Insulation Properties and Interfacial Quantum Chemical Analysis of Cross-Linked Polyethylene Under Different Degassing Time for HVDC Cable Factory Joint Applications

Degassing treatment is critical to remove cross-linking by-products in cross-linked polyethylene (XLPE) cable factory joints and prevent early insulation deterioration. In this article, 500-kV high-voltage direct-current (HVDC) cable factory joint samples were prepared, and the physicochemical and temperature-dependent electrical properties of the samples under different degassing times were investigated. The results show that prolonged degassing treatment increases the crystallinity and reduces the tensile properties of the XLPE samples. The samples without degassing have visible micropores near the interface, and these micropores can be eliminated after a longer degassing time. In addition, degassing treatment can reduce the current density, increase the threshold field strength, and enhance the breakdown strength of the samples. A significant negative charge accumulation appears at the interface of the sample without degassing, which dramatically distorts the electric field. Long-term degassing treatment can improve this phenomenon, and the accumulated charge at the interface at elevated temperatures is significantly reduced. Furthermore, a model of the molecular chain microstructure at the interface before and after degassing is proposed. The molecular chain's energy band structure and electron cloud distribution at the interface are analyzed based on quantum chemical calculation. It is found that cable makers can improve the insulation properties of cable factory joints by appropriately extending the degassing time to avoid interface deterioration.