Polarity Effect on Standard Lightning Impulse in LN2/Insulation Barrier Composite Systems

Impulse breakdown voltage is one of the most important factors for the designation of the R-SFCLs. However, no research about the polarity effect on standard lightning impulse in LN₂/insulation barrier composite systems has been reported. The objective of this paper is to study the polarity effect on the standard lightning impulse in the LN₂/insulation barrier composite systems. The 50% impulse breakdown voltage was measured with a pair of the needle to plane electrodes with the 0.05 mm thick PTFE film. The gap length of the two electrodes was 10 mm. The distance between the needle electrode and the insulation barrier was varied. The time delays during the breakdown progress were recorded by the oscilloscope. The breakdown path was determined by the arc traces on the PTFE films. The results showed that the 50% impulse breakdown voltage of the LN₂/insulation barrier composite increased with the insulation barrier get closer to the needle electrode. The results of the time delays reflect that in negative polarity, all the breakdowns occurred in the tail of the waveform. In positive polarity, most of the breakdowns occurred in the head of the waveform. With the barrier get closer to the insulation barrier, the longer time delay T_delay was in both polarities. And the probability of wave tail breakdown increased in positive polarities. The arc traces on the PTFE showed that all the breakdowns were occurred through the PTFE film.