Streamer propagation and breakdown under AC in mineral oil for gaps up to 80 cm

This paper presents a study of prebreakdown and breakdown phenomena under ac voltage in mineral oil in very large gaps up to 80 cm. The investigations presented concern the measurement of breakdown voltages together with the recording of streamers. Experiments were conducted with rod-plane and sphere-plane electrode systems at different distances. The results describe breakdown mechanisms for moderately divergent fields and large gaps. A breakdown made specific to the ac voltage is evidenced, in which ''bursts'' of streamers initiated at each halfwave can appear and lead to the lowest breakdown fields recorded under ac. Values of the average fields required for ''burst'' or ''direct'' breakdown modes are deduced from the experiments. The influence of particles is also considered.