Fractal analysis of lightning impulse surface discharges propagating over pressboard immersed in mineral and vegetable oils

This paper is aimed at the experimental study of discharges propagating over pressboard immersed in mineral and vegetable oils and the analysis and comparison of the fractal dimension of these discharges. The shape, stopping length (i.e., the maximum extension of creeping discharge) L_f, current and electrical charge of discharges are investigated versus the thickness (e) of pressboard and the type of oil in a point-plane electrode geometry stressed by a standard lightning impulse voltage. It is shown that L_f decreases when e increases and it is longer when the point is positive than with a negative point. The smaller e, the more branched the discharge structures are, indicating the important role of the electric field and capacitive effect in the propagation mechanism. L_f is generally greater of 60% in vegetable oils than in mineral oil when the point is positive and it is of 40% greater when the point is negative. By using the box counting method, we show that the observed discharge patterns present a fractal dimension D which depends on e and the kind of oil. D decreases when e increases. The value of the density of branches increases when the e decreases. D is generally higher in mineral oil than in vegetable oil. This dependency on the thickness of pressboard and the nature of oil reveals the existence of a relation between the fractal dimension and the physicochemical parameters of interface.