Instrumentation for the Detection and Location of Incipient Faults on Power Cables

Since the voltage limiter has to be completely gapless, the metal oxide formulation would be the sole determinant of the arrester characteristics. Accordingly, itwas imperative that the requirements of the formulation be carefully identified. 550 kV and 1200 kV system studies were made and a composite specification was drafted incorporating the more severe case of lightning stroke and transmission line discharge from both systems. Along with the given objective of switching surge, successful pressure relief capability and uniform voltage distribution were also listed. Finally, confidence at 20-year life at maximum continuous line to ground voltage had to be established. These six considerations were paramount in the design and development of the 550 kV and 1200 kV gapless surge arresters. The 1200 kV arrester pole was designed with two parallel stacks of five series units, with additional mechanical support from a third empty porcelain at the lower two levels. The units were selected so that the sum of the parallel unit grading currents was the same ±1% for all the paralleled pairs which were then series connected. The stacked pole was tested with the capacitance bridge to verify optimal positioning of the grading ring as determined theoretically by the computer program. It was also tested for RIV, grading current and 2 ßs to crest current wave with crest magnitude of 100 to 500 amp. Slow front switching impulse tests (250 x 2500 fis) were attempted; however, due to the surge generator limitations and space clearances, meaningful results could not be obtained. It is clear, however, that for the same magnitude of current the slow front discharge voltage will be lower than the fast front discharge voltage. The complete volt ampere curve for the first pole of the 1200 kV gapless surge arrester is shown in Fig. 17. Discharge currents up to 500 amp were tested on the complete pole and the discharge voltages shown are for 1 ¿is rise time to crest. The test point at 26 kA is for the 8 x 20 ks current wave tested on parallel discs and summarized. As can be seen, the objective for 1.5 pu of maximum line to ground voltage at 1000 amp has been met even with the fast front currents. Switching surge at 1000 amp will be approximately 5% lower or about 1.43 pu. At 26 kAthe protective level is 1.83 pu. Fig. 18. 1200 kV completely gapless metal oxide surge arrester on field test. Lyons Substation.Bonneville PowerAdministration.