Positive and negative streamers in oil gaps with and without pressboard interfaces

Inception and propagation of streamers in a point-plane gap, with and without pressboard interfaces parallel to the field, have been studied. Current and light-emission have been recorded. Positive streamers propagate at about 2-4000 m/s at voltages up to two times the minimum breakdown voltage. Above this voltage a fast event (>100 km/s) may occur, preceded by streamers propagating with velocities in the 10-20 km/s range. The negative streamers have about twice as high a breakdown voltage as the positive ones. Also negative streamers may develop into a fast event. Solid field parallel pressboard does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown.