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Transformer engineering

Transformer
Engineering

Copyright © 2004 by Marcel Dekker, Inc.


POWER ENGINEERING
1. Power Distribution Planning Reference Book, H.Lee Willis
2. Transmission Network Protection: Theory and Practice, Y.G.Paithankar
3. Electrical Insulation in Power Systems, N.H.Malik, A.A.Al-Arainy, and
M.I.Qureshi
4. Electrical Power Equipment Maintenance and Testing, Paul Gill
5. Protective Relaying: Principles and Applications, Second Edition, J.
Lewis Blackburn
6. Understanding Electric Utilities and De-Regulation, Lorrin Philipson and
H.Lee Willis
7. Electrical Power Cable Engineering, William A.Thue
8. Electric Systems, Dynamics, and Stability with Artificial Intelligence
Applications, James A.Momoh and Mohamed E.El-Hawary
9. Insulation Coordination for Power Systems, Andrew R.Hileman
10. Distributed Power Generation: Planning and Evaluation, H.Lee Willis and

Walter G.Scott
11. Electric Power System Applications of Optimization, James A.Momoh
12. Aging Power Delivery Infrastructures, H.Lee Willis, Gregory V.Welch,
and Randall R.Schrieber
13. Restructured Electrical Power Systems: Operation, Trading, and
Volatility, Mohammad Shahidehpour and Muwaffaq Alomoush
14. Electric Power Distribution Reliability, Richard E.Brown
15. Computer-Aided Power System Analysis, Ramasamy Natarajan
16. Power System Analysis: Short-Circuit Load Flow and Harmonics, J. C.Das
17. Power Transformers: Principles and Applications, John J.Winders, Jr.
18. Spatial Electric Load Forecasting: Second Edition, Revised and
Expanded, H.Lee Willis
19. Dielectrics in Electric Fields, Gorur G.Raju
20. Protection Devices and Systems for High-Voltage Applications,
Vladimir Gurevich
21. Electrical Power Cable Engineering: Second Edition, Revised and
Expanded, William A.Thue
22. Vehicular Electric Power Systems: Land, Sea, Air, and Space Vehicles,
Ali Emadi, Mehrdad Ehsani, and John M.Miller
23. Power Distribution Planning Reference Book: Second Edition, Revised
and Expanded, H.Lee Willis
24. Power System State Estimation: Theory and Implementation, Ali Abur
and Antonio Gómez Expósito
25. Transformer Engineering: Design and Practice, S.V.Kulkarni and
S.A.Khaparde
ADDITIONAL VOLUMES IN PREPARATION

Copyright © 2004 by Marcel Dekker, Inc.


Transformer
Engineering
Design and Practice
S.V.Kulkarni
S.A.Khaparde
Indian Institute of Technology, Bombay
Mumbai, India

MARCEL DEKKER, INC.


Copyright © 2004 by Marcel Dekker, Inc.

NEW YORK • BASEL


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Copyright © 2004 by Marcel Dekker, Inc.


Foreword

It is a great pleasure to welcome this new book from Prof. S.V.Kulkarni and Prof.
S.A.Khaparde, and I congratulate them for the comprehensive treatment given in
the book to nearly all aspects of transformer engineering.
Everyone involved in or with the subject area of this book, whether from
academics or industry, knows that the last decade has been particularly dynamic
and fast changing. Significant advances have been made in design, analysis and
diagnostic techniques for transformers. The enabling factors for this
technological leap are extremely competitive market conditions, tremendous
improvements in computational facilities and rapid advances in instrumentation.
The phenomenal growth and increasing complexity of power systems have put up
tremendous responsibilities on the transformer industry to supply reliable
transformers. The transformer as a system consists of several components and it is
absolutely essential that the integrity of all these components individually and as
a system is ensured. A transformer is a complex three-dimensional
electromagnetic structure, and it is subjected to variety of stresses, viz. dielectric,
thermal, electrodynamic, etc. In-depth understanding of various phenomena
occurring inside the transformer is necessary. Most of these can now be simulated
on computers so that suitable changes can be made at the design stage to eliminate
potential problems.
I find that many of these challenges in the design and manufacture of
transformers, to be met in fast changing market conditions and technological
options, are elaborated in this book. There is a nice blend of theory and practice in
almost every topic discussed in the text. The academic background of the authors
has ensured that a thorough theoretical treatment is given to important topics. A
number of landmark references are cited at appropriate places. The previous
industry experience of S.V.Kulkarni is reflected in many discussions in the book.
The various theories have been supported in the text by reference to actual
practices. For example, while deliberating on various issues of stray loss
estimation and control, the relevant theory of eddy currents has been first
explained. This theoretical basis is then used to explain various design and
iii
Copyright © 2004 by Marcel Dekker, Inc.


iv

Foreword

manufacturing practices established in the industry to analyze and minimize the
stray losses in the windings and structural components. The design and
manufacturing practices and processes have significant impact on the
performance parameters of the transformers, and the same have been identified in
the text while discussing various topics.
Wherever required, a number of examples and case studies are given which are
of great practical value. The knowledge of zero-sequence characteristics of
transformers is very important for utilities. It is essential to understand the
difference between magnetizing and leakage zero-sequence reactances of the
transformer. These two types of zero-sequence reactances are explained in the
book for three-phase three-limb, three-phase five-limb and single-phase threelimb transformers with numerical examples. One may not find such a detailed
treatment to zero-sequence reactances in the available literature. The effect of
tank on the zero-sequence reactance characteristics is lucidly explained.
The discussions on the sympathetic inrush phenomenon, part-winding
resonance, short-circuit withstand characteristics and noise reduction techniques
should also be quite useful to the readers. With the increase in network complexity
and severity of loads in some cases, the cooperation between the transformer
manufacturers and users (utilities) is very critical. The design reviews with the
involvement of users at various stages of contract should help in enhancing the
reliability of transformers. I am happy to note that such areas of cooperation are
identified at appropriate places in the text.
The book propagates the use of modern computational tools for optimization
and quality enhancement of transformers. I know a number of previously
published works of the authors in which Finite Element Method (FEM) has been
applied for the stray loss control and insulation design of the transformers. The use
of FEM has been aptly demonstrated in the book for various calculations along
with some tips, which will be helpful to a novice in FEM.
The book is therefore a major contribution to the literature. The book will be
extremely helpful and handy to the transformer industry and users. It will be also
useful for teaching transformers to undergraduate and postgraduate students in
universities. The thorough treatment of all-important aspects of transformer
engineering given will provide the reader all the necessary background to pursue
research and development activities in the domain of transformers.
It is anticipated that this book will become an essential reference for engineers
concerned with design, application, planning, installation, and maintenance of
power transformers.
H.Jin Sim, PE
VP, Waukesha Electric Systems
Past Chairman, IEEE Transformers Committee

Copyright © 2004 by Marcel Dekker, Inc.


Preface

In the last decade, rapid advancements and developments have taken place in the
design, analysis, manufacturing and condition-monitoring technologies of
transformers. The technological progress will continue in the forthcoming years.
The phenomenal growth of power systems has put tremendous responsibilities on
the transformer industry to supply reliable and cost-effective transformers.
There is a continuous increase in ratings of generator transformers and
autotransformers. Further, the ongoing trend to go for higher system voltages for
transmission increases the voltage rating of transformers. The increase in current
and voltage ratings calls for special design and manufacturing considerations.
Advanced computational techniques have to be used that should be backed up by
experimental verification to ensure quality of design and manufacturing
processes. Some of the vital design challenges are: stray loss control, accurate
prediction of winding hot spots, short-circuit withstand capability and reliable
insulation design. With the increase in MVA ratings, the weight and size of large
transformers approach or exceed transport and manufacturing capability limits.
Also, due to the ever-increasing competition in the global market, there are
continual efforts to optimize the material content in transformers. Therefore, the
difference between withstand levels (e.g., short circuit, dielectric) and
corresponding operating stress levels is diminishing. Similarly, the guaranteed
performance figures and actual test values are now very close. All these factors
demand greater efforts from researchers and designers for accurate calculation of
various stress levels and performance figures for the transformers. In addition,
strict control of manufacturing processes is required. Manufacturing variations of
components should be monitored and controlled.
Many of the standard books on transformers are now more than 10 years old.
Some of these books are still relevant and widely referred for understanding the
theory and operation of transformers. However, a comprehensive theoretical basis
together with application of modern computational techniques is necessary to
face the challenges of fast-changing and demanding conditions. This book is an
effort in that direction. The principles of various physical phenomena occurring
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Copyright © 2004 by Marcel Dekker, Inc.


vi

Preface

within a transformer are explained elaborately in the text, which could also be
used in a course at the undergraduate or postgraduate level. Wherever required,
adequate references have been cited so that readers can explore the phenomena in
more depth. In fact, a large number of very useful references (more than 400) is one
of the hallmarks of this book. Some of the references—classic sources that date
back to the early part of the last century—explain many of the theories useful in
transformer engineering. Some most recent works are also discussed to give
readers a feel for the latest trends in transformer technology.
The first author worked in the transformer industry for 11 years before joining
academia. He has vast experience in the design and development of transformers,
from the small distribution range to 400 kV class 300 MVA ratings. He had ample
opportunity to investigate problems in transformer operations and sites. A few
case studies and site investigations in which he was actively involved have been
incorporated at appropriate places in the text. Also, he found that some aspects of
transformer engineering had not been given adequate treatment in the books
available. Hence, the emphasis of this book is on these aspects: magnetizing
asymmetry, zero-sequence reactance characteristics, stray losses and related
theory of eddy currents, short-circuit forces and withstand, part winding
resonance phenomena, insulation design, and design aspects of transformers for
rectifier, furnace and HVDC applications. The book will be particularly useful to:
(1)

(2)

(3)

Transformer designers and researchers engaged in optimization and
quality-enhancement activities in today’s competitive
environment
Utility engineers who would like to learn more about the system
interaction aspects of transformers in an interconnected power
system to improve on specifications and employ diagnostic tools
for condition monitoring
Undergraduate and postgraduate students who wish to integrate
traditional transformer theory with modern computing practices

In Chapter 1, in addition to the transformer fundamentals, various types of
transformers in a typical power system are explained along with their features.
There is a trend to use better materials to reduce core losses. Often the expected
loss reduction is not obtained with these better grades. The design and
manufacturing practices and processes that have significant impact on the core
performance are highlighted in Chapter 2. The three-phase three-limb core has
inherent magnetizing asymmetry that sometimes results in widely different noload current and losses in three phases of the transformer during the no-load loss
measurement by the three-wattmeter method. It is shown that one of the three
wattmeters can have a negative reading depending on the magnitude of
asymmetry between phases and the level of excitation. Although the inrush
current phenomenon is well understood, the sympathetic inrush phenomenon—
in which the magnetization level of a transformer is affected by energization of

Copyright © 2004 by Marcel Dekker, Inc.


Preface

vii

another interconnected transformer—is not well known. The factors influencing
the phenomenon are elucidated in the chapter. The phenomenon was investigated
by the first author in 1993 based on switching tests conducted at a site.
Chapter 3 is devoted to reactance of transformers, which can be calculated by
either analytical or numerical methods. Procedures for the calculation of
reactance of various types and configurations of windings, including zigzag and
sandwich windings, are illustrated. The reactance for complex winding
configurations can be easily calculated by the finite element method (FEM),
which is the most widely used numerical method. The chapter gives exhaustive
treatment of zero-sequence characteristics of the transformers. Procedures for
calculation of the magnetizing zero-sequence and leakage zero-sequence
reactances of the transformers are illustrated through examples (such a treatment is
unusual in the published literature). The effect of the presence of delta winding on
the zero-sequence reactance is also explained.
In order to accurately estimate and control the stray losses in windings and
structural parts, an in-depth understanding of the fundamentals of eddy currents
starting from the basics of electromagnetic fields is desirable. The treatment of
eddy currents given in Chapter 4 is self-contained and useful for the conceptual
understanding of the phenomena of stray losses in the windings and structural
components of transformers described in Chapters 4 and 5, respectively. Stray
losses in all the conducting components of the transformers have been given
elaborate treatment. Different analytical and numerical approaches for their
estimation are discussed and compared. A number of useful guidelines, graphs and
equations are given that can be used by practicing engineers. A few interesting
phenomena observed during the load-loss test of transformers are explained (e.g.,
the half turn effect). Various shielding arrangements for effective stray loss control
are discussed and compared.
Failure of transformers due to short circuits is a major concern for transformer
users. The success rate during actual short-circuit tests is far from satisfactory. The
static force and withstand calculations are well established. Efforts are being made
to standardize and improve the dynamic short-circuit calculations. A number of
precautions (around 40) that can be taken at the specification, design and
manufacturing stages of transformers for improvement in short-circuit withstand
are elaborated in Chapter 6. The various failure mechanisms and factors that
determine the withstand strength are explained.
Although methods for calculating impulse distribution are well established,
failures of large transformers due to part-winding resonance and very fast transient
overvoltages have attracted the attention of researchers. After an explanation of
the methods for calculating series capacitances of commonly used windings,
analytical and numerical methods for transient analysis are discussed in Chapter
7. The results of three different methods are presented for a typical winding.
Methods for avoiding winding resonances are also explained.
Chapter 8 examines in detail the insulation design philosophy. Various factors
that affect insulation strength are summarized. The formulae given for bulk oil

Copyright © 2004 by Marcel Dekker, Inc.


viii

Preface

and creepage withstand are very useful to designers. Procedures for the design of
major and minor insulation systems are presented.
Chapter 9 deals with the thermal aspects of transformer design. After a
description of the modes of heat transfer, various cooling systems are described.
The insulation aging phenomenon and life expectancy are also discussed. A
number of recent failures of large transformers have been attributed to the static
electrification phenomenon, which is explained at the end of the chapter.
Various types of loads and tests that determine aspects of structural design are
discussed in Chapter 10. Tank-stiffening arrangements are elaborated. This
material has been scarce in the available literature. Because of increasing
environmental concerns, many users are specifying transformers with lower noise
levels. Different noise level reduction techniques are discussed and compared.
Chapter 11 is devoted to four special transformers: rectifier transformers,
HVDC converter transformers, furnace transformers and phase-shifting
transformers. Their design aspects and features, different from those of
conventional distribution and power transformers, are enumerated.
The text concludes by identifying current research and development trends.
The last chapter is intended to give pointers to readers desirous of pursuing
research in transformers.
Even though the transformer is a mature product, there are still a number of
design, manufacturing and power system interaction issues that continue to
attract the attention of researchers. This book addresses many of these issues and
provides leads to most of the remaining ones. It encompasses all the important
aspects of transformer engineering including the recent advances in research and
development activities. It also propagates the use of advanced computational
tools such as FEM for optimization and quality enhancement of transformers.
S.V.Kulkarni
S.A.Khaparde

Copyright © 2004 by Marcel Dekker, Inc.


ACKNOWLEDGMENTS
We would like to thank our colleagues in the Electrical Engineering Department
of the Indian Institute of Technology, Bombay, for their support and
encouragement. In particular, we are grateful to Profs. R.K.Shevgaonkar, S.A.
Soman, B.G.Fernandes, V.R.Sule, M.B.Patil, A.M.Kulkarni and Kishore
Chatterjee, for their help in reviewing the book. Thanks are also due to Mr. V.
K.Tandon, who suggested editorial corrections.
Research associates Mr. Sainath Bongane, Mr. Sachin Thakkar and Mr. G.
D.Patil helped tremendously, and the excellent quality of the figures is due to their
efforts. Ph.D. students Mr. G.B.Kumbhar, Mr. A.P.Agalgaonkar and Mr. M.U.Nabi
also contributed in the refining of some topics in the book.
Previously, S.V.Kulkarni worked in Crompton Greaves Limited in the area of
design and development of transformers. He sincerely acknowledges the rich and
ample experience gained while working in the industry and is grateful to all his
erstwhile senior colleagues. He would particularly like to express his sincere
gratitude to Mr. C.R.Varier, Mr. T.K.Mukherjee, Mr. D.A.Koppikar, Mr.
S.V.Manerikar, Mr. B.A.Subramanyam, Mr. G.S.Gulwadi, Mr. K. Vijayan, Mr.
V.K.Lakhiani, Mr. P.V.Mathai, Mr. A.N.Kumthekar and Mr. K.V.Pawaskar for their
support and guidance.
Many practical aspects of transformer technology are discussed in the book.
Hence, it was essential to have those sections reviewed by practicing transformer
experts. Mr. V.S.Joshi’s valuable suggestions and comments on almost all the
chapters resulted in refinement of the discussion on many practical points. Mr.
K.Vijayan, with his expertise on insulation design, contributed significantly in
refining Chapter 8. He also gave useful comments on Chapter 9. We thank Mr.
G.S.Gulwadi for reviewing Chapters 1 and 8. Mr. V.D.Deodhar helped to improve
Chapter 10.
We are also thankful to Dr. B.N.Jayaram for reviewing Chapter 7. The efforts of
Dr. G.Bhat in improving Chapter 9 are greatly appreciated. Mr. V.K. Reddy
contributed significantly to Chapter 10. Mr. M.W.Ranadive gave useful
suggestions on some topics. We are grateful to Prof. J.Turowski, Mr. P.
Ramachandran and Prof. L.Satish for constructive comments.
Ms. Rita Lazazzaro and Ms. Dana Bigelow of Marcel Dekker, Inc., constantly
supported us and gave good editorial input.
Finally, the overwhelming support and encouragement of our family members
is admirable. S.V.Kulkarni would like to particularly mention the sacrifice made
and moral support given by his wife, Sushama.

ix
Copyright © 2004 by Marcel Dekker, Inc.


Contents

Foreword Jin Sim
Preface
Acknowledgements

iii
v
ix

1

Transformer Fundamentals
1.1
Perspective
1.2
Applications and Types of Transformers
1.3
Principles and Equivalent Circuit of a Transformer
1.4
Representation of Transformer in Power System
1.5
Open-Circuit and Short-Circuit Tests
1.6
Voltage Regulation and Efficiency
1.7
Parallel Operation of Transformers
References

1
1
5
11
20
23
25
33
34

2

Magnetic Characteristics
2.1
Construction
2.2
Hysteresis and Eddy Losses
2.3
Excitation Characteristics
2.4
Over-Excitation Performance
2.5
No-Load Loss Test
2.6
Impact of Manufacturing Processes on Core Performance
2.7
Inrush Current
2.8
Influence of Core Construction and Winding
Connections on No-Load Harmonic Phenomenon
2.9
Transformer Noise
References

35
36
42
44
46
46
54
56
67
69
72
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xii

Contents

3

Impedance Characteristics
3.1
Reactance Calculation
3.2
Different Approaches for Reactance Calculation
3.3
Two-Dimensional Analytical Methods
3.4
Numerical Method for Reactance Calculation
3.5
Impedance Characteristics of Three-Winding Transformer
3.6
Reactance Calculation for Zigzag Transformer
3.7
Zero-Sequence Reactance Estimation
3.8
Stabilizing Tertiary Winding
References

77
78
85
88
90
98
103
108
121
123

4

Eddy Currents and Winding Stray Losses
4.1
Field Equations
4.2
Poynting Vector
4.3
Eddy Current and Hysteresis Losses
4.4
Effect of Saturation
4.5
Eddy Loss in a Transformer Winding
4.6
Circulating Current Loss in Transformer Windings
References

127
128
133
137
139
141
155
166

5

Stray Losses in Structural Components
5.1
Factors Influencing Stray Losses
5.2
Overview of Methods for Stray Loss Estimation
5.3
Core Edge Loss
5.4
Stray Loss in Frames
5.5
Stray Loss in Flitch Plates
5.6
Stray Loss in Tank
5.7
Stray Loss in Bushing Mounting Plates
5.8
Evaluation of Stray Loss Due to High Current Leads
5.9
Measures for Stray Loss Control
5.10 Methods for Experimental Verification
5.11 Estimation of Stray Losses in Overexcitation Condition
5.12 Load Loss Measurement
References

169
171
182
184
185
187
192
197
199
206
214
216
218
224

6

Short Circuit Stresses and Strength
6.1
Short Circuit Currents
6.2
Thermal Capability at Short Circuit
6.3
Short Circuit Forces
6.4
Dynamic Behavior Under Short Circuits
6.5
Failure Modes Due to Radial Forces
6.6
Failure Modes Due to Axial Forces

231
232
239
240
247
251
254

Copyright © 2004 by Marcel Dekker, Inc.


Contents
6.7
6.8
6.9
6.10
6.11
6.12

xiii

Effect of Pre-Stress
Short Circuit Test
Effect of Inrush Current
Split-Winding Transformers
Short Circuit Withstand
Calculation of Electrodynamic Force Between
Parallel Conductors
6.13 Design of Clamping Structures
References

260
260
261
262
264

Surge Phenomena in Transformers
7.1
Initial Voltage Distrubution
7.2
Capacitance Calculations
7.3
Capacitance of Windings
7.4
Inductance Calculation
7.5
Standing Waves and Traveling Waves
7.6
Methods for Analysis of Impulse Distribution
7.7
Computation of Impulse Voltage Distribution
Using State Variable Method
7.8
Winding Design for Reducing Internal Overvoltages
References

277
277
282
286
298
300
303

8

Insulation Design
8.1
Calculation of Stresses for Simple Configurations
8.2
Field Computations
8.3
Factors Affecting Insulation Strength
8.4
Test Methods and Design Insulation Level (DIL)
8.5
Insulation Between Two Windings
8.6
Internal Insulation
8.7
Design of End Insulation
8.8
High-Voltage Lead Clearances
8.9
Statistical Analysis for Optimization and Quality Enhancement
References

327
328
333
335
348
351
353
356
358
361
362

9

Cooling Systems
9.1
Modes of Heat Transfer
9.2
Cooling Arrangements
9.3
Dissipation of Core Heat
9.4
Dissipation of Winding Heat
9.5
Aging and Life Expectancy
9.6
Direct Hot Spot Measurement
9.7
Static Electrification Phenomenon
References

367
368
370
375
376
380
384
385
387

7

Copyright © 2004 by Marcel Dekker, Inc.

268
270
272

306
314
321


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Contents

10 Structural Design
10.1 Importance of Structural Design
10.2 Different Types of Loads and Tests
10.3 Classification of Transformer Tanks
10.4 Tank Design
10.5 Methods of Analysis
10.6 Overpressure Phenomenon in Transformers
10.7 Seismic Analysis
10.8 Transformer Noise: Characteristics and Reduction
References

389
389
390
392
395
397
401
403
404
409

11 Special Transformers
11.1 Rectifier Transformers
11.2 Converter Transformers for HVDC
11.3 Furnace Transformers
11.4 Phase Shifting Transformers
References

411
411
417
424
428
433

12 Recent Trends in Transformer Technology
12.1 Magnetic Circuit
12.2 Windings
12.3 Insulation
12.4 Challenges in Design and Manufacture of Transformers
12.5 Computer-Aided Design and Analysis
12.6 Monitoring and Diagnostics
12.7 Life Assessment and Refurbishment
References

437
437
438
440
441
443
445
449
449

Appendix A: Fault Calculations
A1
Asymmetrical Fault with No In-Feed from LV Side
A2
Asymmetrical Fault with In-Feed from LV Side

453
454
457

Appendix B: Stress and Capacitance Formulae
B1
Stress Calculations
B2
Capacitance Calculations

459
459
467

Copyright © 2004 by Marcel Dekker, Inc.


Transformer
Engineering

Copyright © 2004 by Marcel Dekker, Inc.


1
Transformer Fundamentals

1.1 Perspective
A transformer is a static device that transfers electrical energy from one circuit to
another by electromagnetic induction without the change in frequency. The
transformer, which can link circuits with different voltages, has been instrumental
in enabling universal use of the alternating current system for transmission and
distribution of electrical energy. Various components of power system, viz.
generators, transmission lines, distribution networks and finally the loads, can be
operated at their most suited voltage levels. As the transmission voltages are
increased to higher levels in some part of the power system, transformers again
play a key role in interconnection of systems at different voltage levels.
Transformers occupy prominent positions in the power system, being the vital
links between generating stations and points of utilization.
The transformer is an electromagnetic conversion device in which electrical
energy received by primary winding is first converted into magnetic energy which
is reconverted back into a useful electrical energy in other circuits (secondary
winding, tertiary winding, etc.). Thus, the primary and secondary windings are not
connected electrically, but coupled magnetically. A transformer is termed as either
a step-up or step-down transformer depending upon whether the secondary
voltage is higher or lower than the primary voltage, respectively. Transformers can
be used to either step-up or step-down voltage depending upon the need and
application; hence their windings are referred as high-voltage/low-voltage or
high-tension/low-tension windings in place of primary/secondary windings.
Magnetic circuit: Electrical energy transfer between two circuits takes place
through a transformer without the use of moving parts; the transformer therefore
has higher efficiency and low maintenance cost as compared to rotating electrical
1
Copyright © 2004 by Marcel Dekker, Inc.


2

Chapter 1

machines. There are continuous developments and introductions of better grades
of core material. The important stages of core material development can be
summarized as: non-oriented silicon steel, hot rolled grain oriented silicon steel,
cold rolled grain oriented (CRGO) silicon steel, Hi-B, laser scribed and
mechanically scribed. The last three materials are improved versions of CRGO.
Saturation flux density has remained more or less constant around 2.0 Tesla for
CRGO; but there is a continuous improvement in watts/kg and volt-amperes/kg
characteristics in the rolling direction. The core material developments are
spearheaded by big steel manufacturers, and the transformer designers can
optimize the performance of core by using efficient design and manufacturing
technologies. The core building technology has improved from the non-mitred to
mitred and then to the step-lap construction. A trend of reduction of transformer
core losses in the last few years is the result of a considerable increase in energy
costs. The better grades of core steel not only reduce the core loss but they also
help in reducing the noise level by few decibels. Use of amorphous steel for
transformer cores results in substantial core loss reduction (loss is about one-third
that of CRGO silicon steel). Since the manufacturing technology of handling this
brittle material is difficult, its use in transformers is not widespread.
Windings: The rectangular paper-covered copper conductor is the most
commonly used conductor for the windings of medium and large power
transformers. These conductors can be individual strip conductors, bunched
conductors or continuously transposed cable (CTC) conductors. In low voltage
side of a distribution transformer, where much fewer turns are involved, the use of
copper or aluminum foils may find preference. To enhance the short circuit
withstand capability, the work hardened copper is commonly used instead of soft
annealed copper, particularly for higher rating transformers. In the case of a
generator transformer having high current rating, the CTC conductor is mostly
used which gives better space factor and reduced eddy losses in windings. When
the CTC conductor is used in transformers, it is usually of epoxy bonded type to
enhance its short circuit strength. Another variety of copper conductor or
aluminum conductor is with the thermally upgraded insulating paper, which is
suitable for hot-spot temperature of about 110°C. It is possible to meet the special
overloading conditions with the help of this insulating paper. Moreover, the aging
of winding insulation material will be slowed down comparatively. For better
mechanical properties, the epoxy diamond dot paper can be used as an interlayer
insulation for a multi-layer winding. High temperature superconductors may find
their application in power transformers which are expected to be available
commercially within next few years. Their success shall depend on economic
viability, ease of manufacture and reliability considerations.
Insulation and cooling: Pre-compressed pressboard is used in windings as
opposed to the softer materials used in earlier days. The major insulation (between
windings, between winding and yoke, etc.) consists of a number of oil ducts

Copyright © 2004 by Marcel Dekker, Inc.


Transformer Fundamentals

3

formed by suitably spaced insulating cylinders/barriers. Well profiled angle rings,
angle caps and other special insulation components are also used.
Mineral oil has traditionally been the most commonly used electrical insulating
medium and coolant in transformers. Studies have proved that oil-barrier
insulation system can be used at the rated voltages greater than 1000 kV. A high
dielectric strength of oil-impregnated paper and pressboard is the main reason for
using oil as the most important constituent of the transformer insulation system.
Manufacturers have used silicon-based liquid for insulation and cooling. Due to
non-toxic dielectric and self-extinguishing properties, it is selected as a
replacement of Askarel. High cost of silicon is an inhibiting factor for its
widespread use. Super-biodegradable vegetable seed based oils are also available
for use in environmentally sensitive locations.
There is considerable advancement in the technology of gas immersed
transformers in recent years. SF6 gas has excellent dielectric strength and is nonflammable. Hence, SF6 transformers find their application in the areas where firehazard prevention is of paramount importance. Due to lower specific gravity of
SF6 gas, the gas insulated transformer is usually lighter than the oil insulated
transformer. The dielectric strength of SF6 gas is a function of the operating
pressure; the higher the pressure, the higher the dielectric strength. However, the
heat capacity and thermal time constant of SF6 gas are smaller than that of oil,
resulting in reduced overload capacity of SF6 transformers as compared to oilimmersed transformers. Environmental concerns, sealing problems, lower
cooling capability and present high cost of manufacture are the challenges which
have to be overcome for the widespread use of SF6 cooled transformers.
Dry-type resin cast and resin impregnated transformers use class F or C
insulation. High cost of resins and lower heat dissipation capability limit the use of
these transformers to small ratings. The dry-type transformers are primarily used
for the indoor application in order to minimize fire hazards. Nomex paper
insulation, which has temperature withstand capacity of 220°C, is widely used for
dry-type transformers. The initial cost of a dry-type transformer may be 60 to 70%
higher than that of an oil-cooled transformer at current prices, but its overall cost
at the present level of energy rate can be very much comparable to that of the oilcooled transformer.
Design: With the rapid development of digital computers, the designers are freed
from the drudgery of routine calculations. Computers are widely used for
optimization of transformer design. Within a matter of a few minutes, today’s
computers can work out a number of designs (by varying flux density, core
diameter, current density, etc.) and come up with an optimum design. The real
benefit due to computers is in the area of analysis. Using commercial 2-D/3-D
field computation software, any kind of engineering analysis (electrostatic,
electromagnetic, structural, thermal, etc.) can be performed for optimization and
reliability enhancement of transformers.

Copyright © 2004 by Marcel Dekker, Inc.


4

Chapter 1

Manufacturing: In manufacturing technology, superior techniques listed below
are used to reduce manufacturing time and at the same time to improve the product
quality:
- High degree of automation for slitting/cutting operations to achieve better
dimensional accuracy for the core laminations
- Step-lap joint for core construction to achieve a lower core loss and noise level;
top yoke is assembled after lowering windings and insulation at the assembly
stage
- Automated winding machines for standard distribution transformers
- Vapour phase drying for effective and fast drying (moisture removal) and
cleaning
- Low frequency heating for the drying process of distribution transformers
- Pressurized chambers for windings and insulating parts to protect against
pollution and dirt
- Vertical machines for winding large capacity transformer coils
- Isostatic clamping for accurate sizing of windings
- High frequency brazing for joints in the windings and connections
Accessories: Bushings and tap changer (off-circuit and on-load) are the most
important accessories of a transformer. The technology of bushing manufacture
has advanced from the oil impregnated paper (OIP) type to resin impregnated
paper (RIP) type, both of which use porcelain insulators. The silicon rubber
bushings are also available for oil-to-air applications. Due to high elasticity and
strength of the silicon rubber material, the strength of these bushings against
mechanical stresses and shocks is higher. The oil-to-SF6 bushings are used in GIS
(gas insulated substation) applications.
The service reliability of on load tap changers is of vital importance since the
continuity of the transformer depends on the performance of tap changer for the
entire (expected) life span of 30 to 40 years. It is well known that the tap changer
failure is one of the principal causes of failure of transformers. Tap changers,
particularly on-load tap changers (OLTC), must be inspected at regular intervals
to maintain a high level of operating reliability. Particular attention must be given
for inspecting the diverter switch unit, oil, shafts and motor drive unit. The
majority of failures reported in service are due to mechanical problems related to
the drive system, for which improvements in design may be necessary. For service
reliability of OLTCs, several monitoring methods have been proposed, which
include measurement of contact resistance, monitoring of drive motor torque/
current, acoustic measurements, dissolved gas analysis and temperature rise
measurements.
Diagnostic techniques: Several on-line and off-line diagnostic tools are available
for monitoring of transformers to provide information about their operating
conditions. Cost of these tools should be lower and their performance reliability

Copyright © 2004 by Marcel Dekker, Inc.


Transformer Fundamentals

5

should be higher for their widespread use. The field experience in some of the
monitoring techniques is very much limited. A close cooperation between
manufacturers and utilities is necessary for developing good monitoring and
diagnostic systems for transformers.
Transformer technology is developing at a tremendous rate. The computerized
methods are replacing the manual working in the design. Continuous
improvements in material and manufacturing technologies along with the use of
advanced computational tools have contributed in making transformers more
efficient, compact and reliable. The modern information technology, advanced
diagnostic tools and several emerging trends in transformer applications are
expected to fulfill a number of existing and future requirements of utilities and
end-users of transformers.

1.2 Applications and Types of Transformers
Before invention of transformers, in initial days of electrical industry, power was
distributed as direct current at low voltage. The voltage drop in lines limited the
use of electricity to only urban areas where consumers were served with
distribution circuits of small length. All the electrical equipment had to be
designed for the same voltage. Development of the first transformer around 1885
dramatically changed transmission and distribution systems. The alternating
current (AC) power generated at a low voltage could be stepped up for the
transmission purpose to higher voltage and lower current, reducing voltage drops
and transmission losses. Use of transformers made it possible to transmit the
power economically hundreds of kilometers away from the generating station.
Step-down transformers then reduced the voltage at the receiving stations for
distribution of power at various standardized voltage levels for its use by the
consumers. Transformers have made AC systems quite flexible because the
various parts and equipment of the power system can be operated at economical
voltage levels by use of transformers with suitable voltage ratio. A single-line
diagram of a typical power system is shown in figure 1.1. The voltage levels
mentioned in the figure are different in different countries depending upon their
system design. Transformers can be broadly classified, depending upon their
application as given below.
a. Generator transformers: Power generated at a generating station (usually at
a voltage in the range of 11 to 25 kV) is stepped up by a generator transformer to
a higher voltage (220, 345, 400 or 765 kV) for transmission. The generator
transformer is one of the most important and critical components of the power
system. It usually has a fairly uniform load. Generator transformers are designed
with higher losses since the cost of supplying losses is cheapest at the generating
station. Lower noise level is usually not essential as other equipment in the
generating station may be much noisier than the transformer.
Generator transformers are usually provided with off-circuit tap changer with a

Copyright © 2004 by Marcel Dekker, Inc.


6

Copyright © 2004 by Marcel Dekker, Inc.

Chapter 1

Figure 1.1 Different types of transformers in a typical power system


Transformer Fundamentals

7

small variation in voltage (e.g., ±5%) because the voltage can always be
controlled by field of the generator. Generator transformers with OLTC are also
used for reactive power control of the system. They may be provided with a
compact unit cooler arrangement for want of space in the generating stations
(transformers with unit coolers have only one rating with oil forced and air forced
cooling arrangement). Alternatively, they may also have oil to water heat
exchangers for the same reason. It may be economical to design the tap winding as
a part of main HV winding and not as a separate winding. This may be permissible
since axial short circuit forces are lower due to a small tapping range. Special care
has to be taken while designing high current LV lead termination to avoid any hotspot in the conducting metallic structural parts in its vicinity. The epoxy bonded
CTC conductor is commonly used for LV winding to minimize eddy losses and
provide greater short circuit strength. Severe overexcitation conditions are taken
into consideration while designing generator transformers.
b. Unit auxiliary transformers: These are step-down transformers with primary
connected to generator output directly. The secondary voltage is of the order of 6.9
kV for supplying to various auxiliary equipment in the generating station.
c. Station transformers: These transformers are required to supply auxiliary
equipment during setting up of the generating station and subsequently during
each start-up operation. The rating of these transformers is small, and their
primary is connected to a high voltage transmission line. This may result in a
smaller conductor size for HV winding, necessitating special measures for
increasing the short circuit strength. The split secondary winding arrangement is
often employed to have economical circuit breaker ratings.
d. Interconnecting transformers: These are normally autotransformers used to
interconnect two grids/systems operating at two different system voltages (say,
400 and 220 kV or 345 and 138 kV). They are normally located in the
transmission system between the generator transformer and receiving end
transformer, and in this case they reduce the transmission voltage (400 or 345 kV)
to the sub-transmission level (220 or 138 kV). In autotransformers, there is no
electrical isolation between primary and secondary windings; some volt-amperes
are conductively transformed and remaining are inductively transformed.
Autotransformer design becomes more economical as the ratio of secondary
voltage to primary voltage approaches towards unity. These are characterized by a
wide tapping range and an additional tertiary winding which may be loaded or
unloaded. Unloaded tertiary acts as a stabilizing winding by providing a path for
the third harmonic currents. Synchronous condensers or shunt reactors are
connected to the tertiary winding, if required, for reactive power compensation. In
the case of an unloaded tertiary, adequate conductor area and proper supporting
arrangement are provided for withstanding short circuit forces under
asymmetrical fault conditions.

Copyright © 2004 by Marcel Dekker, Inc.


8

Chapter 1

e. Receiving station transformers: These are basically step-down transformers
reducing transmission/sub-transmission voltage to primary feeder level (e.g., 33
kV). Some of these may be directly supplying an industrial plant. Loads on these
transformers vary over wider limits, and their losses are expensive. The farther the
location of transformers from the generating station, the higher the cost of
supplying the losses. Automatic tap changing on load is usually necessary, and
tapping range is higher to account for wide variation in the voltage. A lower noise
level is desirable if they are close to residential areas.
f. Distribution transformers: Using distribution transformers, the primary feeder
voltage is reduced to actual utilization voltage (~415 or 460 V) for domestic/
industrial use. A great variety of transformers fall into this category due to many
different arrangements and connections. Load on these transformers varies
widely, and they are often overloaded. A lower value of no-load loss is desirable to
improve all-day efficiency. Hence, the no-load loss is usually capitalized with a
high rate at the tendering stage. Since very little supervision is possible, users
expect the least maintenance on these transformers. The cost of supplying losses
and reactive power is highest for these transformers.
Classification of transformers as above is based on their location and broad
function in the power system. Transformers can be further classified as per their
specific application as given below. In this chapter, only main features are
highlighted; details of some of them are discussed in the subsequent chapters.
g. Phase shifting transformers: These are used to control power flow over
transmission lines by varying the phase angle between input and output voltages
of the transformer. Through a proper tap change, the output voltage can be made
to either lead or lag the input voltage. The amount of phase shift required directly
affects the rating and size of the transformer. Presently, there are two types of
design: single-core and two-core design. Single-core design is used for small
phase shifts and lower MVA/voltage ratings. Two-core design is normally used for
bulk power transfer with large ratings of phase shifting transformers. It consists of
two transformers, one associated with the line terminals and other with the tap
changer.
h. Earthing or grounding transformers: These are used to get a neutral point that
facilitates grounding and detection of earth faults in an ungrounded part of a
network (e.g., the delta connected systems). The windings are usually connected
in the zigzag manner, which helps in eliminating third harmonic voltages in the
lines. These transformers have the advantage that they are not affected by a DC
magnetization.
i. Transformers for rectifier and inverter circuits: These are otherwise normal
transformers except for the special design and manufacturing features to take into
account the harmonic effects. Due to extra harmonic losses, operating flux density

Copyright © 2004 by Marcel Dekker, Inc.


Transformer Fundamentals

9

in core is kept lower (around 1.6 Tesla) and also conductor dimensions are smaller
for these transformers. A proper de-rating factor is applied depending upon the
magnitudes of various harmonic components. A designer has to adequately check
the electromagnetic and thermal aspects of design. For transformers used with
HVDC converters, insulation design is the most challenging design aspect. The
insulation has to be designed for combined AC-DC voltage stresses.
j. Furnace duty transformers: These are used to feed the arc or induction
furnaces. They are characterized by a low secondary voltage (80 to 1000 V) and
high current (10 to 60 kA) depending upon the MVA rating. Non-magnetic steel is
invariably used for the LV lead termination and tank in the vicinity of LV leads to
eliminate hot spots and minimize stray losses. High current bus-bars are
interleaved to reduce the leakage reactance. For very high current cases, the LV
terminals are in the form of U-shaped copper tubes of certain inside and outside
diameters so that they can be cooled by oil/water circulation from inside. In many
cases, a booster transformer is used along with the main transformer to reduce the
rating of tap-changers.
k. Freight loco transformers: These are mounted on the locomotives within the
engine compartment itself. The primary voltage collected from an overhead line is
stepped down to an appropriate level by these transformers for feeding to the
rectifiers, whose output DC voltage drives the locomotives. The structural design
should be such that it can withstand vibrations. Analysis of natural frequencies of
vibration is done to eliminate possibility of resonance.
l. Hermetically sealed transformers: This construction does not permit any
outside atmospheric air to get into the tank. It is completely sealed without any
breathing arrangement, obviating need of periodic filtration and other normal
maintenance. These transformers are filled with mineral oil or synthetic liquid as a
cooling/dielectric medium and sealed completely by having an inert gas, like
nitrogen, between the coolant and top tank plate. The tank is of welded cover
construction, eliminating the joint and related leakage problems. Here, the
expansion of oil is absorbed by the inert gas layer. The tank design should be
suitable for pressure buildup at elevated temperatures. The cooling is not effective
at the surface of oil, which is at the highest temperature. In another type of sealed
construction, these disadvantages are overcome by deletion of the gas layer. The
expansion of oil is absorbed by the deformation of the cooling system, which can
be an integral part of the tank structure.
m. Outdoor and indoor transformers: Most of the transformers are of outdoor
duty type, which have to be designed for withstanding atmospheric pollutants.
The creepage distance of bushing insulator gets decided according to the
pollution level. The higher the pollution level, the greater the creepage distance
required from the live terminal to ground. Contrary to the outdoor transformers,

Copyright © 2004 by Marcel Dekker, Inc.


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