Effect of injected space charges on the positive impulse breakdown of SF6 in a point-plane gap

Positive and negative polarity charges were injected into a point-plane gap by means of four auxiliary needle electrodes surrounding the gap. Experiments showed that the positive impulse breakdown voltage of SF6 in a 20 mm point-plane gap increased when the needles were positively charged up to a certain voltage, and then decreased with further increase of charging voltage. The prebreakdown corona current was virtually unaffected by the injected positive charge. When negative charges were injected from the needles, the prebreakdown corona activity was greatly increased and the positive impulse breakdown voltage was decreased. The findings reported here are consistent with the mechanism of impulse breakdown recently reported for highly non-uniform field gaps in SF6.