Effects of hydrostatic pressure on the prebreakdown phenomena in dielectric liquids

It is commonly accepted that a gaseous phase is involved during the prebreakdown phase of dielectric liquid breakdown. This is indicated by the generation of gas bubbles following streamer propagation, whatever the polarity. It has been known for a long time that increasing the pressure vastly improves the breakdown voltage whether DC or AC. KAO and McMATH [1] have shown that in. a quasi-uniform field, the effects of the pressure on the breakdown voltage became negligible for sufficiently short rise times of the voltage ramp. In point-plane geometry, it has been reported that an increase in pressure reduced the prebreakdown light emission [2]. The rise time and the peak value of prebreakdown current in a parallel plate arrangement with a small protrusion on one electrode decreased with increase in pressure [3]. Time lags to prebreakdown disturbances increased with hydrostatic pressure, the pressure dependence decreasing at very high voltages [4]. The purpose of this paper is to study in the point-plane geometry with rectangular voltage pulses, the influence of the pressure both on the initiation and the propagation phases, by recording the streamer velocity, the transient currents and light emission.