Analysis of creeping discharges activity at solid/liquid interfaces subjected to ac voltage

This paper presents the experimental characterisation of discharges propagating over insulators immersed in mineral oil, under ac using a point-plane electrode arrangement. The influence of voltage magnitude and the nature of insulator on the cumulative number of discharge events and their locations during one voltage cycle are also investigated. It is shown that the nature and the thickness of insulator have a significant influence on the properties of discharges and especially their morphology and stopping length Lf. For a given thickness, Lf increases quasi-linearly with voltage; it decreases when the thickness is increased and/or the dielectric constant of insulator decreases, thus indicating the important role of capacitive effect in the propagation mechanism. The total number of discharges recorded during 500 cycles of applied voltage increases significantly with the voltage magnitude. The threshold voltage Ui of discharges depends on the kind of material; and the average number of discharges (nmoy) increases with the dielectric constant εr, of insulator. For instance, nmoy is the highest for Bakelite (εr=4.8) and the lowest for Polycarbonate (ε_r =2.9). nmoy of negative discharges seems to be slightly higher than that for positive ones irrespective of the solid sample and the amplitude of voltage.