Propagation of electrical discharges in small electrode gap of natural and synthetic ester under negative lightning impulse

This article presents the results of experimental studies on electrical discharge propagation in natural and synthetic ester under lightning impulse voltage of negative polarity. The results concern the small 20 mm gap of point-plane electrode arrangement. The spatial shapes of the developing discharges and oscillograms of light emitted during discharge development were registered at the inception voltage and at the voltages chosen as multiples of the inception voltage. On this basis the propagation velocities of the discharges were statistically evaluated for each level of testing voltage. Two discharge modes were observed during the carried out measurements. The slow discharges developed below the so-called acceleration voltage while fast discharges at acceleration voltage and above it. Observed dependence was related to identical one, registered for mineral oil. The differences between all three liquids were noticed. Fast negative discharges started to develop only in natural ester and synthetic ester while in the case of mineral oil this mode was not observed for the considered electrode gap and testing voltages.