Effect of Thermal Aging on Dielectric Properties of XLPE Insulated HVDC Cable

Space charge characteristics, leakage current, and differential scanning calorimetry (DSC) of the cable insulation were investigated to quantitatively evaluate the thermal aging effects on the dielectric properties of cross-linked polyethylene (XLPE) insulated high-voltage direct current (HVDC) cable. The longest aging time of these cables at 135°C and 17kV/mm DC electric field reaches 1440h. The space charge distribution was obtained under multiple electric fields from 5 kV/mm to 35 kV/mm by a full-scale space charge measurement system for cable sample. It is shown that the amount of space charge and decreases after 720h aging, and then decreases after another 720h aging. At the same time, the leakage current increases with the aging time, while the molecular weight and crystallinity of the insulating material decreases with aging time. Based on the results mentioned above, the effect of thermal stress on XLPE DC cable insulation is similar to that on alternating current (AC) cable insulation. Space charge mainly comes from the dissociation of cross-linking by-products and small molecules generated by aging. In the initial stage of aging before 720h, the degassing process dominates, resulting in a decrease in space charge. Between 720h and 1440h of the aging process, the molecular chain breaks into small molecules, and the amorphous area expands, which increases the space charge. Simultaneously, the decrease in crystallinity and the increase in small molecules increase the trap concentration, which reduces space charge mobility leakage current. Reduced charge mobility leads to a decrease in the leakage current.