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

This paper is aimed at the analysis of some characteristics of discharges (namely shape, stopping length and current) propagating over pressboard immersed in mineral and vegetable oils, under standard lightning impulse voltages in a point-plane 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 these characteristics. For a given thickness, L_f increases quasi-linearly with the voltage; it decreases when the thickness increases. L_f is longer with a positive point than with a negative point. The smaller the thickness of pressboard, the more branched are the discharge structures. The density of discharge branches increases when the pressboard thickness is reduced. This indicates the important role of the electric field and capacitive effects on the propagation mechanism. For both polarities, the L_f values measured in treated vegetable oil and mineral oil are very close.