Fractal analysis of side channels for breakdown structures in XLPE cable insulation

The role of frequency in the range 20 through 300 Hz on the breakdown voltage and the breakdown path is studied in cross-linked polyethylene (XLPE) cable insulation using embedded needle. A maximum breakdown voltage of 25 kV is found at 240 Hz, and side channels are observed on the flank of the main channel of the electrical breakdown path. Fractal analysis of the side channel is carried out and it is induced that the frequency dependence of the fractal dimension D of the side channel are similar to that of the electrical trees before breakdown. It is suggested that the space charge can be injected from the needle tip. This leads to partial discharge causing progress of the electrical tree and the breakdown path. Space charge will also result in field-moderating cloud around the needle tip and turn to sidewall charges in the side channels. The frequency dependence of the breakdown voltage of the XLPE with the embedded needle can be clarified based on the fractal analysis of the side channel and the electrical tree of the XLPE insulation.