Influence of the voltage waveform and hydrostatic pressure on morphology and final length of discharges propagating over solid-liquid interfaces

This paper is aimed at the influence of the voltage waveform (lightning impulse, dc and ac) and hydrostatic pressure on the morphology and final length of creeping discharges propagating over solid/liquid insulating surfaces in a point-plane electrode arrangement. Different solid materials immersed in mineral oil are tested. It is shown that under ac and dc, the discharges do not present a radial shape as observed with the negative discharges under lightning impulse voltages. For a given solid sample, the final length L_f (i.e. the maximum extension) of discharges is longer with ac than with lightning impulse voltage and dc. The currents waveforms are similar to those observed in liquids. On the other hand, the length of discharge branches is reduced when the hydrostatic pressure is increased whatever the voltage waveforms and polarity. L_f increases quasi-linearly with the voltage and decreases when the hydrostatic pressure is increased. And for given voltage and pressure, the thinner the solid sample and/or the higher its dielectric constant, the longer the discharges are, indicating the important role of capacitive effects in the propagation mechanisms of discharges.