Improved DC performance of crosslinked polyethylene insulation depending on a higher purity

Super-clean crosslinked polyethylene (XLPE) has been widely used in DC power cables. In order to understand the nature of purity dependence of electrical insulation performance, two kinds of insulating XLPE materials used in AC and DC high voltage cables, whose abbreviation were AC-XLPE and DC-XLPE respectively, were investigated in this paper. The results showed that DC-XLPE had a relative higher DC electrical conductivity and relative lower thermal activation energy than AC-XLPE, which was governed by the trap characteristics. The thermal activation energies could match the energy depths of the bulk deep traps calculated by thermal stimulated current. Both positive and negative charges can be found in AC-XLPE while there were only positive charges in DC-XLPE. Meanwhile, DC-XLPE showed higher electric breakdown strength than AC-XLPE. It was proposed that the improved DC insulation performance of DC-XLPE was closely related to its higher purity. The Fourier transform infrared spectroscopy results showed AC-XLPE contained more polar groups such as carbonyl groups than DC-XPLE, which were caused by antioxidants. The oxidative induction times verified a higher content of antioxidant in AC-XLPE. X-ray diffraction showed little difference of crystallinity between two samples, but DC-XLPE exhibited a larger spherulite size. As a result, it is suggested that more efforts should be made to achieve higher purity of XLPE in HVDC insulation.