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Construction and Condition Monitoring

Lightning Arresters. Construction and Condition Monitoring . LA Characteristics. Arrester Fundamentals. Need for High Resistance to limit Discharge Current Need Low Resistance during flow of lightning current to limit Surge Voltage . A non-linear resistance material required.

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Construction and Condition Monitoring

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  1. Lightning Arresters Construction and Condition Monitoring

  2. LA Characteristics

  3. Arrester Fundamentals • Need for High Resistance to limit Discharge Current • Need Low Resistance during flow of lightning current to limit Surge Voltage A non-linear resistance material required

  4. EQUIVALENT CIRCUIT OF SURGE ARRESTER

  5. Concept of Leakage Current • Resistive Current • In phase with voltage • Increase with temperature • Increase with aging • Capacitive Current • In 90 Deg phase with voltage • Insignificant change with temperature and voltage • does not affect due to aging

  6. Selection of Surge Arrester Voltage 400/√3 = 230kV During single phase to ground fault, voltage on healthy phase may go upto 1.4 to 1.5 times 230 x 1.4/1.5 = 323 – 346 Temporary O/V = 1.5 pu = 336kV LAs are available at 336kV, 360kV, 372kV and 390kV Higher ratings are selected taking into consideration of ageing of LA elements

  7. Construction of disc • These are made by mixing ZnO with small amount of additives such as Bi2O3, CoO, Cr2O3, MnO and Sb2O3 • ZnO grains (about 10μm dia) have low resistivity and surrounded by a granular layer which is a high resistive Oxide layer(0.1 μm thick). The two are strongly bonded.

  8. Surge Arrester Failures

  9. FAILURE OF LAs

  10. Failures of LAs in POWERGRID • Failures of LA stacks • Failures of Surge Monitors • Failures of LA stacks and LA monitors more in rainy seasons. • Many LAs removed based on Third Harmonic Resistive Current Measurements.

  11. Failed LA

  12. Another failure of LA

  13. Gasket Condition

  14. Conduction of ZnO discs

  15. Contamination on the internal surface of the LA porcelain

  16. Failures of LAs in POWERGRID

  17. Improvements in Manufacturing Quality Meter Terminal Fixing: • Change of the Terminal at the bottom of the Surge Monitor. • Separate grounding terminal is required on surge monitor for connection to 75X12mm GI flat used for LA earthing. Meter Enclosure : • IP – 66 test on the enclosure. • Water dip test on surge counter at 1.5 meter depth for 30 Minutes, Acceptance criteria as per IEC 60068-2-17-1994

  18. Improvement in Sealing Arrangement of Surge Arrester Stacks • Sealing arrangements for Surge Arrester Stacks for all manufacturers were reviewed. • Presently M/s Oblum and CGL are using O Rings (Neoprene Rubber) in the groove whereas M/s Elpro and Alstom are using Rubber gaskets. • Since most of LA failures have taken place due to moisture entry, it was decided to stop use of flat gaskets for sealing.

  19. Moisture Ingress Test of the Arrester : • Dip test routinely for LA stacks at 1.50 meter depth for 30 minutes and thereafter following routine tests shall be conducted: • Partial Discharge Measurement • Reference Voltage • Residual Voltage. • Insulation Resistance measurement. • Tests 1 to 3 shall be conducted as per IEC-60099-4 (Edition 1.2, 2001-12). The Megger values shall be higher than 10GOhm for 66kV and higher rating LAs and more than 1GOhm for below than 66kV rated LAs. • Facilities for water dip test developed by all manufacturers

  20. AS PER IEC-60099(5), TECHNIQUES FOR THE HEALTH MONITORING OF SURGE ARRESTERS IN SERVICE 1. TOTAL LEAKAGE CURRENT MEASUREMENT 2. WATT LOSS MEASUREMENT 3. INSULATION RESISTANCE MEASUREMENT 4. THIRD HARMONIC RESISTIVE CURRENT MONITORING. 5. THIRD HARMONIC RESISTIVE CURRENT (THRC) MONITORING WITH COMPENSATION FOR THIRD HARMONICS IN SYSTEM VOLTAGE.

  21. LA

  22. THE MEASUREMENT PRINCIPLE • HARMONICS ARE CREATED IN LEAKAGE CURRENT ON APPLICATION OF FUNDAMENTAL FREQUENCY, DUE TO NON LINEAR VOLTAGE-CURRENT CHARACTERSTIC OF SURGE ARRESTERS. • THIRD HARMONIC IS THE LARGEST HARMONIC COMPONENT OF THE RESISTIVE CURRENT

  23. EFFECT OF 3RD HARMONICS IN SYSTEM VOLTAGE • THIRD HARMONIC IN SYSTEM VOLTAGE CREATES CAPACITIVE HARMONIC CURRENTS WHICH AFFECT THE MEASURED VALUE • ERROR IN THE MEASURED VALUES MAY BE CONSIDERABLE • AS REPORTED, 1% THIRD HARMONIC IN SYSTEM VOLTAGE MAY INTRODUCE ERROR UPTO 100% IN THE MEASURED VALUE

  24. Basic circuit for LA testing

  25. IDENTIFICATION OF DEFECTIVE SURGE ARRESTERS. • IN POWERGRID, THE UPPER/SAFE LIMIT OF THRC HAS BEEN KEPT AS 350 MICROAMP. • MEASURED VALUE ARE TO BE DIVIDED BY A FACTOR 4.0. • ABOUT 40 SURGE ARRESTERS WERE REMOVED FROM SERVICE TO AVOID MAJOR BLASTING. • CAPACITANCE/ TAN DELTA, IR MEASUEMENT TESTS WERE ALSO CONDUCTED ON REMOVED LAs AND FOUND VIOLATING.

  26. Capacitance and Tan Delta Measurement on LA having 560 micro-Amp

  27. REASONS OF FAILURE OF SURGE ARRESTERS • MOISTURE ENTRY THROUGH SEALING SYSTEM/GASKETS HAS LED TO DEGRADATION OF DISCS AND CONSEQUENT INCREASE IN THRC. • ACCELERATED DEGRADATION OF THE ZnO DISCS, POSSIBLY DUE TO MANUFACTURING DEFECTS/PROCESS PROBLEMS

  28. Thermovision Scanning

  29. Thank You for your attention please

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