Prebreakdown phenomena in insulating oil at large gap distances

The prebreakdown behavior of insulating oil has been studied for strong non-uniform fields at gaps from 10 to 70 mm and at voltages up to 700 kV. Basically, the negative tip has the higher dielectric strength. For the positive predischarge, three consecutive stages can be observed: primary, secondary and tertiary streamers. The primary streamer shows structural variations depending on the growth rate of the electric field. Secondary and tertiary streamers appear in a coniferous branch-like and filamentary pattern. For the negative predischarge the existence of a second, faster propagation stage could be observed and optically recorded. This secondary structure originates from a frequently branched primary streamer and propagates in a less branched, filamentary fashion with significantly higher velocity