DC pre-stress effect on positive streamer initiation and propagation in insulating liquid subjected to step voltages

The effect of dc prestress on positive streamer initiation and propagation was investigated at atmospheric conditions in a small liquid-filled point-plane gap, representing a model for the insulation in power electronic components. Shadowgraphic imaging and photomultipliers to record emitted light were used. The applied voltages were step voltages (100 ns rise time) with or without a dc bias. The results obtained under step voltage show that many features of positive streamers (shape, velocity, initiation voltage) in Midel 7131 fluid are the same as those previously observed in hydrocarbon and mineral oils in similar test conditions. The presence of space charges in the vicinity of electrodes was demonstrated. The superposition of a same polarity dc prestress on the applied step voltage results in the increase of streamer inception voltage and the decrease of propagation lengths. For a polarity reversal for positive steps the initiation voltages were reduced and propagation lengths increased.