A comparison of negative and positive streamers in mineral oil at large gaps

The propagation of negative streamers in mineral transformer oil is investigated in large point-plane gaps up to 35 cm, under impulse voltage. Basic features of streamers are obtained: velocity, stopping length, propagation voltage, electrical charge, photographs of the emitted light. Many features of negative streamers resemble those already observed in positive polarity under similar conditions. The voltage required to initiate fast streamers (velocity higher than 50 km s^-1) is identical in both polarities. The variation of propagation voltage versus distance and streak photographs both suggest that conduction mechanisms within long gaseous streamer channels are identical in positive and negative polarities. Some characteristics are however different: in negative polarity a higher voltage is required for the onset of propagation, propagating streamer heads emit more light and their velocity is lower. These differences are due to different propagation mechanisms occurring at the streamer heads. These mechanisms as well as the implications for practical high voltage insulation are discussed.