THE ROLE OF LEADER RE-ILLUMINATION IN THE DEVELOPMENT OF SURFACE DISCHARGES IN SF6 EXPOSED TO A VERY FAST TRANSIENT OVERVOLTAGE

This paper describes the mechanism of discharge on the surface of an i nsulating prate in SF6 gas exposed to a very fast transient overvoltag e (VFTO). The voltage consists of a sequence of steep short purses on its front. A streak camera and a charge-measuring system were employed for the investigation. The leader channel, which has been formed duri ng the first voltage pulse of the VFTO, flashes repeatedly upon each r ise and tall of the successive voltage pulses. The luminosity at the v oltage's rise consists of the light due to mere flashing of the previo us leader channel and the light of the leader discharge in succession. The former phenomenon is called leader re-illumination in this paper. It plays a notable role in the leader's development. Such re-illumina tion of a leader channel occurs when the steepness of the applied volt age (dV/dt) exceeds a critical value. The critical steepness increases with increasing pressure of gas, the thickness and the reciprocal of the permittivity of the insulating plate. A model for the leader disch arge developing under VFTO stress is proposed.