Streamer and Breakdown Characteristics of Transformer Liquids in Moderately-Uniform Fields under Negative Lightning Impulse

The identification of transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown is important for understanding the breakdown mechanisms of transformer liquids and for the transformer insulation design in practice. Most previous studies focused on either very divergent field or nearly uniform field. This paper reports on breakdown voltage tests with three transformer liquids in moderately-uniform fields. The results show that the transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown under negative lightning impulse is liquid dependent. It is 9.6 for a mineral oil, 7.3 for a GTL oil, and 42.0 for a synthetic ester. The difference of the transition field factor boundary helps explain the observed breakdown voltage distinction among the three transformer liquids. In addition, the characteristics of negative streamers leading to breakdown were also investigated in the moderately-uniform fields. Streamer mode transitions with gap distance were observed and clarified, which is from 1st mode streamer to 2nd mode streamer in both the mineral oil and GTL oil, while it is the 2nd mode streamer to fast streamer in the synthetic ester.