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 L-f and the density of branches rho. For a given thickness' L-f increases 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.