Streamer propagation and breakdown in rape-seed oil at high voltage

In recent years, the use of vegetable-based oils for electrical insulation has been considered, in particular to replace mineral oil in medium voltage system. Up to now, little attention has been paid to the properties of these liquids under high voltage at large gaps, that should be important for their application in very high voltage systems. This paper reports an investigation about discharge propagation and breakdown in rape-seed oil. Streamer propagation is studied in large point-plane gaps up to 20 cm, under impulse voltage of positive polarity. Basic features of positive streamers are obtained: velocity, stopping length, propagation voltage, transient current and photographs of the emitted light. Two main streamer propagation ldquomodesrdquo are observed in rape-seed oil: slow streamers with a velocity about 2 km/s, and fast stepped streamers with a step velocity about 120 km/s. The transition from one mode to another is very steep and occurs over a narrow voltage range. Compared to mineral oil where similar phenomena can be observed, fast streamers appear at much lower voltage in rape-seed oil, which in turn induces lower breakdown voltages in long gaps, and shorter time to breakdown.