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Transformer

Engineering

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

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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

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

v

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) Transformer designers and researchers engaged in optimization and quality-enhancement activities in today’s competitive environment (2) 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 (3) 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 no- load 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

vi 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

viii Preface

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

Transformer

Engineering

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

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 non- flammable. Hence, SF6 transformers find their application in the areas where fire- hazard 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 oil- immersed 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 oil- cooled 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.

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