Experimental characterisation of creeping discharges over solid/gas and solid/gaseous mixture interfaces under lightning impulse voltage

This paper deals with the experimental characterization of discharges propagating over solid insulators immersed in a gas or a gaseous mixture, under lightning impulse voltages (1,2/50 ¿s), using a point-plane electrode arrangement. The morphology of discharges and their final lengths are analyzed versus the amplitude and polarity of the voltage, the kind and pressure of the gas (or mixture). Different kinds of gases (N₂, SF₆, CO₂) and mixtures (SF₆-N₂ and SF₆-CO₂) are considered. It´s shown that the shape of discharges and their final lengths L_f depend significantly on the solid insulator and the type of gas. For given solid and gas, L_f increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges don´t always present a radial structure as reported in literature. For given voltage and pressure, L_f is higher when the point electrode is positive than when it´s negative while the initiation voltage of discharges is higher with a negative point than with a positive one.