Fractal analysis of creeping discharge propagating over pressboard immersed in mineral and vegetable oils

The paper is aimed at the fractal analysis of the real (i.e., observed experimentally) creeping discharge patterns propagating over pressboard immersed in mineral and vegetable rape-seed oils, under lightning impulse voltages, using a pointplane electrode arrangement. By using the box counting method, we show that the discharge patterns present a fractal dimension D which depends on the thickness of pressboard (e) and the kind of oil; D decreases when e increases. The value of the density of branches increases when the thickness of pressboard decreases. D is generally higher in mineral oil than in vegetable rape-seed oil. This dependency on the thickness and nature of oil reveals the existence of a relation between the fractal dimension and the physical parameters.