Effect of surface charge on impulse flashover of insulators in SF 6

The effect of surface charge on the impulse flashover of short spacers in SF₆ was investigated. A split-electrode arrangement allowed scanning of the entire space surface by a capacitive probe. With a heterocharge distribution which facilitates both inception and propagation of the discharge, the flashover level is reduced by as much as 50% for sinusoidally distributed charge of peak density 40 μC/m². With monopolar charge, only positive charge near the cathode has any significant effect on breakdown, the reduction in impulse strength being ~35% at 40 μC/m². Tests with metal insets show that although the shielding effect is reduced in the presence of heterocharge the impulse strength of spacers with insets is still superior to that of plain spacers for the same charge density. The possible effects of surface charge on flashover of longer spacers is discussed in terms of a transition from a streamer to a leader type of breakdown mechanism