Surface discharges over insulators of various materials in presence of SF6, CO2, N2 and SF6-CO2 and SF6-N2 mixtures under DC voltage

This paper is aimed at the experimental characterization of discharges propagating at solid/gas interfaces under DC voltage using a point-plane electrode arrangement. Specifically, we analyze the morphology of creeping discharges and their final (i.e., stopping or maximum extension) length against the type of insulator and its thickness, the amplitude of the applied voltage, the type of gas (mixture) and its pressure. The insulators we investigate are Bakelite and glass; and the considered gases are N₂, SF₆, CO₂ and mixtures SF₆ - N₂ and SF₆ - CO₂. The obtained results show that the discharges can be radial or not depending on the type of interfaces. However, as we observed for AC and lightning impulse voltages, the final length L_f increases quasi-linearly with the voltage; it decreases when the gas pressure increase. L_f is shorter in SF₆ than in CO₂ or N₂; and the increase of SF₆ content in the investigated mixtures leads to a significant decrease of L_f.