IET Power and Energy Series 7

J.S.T. Looms spent over 30 years in research on HV physics and engineering, many of them as head of insulator research at the Central Electricity Research Laboratories, United Kingdom. There he led the section that developed the Salt Fog test and participated in its international standardisation.He was the first man in Europe to work, bare-hand, on overhead conductors at 400,000 volts and was a pioneer in developing the technique.More recently he has been in practice as a consultant to leading manufacturers and has been responsible for numerous inventions in the HV field, some of which are described in the book.

Insulators for High Voltages

Looms

Insulators for High Voltages

J.S.T. Looms

Insulators for High VoltagesThis book is a guide to the whole field of high voltage insulators as used in electrical power networks, traction and production. It covers the historical development of the shapes – sometimes strange ones – of modern types, decribes the principal materials – both ceramic and polymeric – and their fabrication, explains the physical principles of contamination and flashover, and reviews the mass of data on research and testing.

Practical advice is given, with worked examples and many illustrations, on the choice of insulator, including the problems of the environment and nuisance. The remedies that are available to deal with pollution-flashover are compared and precautions are indicated for the potentially dangerous practices of live-washing and live-working.

Useful guiding principles for the designer of power lines and substations are listed with the relevant international standards and tables containing properties of materials. For the student and researcher there are accounts of current basic R&D, and indications of probable future trends. The particularly current topics of optical fibre insulation, direct current and vandal-resistance are covered.

The author’s first-hand experience of most of the matters which he describes enables him to write clearly and to avoid confusing mathematical and technical complexities. This is a book for the intelligent layman as well as for the specialist.

The Institution of Engineering and Technologywww.theiet.org 0 86341 116 9978-0-86341-116-8

IET PowEr and EnErgy SErIES 7

Series Editors: Prof. A.T. Johns G. Ratcliff J.R. Platts

Insulators for High Voltages

Other volumes in this series:

Volume 1 Power circuit breaker theory and design C.H. Flurscheim (Editor)Volume 4 Industrial microwave heating A.C. Metaxas and R.J. MeredithVolume 7 Insulators for high voltages J.S.T. LoomsVolume 8 Variable frequency AC motor drive systems D. FinneyVolume 10 SF6 switchgear H.M. Ryan and G.R. JonesVolume 11 Conduction and induction heating E.J. DaviesVolume 13 Statistical techniques for high voltage engineering W. Hauschild and

W. MoschVolume 14 Uninterruptable power supplies J. Platts and J.D. St Aubyn (Editors)Volume 15 Digital protection for power systems A.T. Johns and S.K. SalmanVolume 16 Electricity economics and planning T.W. BerrieVolume 18 Vacuum switchgear A. GreenwoodVolume 19 Electrical safety: a guide to causes and prevention of hazards

J. Maxwell AdamsVolume 21 Electricity distribution network design, 2nd edition E. Lakervi and

E.J. HolmesVolume 22 Artificial intelligence techniques in power systems K. Warwick, A.O. Ekwue

and R. Aggarwal (Editors)Volume 24 Power system commissioning and maintenance practice K. HarkerVolume 25 Engineers’ handbook of industrial microwave heating R.J. MeredithVolume 26 Small electric motors H. Moczala et al.Volume 27 AC-DC power system analysis J. Arrill and B.C. SmithVolume 29 High voltage direct current transmission, 2nd edition J. ArrillagaVolume 30 Flexible AC Transmission Systems (FACTS) Y-H. Song (Editor)Volume 31 Embedded generation N. Jenkins et al.Volume 32 High voltage engineering and testing, 2nd edition H.M. Ryan (Editor)Volume 33 Overvoltage protection of low-voltage systems, revised edition P. HasseVolume 34 The lightning flash V. CoorayVolume 35 Control techniques drives and controls handbook W. Drury (Editor)Volume 36 Voltage quality in electrical power systems J. Schlabbach et al.Volume 37 Electrical steels for rotating machines P. BeckleyVolume 38 The electric car: development and future of battery, hybrid and fuel-cell

cars M. WestbrookVolume 39 Power systems electromagnetic transients simulation J. Arrillaga and

N. WatsonVolume 40 Advances in high voltage engineering M. Haddad and D. WarneVolume 41 Electrical operation of electrostatic precipitators K. ParkerVolume 43 Thermal power plant simulation and control D. FlynnVolume 44 Economic evaluation of projects in the electricity supply industry H. KhatibVolume 45 Propulsion systems for hybrid vehicles J. MillerVolume 46 Distribution switchgear S. StewartVolume 47 Protection of electricity distribution networks, 2nd edition J. Gers and

E. HolmesVolume 48 Wood pole overhead lines B. WareingVolume 49 Electric fuses, 3rd edition A. Wright and G. NewberyVolume 51 Short circuit currents J. SchlabbachVolume 905 Power system protection, 4 volumes

Insulators for High Voltages

J.S.T. Looms

The Institution of Engineering and Technology

Published by The Institution of Engineering and Technology, London, United Kingdom

First edition © 1988 Peter Peregrinus Ltd Reprint with new cover © 2006 The Institution of Engineering and Technology

First published 1988 Reprinted 1990, 2006

This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any form or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Inquiries concerning reproduction outside those terms should be sent to the publishers at the undermentioned address:

The Institution of Engineering and Technology Michael Faraday House Six Hills Way, Stevenage Herts, SG1 2AY, United Kingdom

www.theiet.org

While the author and the publishers believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgement when making use of them. Neither the author nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed.

The moral rights of the author to be identified as author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

British Library Cataloguing in Publication DataLooms, J.S.T.

Insulators for high voltages– (IEE power engineering series; 7) 1. Electric insulators and insulation 2. High voltages I. Title II. Institution of Electrical Engineers III. Series 621.319’37 TL3401

ISBN (10 digit) 0 86341 116 9 ISBN (13 digit) 978-0-86341-116-8

Printed in the UK by Short Run Press Ltd, Exeter Reprinted in the UK by Lightning Source UK Ltd, Milton Keynes

Contents

Page

Foreword ix

Acknowledgments xi

1 Introduction 1

1.1 Definitions 11.1.2 Functions of insulators 11.1.3 Classification of insulators 2

1.2 History of insulators for transmission systems 31.2.1 Transmission lines 31.2.2 Insulator evolution: Materials and shapes 4

1.3 Critical elements of an insulator 101.3.1 Properties of materials for insulators 101.3.2 Operation in adverse conditions 131.3.3 Costs of insulators 15

2 Insulating materials 172.1 Basic nature of insulator dielectrics 172.2 Properties of electrical porcelain 20

2.2.1 The determinants 202.2.2 Mechanical properties 232.2.3 Electrical properties 25

2.3 Properties of insulator glass 272.3.1 The glassy state 272.3.2 Mechanical properties of insulator glass 292.3.3 Electrical properties of insulator glass 31

2.4 Properties of resin-bonded glass fibre (RBGF) 322.4.1 Fibrous composites: General 322.4.2 Unidirectional fibre materials 352.4.3 Mechanical properties of RBGF 362.4.4 Electrical properties of RBGF 38

2.5 Properties of polymers and polymer concretes 392.5.1 Applications and functions 392.5.2 Polymers for housings 422.5.3 Evaluation of polymers as housing materials 452.5.4 Polymer concretes 50

vi Contents

3 Manufacture of wet process porcelain 533.1 Wet and other processes 533.2 Blending the raw materials 543.3 Dehydration and forming 573.4 Glazing and sanding 583.5 Firing 593.6 Finishing processes 643.7 Other porcelain processes 64

4 Manufacture of tempered-glass insulators 664.1 Scope of manufacturing processes 664.2 Preparation of glass 664.3 Moulding and toughening 68

5 Fibrous cores for polymeric insulators 705.1 General principles 705.2 Glasses and surface treatments 715.3 Sealing of core ends 725.4 Service experience with fibrous cores 74

6 Polymeric housings 756.1 Relationship between shape and material 756.2 Extrusion and bonding 776.3 Casting and moulding 786.4 Other fabrication processes 786.5 Behaviour of polymeric housings: Tests, trials, service 806.6 Profile and performance 84

7 Terminal fittings for insulators 887.1 Terminal materials 887.2 Mechanical design of fittings 90

7.2.1 Fittings for porcelain and glass 907.2.2 Fittings for fibrous composite cores 94

7.3 Effects of transition from metal to insulation 98

8 Finite insulator life: Limiting processes 1028.1 Catastrophic and gradual attack 1028.2 Impact testing and vandal resistance 1038.3 Damage by cycling 1048.4 Cement growth and corrosion 1058.5 Loss of electrical performance 106

9 Aesthetics of insulators 1089.1 Acceptability of transmission lines 1089.2 The inconspicuous insulator 1109.3 Insulators as determinants of tower height: Compaction 1119.4 Unorthodox systems 116

10 Physics of contamination 11810.1 Electrically significant deposits 11810.2 Contaminating processes 11910.3 Purging processes 12410.4 Equilibrium deposit 12610.5 Assessment of required insulation: Severity measurement 127

Contents vii

11 Physics of pollution flashover 13211.1 Flashover paradox 13211.2 Stages of the flashover process 133

11.2.1. Electrolytic layers under electrical stress 13411.2.2 Propagation of discharges 13711.2.3 Voltage waveshape and propagation 142

11.3 Models and empirical theories of complete flashover 143

12 Testing of insulators 14512.1 Classes of test 14512.2 Natural pollution testing: Background 145

12.2.1 Advantages and disadvantages of outdoor testing 14812.2.2 Practice of natural pollution testing 150

12.3 Artificial pollution testing 15212.3.1 Basic philosophies 15212.3.2 Principal artificial tests 156,

12.4 Comparison of artificial-pollution tests 16312.4.1 Severity parameter and voltage/severity function 163

12.5 Source impedance: Effect on test results 16612.6 Principles of mechanical testing 167

13 Conclusions from pollution tests on insulators 17013.1 Scope of Chapter 170

13.1.1 Agreement between artificial and natural test results 17013.1.2 Laws of behaviour of polluted insulators 17113.1.3 Influence of shape on insulator performance 181

13.2 Deterioration: Test results 18813.2.1 Scope of tests 188

13.3 Is testing of insulators valid and valuable? 192

14 Remedies for flashover 19514.1 When are remedies needed? 19514.2 Optimised insulator shapes and creepages 19614.3 Insulator washing 197

14.3.1 Booster shed 19914.4 Surface treatments 20314.5 Use of solid hydrophobes on surfaces 20714.6 Hybrid insulators 20914.7 Resistive glazes 21114.8 Calculated powers in resistive-glazed insulators 215

15 Insulators for special applications 21615.1 Scope of Chapter 21615.2 Railway insulators 21615.3 Insulators for electrostatic precipitators 21915.4 Insulators for direct voltages 219

15.4.1 Basic differences from AC condition 21915.4.2 Relative flashover liabilities, DC and AC 22115.4.3 Relative deterioration rates 224

15.5 Insulators for live working 22615.5.1 Leakage-current limitation 22615.5.2 Hand-held tools - Hot sticks - Struts and ties 22715.5.3 Tensile supports: Ropes, chains, monofilaments 229

viii Contents

16 Interference and noise generated by insulators 23116.1 Generating processes 23116.2 Effects of capacitance 23416.3 Effects of wetting and pollution 23716.4 Acoustic noise from insulators 239

17 Insulator of the future 24117.1 Indicators from known facts 24117.2 Extrapolation from current practices 244

References 249

Appendix A: Glossary of insulator names 259

Appendix B: Testing of insulators 262

Appendix C: Selective bibliography on live washing of insulators 265

Subject Index 267

Name Index 273

Foreword

Although this book is written primarily with the needs of students, practisingelectrical engineers and researchers in mind, I hope that it will be helpful alsoto others who seek information about insulators. There is a mass of publishedwork on the subject, disparate and often contradictory, which makes the searchfor specific answers to questions a frustrating task. The seeker should either findwhat he needs directly, or at least a reliable reference, in these pages.

My text is a distillation of much of a working life's accumulations. Some wasacquired when I worked as a researcher for the Central Electricity GeneratingBoard (CEGB), the UK's national electrical authority and a large user ofinsulators. The rest has come from my days as consultant to manufacturersconcerned with the development and marketing of insulators. Both sides of thecoin are presented here.

After more than a century's development insulators might be expected to havesettled into final forms, but in fact the whole topic continues in a state of flux.Some of what is said here may well be proved wrong, before long. I shall bepleased, however, if I succeed in showing why insulators are made as they areand behave as they do; if my readers acquire a 'feel' for the subject; if I leavewith them a flavour of the fun and excitement which have attended my own daysas an insulator-man.

I have done my best to acknowledge everyone who has helped with my textand apologise for any accidental omissions. I particularly have to thank theCEGB for allowing the inclusion of some of the results obtained in their service,and the International Conference on Large Electrical Networks (CIGRE) forpermitting the inclusion of many internationally acquired results of collabora-tion, under their aegis. IEE publications have been extensively drawn upon.

On the personal level, I express my gratitude to Peter Lambeth, my friend formany years, for a great deal of my material. I thank my 'boys' at the CentralElectricity Research Laboratories (CERL) for their hard and careful work andhappily recall the amicable collaboration of my colleagues in CEGB, in thenational power authorities of France and Italy (EdF, ENEL), in the German,Italian and Czechoslovakian Testing Institutes (FGH, CESI, EGU) and among

x Foreword

the manufacturers in the UK, Germany, France, Japan and the USA. We havefought technical battles but have enjoyed many good times together.

Above all I must thank my Chief, John Samuel Forrest, F.R.S., the father ofinsulator research. He set my hand upon this particular plough and guided mealong the furrow. I dedicate this book to him.

East Molesey

J.S.T. Looms

Acknowledgments

I thank the following gentlemen for their help, in writing this book and thenamed bodies for their courtesy in allowing reproduction of photographs orfigures:

Arthur BarberEwald BauerAlan BradwellRobin ClabburnPaul ClaverieFritz HirschRay HoulgateTony HowardAlan InglesGerard LeroyIan MacleanCarlo Malaguti

Jack MoranMarcel MoreauStan MorganKatsuhiko NaitoHarry NewGeorge OrawskiLaurent PargaminRene ParraudDick PenneckTony PentelowYves PorcheronDon Powell

Frank ProctorCliff RickettsDominique RiviereFarouk RizkBill RobinsonBrian SadlerMario SforziniDon SwiftDerek TaylorLee TeichthesenIan ThorpePrasad Verma

Balfour Beatty Transmission DivisionCentral Electricity Research LaboratoriesCeraverConference Internationale des Grands Reseaux

ElectriquesDoulton Insulators Ltd.Dulmison Pty. Ltd.Electric Power Research InstituteLapp Insulator Co.NGK Insulators Ltd.Raychem UK Ltd.Rosenthal (Hoechst Ceramtec)