Positive streamer propagation in small gap of synthetic ester under lightning impulse

This article describes the experimental studies on streamer propagation in synthetic ester. These studies concern the small 20 mm gap of point-plane electrode arrangement and positive lightning impulse voltage. The spatial shapes of the developing streamers, oscillograms of emitted light and propagation velocities were analyzed and compared with the same features registered for mineral oil. Two streamer propagation modes were observed in both of the investigated liquids. The slow 2^nd mode streamers developed below the socalled acceleration voltage and fast 3^rd mode streamers at acceleration voltage and above it. However, the difference between both the considered liquids was noticed in the threshold value of testing voltage at which the 3^rd mode streamers appeared. Fast positive streamers started to develop in synthetic ester at lower testing voltage (of about 26 kV) than in the case of mineral oil. This means that synthetic esters have a lower ability to protect against overvoltages like standard lightning impulse, even at small (20 mm) electrode gap.