Diagnostics of prebreakdown phenomena in SF6 in non-uniform gaps for very fast transient voltage stress

This paper deals with the predischarge development in SF₆ in strongly non-uniform gaps stressed by very fast transient voltages (VFT) of positive polarity. In our investigations the shape of the applied voltage simulates the voltages that may occur in gas insulated substations (GIS) due to switching operations. Therefore a sinusoidal voltage of 2.6 MHz was superimposed to a voltage step. The maximum voltage amplitude was 215 kV peak. The parameters that have been investigated, are the gap width and the gas pressure. The gap width varied from 15 mm to 25 mm and the gas pressure from 0.1 MPa to 0.3 MPa. For diagnostics both, optical and electrical measuring techniques were used to analyse the prebreakdown and breakdown phenomena. Optical investigations were performed with ultraviolet- and infrared-sensitive photomultiplier tubes. For spatial resolved optical measurements of the predischarge development a high speed camera system with an exposure time of 1 ns was used. The results presented here show that the predischarge development in SF₆ in strongly non-uniform gaps stressed by very fast transient voltages is based on the leader mechanism, even for a gap width of only 15 mm. Depending on the parameters gas pressure and gap width the leader development changes. The transition from streamer to leader occurs according to the Stielbuschel mechanism at low gas pressure or the precursor mechanism at high gas pressure. Furthermore, an increase in the leader radius with increasing gap width was observed