Magnetic-Field-Dependent Electrical Tree Under Impulse-Superimposed Dc Voltage At Low Temperature

Cross-linked polyethylene (XLPE) is an important insulating material in warm dielectric superconducting cable. When it is operated in a high electric field, an electrical tree may occur and cause insulation breakdown. In this paper, the XLPE samples were prepared and a needle-plate electrode was employed. The treeing process at 0, -30, and -60 degrees C was recorded. The magnetic field applied on the samples was 0, 200,400, and 600 mT. An electrical tree was initiated by an impulse-superimposed dc voltage. Two types of tree structures were found and their growth processes were recorded. In addition, the fractal dimension and accumulated damage were both calculated and analyzed. The result indicates that there is a magnetic field threshold to accelerate the tree growth, which increases with the decrease in temperature. Above -60 degrees C, it is below 600 mT and turns out to be higher than 600 mT at -60 degrees C. The magnetic field influences the tree-growth process by affecting the space-charge distribution.