Characterization of Electrical Treeing in XLPE versus Temperature Gradients

Cross-linked polyethylene (XLPE) plays a significant role in AC power cables. In the cables, the difference in temperatures of conductor and surrounding environment produces a temperature gradient in XLPE, and the electrical and thermal aging accelerate the degradation. When defects exist in the dielectric, an electrical tree may be initiated. Hence, it is necessary to research the electrical tree in XLPE under the temperature gradient. In this paper, XLPE samples were tested under 50 Hz AC voltage with multiple temperature gradients, and the electrical tree was recorded by a digital microscope. To simulate an electrical stress concentration, a pair of needle-plate electrodes was utilized in this experiment. There are two types of temperature gradients in this experiment, one is to set the needle electrode temperature to 100 °C and increase the ground electrode temperature from 30 to 70 °C, and the other is the opposite. The tree structure, growth process, fractal dimension, accumulated damage and tree length were recorded and analyzed. The experiment results indicate that the temperature gradient affects the growth rate and the structure of trees. When the ground electrode temperature is higher than the needle tip, the tree grows faster. The fractal dimension and the accumulated damage increase as the temperature gradient increases.