Propagation of 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 te field, have been studied. Curren and light emission have been recorded. Positive streamers propagate at about 2000-4000 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 in the 10-20 km/s range. The negative streamers have about twice as high breakdown voltage as the positive ones, and may also develop into a fast event. Solid, field parallel pressboard does not change the breakdown voltage but make inception of fast events easier.