Streamer propagation in a non-uniform electric field under lightning impulse in short gaps insulated with natural ester and mineral oil

This article describes the comparative experimental studies on streamer propagation in natural ester and mineral oil under a high voltage lightning impulse. These studies were concentrated around the small electrode gaps and the point-plane electrode arrangement. The spatial shapes of the developing streamers, light emission and propagation velocity were analyzed and compared between the two different dielectric liquids. In both of them two streamer propagation modes were registered during the studies performed. Propagation of slow 2nd mode streamers took place below the so-called acceleration voltage while fast 3rd mode streamers developed at acceleration voltage and above. Comparing the streamer shapes corresponding to a given voltage polarity, no visible differences were observed between the liquids tested. Concerning the light emission, higher frequencies of light pulses were registered however in the case of natural ester. The significant differences between both liquids were noticed in the value of the acceleration voltage estimated. In the case of positive polarity streamers started to develop in natural ester as a 3rd mode at lower value of testing voltage than in the case of mineral oil. For negative polarity, within the applied testing voltages, 3rd mode streamers appeared only in natural ester. On this basis, the fundamental conclusion is that natural ester may have a lower ability of preventing the development of fast and energetic 3rd mode streamers, even at small electrode gaps.