POWER ENGINEERING Series Editor H. Lee Willis ABB Electric Systems Technology Institute Raleigh, North Carolina 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. AArainy, 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 DeRegulation, Lorrin Philipson and H. Lee Willis 7. Electrical Power Cable Engineering, William A. Thue 8. Electric Power System Dynamics and Stability, James A. Momoh and Mohamed E. ElHawary 9. Insulation Coordination for Power Systems, Andrew R. Hileman
Insulation Coordination for Power Systems Andrew R Hileman Taylor & Francis Taylor &Francis Group Boca Raton London New York A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa pic Copyright 1999 by Taylor & Francis Group, LLC POWER ENGINEERING Series Editor H Lee Willis ABB Electric Systems Technology Institute Raleigh, North Carolina Power Distribution Planning Reference Book, H Lee Willis Transmission Network Protection: Theory and Practice, Y G Paithankar Electrical Insulation in Power Systems, N H Malik, A A A/-Arainy, and M I Qureshi Electrical Power Equipment Maintenance and Testing, Paul Gill Protective Relaying: Principles and Applications, Second Edition, J Lewis Blackburn Understanding Electric Utilities and De-Regulation, Lorrin Philipson and H Lee Willis Electrical Power Cable Engineering, William A Thue Electric Power System Dynamics and Stability, James A Momoh and Mohamed E El-Hawary Insulation Coordination for Power Systems, Andrew R Hileman ADDITIONAL VOLUMES IN PREPARATION Copyright 1999 by Taylor & Francis Group, LLC Published in 1999 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 1999 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S Government works Printed in the United States of America on acid-free paper The disks mentioned in this book are now available for download on the CRC Web site International Standard Book Number-10: 0-8247-9957-7 (Hardcover) International Standard Book Number-13: 978-0-8247-9957-1 (Hardcover) 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 No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe Library of Congress Cataloging-in-Publication Data Catalog record is available from the Library of Congress Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com informa Taylor & Francis Group is the Academic Division of T&F Tnforma plc Copyright 1999 by Taylor & Francis Group, LLC and the CRC Press Web site at http://www.crcpress.com The disks mentioned in this book are now available for download on the CRC Web site Power engineering is the oldest and most traditional of the various areas within electrical engineering, yet no other facet of modern technology is currently undergoing a more dramatic revolution in technology and industry structure Deregulation, along with the wholesale and retail competition it fostered, has turned much of the power industry upside down, creating demands for new engineering methods and technology at both the system and customer levels Insulation coordination, the topic of this latest addition to the Marcel Dekker, Inc., Power Engineering series, has always been a cornerstone of sound power engineering, since the first interconnected power systems were developed in the early 20th century The changes being wrought by deregulation only increase the importance of insulation coordination to power engineers Properly coordinated insulation strength throughout the power system is an absolute requirement for achieving the high levels of service customers demand in a competitive energy market, while simultaneously providing the long-term durability and low cost required by electric utilities to meet their operating and financial goals Certainly no one is more the master of this topic than Andrew R Hileman, who has long been recognized as the industry's leader in the application of insulation coordination engineering methods His Insulation Coordination for Power Systems is an exceedingly comprehensive and practical reference to the topic's intricacies and an excellent guide on the best engineering procedures to apply At both introductory and advanced levels, this book provides insight into the philosophies and limitations of insulation coordination methods and shows both a rich understanding of the structure often hidden by nomenclature and formula and a keen sense of how to deal with these problems in the real world Having had the pleasure of working with Mr Hileman at Westinghouse for a number of years in the 1980s, and continuously since then in The Pennsylvania State Copyright 1999 by Taylor & Francis Group, LLC vi Series Introduction University's power engineering program, it gives me particular pleasure to see his expertise and knowledge included in this important series of books on power engineering Like all the books planned for the Power Engineering series, Insulation Coordinationfor Power Systems presents modern power technology in a context of proven, practical application It is useful as a reference book as well as for self-study and advanced classroom use The Power Engineering series will eventually include books covering the entire field of power engineering, in all its specialties and subgenres, all aimed at providing practicing power engineers with the knowledge and techniques they need to meet the electric industry's challenges in the 21st century H Lee Willis Copyright 1999 by Taylor & Francis Group, LLC This book is set up as a teaching text for a course on methods of insulation coordination, although it may also be used as a reference book The chapter topics are primarily divided into line and station insulation coordination plus basic chapters such as those concerning lightning phenomena, insulation strength, and traveling waves The book has been used as a basis for a credit hour, 48 contact hour course Each chapter requires a lecture time of from to hours To supplement the lecture, problems assigned should be reviewed within the class On an average, this requires about to hours per chapter The problems are teaching problems in that they supplement the lecture with new material-that is, in most cases they are not considered specifically in the chapters The book is based on a course that was originally taught at the Westinghouse Advanced School for Electric Utility Engineers and at Carnagie-Mellon University (Pittsburgh, PA) After retirement from Westinghouse in 1989, I extensively revised and added new materials and new chapters to the notes that I used at Westinghouse Thus, this is essentially a new edition There is no doubt that the training and experience that I had at Westinghouse are largely responsible for the contents The volume is currently being used for a 48 contact hour course in the Advanced School in Power Engineering at Pennsylvania State University in Monroeville In addition, it has been used for courses taught at several U.S and international utilities For a one-semester course, some of the chapters must be skipped or omitted Preferably, the course should be a two-semester one As may be apparent from the preceding discussion, probabilistic and statistical theory is used extensively in the book In many cases, engineers either are not familiar with this subject or have not used it since graduation Therefore, some introduction to or review of probability and statistics may be beneficial At Copyright 1999 by Taylor & Francis Group, LLC viii Preface Pennsylvania State University, this Insulation Coordination course is preceded by a 48 contact hour course in probability and statistics for power system engineers, which introduces the student to the stress-strength principle The IEEE 1313.2 Standard, Guide for the Application of Insulation Coordination, is based on the material contained in this book I would like to acknowledge the encouragement and support of the Westinghouse Electric Corporation, Asea Brown Boveri, the Electric Power Research Institute (Ben Damsky), Duke Energy (Dan Melchior, John Dalton), and Pennsylvania State University (Ralph Powell, James Bedont) The help from members of these organizations was essential in production of this book The education that I received from engineers within the CIGRE and IEEE committees and working groups has been extremely helpful My participation in the working groups of the IEEE Surge Protective Devices Committee, in the Lightning working group of the IEEE Transmission and Distribution Committee, and in CIGRE working groups 33.01 (Lightning) and 33.06 (Insulation Coordination) has been educational and has led to close friendships To all engineers, I heartily recommend membership in these organizations and encourage participation in the working groups Also to be acknowledged is the influence of some of the younger engineers with whom I have worked, namely, Rainer Vogt, H W (Bud) Askins, Kent Jaffa, N C (Nick) AbiSamara, and T E (Tom) McDermott Tom McDermott has been especially helpful in keeping me somewhat computer-literate I have also been tremendously influenced by and have learned from other associates, to whom I owe much Karl Weck, Gianguido Carrara, and Andy Ericksson form a group of the most knowledgeable engineers with whom I have been associated And finally, to my wife, Becky, and my childern, Judy, Linda, and Nancy, my thanks for "putting up" with me all these years Andrew R Hileman Copyright 1999 by Taylor & Francis Group, LLC Series Introduction Preface Introduction H Lee Willis Specifying the Insulation Strength Insulation Strength Characteristics Phase-Ground Switching Overvoltages, Transmission Lines Phase-Phase Switching Overvoltages, Transmission Lines Switching Overvoltages, Substations The Lightning Flash Shielding of Transmission Lines Shielding of Substations A Review of Traveling Waves The Backflash Appendix Effect of Strokes within the Span Appendix Impulse Resistance of Grand Electrodes Appendix Estimating the Measured Forming Resistance Copyright 1999 by Taylor & Francis Group, LLC Contents Appendix Effect of Power Frequency Voltage and Number of Phases The Incoming Surge and Open Breaker Protection Metal Oxide Surge Arresters Appendix Protective Characteristics of Arresters Station Lightning Insulation Coordination Appendix Surge Capacitance Appendix Evaluation of Lightning Surge Voltages Having Nonstandard Waveshapes: For Self-Restoring Insulations Line Arresters Induced Overvoltages Contamination National Electric Safety Code Overview: Line Insulation Design Appendix Computer Programs for This Book Copyright 1999 by Taylor & Francis Group, LLC Chapter 17 750 Clearances in Stations Based on Switching Surges for Maximum System Voltages of 362 to 800 kV [1] Table Max system Voltage, kV 362 Max SOV per unit Max SOV kV Guard Clearance, m Vertical clearance to live parts, m Horizontal clearance to live parts, m 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 650 680 709 739 768 798 828 857 887 808 853 898 943 988 1033 1078 1123 1167 1212 980 1045 1110 1176 1241 1306 1372 1437 1502 1567 2.13 2.30 2.45 2.60 2.80 3.0 3.2 3.4 3.6 3.2 3.4 3.6 4.0 4.3 4.6 4.9 5.3 5.6 6.0 4.2 4.7 5.2 5.7 6.2 6.8 7.4 8.0 8.6 9.2 4.7 4.9 5.0 5.2 5.4 5.6 5.8 6.0 6.1 5.7 5.9 6.2 6.6 6.9 7.2 7.5 7.9 8.2 8.6 6.8 7.3 7.8 8.3 8.8 9.4 10.0 10.6 11.2 11.8 3.0 3.2 3.4 3.6 3.7 3.9 4.1 4.3 4.5 4.1 4.3 4.6 4.9 5.2 5.5 5.8 6.2 6.6 7.0 5.1 5.6 6.1 6.6 7.2 7.7 8.3 8.9 9.5 10.0 Source: Ref that the BIL is that required by lightning, or better, the clearance could be given as a function of the lightning overvoltage In this regard, NESC states that "where surge protective devices are applied to protect the live parts, the vertical clearances may be reduced provided the clearance is not less than 2.6m (8.5ft) plus the electrical clearance between energized parts and ground as limited by the surge protective devices." The electrical clearance is interpreted to mean the guard clearance Thus, for example, assume that an arrester rated 209 kV MCOV is applied in a 362-kV station The discharge voltage is 665 kV and the maximum voltage in the station is 750 kV Then the vertical clearance is 2.6 m plus the guard or electrical clearance for 750 kV Assuming that the BIL gradient is 500 kV/m, the vertical clearance becomes 2.6 1.5 = 4.1 m To compare, for 1050-kV and 1300-kV BILs, Table 10 gives clearances of 5.2 and 5.7m, respectively Thus, for the usual case of arresters in the station, the clearance can be reduced + Copyright 1999 by Taylor & Francis Group, LLC 75 National Electric Safety Code Table 10 Clearances in Stations Based on Lightning [I] BIL Guard clearance kV mm Vertical clearance to live parts, m Horizontal clerance to live parts, m Source: Ref a For 242-kV max system voltage For a 362-kV max system voltage The guard clearance in terms of the BIL gradient varies from 941 kV/m for 95kV BIL to 484 kV/m for 1550-kV BIL For BILs of 750 to 2050 kV, the BIL gradient ranges from 484 to 568 kV/m and averages 518 kV/m This value is not significantly different from that provided in Chapter 2, where the BIL gradient, taken from IEC Standard 273 [7], is 450 kV/m for BILs of 850 to 2550 kV No increase in clearance is specified for high altitude However, the previous corrections for altitude should be applied CONCLUSIONS Working clearances in stations and lines meet the objective of the NESC, i.e., "practical safeguarding of persons during the installation, operation, or maintenance of electric supply and communication lines and associated equipment." In contrast, however, the specification of design clearances or strike distances as in Section appear to be outside the objective of the NESC Therefore these requirements should be removed If the specification of design clearances is maintained in the NESC, the design swing angle needs revision, see Chapter Working clearances may exceed the strike distance based on a probabilistic design Thus the NESC clearances may be the limiting design criterion Altitude correction factors are in need of coordination throughout the NESC In addition, as in apparatus standards, the assumption is made that no correction is Copyright 1999 by Taylor & Francis Group, LLC 752 Chapter necessary for altitudes below 1000m or 450 m Since all or most data concerning the impulse strength of air or insulators has been corrected to sea level conditions, some correction appears necessary for altitudes above sea level REFERENCES National Electric Safety Code, American National Standard C2, 1997 L Paris and R Cortina, "Switching and Lightning Impulse Discharge Characteristics of Large Air Gaps and Long Insulator Strings," ZEEE Trans on PA&S, Apr 1968, pp 947-957 G Gallet, G LeRoy, R Lacey, and I Kromel, "General Expression for Positive Switching Impulse Strength up to Extra Long Air Gaps," ZEEE Trans on PA&S, Nov/Dec 1975, pp 1989-1993 IEEE Standard 156-1987 and IEEE Standard 156-1995, "IEEE Guide for Maintenance Methods on Energized Power Lines." A R Hileman, W C Guyker, H M Smith, and G E Grosser, Jr, "Line Insulation Design for APS 500-kV System," ZEEE Trans on PA&S, Aug 1967, pp 987-994 E J Yasuda, and F B Dewey, "BPA's New Generation of 500-kV Lines," IEEE Trans on PA&S, 1980, pp 616-624 IEC Standard 273, "Characteristics of Indoor and Outdoor Post Insulators for Systems with Nominal Voltages Greater than 1000V," 1990 Copyright 1999 by Taylor & Francis Group, LLC Overview: Line Insulation Design INTRODUCTION As discussed in the introduction to this book, a study of transmission insulation design results in the following specifications: The phase-to-grounded-tower strike distance, referred to in this chapter as simply the "strike distance" The number and location of overhead shield wires The need for and type of supplemental grounding The number and type of insulators and the insulator string length The need for, rating, and location of line surge arresters The phase-phase strike distance In specifying these quantities, the stresses imposed by lightning, switching surges, and contamination must be considered (For lines having system voltages of 230 kV or less, switching surges need not be considered.) Each of these subjects has been discussed in this book and methods have been suggested so that the design values can be obtained The objective of this final chapter is to compare these specifications for EHV and UHV transmission lines so as to ascertain which stress dominates the design In this comparison, only single circuit lines that have grounded tower members between the phases are considered Therefore the specification of the phasephase strike distance is unnecessary Also, although the option of line surge arresters is a viable option, it will not be considered in the comparison In review, a study of lightning, switching surges, and contamination results in the following specifications: Copyright 1999 by Taylor & Francis Group, LLC Chapter 18 Lightning a The number and location of the overhead shield wires b The need for and the type of supplemental grounding c The insulator string length d The strike distance Switching surges a The strike distance b The insulator string length Contamination a The insulator string length b The number and type of insulators As noted, the strike distance and insulator string length are each specified in the studies of lightning, switching surges, and contamination The maximum value of the strike distance and insulator string length as obtained from each of these three study areas is the design specification Economically desirable is that each of the studies results in identical values of strike distance and insulator string length COMPARISON OF DESIGN Except for the more recent Bonneville Power Administration (BPA) 500-kV design [I], all 500- and 765-kV lines have been designed using the deterministic method of Chapter However, there has never been a reported switching surge flashover on any of these lines [2] The conclusion is that the strike distance can be decreased and that a probabilistic design criterion should be used In Chapter 3, the probabilistic design for switching overvoltages was developed and compared to the deterministic design The SSFOR suggested was one flashover per 100 breaker closings, which is becoming a de facto design standard Using this design value, the strike distance can be significantly decreased from that obtained using the deterministic method In 1980, authors from BPA reported that their new 500-kV lines was designed for one flashover per 10 breaker closings, and again no flashovers have been reported [I] This new design provides further proof that the strike distance can be reduced while maintaining an acceptable switching surge performance Economically, reduction of the strike distance can result in considerable savings, estimated in 1980 at $30,000 to $40,000 per km per one meter reduction in strike distance at 550 kV [2] Assuming that a half-meter reduction in strike distance is possible for a 1000-km system, the savings would be $15million dollars The savings for the entire USA 550-kV system approaches half a billion dollars Thus a strong incentive exists However, the question of whether strike distance can be decreased is not fully answered, since the strike distance as used for a transmission line depends on whether lightning, switching, or power frequency voltage dictates design This question is better considered by the aid of Fig 1, where the approximate design requirements of a tower, specified in terms of strike distance, are shown as a function of maximum system voltage for the three criteria, lightning, switching surge, and power frequency voltage Before reaching any conclusions, each of the design areas will be discussed, after which they will be considered together Copyright 1999 by Taylor & Francis Group, LLC Overview: Line Insulation Design Switching " Contamination Ñ 300 500 700 900 1100 1300 Maximum System Voltage, kV Figure Comparison of requirements from switching surges, lightning, and power frequency voltage 2.1 Lightning Using the CIGRE method, the lightning curve or band is constructed for a flashover rate of 0.6 flashover per 100 km-years and a tower footing resistance of 20 ohms with a soil resistivity of 400 ohm-meters The upper portion of the band assumes a ground flash density of 4.0 flashes/km2-year and the lower portion, 8.0 flasheslkm-year The lightning curve is relatively flat, as it should be since if a personality is ascribed to lightning, it does not care whether it hits a 362-kV line, a 550-kV line, or a 1200-kV line Therefore the lightning requirement should be relatively constant with system voltage However, tower heights increase and coupling factors decrease with increasing system voltage These effects, along with the increase in power frequency voltage, combine to produce a gentle increase in the curve 2.2 Switching Overvoltages Using the techniques of Chapter 3, the strike distance required for switching overvoltages are shown by curves assuming (1) 500 towers and (2) a Gaussian stress distribution and for statistical overvoltages E2 of 2.6, 1.8, and 1.4 per unit An E2 of 2.6 per unit represents a typical value for high-speed reclosing of breakers without a preinsertion resistor; 1.8 per unit represents a typical value for high-speed reclosing Copyright 1999 by Taylor & Francis Group, LLC 756 Chapter 18 with a single preinsertion resistor; and 1.4 per unit represents a value for a breaker with possibly one or two preinsertion resistors or with controlled closing The assumed standard deviation ay of the overvoltage distribution is The switching overvoltage profile is assumed to be Each of the curves sweeps sharply upward portraying the plot of the strike distance as a function of the CFO Interestingly, since 1968, BPA has purchased circuit breakers specified to limit the statistical switching overvoltage to 1.5 per unit In 1976, a report on field test of breakers from six manufacturers was presented [3] All breakers tested limited the statistical overvoltage to 1.5 p.u or less, and four of the breakers limited the statistical overvoltage to less than 1.4p.u Five of the breakers had two-step resistors, one had three-step resistor, and four breakers used synchronously controlled closing devices Using a transient program, studies were performed before these tests but were deemed conservative since they did not simulate all conditions of resistor insertion If these conditions were simulated, the author estimated that the overvoltages obtained by the simulation would have decreased by 0.05 to 0.15 p.u Using the 0.5 p.u decrease, the simulation results indicated that all breakers limited the statistical overvoltage to 1.5p.u and three breakers limited the statistical overvoltage to 1.4p.u or less Thus limitation of the switching overvoltage to within 1.5 p.u has been achieved, and limitation to 1.4p.u is possible 2.3 Power Frequency Voltage Using the IEEE equations of Chapter 16, the power frequency voltage requirements are shown as a function of the IEEE contamination levels of very light (0.03 mg/cm2, 20 mm/kV), light (0.06mg/cm2, 24 mm/kV), to moderate (0.10 mg/cm2, 28 mrn/kV), and heavy (0.30 mg/cm2, 32 mm/kV) Use of ceramic 146 x 254mm insulators in Vstrings is assumed The maximum string length is usually greater than the strike distance For a 90' V-string, the length of the insulator could be 1.414 times the string length However, attachment hardware and gusset plates impinge on this distance so that the string length is decreased Therefore the maximum string length is assumed at 1.25 times the strike distance As noted, the curve rises in a linear fashion and thus a linear relationship is assumed between string length and specific creep distance Two of the curves are labeled with two levels of contamination The first label refers to ceramic insulators, while the second in parentheses applies to nonceramic insulators Assumed is that the string length for nonceramic insulators may be 67% of that for ceramic insulators Copyright 1999 by Taylor & Francis Group, LLC Overview: Line Insulation Design 2.4 Comparison Figure provides the overall concept The strike distance is that for the center phase However, to examine each standard system voltage level, Table is more useful From Fig and Table 1: 362 kV Lightning requires a strike distance of 2.1 to 2.3 m, which is approximately equal to that for a 2.6p.u statistical switching overvoltage, 2.2m This strike distance is also appropriate for moderate contamination using ceramic insulators For heavy contamination, nonceramic insulators appear as an excellent choice Thus, for this voltage, lightning, switching surge, and contamination requirements require about the same strike distance, and an optimum design is achieved 550 kV If the switching overvoltage design is based on a statistical overvoltage of 2.6 p.u., the switching overvoltage dominates the design requiring a strike distance of 4.0 m, whereas lightning requires only 2.5 to 2.7 m To prevent switching surges from dictating the design, a preinsertion resistor is used in the breaker, decreasing the statistical overvoltage to 1.8 p.u and decreasing the required strike distance to 2.4m Note that the switching surge requirement is now less than that for lightning At the strike distance of 2.5 to 2.7 m, ceramic insulators could be used in very light to light contamination severities, or nonceramic insulators could be used for moderate to heavy severities Thus lightning appears to dictate the design Table Required Strike Distance, m Tech area Criteria 362 kV Switching 1FO/100 surges 2.6p.u =2.2 Lightning 550 kV 800 kV 2.6p.u =4.0 1.8p.u = 4.1 1.8p.u = 2.4 1.4p.u = 2.9 1200kV 1.8p.u = 8.1 1.4p.u = 5.4 0.6FO/100 ( N , = 4) = 2.1 ( N g= 4) = 2.5 ( N = 4) = 3.0 ( N , = 4) = 3.5 km-years (Ag= 8) = 2.3 ( N , = 8) = 2.7 ( N g= 8) = 3.2 ( N , = 8) = 3.8 Power Frequency Ceramic Very light Light Moderate Heavy Nonceramic Moderate Heavy SOR unsuccessful 2.6p.u.=2.0 reclose/lO Yrs Source: From Fig Copyright 1999 by Taylor & Francis Group, LLC 2.6p.u.=2.8 1.8p.u = 2.2 1.8p.u.=3.3 1.4p.u = 2.8 1.8p.u.=4.2 1.4p.u = 4.1 Chapter 18 758 800 kV If the switching overvoltage design is based on a statistical overvoltage of 1.8 p.u., the switching overvoltage dominates the design, requiring a strike distance of 4.1 m, whereas lightning requires only 3.0 to 3.2m If the statistical switching overvoltage can be reduced to 1.4p.u., the required strike distance becomes 2.9m, which is slightly less than the lightning requirements of 3.0 to 3.2m Thus again, lightning becomes important If the 1.4 p.u switching surge design is practical, and strike distances of 3.0 to 3.2 are used, nonceramic insulators should be used and are acceptable for light to moderate contamination areas If the 1.8 p.u design is used, the strike distance of 4.1 m encompasses heavy contamination conditions for nonceramic and very light to light for ceramic insulators 1200 kV For a statistical switching overvoltage of 1.8 p.u an 8.1 m strike distance is required, far in excess of that required by lightning, 3.5 to 3.8 m At an 8.1 m strike distance, even ceramic insulation may be used in heavy contamination areas If a statistical switching overvoltage of 1.4 p.u is achievable, a 5.4 m strike distance is estimated This is still greater than that for lightning, and the use of nonceramic insulators is required, except for very light contamination areas To summarize For designs at 362 and 550 kV and possibly at 800 kV, switching surges not dictate design Rather, lightning is the most important requirement and dictates design Because of the innovative development of nonceramic insulators, requirements for contamination have been substantially reduced Unless switching overvoltages are reduced below 1.8 p.u., they become the dominant criterion at 800 kV At 1200 kV, even with the statistical switching overvoltage held at 1.4p.u and the use of nonceramic insulators, switching surges remain as the dominant design criterion Thus the conclusion to this point is that at transmission voltages at 550 kV or less or possibly 800 kV or less, lightning remains the dominant design criterion, and only at 1200 kV does switching overvoltage replace lightning as the dominant criterion From a philosophical viewpoint, this appears reasonable Switching surges are man-made, so they can be man-controlled, while lightning is a phenomenon of nature that must be accepted and mitigating measures employed Returning to the original question of whether strike distances can be reduced, at 550 kV, within the U.S., strike distances of 3.35 to 4.0m are in common use Figure and Table indicate that these distances can be reduced to 2.5 to 2.7m Further reductions may be possible in areas having lower ground flash densities For instance, BPA has announced a new advanced design for single- and double-circuit 550-kV lines located in areas having ground flash densities of 0.7 to 1.2 flashes/km2yr [I] Using V-strings, a minimum clearance of 2.5m is specified This clearance represents a strike distance of 2.24 m plus a hand clearance around tower members of 0.30 m This strike distance is based on a statistical switching overvoltage of 1.7 p.u Based on contamination, eighteen 159 x 280 nlm insulators are used From Fig 1, the estimated required strike distance for a statistical switching overvoltage of 1.8p.u is about 2.4m, which compares favorably to the BPA design Therefore Copyright 1999 by Taylor & Francis Group, LLC Overview: Line Insulation Design 759 the conclusion is that strike distances not only can be reduced by over 0.5 m but are presently being reduced by one utility At 800 kV, in the USA, strike distances of about 4.9m are used for the center phase position From Fig 1, based on a statistical switching overvoltage of 1.8 p.u., a strike distance of 4.1 m is shown, a decrease of 0.8 m Here again, the strike distance can be reduced If the statistical switching overvoltage is reduced to 1.4p.u., a reduction to 3.0 to 3.2m appears possible, a 37% reduction Therefore, in answer to the original question, strike distances can be reduced, resulting in considerable savings COMPARISON BASED ON SOR Another method upon which to base a comparison, introduced in Chapter 3, is the storm outage rate or the number of unsuccessful reclosures This method combines the lightning flashover rate and the switching surge flashover rate That is, the sequence of events is Lightning causes a flashover The breaker opens to clear the fault The breaker recloses, producing a switching surge A flashover occurs caused by the switching surge The breaker locks open Therefore an unsuccessful reclosure occurs, which is called a storm outage The advantage of this method is that it accounts for areas having a low ground flash density, which would result in a lower number of breaker reclosures The concept is not new If there is something new, it is the numerical evaluation The SOR is calculated by the multiplication of the lighting flashover rate for the entire line by the SSFOR If the lightning flashover rate or the BFR is in units of flashovers per year and the SSFOR is in units of flashover per 100 breaker operations, then the SOR is in units of unsuccessful reclosures per 100 years Using the same parameters as before, the curve of Fig demonstrates the process for a 200-km, 550-kV line having a statistical switching overvoltage of 1.8 p.u and Ng of f l a ~ h e s / k m ~ - ~ r Curves similar to those of Fig are shown in Fig for an SOR of one unsuccessful reclose in 10 years, assuming a 200-km line The results are somewhat easier to analyze using Table 362 kV For a 2.6 p.u statistical switching overvoltage, strike distance can be reduced from 2.1-2.3m to 2.0m, a 10% reduction 550 kV For a 1.8p.u statistical switching overvoltage, the strike distance can be reduced from 2.5-2.7 to 2.2m, a 15% reduction 800 kV For a 1.8p.u statistical switching overvoltage, the strike distance can be reduced from 4.1 m to 3.3 m, a 20% reduction For a 1.4p.u statistical switching overvoltage, the strike distance can be reduced from 3.0-3.2m to 2.8m, a 10% reduction Copyright 1999 by Taylor & Francis Group, LLC Chapter 18 Strike Distance, meters Figure Developing the storm outage rate for a 500-kV, 200-km line, l.8p.u statistical overvoltage, Ng = (E2 = 1.8 p.u., sigma/E2 = 7.6%, ES/ER = 0.88, 200 km, 400-m span n = 500, Na = 1200kV For a 1.8 p.u statistical switching overvoltage, the strike distance can be reduced from 8.1 m to 4.2 m, a 25% reduction For a 1.4p.u statistical switching overvoltage, the strike distance can be reduced from 5.4 m to 4.1 m, a 25% reduction However, even the use of nonceramic insulators does not permit the use of strike 6- , , ,, , , ,, 5- , 0 CA ' 4- ,- , d +QB en 3- 2- , 0 , -SOR, 1/10 yrs 1- $00 Figure - - - Contamination 500 700 900 Maximum System Voltage, kV Comparison based on storm outage rate Copyright 1999 by Taylor & Francis Group, LLC 1100 Overview: Line Insulation Design distances of 4.1 to 4.2m, except for light contamination conditions For the lightning criteria employed, except for 1200kV, the use of the storm outage rate of one unsuccessful reclosure in 10 years indicates that strike distance can be further reduced by from 10 to 15% However, the concept of design based on the storm outage rate may require some modification, since faults caused by lightning may result in voltage dips that decrease power quality In this case, both the SOR and the lightning flashover rate need consideration CONCLUSIONS The general curves of Figs and are presented to provide an overall view of line insulation requirements Although admittedly they represent only crude estimates and should not be used for the design of a specific line, they permit an overall conclusion Technically, strike distances can be reduced, and economically, large incentives exist to reduce them The alternate design criterion, the storm outage rate, combines both types of overvoltage, lightning and switching Comparison of this criterion with the requirements of power frequency voltage illustrates the importance of the latter Hopefully this presentation has achieved its unstated primary objective: to show that in general, switching surges should not be and are not the dominant design criterion and that, except at 1200kV, lightning constitutes the primary concern This conclusion is partially due to the innovative development of nonceramic insulators but is also due to the control of switching surges This is not to say that switching surges can be neglected or not considered but is meant to illustrate the progress of the industry within the last 40 years In this period, more information has been amassed concerning the switching impulse insulation strength and the generation of switching surges than is now known about lightning In addition, control measures have been evolved to ameliorate the effect of switching surges In the contamination area, within the last 30 years, research into new materials has concluded with the polymer insulator, which has decreased the length of insulation to the degree that lightning becomes more important The burden of design innovation now rests with lightning, and in this area, the application of arresters to transmission lines has been initiated and will continue to evolve Perhaps in the future, the overall design goal of designing the insulation only for the normal power frequency voltage will be achieved REFERENCES E J Yasuda and F B Dewey, BPA's new generation of 500-kV lines, IEEE Trans on PA&S, 1980, pp 616-624 A R Hileman, Transmission line insulation coordination, Twenty-eighth Bernard Price Memorial Lecture, South African IEE, Sep 1979, pp 3-15 G E Stemler, BPA's field test evaluation of 500 kV PCB's rated to limit switching overvoltages to 1.5 per unit, IEEE Trans on PA&& 1976, pp 352-361 Copyright 1999 by Taylor & Francis Group, LLC Appendix Computer Programs for This Book The following DOS programs appear on the disks supplied with this text These computer programs may be used in some of the problems as specified in the chapters However, in general, the reader is first requested to solve the problems using the simplified methods as presented in the chapters These programs are particularly useful when performing practical engineering studies They may be copied and used as desired The initial versions of some of these programs were written for the Electric Power Research Institute (EPRI) The present versions of these programs have been updated EPRI's permission to include these programs in this book is gratefully acknowledged BFRCIG99 - Backflash Rate, CIGRE Method - Chapter 10 This is the CIGRE method per CIGRE Technical Brochure 63, "Guide to the Procedures for Estimating the Lighting Performance of Transmission Lines", Oct 1991 Help screens are in BFRCIG99.HLP BFR99 - Backflash Rate, CIGRE Method - Chapter 10 An enhanced or investigative program based on the CIGRE method Several options are available (i.e., simplified method, corona, exact LPM method, alternate time-lag curves, exact consideration of power frequency voltage, counterpoise) The file BFR99.HLP contains the help screens which are called by BFR99 FLASH99 - IEEE method, backflash and shielding failure - Chapter 10 A DOS program of the IEEE FLSH17 which was written in BASIC Help screens in FLASH.HLP SFFOR99 - Shielding Failure Flashover Rate - Chapter For transmission lines Calculates the shielding failure flashover rate Copyright 1999 by Taylor & Francis Group, LLC Computer Programs for This Book 763 Options on striking distance equations The file SFFOR99.HLP contains help screens called by SFFOR99 ALPD99 - Shielding Angle Alpha - Chapter For transmission lines Calculates the shielding angle for an inputted or desired SFFOR The file SFFOR99.HLP contains help screens which are called be ALPD99 SRGKON95 - Calculation of Surge Impendances and Coupling Factors Chapter Calculates self and mutual surge impedances and coupling factors Also calculates flashover sequence (e.g., lst, phase A, then phase C, then phase B For each sequence, provides surge impedances of the "ground" wires and coupling factors which may be used in BFR99) Using this output with BFR99 can give the double circuit flashover rates No help screens SRGKON96 - Same as SRGKON95 but does not calculate the flashover sequence SHIELD96 - Station Shielding - Chapter For stations For masts, to 4, and for shield wires For an input value of critical current, calculates the distances so that the shielding diagram can be drawn Help screens are in SHIELD96.HLP, which are called by SHIELD96 SRGBF98 - Incoming surge caused by a backflash - Chapter 11 Calculates the steepness and crest voltage of the incoming surge caused by a backflash Both the simplified method and the more exact method are used Help screens are contained in SRGBF98.HLP, which are called by SRGBF98 SRGSF98 - Incoming surge caused by a shielding failure - Chapter 11 Calculates the steepness and crest voltage of the incoming surge caused by a shielding failure Both the simplified method and the more exact method are used Help screens are contained in SRGSF98.HLP, which are called by SRGSF98 SSFOR97 - Switching Surge Flashover Rate - Chapters and Calculates the switching surge flashover rate Help screens in STRIKE97.HLP STRIKE97 - Switching Surge Strike Distance - Chapters and Calculates the phase-ground strike distance for a given SSFOR Help screens in STRIKE97.HLP PP95 - Phase-Phase Switching Surge Flashover Rate - Chapters and Calculates the combined phase-phase and phase-ground switching surge flashover rate and the separate phase-phase and phase-ground flashover rates Help screens in PP95.HLP ARR97 - Arrester Selection - Chapter 12 Calculates the required minimum arrester MCOV rating TOVEN.DAT and STDRAT.DAT are data files called by the program Also MAJHELP.SCR and PRGDISC.SCR are screens used in the program Help screens are in ARR90.HLP PPSTR97 - Phase-Phase Switching Surge Strike Distance - Chapter and Calculates the phase-phase strike distance and BSL Uses approximate method Help screens in PPSTR97.HLP Copyright 1999 by Taylor & Francis Group, LLC 764 Appendix 16 PPSSF097 -Phase-Phase Switching Surge Flashover Rate -Chapters and Calculates the phase-phase switching surge flashover rate Assumes phaseground SOVs have no effect on flashover rate Help screens in PPSTR97.HLP 17 SIMP99 - Simplified Equations - Chapter 13 Solves the simplified equations developed in Chapter 13 Help screens are in SIMP97.HLP 18 IVFOR99 - Induced Voltage Flashover Rate - Chapter 15 Calculates the voltage induced across the line insulation for a stroke terminating to ground or to trees next to the line The effect of a single line of trees or a forest can be determined Help screens in 1VFOR.HLP 19 OPCB99 - Open Circuit Breaker - Chapter 11 Calculates the probability (or return period) of a surge caused by subsequent stroke which equals or exceeds the open circuit breakers insulation strength Help screens in OPCB99.HLP 20 PROBGAU - Cumulative probability for Gaussian distribution No help screens Copyright 1999 by Taylor & Francis Group, LLC ... important series of books on power engineering Like all the books planned for the Power Engineering series, Insulation Coordinationfor Power Systems presents modern power technology in a context... national standard for power systems -insulation coordination IEC 71-1-1993-12, Insulation coordination Part 1: Definitions, principles and rules IEC Publication 71-1-1976, Insulation coordination, ... Willis Electrical Power Cable Engineering, William A Thue Electric Power System Dynamics and Stability, James A Momoh and Mohamed E El-Hawary Insulation Coordination for Power Systems, Andrew R