INVESTIGATION ON CREEPING DISCHARGES PROPAGATING OVER INSULATORS IMMERSED IN GASES AND GAS MIXTURES UNDER LIGHTNING IMPULSE VOLTAGE

This paper is devoted to the analyze of morphology and final lengths of creeping discharges propagating over solid insulators immersed in gas or gas mixture, under lightning impulse voltage (1,2/50 mu s), using a point-plane electrode arrangement. The gases and mixtures we considered are SF6, N-2, CO2, SF6-N-2 and SF6-CO2. Different kinds of solid materials are also studied. The shape and length of creeping discharges are investigated versus the gas (resp. mixture) pressure, the material thickness, the amplitude and polarity of the voltage, and SF6 content in mixture. It's shown that the discharges don't generally present a radial aspect contrary to the results reported by other researchers. Different shapes of discharges are observed depending on the interface. For a given interface, the final length of discharges increases with the voltage and decreases when the gas pressure or material thickness is increased. The initiation threshold voltage of negative discharges is higher than that of the positive ones.