Modern Power Transformer Practice

MODERN POWER TRANSFORMER PRACTICE

Edited by

R. FEINBERG, Dr.-lng., M.Sc., F.I.E.E.

M

©The Macmillan Press Ltd 1979

Softcover reprint of the hardcover 1st edition 1979

All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission

First published 1979 by THE MACMILLAN PRESS LTD London and Basingstoke Associated companies in Delhi Dublin Hong Kong Johannesburg Lagos Melbourne New York Singapore and Tokyo

British Library Cataloguing in Publication Data

Modern power transformer practice. 1. Electric transformers I. Feinberg, Raphael 621.31'4 TK2551

This book is sold subject to the standard conditions of the Net Book Agreement

ISBN 978-1-349-04089-6 ISBN 978-1-349-04087-2 (eBook) DOI 10.1007/978-1-349-04087-2

Contents

The Contributors

Preface

List of Quantity Symbols

1 General Information 1.1 Power transformer classification 1.2 Review of basic theory 1.3 Transformer reactance 1.4 Transformer losses 1.5 Transformer impedance, resistance and reactance voltages 1.6 Percentage resistance, reactance and impedance

of a transformer 1.7 Transformer designing procedure 1.8 Transformer main parts 1.9 Transformer noise 1.10 Transformer testing 1.11 Transformer transport and site assembly 1.12 Transformer maintenance 1.13 Monitoring of gas-in-oil in large power transformers 1.14 Selection of standard specifications 1.15 Units of measurement used in chapters 1 to 11

2 Theory of Transformer Design Principles 2.1 Introduction 2.2 General considerations: the losses 2.3 General considerations: transformer windings and insulation 2.4 General considerations: cooling of ONAN transformers 2.5 Practical constraints on the design 2.6 Frame and winding proportions 2.7 Considerations of efficiency and cost 2.8 Transformer winding space factors 2.9 Estimation of frame dimensions 2.10 Frame dimensions for a 0.75 MVA three-phase

distribution transformer 2.11 Design of windings and tank Acknowledgements References

IX

xi

XII

5 6 7

8 8 9

II II II 12 12 12 16 18 18 18 22 24 27 32 36 38 41

50 56 60 60

111

VI CONTENTS

3 The Use of the Automatic Electronic Digital Computer as an Aid to the Power Transformer Designer 3.1 Introduction 3.2 Customer's specification 3.3 Designer's specification 3.4 Formulation of the problem 3.5 Method of calculation 3.6 Example 3. 7 General flow diagram 3.8 Cost optimisation 3.9 Advantages and limitation of using computers 3.10 Alternative methods of approach 3.11 Other aspects of transformer design Acknowledgement

4 Transformer Cores 4.1 Introduction 4.2 Materials 4.3 Form 4.4 Construction and manufacture 4.5 Performance 4.6 Future developments Acknowledgements References

5 Windings 5.1 Requirements which control winding design 5.2 Materials 5.3 Common types of winding 5.4 Winding arrangements 5.5 Electric design 5.6 Losses 5.7 Cooling 5.8 Measurement of winding temperature 5.9 Forces 5.10 Clamp design 5.11 Economics of winding design Acknowledgement References

6 On-load Tap-changing Equipment 6.1 Introduction 6.2 Basic conditions of operation 6.3 High-speed resistor tap changer 6.4 General design considerations for a tap changer 6.5 Tapping winding arrangements 6.6 Resistor switching sequence 6. 7 Inductor switching sequence

61 61 62 63 63 65 70 72 80 81 82 83 83 84 84 84 89 96

102 109 110 110 112 112 112 116 120 121 127 130 133 134 137 137 137 138 139 139 139 140 141 142 144 149

6.8 Motor drive mechanisms 6.9 Protective devices 6.10 Maintenance aspects Acknowledgement Reference

CONTENTS

7 Transformer Processing and Testing 7.1 Introduction 7.2 Preliminary tests 7.3 Processing 7.4 Final tests 7.5 Transformer accessories 7.6 Commissioning and site tests 7. 7 Tests on a large power transformer 7.8 Future developments Acknowledgement References

8 Transformer Noise 8.1 Introduction 8.2 Transformer vibration 8.3 Transformer noise 8.4 Fan noise 8.5 Measurement specifications 8.6 Remedial measures 8. 7 Planning 8.8 Prospects for the future Acknowledgement References Additional references

9 Distribution Transformers 9.1 Definition and classification 9.2 Design considerations 9.3 Core construction 9.4 Winding construction 9.5 Tank construction and cooling 9.6 Terminal arrangements 9. 7 Fittings 9.8 A note on USA practice 9.9 Design of a typical distribution transformer 9.10 Calculation of characteristic transformer data 9.11 Thermal calculations 9.12 Short-circuit requirements 9.13 Noise levels Acknowledgement Reference

vii

151 151 152 152 152 153 153 153 155 159 182 182 183 186 186 186 187 187 188 191 193 194 195 202 203 206 207 208 209 209 210 211 213 217 218 218 220 221 230 235 242 242 243 243

Vlll CONTENTS

10 Power System Transformers and Inductors 244 10.1 Introduction-the general power system 244 10.2 Power station transformers 244 10.3 Transmission transformers 248 10.4 Shunt inductors 260 10.5 Series inductors 264 10.6 Stabilising windings 271 10.7 Auxiliary aspects 273 10.8 Transport and site assembly 288 10.9 Future developments 293 Acknowledgements 294 Reference 294

11 Special Transformers 295 11.1 Dry-type transformer 295 11.2 Buried transformers 300 11.3 Coal-mine transformers 304 11.4 Welding transformers 306 11.5 Gas-insulated transformers with class A insulation 311 11.6 Rectifier transformers 314 Acknowledgement 317 Reference 317

12 Transformers in Distribution Systems 318 12.1 Transformer selection-introduction 318 12.2 Transformer selection-constraints of the supply system 318 12.3 Transformer selection-loads supplied from

distribution transformers 322 12.4 Transformer selection-environment 327 12.5 Transformer selection-design philosophy 327 12.6 Economic aspects of transformer selection at the

pre-specification stage 327 12.7 Temperature effects of enclosure of air-cooled transformers 339 12.8 Transformer faults and protection 345 12.9 Maintenance 347 Appendix A12.1 Example of estimation of loss load factor 347 Appendix Al2.2 Example of determination of justifiable costs of

changing a transformer 350 Appendix A12.3 Example of determination of initial rating 351 Appendix A12.4 Example of loss capitalisation 351 Acknowledgement 352 References 353

Bibliography 354

Index 355

The Contributors

Chapter 1 R. Feinberg, Dr.-Ing., M.Sc., F.I.E.E., Consultant, formerly Trans­former Department, Power Division, Ferranti Limited

Chapter 2 A. B. Crompton, M.Sc. Tech., M.I.E.E., Senior Lecturer in Electric Power Engineering, Wigan College of Technology, formerly Designer with the then Distribution Transformer Department, Ferranti Limited

Chapter 3 K. Rowe, B.Sc., Works Manager, Transformer Division, Ferranti Engineering Limited

Chapter 4 S. Palmer, B.Sc.(Eng), F.I.E.E., S.M.I.E.E.E., Canadian Westinghouse Company Limited, formerly Chief Transformer Designer with the then Bruce Peebles Limited

Chapter 5 H. W. Kerr, B.Sc., F.I.E.E., Chief Designer, Transformers, Parsons Peebles Limited

Chapter 6 B. C. Savage, D.S.H., M.I.E.E., formerly Chief Engineer, Tap Changer Department, Power Division, Ferranti Limited

Chapter 7 H. Jackson, M.I.E.E., Deputy Chief Test Engineer, and K. Ripley, Chief Test Engineer, Transformer Division, Ferranti Engineering Limited

Chapter 8 J. Dunsbee, B.Sc., formerly Senior Development Engineer, and M. Milner, B.Sc., F.I.E.E., Manager, Management Services, Transfor­mer Department, Power Division, Ferranti Limited

Chapter 9 Sections 9.1 to 9.8: H. K. Homfray, B.Sc., M.I.E.E., Chief Standard­isation Engineer, GEC Power Transformers Limited, formerly Chief Engineer, Distribution Transformer Division, English Elec­tric Company Limited Sections 9.10 to 9.14: D. Boyle, H.N.C., M.I.E.E., formerly Chief Designer with the then Distribution Transformer Department, Ferranti Limited

Chapter 10 R. Feinberg, Dr.-Ing., M.Sc., F.I.E.E., Consultant, and R. J., Gresley, H.N.C., F:.I.E.E., Consultant, both formerly Transfor­mer Department, Power Division, Ferranti Limited

Chapter 11 T. Kelsall, H.N.C., F.I.T.E., Technical Manager, GEC Distri­bution Equipment Limited

Chapter 12 L. Lawson, B.Sc.Tech., M.I.E.E., Senior Engineer (Plant), North­Western Electricity Board

Preface

Generators and transformers are two major cornerstones in the fabric of any electric power supply system. In 1975 the installed generator capacity was in the region of about 650 GW in Europe and about 585 GW in the USA and Canada. By taking a ratio of about 7:1, the associated installed transformer capacity is about seven times the generator capacity which gives an idea of the magnitude of transformer capacity in service in those parts of the world alone. All over the world the total transformer capacity in service is substantially larger; this signifies the importance and vital duty of power transformers.

The book is intended essentially as a statement on the current state of the art of design, manufacture and operation of power transformers. It arose from a well­attended course of lectures given to practising engineers of the industries of electric power supply and of transformer manufacture. An editorial effort was made to integrate the entire material into a book approached and presented at a standard level.

Standard specifications play an important part in the choice, design, manufac­ture and operation of power transformers. A selection of specifications of the International Electrotechnical Commission (IEC), the British Standards In­stitution (BSI) and the USA Standards is given in section 1.14. Of necessity the list is far from complete. Its primary purpose is to stimulate interest and to provide a pattern of sources for authoritative information also in other countries.

The terminology used is uniform and in line with the current revision of the International Electrotechnical Vocabulary of the IEC; the classification of power transformers within the context of this book is explained in section 1.1. The letter symbols for quantities and for the SI units of measurement are in accordance with the IEC publication Letter Symbols to be Used in Electrical Technology, Part 1: General, 27-1 (1971).

A systematic guide to the contents of the book is given in chapter 1. The book is addressed to a wide range of practising engineers, and students may use it as a factual reference at the initial stages of project or research work.

Cheadle, Cheshire, 1978 R.F.

List of Quantity Symbols

These quantity symbols are common to chapters 1 to 11. Note. For electrical and thermal quantities the subscript 1 refers to the low­voltage and the subscript 2 to the high-voltage winding.

Symbol Meaning Unit Ac external cooling surface of a coil m2

Acu total copper cross-section per phase m2

A Fe core leg net-cross-section area m2

AFe,g core leg gross cross-section area m2

AT external effective tank cooling surface m2

At = Acu +AFe m2

Aw nett core window area between core circles m2

Ae cross-section area for thermal power flow m2

Am heat transfer surface between winding layers m2

Bm peak value of magnetic flux density in core leg T Blm peak value ofleakage flux density in axial duct T ct total cost of active copper and iron urn J,Jl,J2 winding currents A Jk winding current under fault condition A Jm normal full-load winding current A IN = J1 N 1 = 12 N 2 winding ampere-turns A J, Jl> ]2 winding current density Amm- 2

Jk short-circuit winding current density Amm- 2

K see equation 3.15 KA = AFe X Acu KAs output coefficient, see equation 2.55 Kc cost coefficient Kc empirical heat transfer coefficient Ket empirical coefficient for total heat transfer

from a tank surface KT tank heat transfer coefficient, see equation

2.70 Kt = (1/ Ket)0·8

Kvs output coefficient, see equation 2.59 Me total thermal power transfer from unit surface wm- 2

area

Symbol

M""

LIST OF QUANTITY SYMBOLS

Meaning thermal power transfer by convection per unit

area

Unit wm- 2

Mer thermal power transfer by radiation per unit W m- 2

area Met thermal power transfer from tank surface per W m- 2

unit area average of Me, over effective tank cooling

surface N, N 1 , N 2 winding turns Pcu = Pcui + Pcu2 Pcu1,Pcu2 copper loss per phase at 75 oc P cu, t total copper loss Peh vertical thermal power flow PFe total iron loss P1 total load loss at 75 oc PR 12 R loss per phase P,01 total transformer loss %Pcu percentage copper loss %Pi,%Pi1,%Pi percentage conductor eddy current loss R1 , R 2 winding resistance per phase at 75 oc %R percentage resistance Reb horizontal equivalent thermal resistance in

s

winding vertical equivalent thermal resistance in wind-

ing rating per phase winding voltage per phase percentage reactance percentage impedance cross-section area of a conductor strand see equation 9.1 radial clearance between low- and high­

voltage windings radial widths of windings radial clearance between core leg and low­

voltage winding radial clearance between high-voltage windings breadth of rectangular conductor breadth of conductor strand distance between centres of core legs width of vertical duct between coil sections half the width of widest core leg plates width of core window

kW kW kW w kW w kW kW

K w- 1

K w- 1

MVA v

mm mm mm

mm

mm mm mm mm mm mm

Xlll

xiv LIST OF QUANTITY SYMBOLS

Symbol Meaning Unit bx reactive width of windings, see equation mm

2.60 c distance between top and bottom entry mm

pipes Ccu cost per kilogram of copper Umkg-1 Cpe cost per kilogram of core steel Umkg-1 d diameter of circle circumscribing core leg mm de diameter of round conductors mm f frequency Hz h assumed equal height of low- and high- mm

voltage windings ho assumed equal total axial clearance be- mm

tween windings and core yoke

hi' h2 height of low and high-voltage windings, mm respectively

hOI' h02 axial clearance between low- and high- mm voltage windings and core yoke

he height of bare conductor strand mm hd height of horizontal ducts between coil

sections mm hFe height of core mm hw height of core window mm hx meaning h1 or h2 whichever is the greater mm ho height of centre of transformer heating mm k, k1, k2 winding space factors kc cost ratio of copper to total of active

material kc core circle space factor ke = ki2/kil kf = 1/k1 + ljk2 kp see equation 3. 7 kFe space factor of core laminations kh = h2/h1 khx see equation 3.4 kp kil, ki2 conductor eddy current loss factor, see equa-

tion 3.6 kJ = J2jJ1 kJs see equation 3.5 ks fractional load for maximum efficiency ks = 2bpe/d ksx see equation 3.3

kt1' kt2 correction to N 1, N 2 to allow for tappings k. = 110.!11()0 kw window space factor, see equation 2.35

Symbol I Fe

mcu mFe

n

no nb

nh

nsb

nsh

Pcu PFe

P; PR s so

e fl{)

8. ()H

A()H

A()o

fl{)om

fl{)R

LIST OF QUANTITY SYMBOLS

Meaning total length of core legs and yokes total mass of active copper total mass of active iron N 2 /N 1 ; number of copper layers in a coil

cooling only from vertical surfaces number of conductor strands in parallel number of horizontal conductor strands number of vertical conductor strands number of horizontal sections in a winding number of vertical sections in a winding specific copper loss specific iron loss specific conductor eddy current loss specific 12 R loss = f(s1 + s2) length of mean turn of interwinding axial

ducts length of mean turn of winding mean winding circumference on each leg mul-

tiplied by number of legs duration of short circuit peak value of magnetic flux in core leg horizontal insulation thickness between con-

ductor strands

Unit m kg kg

Wkg- 1

Wkg- 1

wm- 3

Wkg- 1

mm

mm mm

m s Wb

mm insulation thickness at sides of a coil section mm external insulation thickness at coil cooling

surfaces mm equivalent horizontal insulation thickness be- mm

tween conductor strands equivalent vertical insulation thickness be- mm

tween conductor strands vertical insulation thickness between con-

ductor strands mm insulation thickness between upper and lower mm

surfaces of coil section thermal emissivity of a surface W m- 2 K- 4

temperature difference between surface and oc cooling medium

ambient temperature oc hot-spot temperature oc hot-spot temperature rise oc top oil temperature rise oc average oil temperature rise oc mean winding temperature rise oc

XV

XVI LIST OF QUANTITY SYMBOLS

Meaning Unit temperature drop at a coil surface oc oil temperature difference between top and oc

bottom of tank winding temperature measured by resistance mean winding temperature rise above oil maximum winding temperature rise above oil thermal conductivity resistivity of copper strip, taken as

21.4 X 10- 3 !lm mass density of copper, taken as 8890 kg m- 3

mass density of core steel, taken as 7650kgm- 3