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Unformatted text preview: ELECTRIC POWER TRANSFORMER ENGINEERING © 2004 by CRC Press LLC © 2004 by CRC Press LLC Library of Congress Cataloging-in-Publication Data Electric power transformer engineering / edited by James H. Harlow. p. cm. — (The Electric Power Engineering Series ; 9) Includes bibliographical references and index. ISBN 0-8493-1704-5 (alk. paper) 1. Electric transformers. I. Harlow, James H. II. title. III. Series. TK2551.E65 2004 621.31d4—dc21 2003046134 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal use, or the personal or internal use of specific clients, may be granted by CRC Press LLC, provided that $1.50 per page photocopied is paid directly to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-1704-5/04/$0.00+$1.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. With regard to material reprinted from IEEE publications: The IEEE disclaims any responsibility or liability resulting from the placement and use in the described manner. Visit the CRC Press Web site at © 2004 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-1704-5 Library of Congress Card Number 2003046134 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper © 2004 by CRC Press LLC Preface Transformer engineering is one of the earliest sciences within the field of electric power engineering, and power is the earliest discipline within the field of electrical engineering. To some, this means that transformer technology is a fully mature and staid industry, with little opportunity for innovation or ingenuity by those practicing in the field. Of course, we in the industry find that premise to be erroneous. One need only scan the technical literature to recognize that leading-edge suppliers, users, and academics involved with power transformers are continually reporting novelties and advancements that would have been totally insensible to engineers of even the recent past. I contend that there are three basic levels of understanding, any of which may be appropriate for persons engaged with transformers in the electric power industry. Depending on dayto-day involvement, the individual’s posture in the field can be described as: • Curious — those with only peripheral involvement with transformers, or a nonprofessional lacking relevant academic background or any particular need to delve into the intricacies of the science • Professional — an engineer or senior-level technical person who has made a career around electric power transformers, probably including other heavy electric-power apparatus and the associated power-system transmission and distribution operations • Expert — those highly trained in the field (either practically or analytically) to the extent that they are recognized in the industry as experts. These are the people who are studying and publishing the innovations that continue to prove that the field is nowhere near reaching a technological culmination. So, to whom is this book directed? It will truly be of use to any of those described in the previous three categories. The curious person will find the material needed to advance toward the level of professional. This reader can use the book to obtain a deeper understanding of many topics. The professional, deeply involved with the overall subject matter of this book, may smugly grin with the self-satisfying attitude of, “I know all that!” This person, like myself, must recognize that there are many transformer topics. There is always room to learn. We believe that this book can also be a valuable resource to professionals. The expert may be so immersed in one or a few very narrow specialties within the field that he also may benefit greatly from the knowledge imparted in the peripheral specialties. The book is divided into three fundamental groupings: The first stand-alone chapter is devoted to Theory and Principles. The second chapter, Equipment Types, contains nine sections that individually treat major transformer types. The third chapter, which contains 14 sections, addresses Ancillary Topics associated with power transformers. Anyone with an interest in transformers will find a great deal of useful information. © 2004 by CRC Press LLC I wish to recognize the interest of CRC Press and the personnel who have encouraged and supported the preparation of this book. Most notable in this regard are Nora Konopka, Helena Redshaw, and Gail Renard. I also want to acknowledge Professor Leo Grigsby of Auburn University for selecting me to edit the “Transformer” portion of his The Electric Power Engineering Handbook (CRC Press, 2001), which forms the basis of this handbook. Indeed, this handbook is derived from that earlier work, with the addition of four wholly new chapters and the very significant expansion and updating of much of the other earlier work. But most of all, appreciation is extended to each writer of the 24 sections that comprise this handbook. The authors’ diligence, devotion, and expertise will be evident to the reader. James H. Harlow Editor © 2004 by CRC Press LLC Editor James H. Harlow has been self-employed as a principal of Harlow Engineering Associates, consulting to the electric power industry, since 1996. Before that, he had 34 years of industry experience with Siemens Energy and Automation (and its predecessor Allis-Chalmers Co.) and Beckwith Electric Co., where he was engaged in engineering design and management. While at these firms, he managed groundbreaking projects that blended electronics into power transformer applications. Two such projects (employing microprocessors) led to the introduction of the first intelligent-electronic-device control product used in quantity in utility substations and a power-thyristor application for load tap changing in a step-voltage regulator. Harlow received the BSEE degree from Lafayette College, an MBA (statistics) from Jacksonville State University, and an MS (electric power) from Mississippi State University. He joined the PES Transformers Committee in 1982, serving as chair of a working group and a subcommittee before becoming an officer and assuming the chairmanship of the PES Transformers Committee for 1994–95. During this period, he served on the IEEE delegation to the ANSI C57 Main Committee (Transformers). His continued service to IEEE led to a position as chair of the PES Technical Council, the assemblage of leaders of the 17 technical committees that comprise the IEEE Power Engineering Society. He recently completed a 2-year term as PES vice president of technical activities. Harlow has authored more than 30 technical articles and papers, most recently serving as editor of the transformer section of The Electric Power Engineering Handbook, CRC Press, 2001. His editorial contribution within this handbook includes the section on his specialty, LTC Control and Transformer Paralleling. A holder of five U.S. patents, Harlow is a registered professional engineer and a senior member of IEEE. © 2004 by CRC Press LLC Contributors Dennis Allan Scott H. Digby James H. Harlow MerlinDesign Stafford, England Waukesha Electric Systems Goldsboro, North Carolina Harlow Engineering Associates Mentone, Alabama Dieter Dohnal Ted Haupert Hector J. Altuve Schweitzer Engineering Laboratories, Ltd. Monterrey, Mexico Maschinenfabrik Reinhausen GmbH Regensburg, Germany Gabriel Benmouyal Douglas Dorr Schweitzer Engineering Laboratories, Ltd. Longueuil, Quebec, Canada Behdad Biglar Trench Ltd. Scarborough, Ontario, Canada Wallace Binder WBBinder Consultant New Castle, Pennsylvania EPRI PEAC Corporation Knoxville, Tennessee Richard F. Dudley Trench Ltd. Scarborough, Ontario, Canada Ralph Ferraro Ferraro, Oliver & Associates, Inc. Knoxville, Tennessee Dudley L. Galloway Galloway Transformer Technology LLC Jefferson City, Missouri TJ/H2b Analytical Services Sacramento, California William R. Henning Waukesha Electric Systems Waukesha, Wisconsin Philip J. Hopkinson HVOLT, Inc. Charlotte, North Carolina Sheldon P. Kennedy Niagara Transformer Corporation Buffalo, New York Andre Lux KEMA T&D Consulting Raleigh, North Carolina Antonio Castanheira Trench Brasil Ltda. Contegem, Minas Gelais, Brazil Anish Gaikwad Arindam Maitra EPRI PEAC Corporation Knoxville, Tennessee EPRI PEAC Corporation Knoxville, Tennessee Armando Guzmán Arshad Mansoor Craig A. Colopy Cooper Power Systems Waukesha, Wisconsin Robert C. Degeneff Rensselaer Polytechnic Institute Troy, New York © 2004 by CRC Press LLC Schweitzer Engineering Laboratories, Ltd. Pullman, Washington EPRI PEAC Corporation Knoxville, Tennessee Shirish P. Mehta Paulette A. Payne Leo J. Savio Waukesha Electric Systems Waukesha, Wisconsin Potomac Electric Power Company (PEPCO) Washington, DC ADAPT Corporation Kennett Square, Pennsylvania Harold Moore H. Moore & Associates Niceville, Florida Michael Sharp Dan D. Perco Perco Transformer Engineering Stoney Creek, Ontario, Canada Dan Mulkey Pacific Gas & Electric Co. Petaluma, California H. Jin Sim Gustav Preininger Consultant Graz, Austria Randy Mullikin Kuhlman Electric Corp. Versailles, Kentucky Trench Ltd. Scarborough, Ontario, Canada Waukesha Electric Systems Goldsboro, North Carolina Robert F. Tillman, Jr. Jeewan Puri Transformer Solutions Matthews, North Carolina Alabama Power Company Birmingham, Alabama Alan Oswalt Loren B. Wagenaar Consultant Big Bend, Wisconsin America Electric Power Pickerington, Ohio © 2004 by CRC Press LLC Contents Chapter 1 Theory and Principles Dennis Allan and Harold Moore Chapter 2 Equipment Types 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Chapter 3 Power Transformers H. Jin Sim and Scott H. Digby Distribution Transformers Dudley L. Galloway and Dan Mulkey Phase-Shifting Transformers Gustav Preininger Rectifier Transformers Sheldon P. Kennedy Dry-Type Transformers Paulette A. Payne Instrument Transformers Randy Mullikin Step-Voltage Regulators Craig A. Colopy Constant-Voltage Transformers Arindam Maitra, Anish Gaikwad, Ralph Ferraro, Douglas Dorr, and Arshad Mansoor Reactors Richard F. Dudley, Michael Sharp, Antonio Castanheira, and Behdad Biglar Ancillary Topics 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 © 2004 by CRC Press LLC Insulating Media Leo J. Savio and Ted Haupert Electrical Bushings Loren B. Wagenaar Load Tap Changers Dieter Dohnal Loading and Thermal Performance Robert F. Tillman, Jr. Transformer Connections Dan D. Perco Transformer Testing Shirish P. Mehta and William R. Henning Load-Tap-Change Control and Transformer Paralleling James H. Harlow Power Transformer Protection Armando Guzmán, Hector J. Altuve, and Gabriel Benmouyal Causes and Effects of Transformer Sound Levels Jeewan Puri Transient-Voltage Response Robert C. Degeneff Transformer Installation and Maintenance Alan Oswalt Problem and Failure Investigation Wallace Binder and Harold Moore On-Line Monitoring of Liquid-Immersed Transformers Andre Lux U.S. Power Transformer Equipment Standards and Processes Philip J. Hopkinson 1 Theory and Principles Dennis Allan 1.1 1.2 1.3 1.4 Magnetic Circuit • Leakage Reactance • Load Losses • ShortCircuit Forces • Thermal Considerations • Voltage Considerations MerlinDesign Harold Moore H. Moore and Associates Air Core Transformer Iron or Steel Core Transformer Equivalent Circuit of an Iron-Core Transformer The Practical Transformer References Transformers are devices that transfer energy from one circuit to another by means of a common magnetic field. In all cases except autotransformers, there is no direct electrical connection from one circuit to the other. When an alternating current flows in a conductor, a magnetic field exists around the conductor, as illustrated in Figure 1.1. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor, as shown in Figure 1.2, then a voltage is induced into the second conductor. The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based. 1.1 Air Core Transformer Some small transformers for low-power applications are constructed with air between the two coils. Such transformers are inefficient because the percentage of the flux from the first coil that links the second coil is small. The voltage induced in the second coil is determined as follows. E = N dJ/dt 108 (1.1) where N is the number of turns in the coil, dJ/dt is the time rate of change of flux linking the coil, and J is the flux in lines. At a time when the applied voltage to the coil is E and the flux linking the coils is J lines, the instantaneous voltage of the supply is: e = ˜2 E cos [t = N dJ/dt 108 (1.2) dJ/dt = (˜2 cos [t 108)/N (1.3) The maximum value of J is given by: J = (˜2 E 108)/(2 T f N) Using the MKS (metric) system, where J is the flux in webers,  © 2004 by CRC Press LLC (1.4) Current carrying conductor Flux lines FIGURE 1.1 Magnetic field around conductor. Flux lines Second conductor in flux lines FIGURE 1.2 Magnetic field around conductor induces voltage in second conductor. E = N dJ/dt (1.5) J = (˜2E)/(2 T f N) (1.6) and Since the amount of flux J linking the second coil is a small percentage of the flux from the first coil, the voltage induced into the second coil is small. The number of turns can be increased to increase the voltage output, but this will increase costs. The need then is to increase the amount of flux from the first coil that links the second coil. 1.2 Iron or Steel Core Transformer The ability of iron or steel to carry magnetic flux is much greater than air. This ability to carry flux is called permeability. Modern electrical steels have permeabilities in the order of 1500 compared with 1.0 for air. This means that the ability of a steel core to carry magnetic flux is 1500 times that of air. Steel cores were used in power transformers when alternating current circuits for distribution of electrical energy were first introduced. When two coils are applied on a steel core, as illustrated in Figure 1.3, almost 100% of the flux from coil 1 circulates in the iron core so that the voltage induced into coil 2 is equal to the coil 1 voltage if the number of turns in the two coils are equal. Continuing in the MKS system, the fundamental relationship between magnetic flux density (B) and magnetic field intensity (H) is: © 2004 by CRC Press LLC Flux in core Steel core Exciting winding Second winding FIGURE 1.3 Two coils applied on a steel core. B = Q0 H (1.7) where Q0 is the permeability of free space | 4T v 10–7 Wb A–1 m–1. Replacing B by J/A and H by (I N)/d, where J = core flux in lines N = number of turns in the coil I = maximum current in amperes A = core cross-section area the relationship can be rewritten as: J = (Q N A I)/d (1.8) where d = mean length of the coil in meters A = area of the core in square meters Then, the equation for the flux in the steel core is: J = (Q0 Qr N A I)/d (1.9) whereQr = relative permeability of steel } 1500. Since the permeability of the steel is very high compared with air, all of the flux can be considered as flowing in the steel and is essentially of equal magnitude in all parts of the core. The equation for the flux in the core can be written as follows: J = 0.225 E/fN (1.10) where E = applied alternating voltage f = frequency in hertz N = number of turns in the winding In transformer design, it is useful to use flux density, and Equation 1.10 can be rewritten as: B = J/A = 0.225 E/(f A N) where B = flux density in tesla (webers/square meter). © 2004 by CRC Press LLC (1.11) 1.3 Equivalent Circuit of an Iron-Core Transformer When voltage is applied to the exciting or primary winding of the transformer, a magnetizing current flows in the primary winding. This current produces the flux in the core. The flow of flux in magnetic circuits is analogous to the flow of current in electrical circuits. When flux flows in the steel core, losses occur in the steel. There are two components of this loss, which are termed “eddy” and “hysteresis” losses. An explanation of these losses would require a full chapter. For the purpose of this text, it can be stated that the hysteresis loss is caused by the cyclic reversal of flux in the magnetic circuit and can be reduced by metallurgical control of the steel. Eddy loss is caused by eddy currents circulating within the steel induced by the flow of magnetic flux normal to the width of the core, and it can be controlled by reducing the thickness of the steel lamination or by applying a thin insulating coating. Eddy loss can be expressed as follows: W = K[w]2[B]2 watts (1.12) where K = constant w = width of the core lamination material normal to the flux B = flux density If a solid core were used in a power transformer, the losses would be very high and the temperature would be excessive. For this reason, cores are laminated from very thin sheets, such as 0.23 mm and 0.28 mm, to reduce the thickness of the individual sheets of steel normal to the flux and thereby reducing the losses. Each sheet is coated with a very thin material to prevent shorts between the laminations. Improvements made in electrical steels over the past 50 years have been the major contributor to smaller and more efficient transformers. Some of the more dramatic improvements include: • • • • • • Development of cold-rolled grain-oriented (CGO) electrical steels in the mid 1940s Introduction of thin coatings with good mechanical properties Improved chemistry of the steels, e.g., Hi-B steels Further improvement in the orientation of the grains Introduction of laser-scribed and plasma-irradiated steels Continued reduction in the thickness of the laminations to reduce the eddy-loss component of the core loss • Introduction of amorphous ribbon (with no crystalline structure) — manufactured using rapidcooling technology — for use with distribution and small power transformers The combination of these improvements has resulted in electrical steels having less than 40% of the noload loss and 30% of the exciting (magnetizing) current that was possible in the late 1940s. The effect of the cold-rolling process on the grain formation is to align magnetic domains in the direction of rolling so that the magnetic properties in the rolling direction are far superior to those in other directions. A heat-resistant insulation coating is applied by thermochemical treatment to both sides of the steel during the final stage of processing. The coating is approximately 1-Qm thick and has only a marginal effect on the stacking factor. Traditionally, a thin coat of varnish had been applied by the transformer manufacturer after co...
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