An experimental study on the effect of DC bias on streamer initiation and propagation in a dielectric liquid under impulse voltage

The role of composite voltage waveforms on prebreakdown processes was studied at atmospheric conditions in a small point-plane gap with a synthetic ester Midel 7131 fluid, representing a model for the insulation in power electronic components. Shadowgraphic imaging and photomultipliers to record emitted light were used to measure streamer initiation voltages, streamer propagation lengths and velocities. The applied voltages were step voltages (100 ns rise time) with or without a dc bias. Results have demonstrated the presence of space charges in the vicinity of electrodes. With a step voltage with the same polarity as the dc prestress, initiation voltages increased and propagation lengths were reduced. For a polarity reversal for positive steps the initiation voltages were reduced and propagation lengths increased as expected, while for negative step the opposite was seen. The latter is explained by an electron "cleaning" effect of the positive dc prestress reducing possibilities for electron avalanches. It is shown that streamers may be initiated by a sudden turn off of a dc stress.