Standby generation has been designed to feed essential services in the very unlikely case of simultaneous loss of both UK and French National Grid supplies. The 3 MVAr reactor shown on the single line diagram is used to compensate for the capacitive effect of the 21 kV cable system. The failed Grid supplies are first isolated from the system. The generators are then run up and initially loaded into the reactor before switching in the cable network. The Channel Tunnel essential loads (ventilation, drainage pumping, lighting, control and communications plant) are then energized by remote control from the Channel Tunnel control centre.

1.3.4 Transient stability 1.3.4.1 A physical explanation and analogy Transient stability describes the ability of all the elements in the network to remain in synchronism following an abrupt change in operating conditions. The most onerous abrupt change is usually the three phase fault, but sudden applications of electrical system load or mechanical drive power to the generator and network switching can all produce system instability. This instability can usually be thought of as an energy balance problem within the system. The analogy of the loaded spring is a useful aid to help visualize the situation. The general energy equation is as follows: Mechanical energy : Electrical energy