Space Charge Characteristics for XLPE Coaxial Cable Insulation under Electrothermal Accelerated Aging

The electrothermal synergistic aging effect on a cross-linked polyethylene (XLPE) cable system can cause strong deterioration of the cable insulation over prolonged service conditions. It is widely accepted that space charge can be the cause and the effect of aging of XLPE insulation under dc stresses. Until now, reports on the correlation between space charge characteristics and the aging effect mainly focus on the XLPE flat samples, and an insight into this correlation for XLPE coaxial cables is still scarce. In this article, space charge in XLPE coaxial cables with different aging durations was measured under polarization and depolarization conditions. The influences of cable coaxial structure and insulation thickness were analyzed. The mobility and trap density were extracted with the depolarization charge profiles. The results show that heterocharge, trap-controlled mobility and shallow trap first increase (0-30 days) and then decrease (30-90 days) with the aging time, while the deep trap increases from 30 to 90 days aging, and for the thicker coaxial cable insulation, due to more deep traps blocking charge migration and extraction, a fraction of residual charge retains near the outer semiconductor after depolarization. In addition, the residual charge density increases with aging time. These phenomena have significant impacts on long-term cable operations and should attract more attentions. The space charge characteristics in coaxial XLPE cable during aging are likely to invoke a modification of the life model for a real cable.