The growth of pre-breakdown cavities in silicone fluids, and the accompanying discharge pulses

Measurements have been made of prebreakdown cavities in silicone fluids, and of the current pulses that accompany cavity growth. These experiments were carried out in silicone fluids of 0.65, 10, 100 and 1000 cSt viscosity. Cavity growth is driven by the electrostatic force on the cavity wall, which is related to the local field and to the space charge density in the liquid adjacent to the cavity; this space charge, in turn arises from the discharges that accompany cavity growth. Discharges occur in well-defined pulse trains: the first pulse in a train is probably an electron avalanche in the liquid, and serves to generate the cavity; subsequent pulses are due to discharges within the cavity. Knowing the scaling laws for cavity growth we can use the time between the first and second pulses to estimate the cavity size when the first cavity discharge occurs; this gives a cavity diameter of about 5-7 mu m and an internal pressure, p(i) similar to 0.3 x 10(5)P(a).