Lighting Impulse Properties in Large Oil Gaps with and without Pressboard Interface

In this paper, negative polarity lighting impulse properties of three kinds of transformer oils in different gaps were investigated. The tested oils included 25# (naphthenic mineral oil), RAPO (natural ester, rape-seed oil) and FR3 (natural ester, soybean oil). Each of oils was tested with and without pressboard under 1.2/50μs standard negative lighting impulse with point-plane electrode system in several gaps. Breakdown voltage (in oil without pressboard) and flashover voltage (in oil with pressboard) were recorded and analyzed. It has been found that the above natural esters represented certain degree of lower breakdown and flashover properties compared to mineral oil; moreover, in small oil gaps, the breakdown voltage was higher than flashover voltage, while in large gaps the opposite, and this trend was increasingly obvious with gap distance increasing. Furthermore, this paper found that other factors had a certain influence on the negative lighting impulse properties, which will be investigated in future.