Very fast transient overvoltage waveshapes in a 500-kV gas insulated switchgear setup

Very fast transient overvoltages (VFTOs) may be generated in SF
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gas insulated switchgear (GIS) due to a series of arc ignitions and extinctions, during the operation of disconnecting switches and circuit breakers. They may also occur during internal flashover in the GIS. Some recent reports of failures in power transformers suggest high internal overvoltages associated with arc restrikes as a possible cause; such internal overvoltages might be generated by resonant frequencies affecting the transformer coils [1]. In order to understand this phenomenon better, Itaipu Binacional, a 14,000-MW Hydropower Plant, in cooperation with LACTEC, a High Voltage Research Laboratory, assembled a 31-meter-long GIS test setup including a disconnecting switch in order to study VFTOs occurring during disconnecting switch operation. It was found that the time to voltage peak, the inner frequency bandwidth, and the voltage peak values around 2.0 to 2.8 times the nominal voltage [2] lay within the ranges suggested in IEC 60071-1 [3]. However, the measured frequencies of the envelope of the transitory overvoltage shape were at least 10 times the upper limit suggested in [2] and [3]. Detailed results are presented below.