Investigations on creeping discharges propagating over pressboard immersed in mineral and vegetable oils submitted to impulse voltage

This paper deals with the experimental characterisation of discharges propagating over pressboard of different thicknesses immersed in mineral and vegetable oils, under impulse voltages, using a point-plane electrode arrangement. It's shown that the thickness of solid insulator and the kind of oil significantly influence the characteristics of creeping discharge and especially the stopping length L-f. For a given thickness, L-f increases quasi-linearly with the voltage; it decreases when the thickness increases. L-f is longer when the point is positive than with a negative point. For given voltage and thickness of pressboard, L-f is greater in vegetable oil than in mineral oil; L-f is generally greater of 60% in vegetable oil than in mineral oil when the point is positive and it's of 40% greater when it's negative. The values of L-f merge into one another in vegetable oils. The smaller the thickness of pressboard, the more branched are the discharge structures. This indicates the important role of the capacitive effect in the propagation mechanism.