Investigation on creeping discharges propagating over pressboard immersed in mineral and vegetable oils submitted to AC and DC voltages

This paper deals with the experimental characterization of discharges propagating over pressboard of different thicknesses immersed in mineral and vegetable oils, under AC and DC voltages using a point-plane electrode arrangement. It's shown that the thickness of pressboard and the type of oil significantly influence the characteristics of creeping discharge and especially the stopping length Lf and the density of branches □. For a given thickness, Lf increases with the voltage; it decreases when the thickness increases. For a given voltage and thickness of pressboard, Lf is longer under AC than under DC whatever the oil type. And under DC, Lf is longer with positive point than with a negative point. However, the characteristics Lf(U) in presence of vegetable oil and mineral oil are very close except for thin pressboard.