Pressure effects on partial discharges in liquid dielectrics

Liquids are routinely used for insulation in a wide range of high voltage applications. These liquids can be subjected to a variety of hydrostatic pressures during normal operating conditions. The work presented here examines the influence of pressure on the behaviour of partial discharges. Details of these events were recorded using simultaneous wide bandwidth current monitoring and high speed imaging of the associated partial discharge streamers that appeared in the liquid. It was determined that the events could be divided into two characteristic classes, with a behaviour that depended not only on the polarity of the voltage, but also on its magnitude. The two classes were found to have widely different pressure dependencies. One group was seen to be strongly pressure dependent, while the other was practically unaffected by pressure over the range examined. While it was found that the lowest inception voltages produced partial discharges that fell predominantly into the pressure sensitive group, it was the pressure insensitive events that ultimately led to breakdown.