Transition to fast streamers in mineral oil in the presence of insulating solids

This paper presents a study of the transition to fast streamers in mineral oil in overvolted point-plane gaps. Streamer propagation is studied in 5 cm and 10 cm gaps, either in the liquid alone or in the presence of solid surfaces parallel or perpendicular to the gap axis. The experiments presented concern the visualization of streamers, the measurement of breakdown and acceleration voltages. In the liquid alone, above some critical voltage, a large increase in propagation velocity is recorded from 2 km/s up to about 100 km/s. The presence of insulating solids parallel to the electric field is greatly in favor of the inception and propagation of fast streamers (velocities up to 300 km/s have been measured). Conversely it is observed that a solid perpendicular to the electric field stops more efficiently fast streamers than slow ones, which easily creep and get round such obstacles