Electrical treeing from needle implants in XLPE during very low frequency (VLF) voltage testing

This paper presents results from experimental investigations performed in order to characterize electrical treeing as a function of frequency and magnitude of the applied ac voltage. All tests were performed using steel needles molded into 2 mm thick samples of cross-linked polyethylene (XLPE) with an insulation distance of 1 mm. The formation of electrical trees from the energized needle tips were studied using optical microscopy video techniques and characterized with respect to growth rate and maximum tree length. The effect of different ac voltages up to 14 kVrms values and frequencies in the range of 0.1-50 Hz were examined. The results are discussed with respect to the electromechanical theory of electrical treeing. A theoretical model is used to qualitatively explain the frequency dependence of the observations, considering the dissipated partial discharge power and the resistive and capacitive voltage distribution along the tree channel. The experiments confirmed that the effect of reducing the voltage frequency below 1 Hz was to reduce the ratio of the applied voltage distributed along the tree channel, to reduce the degree of branching and to facilitate a growth rate proportional to the frequency.