Growth Characteristics of Electrical Tree in Crosslinked Polyethylene under Direct-Current Voltage

Electrical tree is a kind of detrimental phenomenon in the polymer insulation materials of direct-current (DC) power cables. The electrical trees in power cables caused by electric power accidents or insulation defects may keep growing as power cables working normally. To investigate the growth characteristics of electrical trees in cross-linked polyethylene (XLPE) under DC voltage, experiments were carried out with XLPE samples under constant DC voltages of 25, 30 and 35 kV for 4 h at the temperature of 30, 50, 70 and 90 °C. Needle-plane electrode structure was used and the distance between needle tip and plane electrode kept 3 mm. And the curvature radius of needle tip kept 5 μm. Since the initiation of electrical tree under constant DC voltage was very difficult, all electrical trees were firstly initiated by periodic grounded DC voltage of 24 kV. The pictures of electrical trees were acquired every quarter by a camera with micro lens during experiments. The results showed that electrical trees did not grow under the conditions of lower voltages or temperatures such as 25 kV with 30 °C. While under the circumstances of the electrical trees growing apparently, the increment and average growth speeds increased with the temperature or the DC voltage. The growth mechanism of electrical trees under constant DC voltage can be explicated by charge injection and charge trapping. And the growth characteristics of electrical trees under the constant DC voltage is significant to the maintenance of DC power cables.