Investigation on creeping discharges propagating over epoxy resin and glass insulators in the presence of different gases and mixtures

This paper deals with the experimental characterization of discharges propagating over insulators of epoxy and glass, immersed in a gas or a gaseous mixture, under lightning impulse voltages (1.2/50 μs), using a point-plane electrode arrangement. The gases and mixtures we considered are SF6, N2, CO2, SF6-N2 and SF6-CO2. The morphology of creeping discharges and their final lengths are investigated versus the kind of insulator material, the amplitude and polarity of the voltage, the type of the gas (resp. mixture) and its pressure. It is shown that the shape of discharges and their final (stopping) lengths Lf depend significantly on the solid insulator and the type of gas. For given solid and gas, Lf increases quasi-linearly with the voltage and decreases when the gas pressure increases. The discharges do not always present a radial structure as reported in the literature. For given voltage and pressure, Lf is longer when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one; and Lf is longer with glass than with epoxy. Lf is shorter in SF6 than in CO2 or N2. On the other hand, the increase of SF6 content in SF6-CO2 mixture leads to a significant decrease of Lf. Therefore, the addition of small concentration of SF6 in a given gas mixture improves the dielectric strength of insulating structure