Partial Discharge and Growth Characteristics of Electrical Trees in 110kV XLPE Cable Insulation at High Temperature

Abstract: By using real-time microscopy digital image and PD continuous measuring system, we studied the shape properties of electrical tree, initiation, growth law and PD characteristics in 110kV XLPE power cable insulation under different applied voltages at high temperature via the typical needle-plane electrode geometry. The test results indicate that the effect of temperature on the shape, initiation and growth time of typical electrical trees in the XLPE cable insulation is dominant. The electrical tree shapes display the features of diversity under different voltages at an elevated temperature. The shapes of typical tree at 50℃ are branch trees, branch-pine tree and bush tree. The trees at 70℃ are branch trees and the trees at 90℃ are stagnant tree and branch tree. The initial time of electrical tree declines with the increase if the applied voltages, and decreases with the increase of temperature at the same applied voltage, which is due to the melting of the lamella in XLPE insulation at high temperatures, and the increase of the amorphous areas and free volumes in dielectrics. It is found that the electrical growth rate at low voltage (9kV) declines due to the increase of the tree channel conductivity, which restrain the PD activity in the tree structure. The tree growth rate decreases with higher fractal dimension. However, the electrical trees grow rapidly to the opposite electrode in branch-type with the continuous PD activity under high applied voltages (≥11kV). Meanwhile, the elastic modulus of XLPE decreases at high temperature; the dielectric strength declines; the influence of PD activity was more intense, which accelerates the tree growth at high temperature. The fractal dimension of the tree is lower.  

Key words:  XLPE cable insulation, electrical tree, growth characteristics, partial discharge (PD), high temperature, fractal dimension