Streamer initiation in cyclohexane, Midel 7131 and Nytro 10X

Streamer initiation has been studied in cyclohexane, Midel 7131 (ester based dielectric liquid) and Nytro 10X (mineral transformer oil). Experiments were conducted in point-plane geometry under 40 ns/740 μs impulse voltage. The tip electrode was electrochemically etched to approximately 2.0 μm, the electrode gap was approximately 18 mm. A sensitive differential charge measurement technique was used to measure the injected charge. Shadow graphics was used to acquire images of streamers, and a photomultiplier was utilized to study light emission under positive polarity in Nytro 10X. The experimental setup is capable of measuring injected charges down to 0.1 pC, the shadow graphics has an image resolution of 1 μm and the photomultiplier can detect light down to the order of single photons. For all three liquids a small pre-inception current at voltages below streamer inception was found with positive point polarity; no emitted light from this current in Nytro 10X was detected. No preinception current was observed under negative polarity in any of the liquids.