Impulse Breakdown Characteristics in SF6 Gas in the Presence of a Local Spark

The effect of a local spark on the impulse breakdown characteristics of a model gap arrangement which simulates a disconnect switch in GIS has been experimentally investigated. The three-electrode gap consists of two hemispherically capped rods facing each other together with a ground plane, which simulate two switch contacts and a sheath, respectively. One of the rods is energized and one is left floating. The influence of a local spark between the energized and floating rods on the impulse breakdown characteristics to the plane depends on the ratio d/D (d is rodrod distance and D is rod diameter) and gas pressure P. Compared with the breakdown voltage to the plane without a local spark (both rods energized), the local spark reduces the main breakdown voltage particularly for high values of d/D and P. In these conditions, the dielectric strength to ground is reduced to the local spark inception level. Possible mechanisms by which the local spark may influence the main breakdown voltage are discussed, based on time-resolved photographs taken with an image converter camera. The possible mechanism by which the local spark initiates the main breakdown would seem to be associated with the high electric field around the local spark arc channel enhanced by streamers protruding from its surface.