Study on Space Charge Behavior of XLPE after Long-term Aging under Temperature Gradient and DC Stress

In this paper, in order to study the effect of temperature gradient on long-term reliability of high voltage direct current (HVDC) cross-linked polyethylene (XLPE) cable insulation during actual operation, an aging device under DC electrical stress companied with temperature gradient( the high temperature side is 60 degrees C and the low temperature side is 20 degrees C) for multi-group film specimen were designed to carry on three kinds of XLPE insulating materials, which includes pure XLPE, nano-modified XLPE and HVDC XLPE produced by Borealis). Furthermore, traditional thermo-electrical aging at a constant temperature of 60 degrees C were performed on above three kinds of XLPE as reference group. Space charge characteristics of the three kinds of XLPE after aging for different months were measured by pulsed electro-acoustic (PEA) method. Moreover, resistivities of the three kinds of XLPE after aging were measured, respectively. Results show that space charge distributes evenly in the specimens with a constant temperature, while hetero-charge accumulates in the specimens under temperature gradient. Moreover, resistivities of the three kinds of XLPE decrease with the increasing aging time. Furthermore, the resistivities of aged samples under temperatrue gradient were lower than those of thermo-electrical aged samples at the same time. Compared with the other samples, the space charge accumulation in 2# XLPE samples was least in the same operation condition, which shows 2# samples were aged at lowest degree.