IEEE Std 400.1™-2007
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IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Sponsored by the Insulated Conductors Committee 400.1 TM IEEE Power Engineering Society IEEE 3 Park Avenue New York, NY 10016-5997, USA 21 September 2007 Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1™-2007 (Revision of IEEE Std 400™-1991) Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1™-2007 (Revision of IEEE Std 400™-1991) IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Sponsor Insulated Conductors Committee of the IEEE Power Engineering Society Approved 22 March 2007 IEEE-SA Standards Board Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. Abstract: The recommended practices and procedures for direct voltage acceptance and maintenance testing of shielded, laminated dielectric insulated power cable systems rated 5 kV and above are presented in this guide. It applies to all types of laminated power cable systems such as paper insulated, lead covered, pipe-type, and pressurized cables that are intended for the transmission or distribution of electric power. The tabulated test levels assume that the cable systems have an effectively grounded neutral system or a grounded metallic shield. Keywords: cable, cable installation, cable maintenance, cable tests, field test procedures, high direct voltage tests, insulated cable, power cable systems, shielded power cable systems _________________________ 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 21 September 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: PDF: ISBN 0-7381-5579-9 ISBN 0-7381-5580-2 SH95675 SS95675 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. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. 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. 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Introduction This introduction is not part of IEEE Std 400.1-2007, IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage. To say that there is a marked difference of opinion on the matter of cable testing would be a decided understatement. Many users, particularly utilities, while practicing acceptance testing, do not favor maintenance testing, or testing after cable systems have been in service, believing that such tests may shorten cable life. A few utility users, and many industrial users, favor both acceptance and maintenance testing, believing that such testing will contribute to improved service reliability. Others feel that either acceptance or maintenance testing can cause cable damage, resulting in premature failures and customer dissatisfaction. Annex C of this guide gives additional background and more detailed commentary on these attitudes and philosophies. There is undoubtedly much to be said both for and against all viewpoints; only the individual user can determine whether, how frequently, and at what stresses testing is to be conducted. In short, this guide does not suggest that cable system testing be done; it simply provides guidance for such testing, developed by those who have found it useful. Additionally, it provides interpretive information based on many years of experience. Finally, pervading the entire procedure, safety has been a constant consideration in each step of the recommended practices. It is hoped that use of this guide will increase the fund of knowledge on the subject and result in more meaningful testing procedures and methods. Suggestions for improvements to this guide are welcome. They should be sent to the Secretary, IEEE-SA Standards Board, 445 Hoes Lane, Piscataway, NJ 08854, USA. Notice to users Errata Errata, if any, for this and all other standards 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. iv Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. Participants At the time this guide was submitted to the IEEE-SA Standards Board for approval, the Insulated Conductors Committee C-17 Working Group had the following membership: William E. Larzelere, Jr., Chair Vern L. Buchholz, Secretary Torben Aabo John R. Densley Hans R. Gnerlich Martin von Herrmann Stanley V. Heyer Fred B. Koch Jerry L. Landers William M. McDermid James D. Medek Henning H. Oetjen Johannes G. Rickmann Lawrence W. Salberg William A. Thue Joseph Zimnoch The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Torben Aabo S. K. Aggarwal Thomas M. Barnes Earle C. Bascom, III Wallace B. Binder, Jr. Steven R. Brockschink Kent W. Brown Vern L. Buchholz James S. Case Mark S. Clark Michael D. Clodfelder John H. Cooper Tommy P. Cooper Jerry L. Corkran Jorge E. Fernandez Daher John R. Densley Ernest M. Duckworth, Jr. Gary R. Engmann R. B. Gear, Jr. Frank J. Gerleve Harry D. Gianakouros Randall C. Groves Adrienne M. Hendrickson Gary A. Heuston Lauri J. Hiivala Dennis Horwitz David W. Jackson A. S. Jones James H. Jones Lars E. Juhlin Gael Kennedy J. L. Koepfinger Jim Kulchisky William E. Larzelere, Jr. Solomon Lee Jody P. Levine Albert Livshitz William Lumpkins G. L. Luri Glenn J. Luzzi Keith N. Malmedal William M. McDermid Mark F. McGranaghan Nigel P. McQuin John E. Merando, Jr. James R. Michalec Gary L. Michel Rachel I. Mosier Kyaw Myint Shantanu Nandi Michael S. Newman Charles Kamithi Ngethe Ralph E. Patterson Serge Pelissou Johannes G. Rickmann Michael A. Roberts Michael J. Smalley Nagu N. Srinivas James E. Timperley Martin J. von Herrmann Waldemar G. Von Miller Mark D. Walton Joe D. Watson Ernesto Jorge Wiedenbrug William D. Wilkens James W. Wilson, Jr. Donald W. Zipse Ahmed F. Zobaa v Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. When the IEEE-SA Standards Board approved this guide on 22 March 2007, it had the following membership: Steve M. Mills, Chair Robert M. Grow, Vice Chair Don F. Wright, Past Chair Judith Gorman, Secretary Richard DeBlasio Alexander D. Gelman William R. Goldbach Arnold M. Greenspan Joanna N. Guenin Julian Forster* Kenneth S. Hanus William B. Hopf Richard H. Hulett Hermann Koch Joseph L. Koepfinger* John D. Kulick David J. Law Glenn Parsons Ronald C. Petersen Tom A. Prevost Narayanan Ramachandran Greg Ratta Robby Robson Anne-Marie Sahazizian Virginia C. Sulzberger Malcolm V. Thaden Richard L. Townsend Howard L. Wolfman *Member Emeritus Also included are the following nonvoting IEEE-SA Standards Board liaisons: Satish K. Aggarwal, NRC Representative Alan H. Cookson, NIST Representative Lorraine Patsco IEEE Standards Program Manager, Document Development Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development vi Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1 2. Normative references.................................................................................................................................. 1 3. General ....................................................................................................................................................... 2 3.1 Environmental influences .................................................................................................................... 2 3.2 Test equipment .................................................................................................................................... 3 4. Test procedure ............................................................................................................................................ 4 4.1 Test precautions................................................................................................................................... 4 4.2 Safety practices.................................................................................................................................... 5 4.3 Testing procedure ................................................................................................................................ 5 4.4 Recording of test results ...................................................................................................................... 8 5. Evaluation of results ................................................................................................................................... 8 5.1 Current-time relationships ................................................................................................................... 8 5.2 Resistance values................................................................................................................................. 9 Annex A (informative) Reasons for testing.................................................................................................. 11 A.1 Acceptance tests................................................................................................................................ 11 A.2 Maintenance tests.............................................................................................................................. 11 A.3 Corrective actions ............................................................................................................................. 11 Annex B (informative) Protection against possible severe voltage conditions due to flashover .................. 12 B.1 Possible surge voltage conditions ..................................................................................................... 12 B.2 Surge protection requirements .......................................................................................................... 12 Annex C (informative) Discussion of differences of opinions regarding dc testing..................................... 13 Annex D (informative) Bibliography ........................................................................................................... 14 vii Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage 1. Overview This guide provides a description of the methods and practices to be used when field-testing laminated dielectric power cable systems. There is no requirement that any testing be performed either at the time of installation or periodically thereafter. However, it is well known that direct current (dc) testing of laminated dielectric shielded power cables has been performed for many years, especially for new installations or after repairs. If the user decides to have a direct voltage test made on the system, the following information is intended to provide a guide to the methodology, voltages, and concerns to be considered during the testing. 1.1 Scope This guide presents the recommended practices and procedures for direct voltage acceptance and maintenance testing of shielded, laminated dielectric insulated power cable systems rated 5 kV and above. It applies to all types of laminated power cable systems such as paper-insulated lead covered, pipe-type, and pressurized cables that are intended for the transmission or distribution of electric power. The tabulated test levels assume that the cable systems have an effectively grounded neutral system or a grounded metallic shield. 1.2 Purpose The purpose of this guide is to provide uniform practices and procedures for performing direct voltage acceptance and maintenance tests on shielded, laminated power cable systems in the field and to provide guidelines for evaluation of the test results. 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. 1 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Accredited Standards Committee C2, National Electrical Safety Code® (NESC®).1 IEEE Std 4™, IEEE Standard Techniques for High-Voltage Testing.2, 3 IEEE Std 510™-1983 (Withdrawn), IEEE Recommended Practices for Safety in High-Voltage and HighPower Testing.4 3. General 3.1 Environmental influences 3.1.1 Temperature The dielectric strength of some cable insulations is reduced at elevated temperatures. This necessitates a reduction in the test voltages at higher temperatures. Temperature gradients in the cable insulation, caused by heat dissipation from the conductor, can result in abnormal voltage distribution upon application of a high direct voltage. For these reasons, high direct voltage tests should be made with the cable at ambient temperature if possible. 3.1.2 Atmospheric conditions High humidity and conditions favoring condensation on exposed surfaces can affect test results to a marked degree. Contamination of termination surfaces can greatly increase conduction or leakage current and reduce flashover levels. Relative air density affects the measurement of a test voltage by use of rod gaps and may also affect the flashover voltage of cable terminations. At elevations higher than 1000 m (3280 ft), additional insulation and clearance may be required to withstand both working voltages and the prescribed test voltages. If excessive corona or air discharges exist during a test, a reduced test voltage may result and high leakage current readings will be present. This factor can be compounded by high wind conditions. 3.1.3 Extraneous electric fields Although field tests on cable are often made in the vicinity of energized equipment, extraneous electrical fields usually will have little influence on direct voltage test results as long as the voltage measurement circuit is well shielded. A simple test to see the influence can be made by ungrounding the test system prior to energization and observing any indications on the test system instrumentation on the lowest metering range. It is always important to maintain adequate clearances for the dc test voltages anticipated also taking into consideration the recommended clearance for the energized adjacent circuits. CAUTION Care should always be taken to avoid sharp connections on the high-voltage circuit to maintain a low electric field value. 1 The NESC is available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08855-1331, USA (http://standards.ieee.org/). 2 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 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 IEEE Std 510-1983 has been withdrawn; however, copies can be obtained from Global Engineering, 15 Inverness Way East, Englewood, CO 80112-5704, USA, tel. (303) 792-2181 (http://global.ihs.com/). 2 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage 3.2 Test equipment 3.2.1 High direct voltage test equipment The test voltage source should: ⎯ Provide the maximum (usually negative polarity) test voltage required plus some margin. ⎯ Provide a means of increasing voltage continuously or in small steps. ⎯ Have a current capability sufficient to charge the test object in a reasonable time. ⎯ Maintain a ripple factor as defined in IEEE Std 45 of less than 3%. Normally the capacitance of the test object reduces the ripple voltage of the dc source to low levels. ⎯ Provide a sufficiently powerful source to maintain voltage drops during transient current pulses to a value of less than 10%. Care should always be taken to avoid strong corona discharges from high-voltage connections that cause transient current pulses. ⎯ Provide voltage and current instrumentation that meet the requirements of IEEE Std 4. Ammeters used to measure leakage current should have a low range that can resolve currents of 10 µA to 100 µA. An ammeter of sufficient range and scale should be provided to measure the leakage current accurately. Often, ammeters are provided with guard circuits to isolate the measurement of the current through the test object from stray leakage current. 3.2.2 Power supply A well-stabilized power supply for the high direct voltage generator is essential. Minor variations in the mains voltage or the test voltage supply can cause major variations in the output current indication. The test voltage should remain stable to within 3% during the test time. 3.2.3 Discharge resistor A resistor with a resistance not less than 10 000 Ω/kV of test voltage is recommended to be used to discharge the cable system after testing. This resistor should be designed to withstand the full test voltage without flashover and to handle the discharge energy without overheating. An insulating grounding stick rated for the full test voltage and a flexible conductor should be provided to connect the resistor across the cable circuit and ground. Remote control grounding devices that avoid operator intervention are preferred over manual grounding devices. CAUTION At the end of a test, a solid, visible ground must be connected to the test cable after the test voltage is reduced to a negligible value. The energy to be dissipated by the discharge resistor after the test may be calculated from Equation (1) as follows: E = 1/2 CV2 5 (1) For information on references, see Clause 2. 3 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage where E is the energy (joules) (watt-seconds) V is the test voltage (volts) C is the capacitance of the test circuit (farads) 4. Test procedure 4.1 Test precautions The following precautions should be considered when performing tests with high dc voltage in the field: ⎯ All components require de-energizing and solid grounding before testing. Checking with a reliable voltage indicator that responds to alternating current (ac) and dc voltage is recommended. While the indicator is in contact with each component and indicates no voltage, a ground connection should be applied to the component and remain attached at all times except when the test voltage is being applied. All unenergized metallic parts in the vicinity of the highvoltage connections should always remain grounded. ⎯ All cable termination ends as well as all connecting leads of components being tested require guarding from accidental contact by such means as barriers, enclosures, or a watchman at all points. The cable ends require separation from all elements not to be subjected to test and by distances at least 0.25 cm/kV (0.1 in/kV) of test potential for voltages up to 100 kV and at least 0.5 cm/kV (0.2 in/kV) for higher test voltages. Please note that the test electrode geometry has a great influence on the safety clearances. The values recommended in this paragraph are only for conditions where high-voltage and ground electrodes are smooth and uniform and where highvoltage connections are made with round conductors of adequate size to avoid corona. If the test connections are not smooth, larger clearances should be provided. ⎯ Breakdown or terminal flashover may generate traveling waves up and down the cable that can be of a magnitude great enough to cause degradation of the insulation of the cable or accessories or breakdown of the insulation. Consideration should be given to the installation of suitable rod gaps according to IEEE Std 4 for dc voltages to provide protection. A damping resistor rated for the voltage and energy of the cable can be installed in the series circuit to reduce oscillations and reflection voltages. ⎯ For circuits with open-circuit (split) shields, the shield gaps should be short circuited (jumpered connection), and this jumper conductor connected to ground. Surge protectors should be disconnected and grounded. Grounding in this manner reduces overvoltages on gap insulation and surge protectors in the event of a test failure. ⎯ High direct voltage field-testing of cable systems involves all of the concerns normally associated with working on energized circuits, as well as several unique concerns that must be addressed. Cable circuits will normally have one or more ends remote from the location of the test equipment and the test operator. These ends must be cleared and guarded to ensure the safety of personnel. Reliable voice communication should be established between all such locations and the test operator. ⎯ Cables have high capacitance and dielectric absorption characteristics with long time constants. Particular attention must be directed to the special techniques required for discharging cables after testing to minimize the possibility of personal injury. 4 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage CAUTION Cables subjected to high-voltage testing that are not grounded for sufficiently long periods of time following such tests can experience dangerous charge buildups as a consequence of the very long time constant associated with dielectric absorption currents. For this reason, the grounding procedures recommended in 4.3 shall be followed. 4.2 Safety practices When testing, personnel safety and service reliability of the electrical systems are of ultimate importance. All cable and equipment tests must be performed on isolated and de-energized systems except where otherwise specifically required and authorized. The safety practices should include at least the following requirements: a) Applicable user safety operating procedures. b) Recommended practices for safety in high-voltage testing (see IEEE Std 510-1983). c) Applicable state and local governing safety operating procedures. d) Part 4 of the National Electrical Safety Code® (NESC®) (Accredited Standards Committee C22002) where applicable. e) Protection of utility and customer property by such means as barriers, enclosures with warning signs, and safety watchers at all points. The test program of each safety system should be designed to provide for minimum interference, as much as practicable, with related operations channels, systems, or equipment. f) Cables must be de-energized and grounded before testing is begun. g) While testing, one or more cable ends will be remote from the testing site; therefore cable ends must be cleared and guarded. h) At the conclusion of high-voltage testing, cables and cable systems should be discharged and careful consideration must be given to eliminate the aftereffects of the cables’ dielectric absorption and capacitance characteristics. Those effects can be reduced by leaving both the conductor and sheath of the cable grounded until it is placed in service. 4.3 Testing procedure Disconnect all equipment not to be included in the test, but leave all ground connections intact. Any temporary struts, ties, leads, spacers, and terminations or connecting leads must be capable of sustaining the test voltage without undue leakage or heating. Prepare the cable system for testing in accordance with manufacturer or utility recommendations. Clean insulator surfaces with a dry cloth and, if necessary (in severely polluted areas), apply silicone grease to minimize leakage currents and prevent flashover. An output connection is required to connect the power supply to the termination of the cable under test. This connection should be smooth and free of surface irregularities if possible. The test lead itself as well as the entire measuring circuit should be corona-free. If a bare conductor is used, it should be of a sufficient diameter to prevent partial discharges (corona) from its surface. A general guideline of 2.5 cm (1 in) conductor diameter for every 100 kV of test voltage is usually sufficient for smooth conductors and cable terminations with stress relief electrodes. If an insulated conductor is used, the insulation should be thick enough or have a large enough diameter to prevent pinholes from developing, due to the external field that would cause corona. If a shielded conductor is used, its insulation and terminations should be adequate to withstand the test voltage, and the shield should be at ground or at the test equipment guard potential. 5 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Check the operation of the test equipment in accordance with the manufacturer’s recommendations prior to connecting the test cable. If a portable voltage reference is available, the test equipment voltmeter can be checked by comparison before connecting the load. When concern exists about accidental flashover, a check of the test equipment should include a voltage withstand test with a suitable margin. The current indication can be checked by connecting a short piece of small wire to the test lead and through a suitable milliammeter to ground and raising the voltage slightly until the two instruments can be compared at various current levels. If the leakage current in the test equipment is a substantial portion of the test value to be measured, this current should be measured and subtracted from the test current readings. Alternatively, guard current metering circuits may be used that only measure the leakage current of the test cable. The current measurement may be repeated at the end of the test. The source of extraneous leakage should be determined and eliminated if possible. The ground lead for the test equipment should be connected to a local ground or in the absence of a local ground to the metallic shield of the cable that must be known to be grounded. All ground connections must be observed to be solid mechanical connections prior to beginning any high-voltage test. For ungrounded cable terminations, the metallic shield should be connected to a local ground, during high direct voltage testing. Connect the test lead to the first conductor or conductors to be tested. Remove any safety ground rods from the conductor to be tested. When multiconductor or belted cables are tested, each conductor should be tested separately, with the remaining conductors and shields grounded. If the cable system has been operating under sufficient load to raise its temperature, it should be allowed to cool to ambient temperature before applying the test voltage. The initially applied voltage should not exceed 1.8 times the rated ac rms phase-to-phase voltage of the cable. The voltage may be increased continuously or in steps to the maximum test value. Apply voltage slowly enough to prevent overloading and/or tripping of the power supply or overshooting the test level. If the voltage is increased continuously, the rate of increase should be approximately uniform and should result in the maximum test voltage being reached in a time period of not less than 10 s and not more than 60 s. In cases where extremely long installations are to be tested, the rate of voltage rise may be slower due to practical considerations of the test equipment. If the step method of voltage increase is employed, a minimum of five steps is desirable. Duration at each step should be long enough for the current to reach a steady value (1 min suggested). Current readings at each voltage step should be recorded at the end of the step duration. The maximum test voltage should be maintained for 15 min. After reaching the maximum test voltage, the current magnitude should be recorded at least twice: once at approximately 2 min and again at the end of the test period (15 min). Recommended test voltages for shielded cable systems 5 kV and above are set forth in Table 1. When, in the opinion of the user/owner, it is necessary to use more stringent test voltages, the higher level should be determined in consultation with the suppliers of the cable and cable accessories. If any equipment is included beyond the cable and its terminations, the dielectric strength of such equipment must be taken into consideration when establishing the test voltage. NOTE—If an external flashover occurs during a high-voltage test of a shielded cable, it is possible to develop fast transients and voltage reversals of high magnitude that may damage the cable or accessories. Precautions should be taken to provide high-voltage connections that are suitable for the testing voltage.6 6 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard. 6 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Table 1 —Field test voltages for shielded power cables from 5 kV to 500 kV system voltage System voltage, kV rms, phase-to-phase Acceptance test, kV dc, phase-to-ground System BIL, kV crest Maintenance test, kV dc, phase-to-ground 5 75 28 23 8 95 36 29 15 110 56 46 25 150 75 61 28 170 85 68 35 200 100 75 46 250 125 95 69 350 175 130 115 450 225 170 115 550 275 205 138 650 325 245 161 750 375 280 230 1050 525 395 345 1175 585 440 345 1300 650 488 500 1425 710 535 500 1550 775 580 500 1675 838 629 NOTE 1—Voltages higher than those listed, up to 80% of system BIL, may be considered, but the age and operating environment of the system should be taken into account. The user is urged to consult the suppliers of the cable and any/all accessories before applying the high voltage. NOTE 2—When older cables or other types/classes of cables or other equipment, such as transformers, switchgear, motors, etc. are connected to the cable to be tested, voltages lower than those shown in this table may be necessary to comply with the limitations imposed by such interconnected cables and equipment. See IEEE Std 95™ [B5]7 and Table 1 of IEEE Std C37.20.2™-1999 [B7]. NOTE 3—If the test voltage exceeds 50% of system BIL, surge protection against excessive overvoltages induced by flashovers at the termination should be provided. NOTE 4—It is strongly recommended that the user consult with the manufacturer(s) of all components that will be subjected to such testing before performing any tests on cables and cable accessories rated 115 kV and higher. NOTE 5—It should be noted that this table and the test procedures suggested in this guide do not necessarily agree with the recommendations of other organizations, such as those of the Association of Edison Illuminating Companies [B1], [B2], [B3], [B4]. Where there is concern, a user should consult the supplier of the cable and accessories to ascertain that the components will withstand the test. 7 The numbers in brackets correspond to those of the bibliography in Annex D. 7 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage CAUTION The test polarity should not be reversed unless the circuit is grounded for a prolonged period following a test. In cases where laminated cables are designed for polarity reversal duty, the relevant apparatus committee shall define polarity reversal tests. At the completion of the test period, the voltage can be reduced by returning the voltage control of the test equipment to zero. The voltage on the cable will discharge through the internal resistance of the test equipment. Normally this resistance has a very high ohmic value and the discharge time will be very long, especially for cables longer than a few 100 m (300 ft). A long discharge time may be impractical. Further, directly grounding the cable with zero resistance while the voltage is at an elevated voltage can damage the cable. In order to discharge longer lengths of cable safely, separate automatic grounding systems with built-in high-voltage discharge resistance designed to provide a time constant of several seconds can be used. In all cases, discharge mechanisms should be designed to safely handle the test voltage and energy stored in the cable under test. After the test voltage is reduced to a low level, the high-voltage conductor should be solidly grounded. The grounding provision used after a test should have no resistance included and be rated for the maximum fault current and fault duration of the system. The cable should remain grounded until ready for service or further testing. A retest should not be started until the cable has been grounded for a period of at least four times the duration of the previous test. 4.4 Recording of test results It is recommended that test data be recorded for future reference. Such data should include the date, time of day, location, ambient temperature, relative humidity or weather condition, cable description, phase, and circuit identification, as well as the name of the test operator and the test equipment used. The record of the test schedule used should include the time of voltage application as well as the voltage and current readings. Whenever available, the name of the manufacturer of the cable, its terminations, and its date of installation should be added to the record. 5. Evaluation of results 5.1 Current-time relationships The test current will momentarily increase for each voltage increment due to the charging of the capacitance and the dielectric absorption characteristics of the cable. Both of these decay, the first in a few seconds, the latter more slowly, ultimately leaving only the conduction current plus any external surface leakage or corona currents. The time required to reach this steady-state current depends on the insulation temperature and material. This time could be on the order of hours for laminar oil impregnated insulation. One criterion of a satisfactory test in direct voltage testing is a steady current value or a decrease of current with time at a fixed voltage application. While this may be partially obscured by corona current, voltage regulation, and insufficient meter damping, the absence of an increase in current with time is generally a practical criterion for acceptance. If the current starts to increase, slowly at first but at an increasing rate, without any increase in applied voltage, gradual insulation failure may be in progress. This process will probably continue until the cable or accessories eventually fail unless the voltage is rapidly reduced. Non-pressurized impregnated paper insulations typically exhibit this type of insulation failure. 8 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage If the test equipment overload system trips at any time during a test, it may indicate one of the following: ⎯ A very rapid increase in current ⎯ A flashover of the test equipment, the leads, or a termination ⎯ A failure of the circuit under test, the cable, a splice, or a termination A failure can be confirmed by the inability to sustain another application of the test voltage. In the event of such an apparent failure, it is recommended that the source of the failure be determined prior to re-energization of the cable. 5.2 Resistance values Readings of voltage (V) and conduction current (I) observed during the high direct voltage test may be used to calculate the effective insulation resistance (R) of the cable system by means of Ohm’s Law, R = V/I. A useful relation is given in Equation (2) as follows: DC test voltage (V) / Test current (µA) = Insulation resistance (MΩ) (2) Calculating and plotting resistance versus voltage, in conjunction with a step-voltage test, is an aid in evaluating the insulation condition and is frequently an even more sensitive indicator of an approaching current avalanche failure than is the dynamic behavior of the microammeter itself. A reduction in insulation resistance with increasing voltage can be such an indication, particularly in tests on laminar dielectric cables. A decrease in calculated insulation resistance may also be the result of higher leakage currents due to corona at increased voltage levels. The cable terminals should be checked for excessive corona, and steps should be taken to mitigate the discharges, if present, as corona leakage currents can increase rapidly with higher voltages after inception. Comparison of the calculated resistance of the three conductors of a circuit can be a very useful indicator of an anomalous condition in the insulation of one or more of the cables. Although no clear guidelines for such comparison can be given, an anomalous condition is likely if this ratio of insulation resistance is greater than 3:1 for cables longer than 1000 m (3000 ft). Somewhat higher resistance ratios can be permitted for cables less than 300 m (1000 ft) in length. The insulation resistance characteristics and terminating conditions vary so widely that a statement here of absolute values of resistance would be misleading. Comparison of resistance values with those obtained when the cable system was installed or last tested can be useful to “fingerprint” an installation over time. All cable insulations exhibit a negative temperature-resistance coefficient; increased temperature will therefore always result in lower insulation resistance. Several types of compounds used for filling cable terminations exhibit much lower resistance and higher negative temperature-resistance coefficients than those of the oil-paper cables they terminate. For this reason, terminations should be allowed to cool to ambient temperature before testing takes place. Some factory-prefabricated terminators may also exhibit relatively low resistance and moderately high negative temperature- resistance coefficients. Humidity, condensation, and precipitation on the surface of a termination can increase the leakage current by several orders of magnitude and give rise to an undesirable flashover. Extremely low humidity levels may also increase corona levels. Very high or very low humidity may increase the corona current, which is indicated in the total leakage current. 9 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Wind prevents the accumulation of space charges at bare energized terminals. This results in an increase of corona. An insulating envelope surrounding a termination tends to retain this space charge even in the presence of wind and can increase flashover voltages. When a test includes other connected high-voltage apparatus in addition to the cable being tested, the current measurement sensitivity required to note approaching avalanche conditions outlined in 5.1 may not be observed due to masking by other extraneous current values of higher magnitudes. In such cases, avalanche or runaway conditions may be noted only when the failure mechanism is far advanced. Reduction of voltage may not be possible before actual breakdown occurs. All of the above factors should be considered when comparing or evaluating the apparent insulation resistance of a cable circuit. 10 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Annex A (informative) Reasons for testing A.1 Acceptance tests An acceptance test before placing the cable in service normally will reveal gross dielectric defects, weaknesses, or errors that would result in failure within the first year or so. A.2 Maintenance tests Laminar dielectric cables, when pressurized, are much less susceptible to progressive deterioration from partial discharge action at imperfections. When the dielectric is not pressurized, however, migration of oil and the subsequent appearance of voids in the insulation will permit degradation from partial discharge activity. A major cause of failure of these types of cable is failure of the enclosing sheath and a consequent ingress of water. This condition is aggravated by cyclic loading of the cable. However, actual failure does not occur immediately after a breach of the sheath. Insulation resistance decreases and dielectric losses increase progressively after the initial moisture encroachment. Actual failure may not occur for several months. The usefulness of maintenance tests on this cable type depends largely upon the frequency of the testing. Results published to date indicate the intervals of less than one year may be necessary to obtain substantial improvements in service reliability. CAUTION The consequences of experiencing a failure during performance of a maintenance test should be considered prior to undertaking any such test. The faulted circuit would be out of service until repairs or possibly even cable replacement could be completed. This could result in a prolonged delay for circuit availability. A.3 Corrective actions When a failure of a cable occurs, corrective actions can include: cable replacement, replacement or rework of terminations, and/or replacement of accessories. Additional test methods may be required to determine the location of the cause of unacceptable test results and corrective actions. DC high-voltage tests assess the overall condition of an insulation system and cannot necessarily identify individual flaws except when those flaws result in a test failure. 11 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Annex B (informative) Protection against possible severe voltage conditions due to flashover B.1 Possible surge voltage conditions If a flashover should occur during the course of a high direct voltage test, either in the cable itself or at the terminations, voltage surges of a polarity opposite to the test voltage are initiated. These travel along the cable and produce reflections at the terminals as described by traveling wave theory. Before any reflections occur, the traveling wave voltage tends to neutralize the cable test voltage and relieve the prevailing voltage stress. However, the surge voltage doubles with the same polarity at an open-circuit terminal and therefore produces a polarity reversal at the terminal. This polarity is subsequently imposed on the shielded cable system as well. While the maximum reversed voltage from conductor to ground would appear to be less than equipment BIL, the effects of the reversals on stress between conductors and shield are uncertain. Cables and accessories could be subject to damage or multiple failures when terminals are not surgeprotected and an initial flashover or failure takes place. Protective rod–rod gaps may be employed to limit the voltage at an end terminal (see IEEE Std 4). B.2 Surge protection requirements The effects of test flashover occurrences can be minimized by preventing reflections at the terminals. This could be accomplished by installing a protective device that will withstand test voltage but will flash over if subjected to a voltage of opposite polarity and of the same magnitude or significantly less, say 60% or 70%. Such a device can be connected to ground through a resistor approximately equal to the cable surge impedance, thereby minimizing reflections at terminals. 12 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Annex C (informative) Discussion of differences of opinions regarding dc testing This annex sets forth some of the areas of debate among cable engineers on the merits of conducting high direct voltage tests on shielded cable systems having laminar dielectric insulations. There is undoubtedly much to be said both for and against all viewpoints. Inept testing can certainly give a false indication of the cable condition and may cause damage, particularly when voltmeter range switches are in the wrong position or when the tested cable is discharged by short-circuiting through essentially zero resistance. Unskilled testing can also cause serious injury through unsafe practices. On the other hand, skillful techniques and proper care, as outlined in this guide, can be of much help in improving service reliability of cable systems. The following should be noted when considering these issues: ⎯ In some cases, practical insulating systems may have significantly higher thresholds of damage initiation when stressed with dc potential than when stressed with ac potential. This has lead to use of dc test potentials several times a comparable ac test level for cables and electric machines. ⎯ Under ac stress, the potential distribution across the insulation system is determined by unit capacitances; under dc stress, the potential distribution is determined by unit resistances. It will be noted that the test voltages shown in Table 1 of this guide are lower than the values shown in IEEE Std 404™ [B6]. It should also be noted that the test voltages and procedures recommended in this guide do not necessarily agree with those recommended by other organizations. Where there is concern about testing up to these voltage levels, a user should consult the suppliers of the cable and accessories or even test a few accessories to ascertain that these components will withstand the test. Still lower test voltages and shorter periods between tests may produce the same overall reliability as higher test voltages and longer periods between tests, although no clear comparison has been established. More frequent testing may be more costly, however, in both testing cost and out-of-service cost of the circuit. Some users may find it desirable to apply even more rigorous test potentials than those listed in the guide. Some of these have found testing at dc test voltage on the order of 80% of system BIL to be desirable and useful for their purposes. It should be noted that if the higher test voltages advocated by some are used, acceptance testing after installation may employ the same voltages used in factory tests. Good installation practice assures that the rigors of installation, splicing, and terminating have not lowered the dielectric strength of the system below the factory test level. Maintenance test voltages should be 75% of the acceptance (installation) test voltages. This provides for a normal degree of deterioration in dielectric strength before replacement or repair is indicated. The real purpose of a test is to apply a high enough voltage to detect in the insulation any weak spots likely to cause a service failure before the next scheduled test. At the same time, the test voltage should not be so high as to damage any sound insulation. Acceptance test voltage duration is normally 15 min. Maintenance test voltage duration is normally not less than 5 min or more than 15 min. 13 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply. IEEE Std 400.1-2007 IEEE Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage Annex D (informative) Bibliography [B1] AEIC CS1-90, Specifications for Impregnated-Paper-Insulated Metallic Sheathed Cable, Solid Type.8 [B2] AEIC CS2-97, Specifications for Impregnated Paper and Laminated Paper Polypropylene Insulated Cable, High Pressure Pipe Type. [B3] AEIC CS3-90, Specifications for Impregnated-Paper-Insulated Metallic Sheathed Cable, Low Pressure-Gas Filled Type. [B4] AEIC CS4-93, Specifications for Impregnated-Paper-Insulated Low and Medium Pressure SelfContained Liquid Filled Cable. [B5] IEEE Std 95™-2002, IEEE Recommended Practice for Insulation Testing of AC Electric Machinery (2300 V and Above) With High Direct Voltage.9, 10 [B6] IEEE Std 404™-2006, IEEE Standard for Extruded and Laminated Dielectric Cable Joints Rated 2 500 V to 500 000 V. [B7] IEEE Std C37.20.2™-1999, IEEE Standard for Metal-Clad Switchgear. 8 AEIC publications are available from the Association of Edison Illuminating Companies, 600 N. 18th Street, P. O. Box 2641, Birmingham, AL 35291-0992, USA (http://www.aeic.org/). AEIC publications are also available from Global Engineering Documents, 15 Inverness Way East, Englewood, Colorado 80112-5704, USA (http://global.ihs.com/). 9 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 10 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 14 Copyright © 2007 IEEE. All rights reserved. Authorized licensed use limited to: Wuhan University. Downloaded on November 8, 2008 at 14:30 from IEEE Xplore. Restrictions apply.