Characteristics of Partial Discharge in XLPE during Electrical Tree Initiation Process under Different Temperatures

In order to research the characteristics of partial discharge (PD) during electrical tree initiation process under different temperatures, an AC electrical tree experiment with PD detection and pulse sequence analysis (PSA) was carried out in this paper. The pin-plate electrode was applied to simulate the field concentration areas in cable insulation. Samples of crosslinked polyethylene (XLPE) were prepared and the experiment was carried out under 20°C, 60 °C and 90°C. The applied voltage started from 3 kV and increased at a step of 2 kV until the electrical tree was initiated or the voltage reached 15 kV, where each voltage was kept steady for 30 minutes. The results showed that no PD signal was detected and electrical tree was not initiated at 20°C. At 60 °C, the magnitude of PD increased with the increased voltage but the electrical tree was still not initiated. As the temperature increased to 90°C, the magnitude of PD was significantly greater than that at 60°C, and the tree was initiated at 9 kV. The point clusters in the PSA spectrum at 90°C are more scattered than those at 60 °C that is because the initiation of electrical tree causes the change of discharge channel, so PD changes easily at 90°C. Therefore, under the given conditions, the increase in temperature enhances the magnitude of PD and promotes the initiation of electrical tree. When temperature rises, the molecular segments of XLPE move violently, making them disperse, bend, fold, and even fracture. Then space charge accumulates in the polymer and causes local electric field distortion. Local high electric field causes indistinguishable tiny cavities in the polymer, resulting in minute PD but no tree. Moreover, as the temperature further increases, the material would inevitably soften or even deform. The invisible tiny cavities near the needle are thermally expanded, eventually leading to the PD increases and the initiation of electrical tree.