The initial stages of positive and negative corona DC breakdown of transformer oil

The initial stages of spontaneous electrical DC breakdowns in transformer oil in 1-10 mm positive and negative corona gaps were studied using simultaneous recording by an oscilloscope, photomultipliers, and image converter/intensifier cameras of nanosecond time resolution. The phenomena were untriggered, and were observed by their own light. Special care was taken to ensure observation of eventual semicontinuous current and light emission occurring between the more easily observed pulse phenomena. A 60-ns image delay mirror system was used when necessary to observe the very beginning of spontaneous streamers or breakdowns. The mobilities of the dominant positive and negative ions involved were estimated from DC current-voltage characteristics. The current-voltage characteristics indicate that negative DC coronas conduct current unipolarly up to breakdown, while positive coronas are dominated by bipolar streamers. The estimated dominant positive and negative ion mobilities are around 10^-7m²/Vs