Voltage-time characteristics of breakdown in $SF_6$

The impulse breakdown voltage-time to flashover (V-t) characteristics of a point-plane gap in SF6 has been investigated using impulse wave fronts in the range of 1.2 μs to 250 μs. Also, field nonuniformity was systematically varied to study the effect of corona preceding breakdown on time delays. The V-t characteristics are categorized into three types depending on the field nonuniformity and gas pressure. The properties of the characteristics in these types are generalised and will be useful in the insulation co-ordination studies for abnormal voltage protection of $SF_6$ gas insulated power equipment and systems (GIS). It has been deduced that the lightning impulse (LI) voltage appears to be the most effective on-site test voltage to detect any defects like fixed particles or protrusions in the GI