Abstract
The voltage-time curves of flashovers along the insulating spacer in SF6 gas become flat in the time range shorter than 1μs, compared with the V-t curves in the gas space. The paper tries to give a physical interpretation to the phenomenon by studying the initiation and development of the surface discharges optically. We use a photomultiplier to detect the initiation of streamer discharge and an image converter camera to measure the speed of the leader propagation. The test sample is so arranged that the flashover starts from the triple junction. The optical measurements show that the speed and the light intensity of the leader discharge increase with the discharge initiation voltage and the steepness of the impulse wavefront. We account for the flatness of the V-t curves in the fast time region by the enhancement of the electric field at the triple junction and the increase of the propagation speed of the discharge. We propose an equation which describes the propagation speed of the discharge in the time range of switching to steep-front impulses.
