Streamer Characteristic and Breakdown in a Mineral Oil and a Synthetic Ester Liquid under DC Voltage

This paper presents experimental studies of streamer propagation and breakdown in a mineral oil and a synthetic ester liquid under both positive and negative DC voltages. A point-plane electrode configuration with a point tip radius of 10 mu m was used. Characteristics of streamer including length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm using a multi-channel ultra-high speed camera. Under positive polarity, the streamers in both liquids propagate in the 2nd mode; the stopping length of the synthetic ester is longer than that of the mineral oil at the same applied voltage level. However there is no obvious streamer propagation (less than 10% of the gap distance) under negative polarity even when the applied voltage approaches to the breakdown voltage; the streamers in both liquids stay in the 1st mode. Breakdown tests at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. 50% breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities.