Diagnostic Measurement in GIS based on Frequency Analysis of Partial Discharge in SF_6 Gas

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To investigate the fundamental properties about the detection of abnormal discharge in GIS, short-gapdischarge phenomena were studied in air and SF_6 gas. The results can be summarized as follows:(1) In the spark-type discharge, pulse width of the current wave shape is several nano seconds, and the peak value of it is several amperes. In this case, the radiated electromagnetic wave, of which frequency spectrum extends to more wider than 1 GHz, is strong enough to be detected with an antenna. in the corona-type discharge, the peak value of the current wave shape is smaller than several milli amperes, and the level of its radiated electromagnetic wave is comparable to that of the background noise. Therefore, it is more difficult to detect the corona-type discharge than the spark-type short gap discharge.(2) Current wave shape of the corona-type discharge in SF_6 gas is remarkably different from that in air. The wave tail is about 2 nano seconds in SF_6 gas, and the one in air is several ten nano seconds. To clear the cause of these differences, a calculation of a single electron avalanche model was carried out.In SF_6 gas, negative ion cloude of high density is produced very near the tip of the negative needle, in a time shorter than 1 nano second after the generation of trigger electron. In air, the negative ion cloud of low density is produced at a distance from the tip of the negative needle. Therefore, it can be saide that suppression of the discharge requires repetition of many avalanche developments in air, and as the result the wave tail of the discharge current is longer.(3) The digital processing of the frequency spectrum constitutes an effective way to detect the corona-type discharge. In this way it is possible to detect a positive corona discharge of several ten pico Coulombs. On the other hand, detection of a negative corona discharge is difficult even with digital processing, due to small charge pulse, and more studies are required.