Influence of Electrical Tree Aging on the Dielectric and Trap Properties of XLPE Insulation

The dielectric and charge properties of XLPE insulation with different service periods are important to estimate the aging state of high voltage cables. The accelerated aging experiment was carried out to obtain the aged XLPE samples with the needle-plate electrodes using power frequency AC source. The surface composition and dielectric parameters of XLPE samples were measured and analyzed with the Fourier Transform Infrared Spectroscopy (FTIR) and impedance analyzer. The surface potential and trap distribution of XLPE samples were measured by the isothermal surface potential decay (ISPD) method. The tested objects include the new XLPE sample, the samples with different electrical tree lengths, and the one with electrical tree breakdown. The results show that the aging of the electrical tree changes the dielectric parameters and trap distribution of XLPE samples. The dielectric constant increases with the length of electrical tree. The deeper and shallower traps appear simultaneously in the surface layer of XLPE insulation after electrical tree aging, and the trap density increases with the length of electrical tree. The trap energy level is the lowest and the trap density is the largest in the sample with electrical tree breakdown. The FTIR result shows that the absorption peaks of XLPE backbone structure decline obviously after electrical tree aging. The fracture of molecular chain and formation of low density area in XLPE sample are the main reasons for the change of dielectric and trap after electrical tree aging.