IEEE Std 1427-2006, IEEE Guide for Recommended Electrical

IEEE Std 1427™-2006

IEEE Guide for RecommendedElectrical Clearances and InsulationLevels in Air-Insulated ElectricalPower Substations

I E E E3 Park Avenue New York, NY 10016-5997, USA

4 May 2007

IEEE Power Engineering SocietySponsored by theSubstations Committee

IEEE Std 1427TM-2006

IEEE Guide for Recommended Electrical Clearances and Insulation Levels in Air-Insulated Electrical Power Substations

Sponsor Substations Committee of the IEEE Power Engineering Society

Approved 6 December 2006 IEEE-SA Standards Board

Abstract: This guide, covering three-phase ac systems from 1 kV to 800 kV, provides recommended electrical operating and safety clearances and insulation levels in air-insulated electric supply substations; addresses insulation coordination procedures; provides design procedures for the selection and coordination of the insulation levels within the station as they relate to substation clearances; and addresses how reduced clearances in high-voltage ac substations will allow for compact bus arrangements and substation voltage uprating applications.

Keywords: basic lightning impulse insulation level (BIL), basic switching impulse insulation level (BSL), clearances, insulation coordination, insulation levels, substation _________________________ The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2007 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 4 May 2007. Printed in the United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by the Institute of Electrical and Electronics Engineers, Incorporated. National Electrical Safety Code and NESC are registered trademarks in the U.S. Patent & Trademark Office, owned by The Institute of Electrical and Electronics Engineers, Incorporated. Print: ISBN 0-7381-5310-9 SH95611 PDF: ISBN 0-7381-5311-7 SS95611 No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher.

IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Committees of the IEEE Standards Association (IEEE-SA) Standards Board. The IEEE develops its standards through a consensus development process, approved by the American National Standards Institute, which brings together volunteers representing varied viewpoints and interests to achieve the final product. Volunteers are not necessarily members of the Institute and serve without compensation. While the IEEE administers the process and establishes rules to promote fairness in the consensus development process, the IEEE does not independently evaluate, test, or verify the accuracy of any of the information contained in its standards. Use of an IEEE Standard is wholly voluntary. The IEEE disclaims liability for any personal injury, property or other damage, of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, or reliance upon this, or any other IEEE Standard document. The IEEE does not warrant or represent the accuracy or content of the material contained herein, and expressly disclaims any express or implied warranty, including any implied warranty of merchantability or fitness for a specific purpose, or that the use of the material contained herein is free from patent infringement. IEEE Standards documents are supplied “AS IS.” The existence of an IEEE Standard does not imply that there are no other ways to produce, test, measure, purchase, market, or provide other goods and services related to the scope of the IEEE Standard. Furthermore, the viewpoint expressed at the time a standard is approved and issued is subject to change brought about through developments in the state of the art and comments received from users of the standard. Every IEEE Standard is subjected to review at least every five years for revision or reaffirmation. When a document is more than five years old and has not been reaffirmed, it is reasonable to conclude that its contents, although still of some value, do not wholly reflect the present state of the art. Users are cautioned to check to determine that they have the latest edition of any IEEE Standard. In publishing and making this document available, the IEEE is not suggesting or rendering professional or other services for, or on behalf of, any person or entity. Nor is the IEEE undertaking to perform any duty owed by any other person or entity to another. Any person utilizing this, and any other IEEE Standards document, should rely upon the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Interpretations: Occasionally questions may arise regarding the meaning of portions of standards as they relate to specific applications. When the need for interpretations is brought to the attention of IEEE, the Institute will initiate action to prepare appropriate responses. Since IEEE Standards represent a consensus of concerned interests, it is important to ensure that any interpretation has also received the concurrence of a balance of interests. For this reason, IEEE and the members of its societies and Standards Coordinating Committees are not able to provide an instant response to interpretation requests except in those cases where the matter has previously received formal consideration. At lectures, symposia, seminars, or educational courses, an individual presenting information on IEEE standards shall make it clear that his or her views should be considered the personal views of that individual rather than the formal position, explanation, or interpretation of the IEEE. Comments for revision of IEEE Standards are welcome from any interested party, regardless of membership affiliation with IEEE. Suggestions for changes in documents should be in the form of a proposed change of text, together with appropriate supporting comments. Comments on standards and requests for interpretations should be addressed to:

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Copyright © 2007 IEEE. All rights reserved.

Introduction

This guide was revised by members of Working Group D1—Recommended Minimum Clearances in Substation and is under the sponsorship of the Transmission and Distribution Substations Subcommittee of the IEEE/PESCS, Substations Committee.

Notice to users

Errata

Errata, if any, for this and all other guides can be accessed at the following URL: http:// standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for errata periodically.

Interpretations

Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/ index.html.

Patents

Attention is called to the possibility that implementation of this guide may require use of subject matter covered by patent rights. By publication of this guide, no position is taken with respect to the existence or validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying patents or patent applications for which a license may be required to implement an IEEE standard or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention.

Participants

At the time this guide was completed, the Working Group D1—Recommended Minimum Clearances in Substation had the following membership:

Kenneth White, Chair

Hanna Abdallah Abbas Abed Ken Aldridge Joseph Bell Steven Brown Alan R. Byrd Alton Comans Richard N. Crowdis Gary Engmann

Dennis R. Falkenheim William B. Kahanek Charles Koenig Debra Longtin Jeffrey Merryman Jeffrey Nelson Robert Nowell Edward J. O’Donnell Janusz Polak Mike Portale

John Randolph Donald R. Rogers Alan Rotz Steve Rozinka Anne-Marie Sahazizian Hamid Sharifnia Boris Shvartsberg Garry Simms Roland Youngberg

This introduction is not part of IEEE Std 1427-2006, IEEE Guide for Recommended Electrical Clearances and Insulation Levels in Air Insulated Electrical Power Substations.

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The following members of the balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. William Ackerman Roy Alexander Marcos Andrade Stan J. Arnot Ali Al Awazi Anthony Baker Thomas Barnes W. J. Bergman Enrique Betancourt Philip Bolin Stuart Bouchey Eldridge R. Byron William Chisholm R. Daubert Byron Davenport Matthew Davis Frank Denbrock Guru Dutt Dhingra Randall Dotson Gary Engmann Rabiz Foda Frank Gerleve Josephe Greco Erik Guillot Ajit K. Gwal Edwatd Horgan, Jr.

James Houston David W. Jackson Clark Jacobson Joseph Jancauskas Mark Kempker Hermann Koch Joseph Koepfinger Alan Kollar David Krause Chandra Krishnayya Luther Kurtz Stephen R. Lambert Gerald Lee George N. Lester Jason Lin Peter Lips Albert Livshitz William Lowe Gregory Luri Keith Malmedal Frank Mayle Mark McGranaghan Bryan Melville Gary L. Michel Daleep Mohla Abdul Mousa

Jeffrey Nelson Art Neubauer Joe Nims Robert Nowell T. W. Olsen Carlos Peixoto Michael Pehosh John Randolph Anne-Marie Sahazizian Carl Schneider Michael Sharp H. Jin Sim Garry Simms Harinderpal Singh James Sosinski Allan St. Peter Brian Story Malcolm Thaden Eric Udren Jane Ann Verner Hemant Vora Joe Watson William Wessman Kenneth D. White James W. Wilson, Jr. Roland Youngberg

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When the IEEE-SA Standards Board approved this standard on 6 December 2006, it had the following membership:

Steve M. Mills, Chair Richard H. Hulett, Vice Chair

Don Wright, Past Chair Judith Gorman, Secretary

Mark D. Bowman Dennis B. Brophy Joseph Bruder Richard Cox Bob Davis Julian Forster* Joanna N. Guenin Mark S. Halpin Raymond Hapeman

William B. Hopf Lowell G. Johnson Herman Koch Joseph L. Koepfinger* David J. Law Daleep C. Mohla Paul Nikolich

T. W. Olsen Glenn Parsons Ronald C. Petersen Gary S. Robinson Frank Stone Malcolm V. Thaden Richard L. Townsend Joe D. Watson Howard L. Wolfman

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Satish K. Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Alan H. Cookson, NIST Representative

Don Messina IEEE Standards Program Manager, Document Development

Matthew Ceglia

IEEE Standards Program Manager, Technical Program Development

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Contents

1. Overview .................................................................................................................................................... 1

1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1

2. Normative references.................................................................................................................................. 2

3. Definitions .................................................................................................................................................. 2

4. Criteria........................................................................................................................................................ 4

4.1 Safety codes and regulations ............................................................................................................... 4 4.2 Basic lightning impulse insulation level criteria.................................................................................. 5 4.3 Radio interference voltage and corona criteria .................................................................................... 5 4.4 Operating history ................................................................................................................................. 5 4.5 Economic aspects ................................................................................................................................ 5 4.6 Community acceptance........................................................................................................................ 5

5. Substation insulation coordination ............................................................................................................. 5

5.1 Protective margins and arrester maximum continuous operating voltage ........................................... 6 5.2 Selection of basic lightning impulse insulation level and basic switch impulse insulation level ........ 6 5.3 Selection of electrical clearances......................................................................................................... 9

6. Electrical operating/design clearances...................................................................................................... 10

6.1 General discussion............................................................................................................................. 10 6.2 Historical background........................................................................................................................ 11 6.3 Clearances based on lightning impulse conditions ............................................................................ 12 6.4 Clearances based on switching surge conditions ............................................................................... 14

7. Electric maintenance/safety clearances .................................................................................................... 19

8. Substation voltage uprating and compact design...................................................................................... 19

8.1 BIL/System voltage ratio concept...................................................................................................... 20 8.2 Other considerations .......................................................................................................................... 21

Annex A (informative) Bibliography ........................................................................................................... 22

Annex B (informative) Example calculations .............................................................................................. 28

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IEEE Guide for Recommended Electrical Clearances and Insulation Levels in Air-Insulated Electrical Power Substations

1. Overview

1.1 Scope

This guide, covering three-phase ac systems from 1 kV to 800 kV, provides recommended electrical operating and safety clearances and insulation levels in air-insulated electric supply substations; addresses insulation coordination procedures; provides design procedures for the selection and coordination of the insulation levels within the station as they relate to substation clearances; and addresses how reduced clearances in high-voltage ac substations will allow for compact bus arrangements and substation voltage uprating applications. This guide addresses insulation coordination procedures, including the choice of insulation levels and arrester specification, in limited detail and only as relevant to clearance requirements. Detailed and expanded coverage of insulation coordination procedures is provided in other ANSI and IEEE guides and standards (see Clause 2). This guide focuses on open-air bus assemblies and configurations and excludes apparatus clearances (i.e., bushing clearances for transformers, and breakers). Detailed coverage of apparatus clearances is provided in other applicable guides and standards.

1.2 Purpose

Proper electrical clearances are necessary for the design, construction, and operation of electric supply substations. This document develops guidelines for the application of recommended electrical clearances and insulation levels in air-insulated substations. The recommended clearances incorporate both design/operating clearances and safety clearances.

IEEE Std 1427-2006 IEEE Guide for Recommended Electrical Clearances and Insulation Levels in Air Insulated Electrical Power Substations

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2. Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. Accredited Standards Committee C-2, National Electrical Safety Code® (NESC®).1 IEEE Std 1313.1TM-1996, IEEE Standard for Insulation Coordination—Definitions, Principles, and Rules.2, 3 IEEE 1313.2TM-1999, IEEE Guide for the Application of Insulation Coordination.

3. Definitions

For the purposes of this guide, the following terms and definitions apply. The Authoritative Dictionary of IEEE Standards Terms [B68]4 should be referenced for terms not defined in this clause.

3.1 air-insulated substation: An assembly of structures supporting equipment and buses exposed to atmospheric conditions that uses air as the external insulating medium.

3.2 ambient air temperature: The temperature of the surrounding air that comes in contact with the substation.

3.3 atmospheric correction factor: A factor applied to account for the difference between the atmospheric conditions in service and the standard atmospheric conditions.

NOTE—In terms of this guide, it applies to external insulation only.5

3.4 basic lightning impulse insulation level (BIL): The electrical strength of insulation expressed in terms of the crest value of a standard lightning impulse under standard atmospheric conditions. BIL may be expressed as either statistical or conventional.

3.5 basic switching impulse insulation level (BSL): The electrical strength of insulation expressed in terms of the crest value of a standard switching impulse. BSL may be expressed as either statistical or conventional.

3.6 bus support: An insulating support for a bus. It includes one or more insulator units with fittings for connection to the support structure and for receiving the bus.

3.7 clearance, minimum phase-to-ground: The shortest distance between any energized part(s) and the adjacent grounded part(s).

3.8 clearance, minimum phase-to-phase: The shortest distance between any energized parts where the parts are of different phases, including phases of different voltages.

1 The NESC is available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 2 IEEE publications are available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 3 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 4 The numbers in brackets correspond to those in the bibliography in Annex A. 5 Notes in text, tables, and figures are given for information only and do not contain requirements needed to implement the standard.


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NOTE—Cautionary differentiation should be made between surface-to-surface clearance and bus-to-bus centerline distance.

3.9 clearance, center-to-center phase spacing: The distance between centerlines of the energized parts of adjacent phases of bus.

3.10 conventional withstand voltage: The voltage that an insulation is capable of withstanding without failure or disruptive discharge under specified test conditions.

3.11 energized parts: Those parts that are designed to operate at voltage different from that of the earth.

3.12 external insulation: The distances between electrodes in open air and across the surfaces of the solid insulation of equipment in contact with open air that are subjected to dielectric stresses and to the effects of atmospheric and other external conditions such as contamination, animals, etc.

3.13 insulation configuration: The complete geometric configuration of the insulation, including all elements (insulating and conducting) that influence its dielectric behavior. The following insulation configurations are identified:

a) Phase-to-ground: An insulation configuration between an energized part and the neutral or ground.

b) Phase-to-phase: An insulation configuration between two phases of energized conductors or parts.

c) Longitudinal: An insulation configuration between energized conductors or parts belonging to the same phase, which are temporarily separated into two independently energized parts (e.g., open switching device).

3.14 insulation coordination: The process of bringing the insulation strengths of electrical equipment and buses into the proper relationship with expected overvoltages and with the characteristics of the insulating media and surge protective devices to obtain an acceptable risk of failure.

3.15 lightning overvoltage: A fast front voltage produced by lightning. Such overvoltages are usually unidirectional and of very short duration.

3.16 maximum system voltage (Vm): The highest root mean square (rms) phase-to-phase power frequency voltage that occurs on the system under normal operating conditions for which equipment and other system components are designed for continuous operation without derating of any kind.

3.17 nominal system voltage: The root mean square (rms) phase-to-phase power frequency voltage below the maximum system voltage by which the system is designated and to which certain operating characteristics are related.

3.18 non–self-restoring insulation: An insulation that loses its insulating properties or does not recover completely after a disruptive discharge. This type of insulation is generally, but not necessarily, internal insulation.

3.19 overvoltage: Any time-dependent voltage between one phase and ground or between phases having a value exceeding the maximum system voltage. Unless otherwise indicated, such as for surge arresters, overvoltages are expressed in per unit with reference to 3V2 m (maximum system crest voltage). Overvoltages are classified by their shape and duration. See also: temporary overvoltage; transient overvoltage.

3.20 protective margin (PM): The protective ratio (PR) minus one, expressed as a percentage. PM = (PR – 1) × 100.


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3.21 protective ratio (PR): The ratio of the insulation strength of the protected equipment to the overvoltages appearing across the insulation.

3.22 self-restoring insulation: Insulation that completely recovers its insulating properties after a disruptive discharge. This type of insulation is generally, but not necessarily, external insulation.

3.23 statistical withstand voltage: The voltage that an insulation is capable of withstanding with a given probability of failure, corresponding to a specified probability of failure (e.g., 10% and 0.1%).

3.24 switching overvoltage: A slow front, short duration, unidirectional or oscillatory, highly damped voltage. These overvoltages are usually generated by switching or faults.

3.25 temporary overvoltage: An oscillatory overvoltage that is of relatively long duration and undamped or only slightly damped. Temporary overvoltages usually originate from switching operations or faults (for example, load rejection and single-phase faults), nonlinearities (ferro-resonance effect and harmonics), or both. Load rejection overvoltages may arise in the substation mainly due to a fault in the distant substation. Depending on the protection scheme, either all parts or the parts between circuit breaker and transformer will be stressed.

3.26 transient overvoltage: A short-duration highly damped, oscillatory or non-oscillatory overvoltage, having a duration of few milliseconds or less. See also: lightning overvoltage; switching overvoltage; very fast front overvoltage.

3.27 very fast front overvoltage: A transient overvoltage with a very fast rise time, usually of short duration, and unidirectional voltage (often by GIS disconnect switch operation or when switching motors). High-frequency oscillations are often superimposed on the unidirectional wave.

3.28 withstand voltage: The voltage that the insulation is capable of withstanding with a given probability of failure. See also: conventional withstand voltage; statistical withstand voltage.

4. Criteria

The proper electrical clearance levels for each user can be based on several criteria. Although initial clearance levels will probably be based on insulation coordination studies (see Clause 5), additional factors can influence the final levels selected, such as the latest regulatory requirements, added clearance for worker safety, animals/birds on conductors or structures, and an unacceptable level of clearance-related outages at existing installations. Acceptable minimum clearances will incorporate even greater concerns. Typical criteria may incorporate regulatory requirements, corporate policy, frequency and duration of outages, cost of each occurrence, safety issues, severity of damage, applicable equipment standards, potential impact on nearby customers, substation location, and quality-of-service requirements. The following criteria outlined in 4.1 through 4.6 should be considered when evaluating the minimum clearance requirements.

4.1 Safety codes and regulations

Due to the nature of the National Electrical Safety Code® (NESC®) (Accredited Standards Committee C-2) and other governmental regulations, some clearance levels determined through the use of this guide could come into conflict with governmental regulations or overlapping jurisdictions. Determination of the acceptable operating and safety clearances should include application of applicable laws and regulations, such as the NESC.


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4.2 Basic lightning impulse insulation level criteria

The performance criterion upon which the insulation strength or withstand voltages and clearances are selected is based on an acceptable probability of insulation failure and is determined by the consequence of failure, required level of reliability, expected life of equipment, economics, and operational requirements. The criterion is usually expressed in terms of an acceptable failure rate (number of failures per year, years between failures, risk of failure, etc.) of the insulation configuration.

4.3 Radio interference voltage and corona criteria

As phase-to-ground and phase-to-phase clearances in substations are reduced, the higher electric gradients surrounding conductors, especially at bus hardware and irregular-shaped equipment parts, may initiate corona generation and corresponding radio interference voltage (RIV). Audible and visible (at night) corona, as well as RIV, may be environmentally objectionable to the community or nearby residents. In general, buses should be designed in accordance with IEEE Std 605TM-1998 [B69] to limit electric gradients below corona/RIV onset. If specific problems develop at fittings or irregular parts due to reduced (minimum) substation clearances, bolt covers or corona rings/shields can be installed.

4.4 Operating history

If an unacceptable level of clearance-related outages has been identified, the specific causes need to be determined. Outage records, maintenance reports, and routine inspections of facilities will afford information as to the cause and extent of the problem. Typical information found in these reports includes facility name, date, time, equipment affected, outage cause and duration, and number of customers affected.

4.5 Economic aspects

Economic aspects can be considered when determining which alternative design to employ, and severe cost constraints may result in the use of a less-effective design.

4.6 Community acceptance

Community acceptance of the potential increased noise levels when reduced clearances are used should also be considered, as reduced clearances (i.e., compact designs) can result in increased corona-related audible or RIV noise. The acceptability of certain levels of noise is intangible and may be dictated by company policies, community acceptance, customer relations, and so on. The impact on adjacent property owners should be addressed. See IEEE Std 1127TM-2004 [B72].

5. Substation insulation coordination

Electrical clearances in air-insulated substations, from an operational viewpoint, are closely tied to the insulation levels in the station. For example, the strike distance across a station post insulator can be directly correlated to its BIL. Establishment of recommended operating clearances necessitates an understanding of the insulation coordination process and the selected insulation levels for a given system voltage. Insulation coordination is a method for providing adequate protective margin between the insulation withstand levels of the equipment, bus, and insulators and the voltage stresses that can arise in the substation. At a basic level, insulation coordination becomes a selection of the BIL and BSL withstand ratings for various pieces of major equipment and substation bus/insulator assemblies that will provide the


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desired protective margin(s). The protective margin for a given BIL/BSL withstand voltage is usually tied closely to the rating of the surge arrester employed. A lower arrester rating provides a greater protective margin or (at the user’s discretion) allows for the use of lower BIL/BSL levels. Lower BIL and BSL levels can translate into reduced capital costs. The complete methodology is very complex and probabilistic in nature and often involves analog studies or computer simulations of the proposed system. It involves many detailed steps, many of which are beyond the scope of this guide. For a detailed description of the methods available, the user is encouraged to study IEEE Std 1313.1-1996 and IEEE Std 1313.2-1999.6 Substation insulation coordination comprises the following activities:

a) Selection of the arrester maximum continuous operating voltage (MCOV) rating and establishment of the minimum protective margin requirements.

b) Selection of BIL/BSL levels that provide the required protective margin.

c) Establishment of electrical clearances based on BIL/BSL levels.

5.1 Protective margins and arrester maximum continuous operating voltage

Historically, the minimum protective margins have been set at 15% for the BSL and 20% for the BIL. However, with the application of modern metal oxide surge arresters, protective margins of 50% or greater are common. The quantity and location of arresters within the substation is also important, and the BIL/BSL selection should take into account the voltage level, the separation distance between the arresters, and the protected equipment/assemblies. For large separations, it may be necessary to increase the protective margins. In the simplest of terms, selection of the surge arrester MCOV rating for any given system voltage is based on the lowest rating possible for which the arrester will survive the expected system overvoltages. Arrester energy dissipation capabilities for lightning and switching surge conditions may govern the MCOV rating at higher voltages (230 kV and above). However, it is the capability of the surge arrester to handle long-term sinusoidal temporary overvoltages (temporary system overvoltages for metal-oxide arresters and unfaulted phase overvoltages during faults for silicon carbide arresters), which will usually establish the minimum MCOV rating at which the arrester can survive. See IEEE Std C62.22TM-1997 [B77] for proper application.

5.2 Selection of basic lightning impulse insulation level and basic switch impulse insulation level

5.2.1 Statistical insulation levels

Statistical methods are used to classify and analyze insulation withstand levels and various overvoltages, as described in 5.2.2. These methods are generally applied when first establishing the insulation levels for a given system and later at specific substations where unusual conditions or proposed modifications warrant detailed study.

6 Information on references can be found in Clause 2.


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The basic lightning impulse insulation level may be either a conventional BIL or a statistical BIL, classified as follows:

a) Conventional BIL: The crest value of a standard lightning impulse for which the insulation shall not exhibit disruptive discharge when subjected to a specific number of applications of this impulse under specified conditions, which is applicable specifically to non–self-restoring insulations.

b) Statistical BIL: The crest values of a standard lightning impulse for which the insulation exhibits a 90% probability of withstand (or a 10% probability of failure) under specified conditions, which is applicable specifically to self-restoring insulations.

The basic switching impulse level may be either a conventional BSL or a statistical BSL, classified as follows:

⎯ Conventional BSL: The crest value of a standard switching impulse for which the insulation does not exhibit disruptive discharge when subjected to a specific number of impulses under specified conditions, which is applicable to non–self-restoring insulations.

⎯ Statistical BSL: The crest value of a standard switching impulse for which the insulation exhibits a 90% probability of withstand (or a 10% probability of failure), under specified conditions, which is applicable to self-restoring insulations.

Switching impulses often represent the more severe air insulation withstand condition at system voltages 362 kV and over.

5.2.2 Standard insulation levels

Based on many years of historically successful operating experience, standard BIL and BSL have been established for given maximum system voltage levels. Table 1 and Table 2 (from IEEE Std 1313.1-1996) show several possible withstand voltages for a given maximum rated voltage. The withstand voltage selected should be based on the results of the user’s insulation coordination analysis. For example, in Table 1, 650 kV BIL has been established as the historical, conservative insulation level at 145 kV. With improvements in surge protection, and for other reasons, successful operation at a BIL of 550 kV or 450 kV is possible, depending on user criteria and the desired protective margin. When other considerations are incorporated (see Clause 8), even further reductions in BIL may be possible. The withstand voltages in Table 1 and Table 2 are phase-to-ground voltages. To establish the phase-to-phase withstand voltage, and associated clearances, it is necessary to continue the insulation coordination analysis. With some equipment, the phase-to-phase withstand voltage (based on test voltages) can be the same as the phase-to-ground withstand voltage (for example, three-phase transformers). With other equipment, the phase-to-phase insulation level is undefined (for example, support insulators) and the withstand voltage is dictated by the design of the assembly (i.e., air clearances between phases and from phase-to-ground).


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Table 1 —Standard withstand voltage for am kV 242V kV 1 ≤≤

Maximum system voltage

(phase-to-phase) Vm

kV, rms

Low-frequency, short-duration withstand

voltageb (phase-to-round)

kV, rms

BIL (phase-to-ground)

kV, crest 1.2 30

45 5 60

75 15 34 95

110 26.2 50 150 36.2 70 200 48.3 95 250 72.5 95

140 250 350

121 140 185 230

350 450 550

145 185 230 275

450 550 650

169 230 275 325

550 650 750

242 275 325 360 395 480

650 750 825 900 975 1050

aFrom p. 11 of IEEE Std 1313.1-1996, except for the 1.2 kV and 5 kV values. bSee relevant apparatus standards for specific equipment values.


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Table 2 —Standard withstand voltage for Vm > 242 kVa Maximum system

voltage (phase-to-phase)

Vm

kV, rms

BILc (phase-to-ground)

kV, crest

BSLb,c (phase-to-ground)

kV, crest

362 900 975 1050 1175 1300

650 750 825 900 975 1050

550 1300 1425 1550 1675 1800

1175 1300 1425 1550

800 1800 1925 2050

1300 1425 1550 1675 1800

aFrom p. 12 of IEEE Std 1313.1-1996. bSee relevant apparatus standards for specific equipment values. cThe selected combination of BIL and BSL insulation levels is defined by the user and is usually based on insulation coordination studies. The reader is cautioned to not relate BIL to BSL only linearly across the table. For example, this table is not intentionally matching up a BSL of 1300 kV with a BIL of 1800 kV for 800 kV systems. (BIL, BSL) combinations of (1800 kV, 1300 kV); (1800 kV, 1425 kV); (1925 kV, 1300 kV), etc. may be acceptable. The BIL and BSL levels in Table 2 were derived initially for individual pieces of equipment where the BIL is demonstrated by testing. It is common practice to extend the BIL/BSL concept to an assembly of individual components (such as bus, connectors, insulators, switches, and arresters) in order to establish a BIL or BSL for the entire station design. Typically, each user will establish one station BIL and BSL for each system voltage level. Generally, the substation as a whole is not tested. In addition, the station withstand is a function of the air clearances as well as of the BIL of the equipment. It is assumed when referring to a station BIL that the clearances are set to maintain this BIL.

5.3 Selection of electrical clearances

Proper insulation coordination should consider the total design of the insulation-air gap system needed to achieve the desired reliability and economic design, with appropriate allowance for safety, switching and impulse levels, altitude variations, contamination, weather variables, and maintenance requirements. Selection of the appropriate clearances directly follows the insulation coordination analysis. Insulation systems for air-insulated substations generally use porcelain or polymer insulators and bushings for mechanical support and equipment termination. The limits of normal (system) and abnormal (impulse, lightning, and switching) voltages, which can be applied to these insulation systems, are usually determined by the disruptive discharge (flashover) in air. Time is required upon application of voltage across an air gap before discharge occurs, as breakdown is by electron/free ion-pair collision. This volt-time (VT) characteristic of air gaps and all insulators allows short-time overvoltages to occur without flashover; insulation coordination takes advantage of this principle.


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5.3.1 Air gap characteristics

Air gap withstand characteristics vary widely with gap length and the polarity, duration, and waveshape of the applied voltage. For medium- and high-voltage systems typically up to 230 kV, the impulse-type (lightning) and very fast front overvoltages normally result in the severest stress. For EHV systems (345 kV and above), longer duration switching overvoltages normally result in a more severe, controlling stress for air gaps. Insulators placed in air gaps also tend to degrade the performance of the air gap. The critical flashover overvoltage (CFO), the voltage at which an air gap flashes over 50% of time under defined test conditions, generally increases with increased air density, as described in 5.3.2. Air gap flashover is also influenced by the shape of the energized and grounded parts or planes, as electric field gradients are dependent on geometric shapes. High fields on irregularly shaped parts of conductors or fittings ionize the air near the conductor, which affects gap strength. This partial pre-ionization of the air gap at reduced clearances can result in a disruptive discharge (flashover). A wealth of data on the flashover of various gap configurations has been measured and published in various forms. Sufficient testing of air gaps has been performed over the years using standardized types of gaps and voltage waveshapes to provide acceptable confidence levels for predicting air gap withstand versus gap length for impulse and switching voltages.

5.3.2 Atmospheric conditions

As air gap breakdown is precipitated by electron collision and ion-pair formation, factors affecting air molecule density and types of molecules in the air also affect the breakdown voltage for a given gap and configuration. The major factors include vapor pressure (humidity) and density, the altitude above sea level, and barometric pressure. Other airborne contaminants such as chemical vapors, products of combustion, and dust can also affect the gap breakdown properties. Most air gap withstand testing has been performed at the following standard reference atmospheric conditions.

Temperature 20 ºC (68 ºF) Barometric Pressure 101.3 kPa (1013 hPa, 760 mm of mercury, 29.92 in of mercury) Absolute humidity 11 g/m3 Altitude Sea level

In air gap withstand computations, factors may need to be added to compensate for “nonstandard” air. For elevations higher than sea level, the insulation strength decreases as a linear function of the relative air density (see 6.4.2). The sea level BILs and clearances must be divided by the relative air density.

6. Electrical operating/design clearances

Once the insulation levels are known, electrical clearances can be established. For several reasons or concerns (system configuration, switching surge levels, arrester energy capabilities, transient recovery voltage, resonance, etc.), detailed insulation coordination studies involving digital computer simulations may be required at any specific substation. In general, however, the insulation levels and associated recommended electrical clearances are often standardized at specific values common throughout a user’s entire power system, which is historically based on nominal system voltage levels. The associated operating electrical clearances are determined by alignment to the strike distances associated with the insulator BIL or BSL ratings.

6.1 General discussion

Table 3 and Table 5 establish recommended minimum phase-to-ground and phase-to-phase clearances for various levels of BIL and BSL insulation. These clearances are governed only by the electrical operating conditions to meet the system insulation coordination requirements, and they are based on the equations


11


developed in 6.3 and 6.4. Additional considerations for safety clearances must be evaluated separately (see Clause 7). It is not expected that these minimum clearance values will find great use in substation design as standard spacings. The larger clearances now often in use have a long history of successful experience behind them and are not likely to be easily changed. It is expected that these minimum clearance values will find application for users considering voltage uprating at existing substations for system voltages of 69 kV to 230 kV, and in situations where the clearances for a physically larger piece of equipment (switch, transformer, etc.) in an existing station no longer meet the users standard design clearances. Some conservatism is recommended when applying these values, especially for the reduced clearances developed in Clause 8. The minimum electrical clearances between live parts of different voltages should be the value for phase spacing at the higher voltage. An overriding rule for parallel buses at the same elevation is that the minimum distance between the conductors of different voltages should be the sum of the phase-to-ground minimum striking distances for each voltage. This rule results in a larger distance between circuits than between phases and enables field personnel to tell at a glance what conductors belong to what circuit. Other considerations that may result in the need for higher clearances and should be accounted for in the final design include

a) The arc that can result during operation of air-break switches and expulsion fuses.

b) The reduced air gap that can result from the presence of corona rings, insulator/bushing caps, and other hardware.

c) The effect of higher altitude.

d) The presence of contamination.

e) The presence of animals on energized conductors.

f) The effect of high fault current levels and the resulting mechanical stress on closely spaced bus configurations.

g) Areas of high keraunic level.

h) Operating constraints.

i) Maintenance access constraints.

j) The location of surge arresters.

This guide should not be taken to imply that the lowest clearances presented must be used or are even preferred. Instead, the values provided should be taken as acceptable options, to be used when economics, space limitations, or other considerations justify the resulting benefits.

6.2 Historical background

Apparatus-based electrical clearances, verified by laboratory testing, have existed for many years. The same cannot be said for substation air clearances, as substations are typically composed of assemblies of insulators, bus, and connectors not often tested in a factory setting. In 1954, the AIEE Substations Committee [B32] published a guide for minimum electrical clearances in substations. It presented clearances governed by the phase-to-ground, lightning overvoltage flashover of a 1.3 cm (0.5 in.) rod gap at various BIL levels. It accounted for altitude and, before safety factor adjustments, reflected a breakdown gradient for air of 600–850 kV/m. Switching surge-based clearances for EHV voltage class substations had not yet been developed in 1954. With the development of EHV operating voltages above 242 kV, switching surge-induced flashovers became a greater concern. The IEEE Substations Committee published reports on minimum electrical


12


clearances based on switching surge requirements in 1965 [B46] and again in 1972 [B47]. Based on accumulated test data, the committee found that phase-to-ground electrical clearances above 242 kV maximum system voltage were dictated by the magnitude of the switching surge overvoltage. The reports presented a range of minimum clearance requirements for switching surge levels corresponding to 1.5 through 3.0 per unit. Below 362 kV, the committee concluded that air clearances were adequately dictated by the impulse (BIL) based withstand characteristics established earlier. Extensive research and testing at EHV system voltage levels continued. In November/December 1975, Georges Gallet et al. [B10] published what is now the accepted general expression for the positive switching impulse strength of air insulation, which is valid even for extra long air gaps (see 6.4). Previous work had introduced the concept of a gap factor, kg, which was found to be valid for all practical air gap configurations between 1 m (3.3 ft) and at least 30 m (100 ft).

6.3 Clearances based on lightning impulse conditions

6.3.1 Phase-to-ground clearances based on lightning impulse conditions

For self-restoring insulation, the calculation of air clearances to withstand lightning impulse overvoltages has been well understood for many years. The method is effectively unchanged from the 1954 AIEE paper. Clearances are calculated by dividing the highest equivalent crest voltage for the 1.2 × 50 µs impulse wave shape (BIL) by the negative polarity critical breakdown gradient for air. This critical gradient is a function of the gap configuration and testing has shown that it varies from about 540 kV/m to 750 kV/m, the latter being for a rod-plane gap. The minimum clearances shown in Table 3 are based on a CFO gradient value of 605 kV/m, a value which has been found to represent the typical geometry for an air-insulated substation (see IEEE 1313.2-1999 Table 18). It can be reduced to as low as 500 kV/m depending on the conservatism of the user. As a reference, the equivalent CFO gradient for the clearances shown in Columns 4 and 7 of Table II in the 1954 AIEE report vary from 718 kV/m (for 95 kV BIL) to 576 kV/m (for 1050 kV BIL). The value for 550 kV BIL was 594 kV/m. The relevant equation is

GradientCFOgphcrestV −=S (1)

where S is the metal-to-metal strike distance in meters Vcrest ph-g is in kilovolts CFOGradient is the critical flashover gradient in kilovolts/meter

As the

1.15gphcrestV

BIL −= (2)

A substitution of Equation (2) into Equation (1) gives

526BIL

605BIL 1.15 ==S (3)

The resulting minimum phase-to-ground air clearances are shown in Table 3. For elevations higher than sea level, the insulation strength decreases as a linear function of the relative air density (see 6.4.2). The sea


13


level BILs and clearances must be divided by the relative air density. The clearances required by applicable safety codes may vary from the values calculated here (see Clause 7).

Table 3 —Recommended minimum electrical clearances for air-insulated substations when lightning impulse conditions governa,b

Maximum systemc voltage phase-to-phase

Basic BILc Minimum phase-to-groundd,f clearances

Minimum phase-to-phased,e,f clearances

(kV, rms) (kV, crest) mm (in) mm (in) 1.2 30

45 57 86

(2.3) (3.3)

63 95

(2.5) (3.6)

5 60 75

115 145

(4.5) (5.6)

125 155

(5) (6.2)

15 95 110

180 210

(7) (8)

200 230

(8) (9)

26.2 150 285 (11) 315 (12) 36.2 200 380 (15) 420 (16) 48.3 250 475 (19) 525 (21) 72.5 250

350 475 665

(19) (26)

525 730

(21) (29)

121 350 450 550

665 855 1045

(26) (34) (41)

730 940 1150

(29) (37) (45)

145 350 450 550 650

665 855 1045 1235

(26) (34) (41) (49)

730 940 1150 1360

(29) (37) (45) (54)

169 550 650 750

1045 1235 1325

(41) (49) (56)

1150 1360 1570

(45) (54) (62)

242 650 750 825 900 975 1050

1235 1425 1570 1710 1855 2000

(49) (56) (62) (67) (73) (79)

1360 1570 1725 1880 2040 2200

(54) (62) (68) (74) (80) (86)

362 900 975 1050 1175 1300

1710 1855 2000 2235 2470

(67) (73) (79) (88) (97)

1880 2040 2200 2455 2720

(74) (80) (86) (97)

(105) 550 1300

1425 1550 1675 1800

2470 2710 2950 3185 3420

(97) (105) (115) (125) (135)

2720 2980 3240 3500 3765

(105) (115) (130) (140) (150)

800 1800 1925 2050 2300

3420 3660 3900 4375

(135) (145) (155) (170)

3765 4025 4285 4815

(150) (160) (170) (190)

aClearances shown are based on a 605 kV/m flashover gradient. See 6.3.1 for other choices. bSwitching surge conditions normally govern for system voltages above 242 kV. See Table 5. cValues for maximum system voltages and BIL levels are from Table 1 and Table 2 of IEEE Std 1313.1-1996, except for the 1.2 kV and 5 kV system voltage and the 30 kV, 45 kV, 60 kV, 75 kV, and 2300 kV BIL values. dFor specific equipment clearance values, see relevant apparatus standards. ePhase-to-phase clearances shown in this table are metal-to-metal clearances not bus-to-bus centerlines. fAdditional considerations for safety clearances must be evaluated separately (see Clause 7).


14


6.3.2 Phase-to-phase clearances based on lightning impulse conditions

When a lightning strike occurs to a single phase of a transmission line, a coupled voltage of the same polarity appears on the other phases. The phase-to-phase impulse voltage at the struck point is less than the phase-to-ground impulse voltage. The phase-to-phase impulse voltages seldom exceed the phase-to-ground impulse voltages. As a result, the minimum phase-to-phase clearances based only on lightning impulses can be equal to the phase-to-ground clearances as calculated in 6.3.1. It has been shown, however, that because of the severe system disturbances they cause, phase-to-phase faults are less desirable. All that is required is enough increased spacing to cause flashover to occur phase-to-ground instead of phase-to-phase. It is therefore necessary that the distance between phases exceeds that to ground, ensuring a flashover to ground in most cases. An addition of 10% to the phase-to-ground clearances has been shown to be sufficient to reasonably ensure that the flashover will be to ground. The phase-to-phase air clearances shown in Table 3 where lightning conditions govern reflect the 10% safety factor.

6.4 Clearances based on switching surge conditions

6.4.1 Phase-to-ground clearances based on switching surge conditions

For self-restoring insulation, the probability of flashover may be described by an insulation strength characteristic curve that has two basic parameters, as follows:

a) The CFO, corresponding to the 50% probability of flashover for a single impulse application.

b) The standard deviation σf or equivalently the coefficient of variation σf /CFO.

This characteristic is modeled by a cumulative Gaussian distribution function and is considered valid to at least 4 standard deviations below the CFO. The actual truncation point, below which no flashover can occur, is not known. The insulation strength characteristic may be estimated by established statistical test methods. The statistical BSL, reflecting the 10% probability of flashover, is mathematically defined as

( )[ ]CFOσ1.281CFOσ 1.28CFOBSL ff ×−×=×−= (4)

For self-restoring insulations, the value of the coefficient of variation is smaller for lightning impulses than for switching impulses. For lightning impulses, this coefficient has been shown to be about 3% or 0.03. For switching impulses on line or tower insulation, the coefficient increases to an average of about 0.05, and for station insulation, it is generally assumed to be about 0.06 or 0.07. With a value of 0.07 for σf /CFO, Equation (4) becomes

CFO 9104.0BSL g-ph = (5)

Above 230 kV, switching surge conditions normally dominate. Based on the work of Gallet, Paris, and others, it has been shown that the following relationship holds between the flashover of the air gap and the strike distance for various air gap configurations up to at least 30 m (100 ft). For a more detailed explanation for the basis of the numerical constants 8 and 3400, which are based on interpretation of laboratory test data, see Gallet et al. [B10] and Hileman [B24].


15


1CFO

δ34008

mg −

××=

kS (6)

where S is the metal-to-metal strike distance in meters kg is the gap factor δm is the altitude correction factor (1.0 at sea level) CFO is the critical flashover voltage in kilovolts

Typical values of the gap factor kg for phase-to-ground insulation and various configurations are shown in the Table 4. Where the gap factor for a specific configuration is unknown, a value of 1.3 for kg is suggested for substation air clearance calculations (see Table 10 and p. 42 of IEEE 1313.2-1999).

Table 4 —Switching surge factors for phase-to-ground insulations Gap configuration Gap factor

(kg) Coefficient of variation

(σf /CFO) Rod-plane 1.00 0.07 Rod-rod (vertical) 1.30 0.07 Rod-rod (horizontal) 1.35 0.07 Conductor-lateral structure 1.30 0.07 Conductor-plane 1.15 0.07 At sea level, δm = 1.0. Substituting Equation (5) into Equation (6) results in the following equation for switching surge-based air clearances in substations at sea level:

1)BSL(40248

10.9104)(BSL

3.134008

−=

−×=S (7)

The resulting minimum phase-to-ground air clearances are shown in Table 5. Two phase-to-ground air clearance columns are provided, one each for kg = 1.3 and kg = 1.0. The kg = 1.3 column is based on the assumptions described above. The kg = 1.0 column, which reflects a rod-plane gap configuration, will result in more conservative clearances.


16


Table 5 —Recommended minimum electrical clearances for air-insulated substations when switching surge conditions governa,b

Maximum system voltage phase-to-phasec

BSL Equivalent PUi

Minimum phase-to-ground clearances (kg =

1.3)d,e,h

Minimum phase-to-ground

clearances (kg = 1.0)d,e,h

Minimum phase-to-phase

clearances (kg = 1.3)d,f,g,h

(kV, rms) (kV, Ph-g,crest) SSF mm (in) mm (in) mm (in) 362 550

650 750 825 900 975 1050

1.86 2.20 2.54 2.79 3.04 3.30 3.55

1265 1540 1835 2065 2305 2560 2825

(50) (61) (72) (81) (91)

(100) (110)

1730 2125 2560 2910 3280 3680 4110

(68) (84) (100) (115) (130 (145) (160)

1630 2000 2405 2725 3065 3505 3905

(64) (79) (95) (105) (120) (140) (155)

550 900 975 1050 1175 1300 1425 1550

2.00 2.17 2.34 2.62 2.89 3.17 3.45

2305 2560 2825 3300 3820 4385 5010

(91) (100) (110) (130) (150) (175) (195)

3280 3680 4110 4895 5795 6825 8025

(130) (145) (160) (190) (230) (270) (315)

3065 3505 3905 4640 5475 6420 7840

!120) (140) (155) (180) (215) (250) (310)

800 1175 1300 1425 1550 1675 1800

1.80 2.00 2.18 2.37 2.56 2.76

3300 3820 4385 5010 5705 6475

(130) (150) (175) (195) (225) (255)

4895 5795 6825 8025 9435

11120

(190) (230) (270) (315) (370) (440)

4540 5475 6420 7840 9200

10815

(180) (215) (250) (310) (360) (425)

aClearances shown are based on specific gap factors. See Table 4 and Table 7 for other choices. bLightning impulse conditions may govern when low BSL levels are used. See Table 3. cValues for maximum system voltages are from Table 2 of IEEE Std 1313.1-1996. dSee relevant apparatus standards for specific equipment clearance values. eAssumptions for phase-to-ground clearances: altitude = sea level, coefficient of variation = 0.07. fAssumptions for phase-to-phase clearances: altitude = sea level, coefficient of variation = 0.035. BSLph-ph/BSLph-g = 1.56 to 1.74. gPhase-to-phase clearances shown in Table 5 are metal-to-metal clearances not bus-to-bus centerlines. hAdditional considerations for safety clearances must be evaluated separately (see Clause 7). iEquivalent SSF = BSL ÷ Vcrest ph-g, where Vcrest ph-g = 3V2 m .

6.4.2 Altitude adjustments

At sea level, the factor δm = 1.0. Above sea level, the factor comes into play to account for nonstandard atmospheric conditions. Equation (8) through Equation (10) apply:

)0.106(-0.997m A=δ (8)

where δ is the relative air density A is the altitude in kilometers m is a constant defined by

).-(G G.m 20251 00= (9)

where

)( 467BSL

)( 500BSL 1.07

)( 500CFO

0 SSSG === (10)


17


where

S is the metal-to-metal strike distance in meters Unfortunately, the solution requires an iterative process where an initial guess is made for S.

6.4.3 Phase-to-phase clearances based on switching surge conditions

Phase-to-phase BSLs have not yet been established for apparatus in the current ANSI/IEEE standards. When developing IEC 71.1 [B25], it was shown that the maximum phase-to-phase switching impulse voltage is related to its associated phase-to-ground level and that the ratio between them varied from 1.5 to 1.7, with higher ratios associated with higher system voltages. Hileman [B24] developed this concept further and found that, when the phase-to-phase switching surge overvoltages are calculated more accurately, the BSL phase–phase to phase–ground ratio for air clearance calculations is closer to 1.25–1.40 (see 6.2.3 and 6.2.4 of IEEE 1313.2-1999). Computer simulations will usually be required, and one of the computer programs available can be found in Hileman’s book [B24]. The phase-to-phase switching surge ratios are shown in Table 6.

Table 6 —Phase-to-phase switching surge factors Maximum system voltage

(kV, rms) Rated BSLph-h

(kV, crest) Rph-ph ratio

362 550 650 750 825 900 975 1050

1.35 1.35 1.35 1.35 1.35 1.37 1.37

550 900 975 1050 1175 1300 1425 1550

1.35 1.37 1.37 1.37 1.37 1.37 1.4

800 1175 1300 1425 1550 1675 1800

1.37 1.37 1.37 1.4 1.4 1.4

Selection of the phase-to-phase air clearances for switching surge conditions follows the same concepts and equations used in the selection of phase-to-ground air clearances with the substitution of appropriate phase-to-phase factors (see 6.4.1). The phase-to-phase gap factor and coefficient of variation are again used, with phase-to-phase values for these factors presented in Table 7.


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Table 7 —Switching surge factors for phase-to-phase switching impulses Gap configuration Gap factor

(kg) Coefficient of

variation (σfp/CFOo)

Conductor-conductor, 10 m length 1.35 0.035 Rod-rod 1.35 0.05 Conductor-conductor, 300 to 400 m length 1.26 0.02 Crossed conductors 1.34 0.05 Ring-ring or large eElectrode 1.53 0.05 Asymmetrical gaps rod-conductor 1.21 0.02 The conductor-conductor, 10 m length, may be used for the bus–bus clearance. The ring-ring applies to spacing between large grading rings. As an example, these types of rings are used on high-voltage (500 kV) current transformers. The rod-conductor applies to the top of a vertical-break-disconnecting switch to the upper bus. The crossed conductors apply to the spacing between the upper and the lower buses. The rod-rod applies to the spacing between small diameter grading rings. The equation relating CFO to BSL is again

( )[ ]CCFOσ1.281CFOBSL fpph-phph-ph ×−×= (11)

From Table 7, a value of 0.035 for conductor-conductor (10 m) is used for σfp/CFO and Equation (11) becomes

ph-phph-ph CFO955.0BSL ×= (12)

The air clearance is again calculated from Gallet et al.’s [B10] general expression

1CFO

δ34008

ph-ph

mg

ph-ph

−××

=K

S (13)

Substituting

ph-phgrd-phph-phph-ph BSL047.1BSL047.1CFO R××=×=

where Rph-ph is the ratio of BSLph-[h/BSLph-grd into Equation (13), using δm = 1.0, kg = 1.35 for conductor-conductor (10 m) from Table 7, and the values for Rph-[h from Table 6, the phase-to-phase air clearances based on switching surges at sea level can be calculated by

1BSL

43848

1BSL047.1

0.135.134008

ph-phgrd-ph

ph-phgrd-ph

[h-ph

−×

=

−××

××=

R

R

S

(14)

The resulting minimum phase-to-phase air clearances are shown in Table 5.


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7. Electric maintenance/safety clearances

Phase-to-phase and phase-to-ground clearances required for personnel safety are defined by various national standards, such as the NESC used in the United States. Once operating clearances (based on insulation coordination) have been established, the resulting clearances must be compared with safety code requirements (such as the NESC) as well as each user’s maintenance and operating policies and practices. Maintenance and safety clearances incorporate an additional dimension, the presence of humans working above the normal working grade on de-energized equipment, bus, and so on, but in close proximity to nearby energized parts. Safety aspects must extend insulation coordination-based clearances developed in this guide to account for the presence of operating and construction personnel. If the reduced insulation coordination clearances presented in this guide are used, care must be taken to adhere to all applicable standards to ensure that safety and working clearances are maintained. Barriers, such as enclosures or railings, may be required to keep personnel away from energized live parts. Some substation safety clearance requirements that should be considered include

a) Clearances from earth, taking into account several factors such as voltage class, height of a person, depth of snow where applicable, and height of footings.

b) Clearances to vehicles, taking into account the height of typical maintenance vehicles, as well as the height of floats and trucks that are used for the transportation of major equipment.

c) Clearances to fences.

d) Clearances to substation buildings.

e) Working clearances required to perform equipment maintenance.

8. Substation voltage uprating and compact design

The conversion of existing transmission lines and substations to higher voltages is a viable alternative to meeting the requirements of increased system demand. Uprating of 69 kV to 230 kV transmission lines has been implemented with various degrees of utilization of the original investment. A corresponding practice for substations, particularly in the high-voltage class (72.5 kV to 242 kV maximum system voltage), has been slower to develop. Voltage uprating involves increasing the operating voltage while maintaining the original insulation level. Existing substations can be uprated to a higher voltage level by replacing circuit breakers, transformers, and other voltage-sensitive equipment without having to completely rebuild the substation. By installing MOV surge arresters in specific locations such as the line entrances, the clearances at the lower voltage level will be acceptable at the uprated voltage level. This result has been accomplished at several locations in the United States. Examples include

a) 115 kV, 550 kV BIL class substations converted to 230 kV

b) 69 kV, 350 kV BIL class substations converted to 138 kV

c) 69 kV, 350 kV BIL class substations converted to 115 kV

The benefits provided with uprating can be significant. For example, the costs associated with uprating a substation to a higher voltage level are generally less than rebuilding the entire substation. New substations with compact bus spacing can be constructed in smaller areas, which may have economic benefits in areas where land costs are expensive. Where the land necessary to build a substation with conventional clearances is not available, the compact bus substation could be a viable alternative. Community acceptance of a new substation may be enhanced by using a compact bus design, which occupies a smaller footprint than that for an equivalent conventional substation. The clearances in this guide can also lead to lower bus heights, which can lead to lower costs for aesthetic treatments such as walls and other barriers if


20


these measures are required. Additions to existing substations are in some cases difficult to make because of lack of space for installing the new equipment. Employing the clearances presented in this guide will make it possible to add new equipment with the assurance that proper electrical clearances have been maintained.

8.1 BIL/System voltage ratio concept

The feasibility of voltage uprating for high-voltage class substations (72.5–242 kV) is established by insulation coordination of the various voltage stresses observed by the substation (power frequency overvoltages, lightning surge voltages, and switching surge voltages). The ratio of insulation level, or BIL, to maximum system operating voltage for high-voltage class substations is much larger than it is for EHV class substations (362 kV and above). Table 8 shows a comparison between various maximum system voltages and the BILs associated with these voltages. The comparison is intended ONLY to illustrate that the ratio has decreased with use of higher system voltages.

Table 8 —Ratio of BIL to maximum system voltage Maximum

system voltage phase-to-phase

(kV, rms)

Typical BIL

(kV, crest)

Ratio of BIL to maximum system voltage

72.5 350 4.83 121 550 4.55 145 650 4.48 169 750 4.44 242 900

1050 3.72 4.34

362 1050 1300

2.90 3.59

550 1550 1800

2.82 3.27

800 1800 2050 2300

2.25 2.46 2.88

Table 8 is not meant to be representative of all aspects of insulation coordination. For example, EHV substation clearances are often dictated by the switching surges rather than by the lightning overvoltages, and clearances below 72.5 kV are often governed by operating accessibility and animal-related outage mitigation requirements. However, the comparison of relative insulation levels does serve to illustrate the possibilities of a reduction in insulation level (and associated clearances) for substations in the high-voltage class. With the application of MOV surge arresters, insulation levels may be decreased by two or three steps up to 242 kV. This decrease may call for the addition of line entrance arresters (in those cases where none are currently employed) and possibly additional arresters at various locations on the substation bus. From Table 8, it can be concluded that the margins used in high-voltage class substations are overly conservative and that the insulation levels in high-voltage substations can be reduced. The level of reduction must be determined by the user. This conclusion is supported by the fact that the clearance standards for high-voltage substations were developed in the 1950s when available data and experience required larger safety factors, MOV surge arresters were not available, and economic savings attributed to reduced bus spacings were not justified for high-voltage substations. Furthermore, the reliability of EHV stations with ratios of BIL to system voltage under 3.00 has proven to be good. As the withstand of the substation assembly may be less than the withstand of its individual components, performance data are desirable if margins are to be reduced. Laboratory testing, full-scale testing of a substation assembly, and actual utility applications have demonstrated the feasibility of voltage uprating.


21


8.2 Other considerations

In addition to insulation coordination aspects, the following considerations should be explored when making the decision to design a new compact substation or to uprate an existing substation. All factors listed below should be factored into an analysis to determine the feasibility of the potential uprating or compact design project.

a) The clearances represent the minimum electrical clearances and may not take into account allowances for objects or animals which could appear between phases or phase-to-ground. See IEEE Std 1264TM-1993 [B73].

b) Consideration should be given to the altitude above sea level as the dielectric strength of air decreases as air density decreases (i.e., at higher elevations). The standard BIL and BSL are applied at altitudes up to 1000 m. Adjustments for altitude can be calculated from the equations given in IEEE 1313.2-1999.

c) Lightning overvoltages are the most important of the overvoltages that affect the requirements for uprated substations up to 242 kV. The direct stroke shielding of the substation should be checked to determine whether the bus and equipment have adequate protection. See IEEE Std 998TM-1996 [B71].

d) Creepage and strike requirements for insulators and bushings are influenced by the contamination levels and icing conditions present at the substation.

e) Decreased clearances result in increased electrical gradients at the conductor or hardware surface, which increases corona. This result is normally a concern only at EHV substations. However, the decreased clearances of an uprated or compact substation could result in corona levels higher than would be acceptable. Excess corona in substations leads to radio interference and audible noise and could adversely affect polymeric insulators. Corona from conductors can be controlled by ensuring that the bus conductors, including jumpers to equipment, are of adequate diameter. Corona from hardware surfaces can be reduced by the use of welded fittings, and in extreme cases EHV fittings, instead of bolted fittings. It is suggested that radio interference measurements be taken before and after the uprating takes place.

f) The operation of certain substation equipment when arcing in air is possible, such as horn gap air switches or power fuses, should be reviewed to determine whether switching operations can be continued in the same way with no change.

g) The mechanical stress on buses and insulators during fault conditions should be taken into account.

h) Maintenance practices and procedures may be hindered with the reduced clearances in uprated stations.


22


Annex A

(informative)

Bibliography

A.1 Insulation characteristics

[B1] AKA Electrical Engineering, “Flashover characteristics of rod gap and insulators,” AIEE Transactions, pp. 712–713, June 1937.

[B2] Bellaschi, P. L., “The measurement of high surge voltages,” AIEE Transactions, vol. 52, pp. 544–552, 1933.

[B3] CIGRE WG23.01, J. Parizy, Insulation Characteristic of Substations With a Nominal Voltage Up to 765 kV. ELECTRA No. 39, pp. 31–46, Mar. 1975.

[B4] CIGRE WG 33.03.03, G. Carrara, Switching Impulse Performance of Post Insulators. ELECTRA No. 109, pp. 115–132, Dec. 1986.

[B5] CIGRE Working Group on Overvoltages and Insulation Coordination, “A critical comparison of artificial pollution test methods for HV insulators,” CIGRE International Conference on Large High Voltage Electra Systems, No. 64, pp. 117–136, May 1979.

[B6] CIGRE Working Group on Overvoltages and Insulation Coordination, “The measurement of site pollution severity and its application to insulator dimensions for AC systems,” CIGRE International Conference on Large High Voltage Electra Systems, No. 64, pp. 101–116, May 1979.

[B7] Dowell, J. C. and Foust, C. M., “Laboratory measurement of impulse voltages,” AIEE Transactions, vol. 52, pp. 537–543, 1933.

[B8] EEI-NEMA Subcommittee on Correlation of Laboratory Data, “Recommendations for high voltage testing,” AIEE Transactions (AKA Electrical Engineering), vol. 59, pp. 598–602, Oct. 1940.

[B9] Feser, K., and Pigini, A., “Influence of atmospheric conditions on the dielectric strength of external insulation,” ELECTRA No. 112, pp. 83–95, May 1987.

[B10] Gallet, G., et al., “General expression for positive switching impulse strength valid up to extra long air gaps,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-94, pp. 1989–1993, Nov./Dec. 1975.

[B11] Gallet, G., et al., “Switching impulse results obtained on the outdoor testing area at Renardieres,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-95, pp. 580–585, Mar./Apr. 1976.

[B12] Gallet, G., et al. “Analysis of the switching impulse strength of phase-to-phase air gaps,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-97, pp. 485–494, Mar./Apr. 1978.

[B13] Hileman, A. R. “Weather and its effect on insulation specifications,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-103, pp. 3104–3116, Oct. 1984.


23


[B14] IEEE WG on Characteristics of Protective Gaps, “Sparkover characteristics of high voltage protective gaps,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-93, pp. 196–205, Jan./Feb. 1974.

[B15] Isa, H., and Hayashi, M., “Breakdown phenomena in non-uniform short air gap under impulse voltage,” Proceedings of Third International Symposium on High Voltage Engineering, August 28–31, 1979, Milan, Italy.

[B16] Moran, J. H., “Switching surge study of EHV station posts: II,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-88, pp. 238–244, Mar. 1969.

[B17] Moran, J. H., and Alton, R. J., “Switching surge study of EHV station posts: I,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-87, pp. 1471–1480, June 1968.

[B18] Paris, L. “Influence of air gap characteristics on line-to-ground switching surge strength,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-86, pp. 936–974, Aug. 1967.

[B19] Paris, L., and Cortina, R., “Switching and lightning impulse discharge characteristics of large air gaps and long insulator strings,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-87, pp. 47–57, Apr. 1968.

[B20] Thione, L., “Evaluation of switching impulse strength of external insulation,” Electra, pp. 77–95, May 1984.

A.2 Basic substation insulation coordination procedures

[B21] CAN3-C308-M85 (1985), The Principles and Practice of Insulation Coordination.

[B22] Diesendorf, W., Insulation Coordination in High Voltage Electric Power Systems. London, U.K.: Butterworth, 1974.

[B23] Hileman, A. R., “Insulation Coordination in Air Insulated Stations,” in Surges in High Voltage Networks, Ragaller, K., ed. New York: Plenum Press, 1980, pp. 323–344.

[B24] Hileman, A. R., Insulation Coordination for Power Systems. New York: Marcel-Dekker, June 1999.

[B25] IEC-71.1 (1994), Insulation Coordination, Part 1: Definition/Principles and Rules.7

[B26] IEC-71.2 (1994), Insulation Coordination, Part 2: Application Guide.

[B27] IEC-71.3 (1982), Insulation Coordination, Phase-to-Phase Insulation Coordination.

[B28] IEC 99-1 (1991), Surge Arresters, Part 1, Non-Linear Resistor-Type Gapped Arresters for AC Systems.

[B29] IEC 99-3 (1990), Surge Arresters, Part 3, Application Guide to IEC 99-1 and 99-4.

[B30] IEC 99-4 (1991), Surge Arresters, Part 4, Metal-Oxide Arresters Without Gaps for AC Systems.

7 IEC publications are available from the Sales Department of the International Electrotechnical Commission, Case Postale 131, 3, rue de Varembé, CH-1211, Genève 20, Switzerland/Suisse (http://www.iec.ch/). IEC publications are also available in the United States from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/).


24


A.3 Electrical operating/design clearances

[B31] AIEE Working Group of the Lightning Protective Devices Subcommittee, “Simplified method for determining permissible separation between arresters and transformers,” IEEE Transactions on Power Apparatus and Systems, vol 582, pp. 35–571, 1963.

[B32] AIEE Committee Report, “A guide for minimum electrical clearances for standard basic insulation levels,” AIEE Transactions, Part IIIA, vol. 73, pp. 636–641, June 1954.

[B33] AIEE Committee Report, “Standard basic impulse insulation levels,” AIEE Transactions Part IIIA, vol. 78, pp. 10–12, 1959.

[B34] AIEE Committee Report, “Interim report—minimum electrical clearances for substations based on switching surge requirements,” AIEE Transactions, pp. 1072–1076, Dec. 1963.

[B35] AIEE Transactions (Power Apparatus and Systems), Special Supplement no. 81, pp. 33–57, 1963.

[B36] Boehne, E. W., and Carrara, G. Switching Surge Insulation Strength of EHV Line and Station Insulation Structures, CIGRE Paper 415, 1964.

[B37] Bruer, G. D., Hopkinson, R. H, Johnson, I. B., and Shultz, A. J., “Arrester protection of high voltage stations against lightning,” AIEE Transactions (Power Apparatus and Systems), no. 79, pp. 414–422, 1960.

[B38] Carpenter, T. J., Johnson, I. B., and Saline, L. E., “Evaluation of lightning—arrester lead length and separation in coordinated protection of apparatus against lightning,” AIEE Transactions, vol. 69, pp. 933–944, 1950.

[B39] CIGRE Working Group 07 of Study Committee 23, Insulation Characteristics of Substations With a Nominal Voltage Up to 765 kV. Electra No. 39, pp. 31–46.

[B40] CIGRE Working Group on Overvoltages and Insulation Coordination, “Phase-to-phase insulation coordination,” CIGRE International Conference on Large High Voltage Electric Systems, No. 64, pp. 137–236, May 1979.

[B41] Clayton, J. M., and Powell, R. W., “Application of arresters for complete lightning protection of substations,” AIEE Transactions, vol. 77, pp. 1608–1614, Feb. 1959.

[B42] Clayton, J. M., and Young, F. S., “Application of arresters for lightning protection of multi-line substations,” AIEE Transactions, vol. 79, pp. 566–575, 1960.

[B43] Clem, J. E., Meador, J. R., Rudge, W. J., and Powell, A. H., “Proposed basic impulse insulation levels for high voltage systems,” AIEE Transactions, vol. 69, pp. 953–963, 1950.

[B44] Gert, R., Jirku, J., Lohlanina, H. I., and Dashkes, V. S., “Phase-to-phase switching overvoltages in EHV systems,” CIGRE International Conference on Large High Voltage Electric Systems, No. 33-03, 1978.

[B45] Hertig, G. E., and Kelly, W. B., “Switching surge test results EHV substation bus configurations,” IEEE Transactions on Power Apparatus and Systems, pp. 846–858, Aug. 1966.

[B46] IEEE Committee Report, “Second interim report—minimum electrical clearances for substations based on switching surge requirements,” IEEE Transactions on Power Apparatus and Systems, pp. 415–417, May 1965.


25


[B47] IEEE Working Group 59.1 Transmission Substation Subcommittee IEEE Substations Committee, “Minimum line-to-ground electrical clearances for EHV substations based on switching surge requirements,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-91, no. 3, pp. 1924-1930, May/June 1972.

[B48] Kishizina, I., et al., “New facilities for phase-to-phase switching impulse tests and some test results,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-103, no. 6, pp. 1211–1216, June 1984.

[B49] Melvin, H. L., and Pierce, R. E., “Application of spill gaps and selection of insulation levels,” AIEE Transactions, pp. 689–694, June 1937.

[B50] Musgrave, D. C., Agenda for the April 30, 1979, Meeting of Working Group 59.1 of the Transmission Substation Subcommittee in St Petersburg, Florida.

[B51] Menemenlis, C., Avis, H., and Harbec, G., “Phase-to-phase insulation Part I: Generalized effects of stress parameters and gap geometry,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-95, no. 2, pp. 643–650, Mar./Apr. 1976.

[B52] Paris, L., Taschim, A., Schneider, K. H., and Weck, K. H., “Phase-to-ground and phase-to-phase air clearances in substations,” CIGRE Committee on Substations and Overvoltages and Insulation Coordination, Electra, No. 29, pp. 29–44, July 1973.

[B53] Task Force on External Clearance Requirements, Minimum External Clearances Between Live Parts of Different Phases of the Same Voltage (345 and 500 kV Nominal System Voltages), 1987.

[B54] Udo, T., “Minimum phase-to-phase electrical clearances for substations based on switching surges and lightning surges,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-85, no. 8, pp. 838–845, Aug. 1966.

[B55] Vogel, F.J., “Margins between lightning arrester protection levels and transformer insulation,” AIEE Transactions, vol. 69, pp. 110–114, 1950.

[B56] Watson, Jr., T. F., and Hiatt, R., “Line entrance gaps for protection of substation insulation,” AIEE Transactions, pp. 43-55, Apr. 1961.

[B57] Whitehead, E. R., Final Report on Mechanism of Lightning Flashover Research Project, EEI Report RP50, Pub 72-900, Feb. 1971.

[B58] Witzke, R. L., and Bliss, T. J., “Coordination of lightning arrester location with transformer insulation level,” AIEE Transactions, vol. 69, pp. 964–975, 1950.

A.4 Substation voltage uprating/compact design

[B59] Anderson, J. G., Minimum Clearance and Insulation Characteristic of Uprated 230 kV Substation. Engineering Report prepared for LP&L by General Electric Company, Oct. 1964.

[B60] Anderson, J. G., and DiPietro, J., 138/230 kV Substation Uprating Study for Central Louisiana Electric Company. General Electric Company/EUSED report, 1974.

[B61] Barthold, L. O., et al., Transmission Line Reference Book 115-138 kV Compact Line Design. EPRI, 1978.


26


[B62] Carter, R. T., et al., “Analysis of radio interference and substation modifications for uprating a 115 kV substation to 230 kV,” IEEE Transactions on Power Delivery, vol. PWRD-2, no. 2, pp. 544–555, Apr. 1987.

[B63] Chaply, D., “Link substation upratings with reduced insulation levels to realize cost and schedule savings,” EEI Meeting, Sept. 1997.

[B64] Elahi, H., Panek, J., et al., “Substation voltage uprating: Design and experience,” IEEE Transactions on Power Delivery, vol. 6, no. 3, pp. 1049–1057, July 1991.

[B65] EPRI Report EL-6474, Project 2794-1, Substation Voltage Upgrading, General Electric, Aug. 1989.

[B66] EPRI Report EL 3147, Phase-to-Phase Switching Surge Design. Addendum to Transmission Line Reference Book 115-138 kV Compact Line Design, Grant, I. S., et al., June 1983.

[B67] IEEE Std 4TM-1995, IEEE Standard Techniques for High-Voltage Testing.8, 9

[B68] IEEE Std 100TM, IEEE Standard Dictionary of Electrical and Electronics Terms.

[B69] IEEE Std 605TM-1987, IEEE Guide for Design of Substation Rigid-Bus Structures (withdrawn).

[B70] IEEE Std 605TM -1998, IEEE Guide for Design of Substation Rigid-Bus Structures.

[B71] IEEE 998TM-1996 (Reaff 2002), IEEE Guide for Direct Lightning Stroke Shielding of Substations.

[B72] IEEE Std 1127TM-2004, IEEE Guide for the Design, Construction and Operation of Safe and Reliable Substations for Environmental Acceptance.

[B73] IEEE Std 1264TM-1993, IEEE Guide for Animal Deterrents for Electric Power Supply Substations.

[B74] IEEE Std C62.1TM-1989 (Reaff 1994), IEEE Standard for Gapped Silicon-Carbide Surge Arresters for AC Power Circuits.

[B75] IEEE Std C62.2TM-1987 (Reaff 1994), IEEE Guide for the Application of Gapped Silicone-Carbide Surge Arresters for Alternating Current Systems.

[B76] IEEE Std C62.11TM-1999, IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current Power Circuits.

[B77] IEEE Std C62.22TM-1997, IEEE Guide for the Application of Metal Oxide Surge Arresters for Alternating-Current Systems.

[B78] IEEE/ANSI Std C84.1TM-1995 (Reaff 2001), American National Standard for Electric Power System and Equipment - Voltage Rating (60 Hz).

[B79] IEEE Substation Committee WG E1: Recommended Minimum Clearances in Substations, “Safety aspects in substation voltage uprating,” IEEE Transactions on Power Delivery, vol. 7, no. 3, pp. 1250–1255, July 1992.

[B80] Kennon, R., “Uprating options for lines and rights-of-way,” EPRI document presented at IEEE PES 1987 Winter Meeting, New Orleans, LA, Feb. 1–6.

8 IEEE publications are available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 9 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc.


27


[B81] LaForest, J. J., and Durbak, D. W., “525 kV reliability analysis for PSNM (Insulation and Voltage Uprating Study),” General Electric Company/EUSED report, 1982.

[B82] LaForest, J. J., et al., Transmission Line Reference Book 345 kV and Above/Second Edition. EPRI, 1982.

[B83] Lannes, W. J., “Modification of 115 kV substation to operate at 230 kV,” Paper presented to EEI Systems and Equipment Committee Meeting, Denver, CO, Feb. 1984.

[B84] Lannes, W. J., “Operational Experience of Louisiana Power and Light Company’s Upgraded Pontchartrain Substation,” Paper presented at Edison Electric Institute. Electrical System and Equipment Committee Meeting, Shreveport, LA, Feb. 24, 1987.

[B85] Lannes, W. J., et al., “230 kV operation of a substation designed for 115 kV by controlling voltage transients,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-90, pp. 1698–1718, 1990.

[B86] Neibukr, W. D. and Reid, W. E., “Overvoltage analysis to determine the surge arrester ratings and locations at the Louisiana Power and Light Company, Sterlington 500 kV gas insulation substation,” Engineering report prepared for LP&L by McGraw Edison Company, Apr. 1980.

[B87] Panek, J., and Elahi, H., “Substation Voltage Uprating Study 4: PSCC Western Avenue 115 kV—1990 system,” General Electric Company/SDED report, 1987.

[B88] Panek, J., and Elahi, H., “Substation voltage upgrading,” IEEE Transactions on Power Apparatus and System, vol 4, no. 3, pp. 1715–1724, July 1989.

[B89] Panek, J., et al., Substation Voltage Uprating. CIGRE 1992 Session, No. 33-207.

[B90] Saavedra, R., “Experience with uprated substations,” IEEE PES Summer Meeting, July 1989.

[B91] Wilkinson, G. A., et al., “Uprating transmission voltages—a coordinated design approach,” Proceedings of the American Power Conference, p. 1245, 1973.


28


Annex B

(informative)

Example calculations

B.1 Introduction

The purpose of this example is to present a procedure that can be used to estimate the following:

a) The BIL, BSL of the substation

b) The phase-to-ground and phase-to-phase clearances

The BIL and BSL calculations, which are presented in this example, are based on the method outlined in the Insulation Coordination for Power Systems by A. R. Hileman [B24]. This guide does not cover the BIL and BSL calculations. The phase-to-ground and phase-to-phase clearances calculations use the procedure outlined in this guide.

B.2 Given

Figure B.1 represents a 230-69 kV substation with two 230 kV lines and a 230-69 kV transformer. All distances between equipment are given in meters. Lightning arresters are installed on the high side of the transformer.

Figure B.1—230-69 kV substation insulation coordination calculations example


29


Time for the surge wave travel is measured from the device to the junction point at which the arrester leads are connected to the bus, point J. The time is calculated by the following equation:

μsm 300(meter) Distance=T

where 300 m/μs is the velocity of propagation. The time can be calculated and summarized in Table B.1 and Table B.2.

Table B.1—Summary of time for the surge travel Time to point “J” From Time

(μs) TT Transformer 0.01 TA Arrester 0.0067 TB1 Bus Point 1 0.033 TB2 Breaker 2 0.076 TB3 Switch 3 0.0933 TB4 Switch 4 0.067 TB5 Breaker 5 0.13 TB6 Switch 6 0.146

Table B.2—Given data for BIL calculations Data description Data for this

example Symbols

Incoming surge “Em” 1556 kV Ea,b Substation elevation-meter 2000 A

Substation data

Safety factor for BIL calculation 15% SF MCOV (kV) 140 Arrester discharge voltage kV 426 Ed

Transformer arresterc

Arrester voltage kV 557 Incoming line voltage (phase-to-phase) kV 230 KVLL Incoming lines number of conductors/phase 2 Number of lines connected to the bus 2 n Line span length (meter) 250 Line configurations Horizontal KPF Line surge impedance (phase conductor) 488 Ω Z Transformer capacitance 4nF CT Breaker capacitance 4nF MTBF (mean time between failure) years 100 MTBF Back flash rate of the line-BFR(FO/100 km-years) 2 BFR aThe calculation of the incoming surge is beyond the scope of this guide. The magnitude of the incoming surge depends on such factors as the critical flashover (CFO) of the line, the back flash rate (BFR), mean time between failure (MTBF), soil resistivity, tower ground resistance, and line configurations. The incoming surge can be estimated at 120% of the CFO of the line. bThe maximum steepness of the incoming surge is calculated based on the MTBF, the BFR, and a constant depending on the number of conductors per phase. cArrester selection is also beyond the scope of this guide. The arrester maximum continuous operating voltage (MCOV) is normally based on maximum system voltage, the temporary overvoltage, lightning voltage crests, and the switching surge over voltage. Based on the arrester MCOV, the surge impedance of the line and the incoming surge, the voltage at the arrester EA can be calculated.


30


B.3 Calculations

The process of determining the BIL of the substation consists of first calculating the surge voltages that will develop at different points of the substation bus and equipment caused by the incoming surge. The surge voltages can be calculated by the use of a computer program. In this example, a simplified method will be used. This method is beyond the scope of this guide. It has been presented in this example to give the user of this guide the background of a method that can be used to determine the BIL of the substation. See the references cited in the main body of the guide. Once the maximum surge voltage at the substation equipment and bus is determined, the BIL can be calculated. As shown in Figure B.1, the surge voltages are defined as follows:

EA is the surge voltage at arrester Ed is the arrester discharge voltage (phase-to-ground) Eb is the surge voltage at equipment/bus/switches not including power frequency voltage EB is the surge voltage at equipment/bus/switches including power frequency voltage Et is the surge voltage at transformer not including power frequency voltage ET is the surge voltage at transformer including power frequency voltage Ej is the surge voltage at the arrester to bus connection not including power frequency voltage VPF EJ is the surge voltage at the arrester to bus connection including power frequency voltage VPF

The above voltages are related by the following equations:

EA = Ed + VPF

EB = Eb + VPF

ET = Et + VPF

EJ = Ej + VPF

where the voltage

KPFV32V LLPF ××= K

where KPF is 0.7 for horizontal phase configuration 0.4 is vertical phase configuration 0.7 is uncertain

For this example

kV 131.45VPF =


31


The incoming surge steepness (S) is required for the calculations of the maximum surge voltages that are generated on the substation bus and equipment. The surge steepness (S) is calculated by the following equation:

m

SdKS =

where Ks is given in Table B.5 and dm is calculated by the following equation:

MTBSBFR1

m ×=d

where BFR is given in Table B.2 and MTBS (mean time between surges) is equal to

MTBST (Transformer) = n MTBF

MTBSB (Breaker) = MTBF

where MTBST is used to calculate the surge steepness for the transformer and MTBSB is used to calculate the surge steepness for other substation equipment, such as breakers, switches, and other substation equipment. For this example:

n (number of lines connected to the bus) = 2

2002100TMTBS =×=

100BMTBS =

Surge steepness for transformer:

BFR = 2 FO/100 km-years or BFR = 0.02 FO/km-year

m 250km 0.252000.02

1m ==

×=d

because the span length is equal to 250 m:

= 0.25 km dm/span length must be rounded to integer

S = 1000/0.25 = 4000 kV/µs


32


Table B.3 indicates the voltage calculations used to determine BIL. Surge steepness for other substation equipment:

dm = 1/0.02 × 100 = 0.5 km

S = 1000/0.5 = 2000 kV/µs

Using the above equations, the voltage magnitudes at different point of the substation can be calculated. For this example, using a spreadsheet, a summary of these voltage magnitudes is shown in Table B.6.


33


Table B.3—Voltage calculations Surge voltage calculation Location

Transformer capacitance CT = 0 (open circuit)

Transformer capacitance CT ≠ 0

Transformer Condition 1: S T T

Eto

nT A

A

( )+= 0

6 E K E S

nT TT A T A= + +

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

22 ( )

Condition 2: S T T

En

tonT A

A

( )+=

+6

14

( )[ ]ATA2

T 23

4 TTSEn

KE +++

=

n is the number of lines connected to bus

E E AB K

T A= ++

⎛

⎝

⎜⎜⎜⎜

⎞

⎠

⎟⎟⎟⎟

11 1

where A and B are given in Table B.4: ( )K

S T TET A

A1 =

+

Time to crest (µs) of the transformer voltage can be estimated by the following equation:

( )( )t T T ZC T TES

T T A T T AA

= + + + +π

Based on time tT

Arrester Bus Connection A2kJ STEE +=

211J KB

AE+

+= — AA

2 EST

K =

Breaker/Switch and Bus

)TS(TEE AB2AB ++=

where TB = TB1, TB2, TB3, etc. See Table B.1.

Calculate the time tA for the arrester to operate using the following equation:

⎟⎟⎠

⎞⎜⎜⎝

⎛ −−−=

τττ

A1A2

A te

tS

nE

where

nZCT=τ

n is the number of line connected to the bus Z is the surge impedance of the phase conductor tA can be calculated using the iteration method. Based on the calculated time tA, the voltage behind the arrester is calculated: For:

( )[ ]AB2fA TTtt +−≤

where

SEt =f

E is the incoming surge voltage magnitude. S is the incoming surge steepness. See above for steepness calculations.

B2JB STEE += For:

( )[ ]AB2fA TTtt +−≥

( )[ ]⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡−

−−+−−=

⎟⎠⎞

⎜⎝⎛ +−−ττ

AB2f12222

BAB

TTt

enSTTS

nn

nEE


34


Table B.4—Values for A, B, and K2 Number of lines

connected to substation “n”

A B K2

1 1.0 0.14 0.91 2 0.98 0.16 0.93 3 0.84 0.18 0.95 4 0.68 0.25 0.97

Table B.5—Suggested values for the constant Ks Number of conductors/phase Ks

1-conductor 700 2-conductor 1000 3 or 4-conductor 1700 6 or 8-conductor 2500

Table B.6—Voltage calculations Voltage location Voltage magnitude (kV)

Transformer internal busing 660 Transformer external busing 660 Point 1 (EB1) 564 Point 2 (EB2) 590 Point 3 (EB3) 606 Point 4 (EB4) 581 Point 5 (EB5) 644 Point 6 (EB6) 662

B.4 BIL calculations

The BIL calculations consist of the following.

B.4.1 Non-self-restoring insulations (transformer internal insulations)

To calculate the internal BIL of the transformer, the incoming surge time to crest tc must be estimated using the following equation:

⎟⎠

⎞⎜⎝

⎛ +−=E

Et2

V77.0ln PFdc τ

where Ed, VPF, and E are defined or given above.


35


Two conditions are considered as follows: For an incoming surge with time to crest tC less than 60 µs, the BIL is given by the following equation:

( ) dFBIL ES=

If tT = 3.0 µs and Et/Ed = 1.1

( )1.1

BIL dF

ES=

If tT = 3.0 µs and Et/Ed > 1.1

( )ESFBIL =

If tT > 3.0 µs, where SF is the safety margin, a value of 1.2 is suggested.

For an incoming surge with time to crest tC is greater than 60 µs for incoming surge caused by a shielding failure without a flashover, the BIL is given by the following equations:

( )d

F83.0

BIL ES=

B.4.2 Self-restoring insulators

The BIL is calculated by the following equations:

δdBIL E= 15.1If

d

b >EE

δBIL bE= 15.1If

d

b >EE

6.8A

e−

=δ

where A is the elevation in kilometers.


36


Table B.7 shows the results for the example given in Figure B.1.

Table B.7—Summary of voltage and BIL calculation Voltage

magnitude Calculated BIL Standard required BIL Voltage

location Sea level Sea level

elevation 2000 m

elevation Sea level elevation

2000 m elevation

Selected BIL

Transformer internal busing

660 690 690 750 750 750

Transformer external busing

660 690 731 750 750 750

Point 1 (EB1) 564 492 625 550 650 900 Point 2 (EB2) 590 515 654 550 750 900 Point 3 (EB3) 606 529 672 550 750 900 Point 4 (EB4) 581 506 643 550 650 900 Point 5 (EB5) 644 562 713 650 750 900 Point 6 (EB6) 662 578 734 650 750 900

B.4.3 Phase-to-ground clearance calculation based on the selected BIL, which covers both sea level and 2000 m elevations

Using Equation (3) of the guide, the minimum phase-to-ground clearance is given by the following equation:

Sph-grd = BIL/526 = 900/526 = 1.71 m (5.61 ft)

B.4.4 Phase-to-phase clearance calculations

Using 6.3.2 of the guide, the phase-to-phase clearance is equal to

Sph-ph = 1.1 Sph-grd = 1.1 × 1.71 m = 1.88 m (6.17 ft)

B.4.5 Phase-to-ground clearances based on switching surge

To calculate the phase-to-ground clearance based on switching surge, the BSL must be determined. The following data are required for the calculations of the BSL:

⎯ Statistical withstand voltage (V3) is the minimum insulation strength required and is set three standard deviation below the critical flash over in equation form:

f3 3CFOV σ−=


37


⎯ The switching surge voltage E2 is the maximum surge (phase-to-ground) voltage applied to the substation equipment, and insulators for this example assume E2 = 2.59 p.u.

⎯ The switching Surge Flashover Rate (SSFOR) is the number of flashover per number of applied switching. For example for 1001SSFOR = means that there will be one flashover per hundred switching. For this example, 1001SSFOR = .

⎯ For a given SSFOR, the 23V E can be calculated for this example and assume 9.0V 23 =E .

⎯ Assume 07.0CFOf =σ .

⎯ Station Elevation = 2000 m

⎯ Maximum system voltage = 242 kV

⎯ Gap factor = 1.3

Calculations:

3

2VV LLbase

×= K

56.19732242Vbase =×=

Calculate V3 at the substation:

9.0V

2

3 =E

.p.u33.29.059.2V3 =×=

kV31.46056.19733.2V3 =×=

Assuming the transmission line phase-to-ground withstand voltage V3 (line) is equal to 2.53 p.u. because no arresters are installed at the line entrance, the substation must be designed based on the line phase-to-ground withstand voltage as it is higher than the substation withstand voltage. Based on this design, the substation phase-to-ground withstand voltage is equal to

p.u.53.2V3 =

kV05.49956.19753.2V3 =×=

The CFO at the substation elevation is calculated by the following equation:

f

331

VCFOσ−

=

CFO at the station elevation is equal to p.u.20.3)07.031(53.2 =×− :

kV63256.19720.3CFO =×=


38


Using Equation (5):

kV5766329104.0CFO9104.0BSL =×=×=

Using Equation (7) in the guide, the phase-to-ground clearance at sea level is equal to 1.34 m (4.41 ft). If the substation is at 2000 m elevation altitude, adjustment is required per 6.4.2 of the guide. To calculate the phase-to-ground clearance S, the solution requires an iterative process. Assuming, m = 0.5, then δm = 0.915, and using Equation (6), the phase-to-ground clearance is calculated with the last five iterations shown as follows:

S CFOs G0 m δm S 1.54 713.53 0.93 0.84 0.82 1.70 1.70 775.04 0.91 0.81 0.82 1.69 1.69 769.16 0.91 0.81 0.82 1.69 1.69 769.72 0.91 0.81 0.82 1.69 1.69 769.66 0.91 0.81 0.82 1.69

From these calculations, the phase-to-ground clearance based on switching surge is equal to 1.69 m (5.53 ft).

B.4.6 Phase-to-phase clearances based on switching surge

Station elevation is at sea level. Assume that the phase-to-phase withstands voltage V30 is equal to 2.8 p.u., the positive critical flashover (when negative component equals to zero) CFO0 is given by the following equation:

0CFOfp31

30V0CFO σ

−

=


39


For conductor-conductor 10 m long or rod-rod, the σfp/CFO0 = 0.035 and therefore

CFO0 = 1.117 V30 = 1.117 × 2.8 = 3.13 = 3.13 × 197.6 = 618.5 kV

The phase-to-phase CFOp is given by the following equation:

)1(1CFOCFO

L

0p K−−

=α

For α = 0.5, KL = 0.67 (conductor to conductor 10 m long), the CFOp is calculated to equal

kV 741p.u.75.3CFO 197.1CFO 0p ===

The phase-to-phase BSL is given by the following equation:

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

p

fppp CFO

28.11CFOBSLσ

For σfp/CFOp = 0.035 (conductor to conductor 10 m long):

BSLp = 0.955 CFOp = 0.955 × 3.75 = 3.58 p.u.

BSLp = 3.58 × 197.6 = 708 kV

The gap factors used in the above equation are given in Table B.8.

Table B.8—Gap factors for phase-to-phase switching impulses Alpha method V+ – V– Gap configuration

α kg σf/CFOp KL kg σf/CFOp Conductor-conductor, 10 m 0.33

0.5 1.52 1.62

0.035 0.035

0.67 1.35 0.035

Rod-rod 0.33 0.5

1.52 1.62

0.05 0.05

0.67 1.35 0.05

Conductor-conductor, 300 m to 400 m 0.33 0.5

1.53 1.65

0.05 0.05

0.62 1.34 0.05

Crossed conductors 0.33 0.5

1.53 1.65

0.05 0.05

0.62 1.34 0.05

Ring-ring or large electrode 0.33 0.5

1.7 1.8

0.05 0.05

0.7 1.53 0.05

Asymmetrical gaps rod conductor 0.33 0.5

1.36 1.45

0.05 0.05

0.67 1.21 0.05

Supported bus bar fitting 0.33 0.5

1.4 1.5

0.05 0.05

0.65 1.23 0.05

Jumper-shield ring 0.33 0.5

1.57 1.68

0.04 0.04

0.66 1.39 0.04

Conductor-conductor 47 m 0.33 0.5

1.56 1.66

— —

0.68 1.4 —


40


Using Equation (13) to calculate phase-to-phase clearance at sea level where δm = 1:

1CFO

)(34008

phph

mg

phph

−

=

−

− δKS

S = 1.54 m (5 ft) At 2000 m using the formula in the guide, the required BSL and phase-to-phase clearances are calculated using the guide equations to equal:

BSL = 873 kV

S = 1.98 m (6.49 ft)

Table B.9 lists the results of the preceding calculations.

Table B.9—Calculations summary Phase-to-ground clearance

(m) Phase-to-phase clearance

(meter) BSL BSL BIL

for both sea level and 2000 m elevation

Sea level 2000 m elevation

BIL for both sea level and

2000 m elevation Sea level 2000 m

elevation 1.71 1.34 1.69 1.88 1.54 1.98

Documents

IEEE Std 1427-2006, IEEE Guide for Recommended Electrical