Frequency dependence of breakdown performance of XLPE with different artificial defects

In this paper, the effect of the applied field frequency on the breakdown performances of cross-linked polyethylene (XLPE) insulation was investigated. Three kinds of artificial defects, namely an inserted needle, a needle void and a knife void, were introduced into a 10 kV commercial XLPE cable, to simulate real defects that may be introduced during on-site service. Breakdown tests were conducted in transformer oil at room temperature using a frequency-tuned resonant test system in a frequency range from 20 to 300 Hz. It was found that the breakdown voltage of specimens with the inserted needle was sensitive to the applied field frequency, and the breakdown voltage showed a maximum value at about 240 Hz. However, for other two kinds of specimens with the needle void and knife void, the breakdown behavior shows a weak relation with the applied field frequency. The breakdown voltages of the specimens with the needle and void are 25 kV and 20 kV respectively. It was further found that side channels appeared on the flank of the main channel of the breakdown path in the specimen with the inserted needle, and the frequency dependence of fractal dimension D of the side channel in the breakdown path are similar to that of electrical trees before breakdown, which may consequently lead to the maximum value in the experimental results.