Propagation of positive and negative streamers in oil with and without pressboard interfaces

Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at similar to 2 to 4 mm/mu s at voltages to 2x the minimum breakdown voltage. Above this voltage, a fast event (> 100 mm/mu s) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm/mu s range. The negative streamers have similar to 2x the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field 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.