Optical and electrical characterization of creeping discharges over solid/liquid interfaces under lightning impulse voltage

This paper deals with the experimental characterization of discharges propagating over solid insulators immersed in mineral oil, under impulse voltages using a point-plane electrode arrangement. It's shown that the nature and the thickness of the solid insulator significantly influence the characteristics of creepage discharge and especially the final length Lfand the density of branches rho. For a given thickness, Lfincreases quasi-linearly with the voltage; it decreases when the thickness increases. So rho increases with the dielectric constant of the solid insulator; it also increases when the thickness is reduced. This indicates the important role of the capacitive effect in the propagation mechanism. The main discharge is followed by a secondary one of opposite sign to that of the main discharge