Positive Backstrokes from a Propagating Negative Streamer in Transformer Oil in an 80-mm gap

Negative streamers with an applied step voltage in an 80-mm point-to-plane gaps in transformer oil have been studied with a fast 16-frame camera. The negative streamers propagated for around 200 µs – 300 µs before breakdown occurred or the streamer channel disappeared. The streamer velocity was approx. 0.4 mm/µs. For some of the captured images, we have seen that faster positive streamers have propagated in the opposite direction from the propagating negative streamer head. This clearly shows there is little or no contact back to the electrode behind the head of the propagating negative streamer, and a positive field towards the electrode must exist, likely due to charge separation in the streamer head.