Differences in Characteristics of Long-Gap Lightning Impulse Breakdown between Vegetable and Mineral Insulating Oil

Abstract

Vegetable insulating oils, with higher fire point, lower biological hazard and better availability compared to traditional mineral insulating oils, have enormous potential to become the substitute of the latter, given the trend of green development of the power equipment, for example, the transformers. However, the obviously different constitution of the vegetable oils makes the circumstance hard when considering their application on the insulation for actual transformers. One of the significant deficiencies of the vegetable oils in working condition is their insufficient lightning impulse endurance under partial high electric fields and large discharge gaps. In this article, three kinds of transformer insulating oils, rapeseed oil, FR3 and 25# mineral oil were chosen to investigate the performances of these insulation materials under long-gap lightning impulses with positive and negative polarity respectively. The variation rules of breakdown voltage and velocity with the gap length were obtained. And the difference of the lightning impulse voltage endurance between the vegetable and mineral insulating oils as well as its causes based on theories of streamer and ionization potential (IP) were analyzed, which led to the conclusion that the gap increase causes more threat to the positive lightning impulse breakdown of vegetable insulating oils, compared to its counterpart.