Investigation on creeping discharges propagating over pressboard immersed in mineral and vegetable oils under AC, DC and lightning impulse voltages

This paper deals with the experimental characterization of discharges (namely the shape and stopping length) propagating over pressboard of different thicknesses immersed in mineral and vegetable oils, under AC, DC and lightning impulse voltages (1.2/50 μs) using a pointplane electrode arrangement; the point being perpendicular to pressboard. It's shown that the thickness of pressboard and the type of oil as well as the magnitude and polarity of voltage significantly influence the characteristics of discharges. For a given thickness, the stopping length of discharges Lf increases quasi-linearly with the voltage (U); and it is longer with a positive point than with a negative point whatever the type of voltage. Lf decreases when the thickness increases. The thinner the pressboard, the more branched are the discharge structures. The density of discharge branches increases when the pressboard thickness is reduced. On the other hand, for a given voltage and thickness of pressboard, Lf is longer under AC than under DC whatever the type of oil. Also, the characteristics Lf(U) in presence of processed vegetable oil and mineral oil are very close except for thin pressboard. The influence of electric field and capacitive effects on the propagation mechanism is also evidenced.