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Ultra high speed camera studies of paper puncture in oil-cellulose insulation systems under LI testing

This paper aims at presenting results related to electrical breakdown of oil paper insulation systems by puncturing of paper insulation. In the investigations the used experimental setup is a needle-plane geometry with sheets of paper inserted in the oil gap. The voltage applied to the needle is a Lightning Impulse (LI) of positive polarity. The propagation of streamers was studied by an ultra-high-speed frame camera. The streamers were always initiated at the needle tip and were classified as 2nd mode streamers. In some cases the streamer spread out along the top of the paper without puncturing it. In other cases the streamer punctures the paper and continues to propagate below the paper. Differences between the two cases are discussed and analyzed. It is found that the paper puncture voltage coincide with the voltage at which the Laplace field below the paper sheet reach the critical field for streamer initiation.