Effects of Silicone Oil on Interfacial Conductivity and Interfacial Breakdown Voltage of XLPE/EPDM Double-Layer

The crosslinked polyethylene (XLPE)/ethylene propylene diene monomer (EPDM) interface structure is one of the weakest points in the high-voltage direct current (HVDC) cable insulation systems. The interfacial conductivity is an essential parameter for evaluating the insulating performance of the interface, as it is closely related to interfacial breakdown. This study first designed a four-electrode system for measuring the interface conductivity of XLPE/EPDM and used a needle-plate electrode to test the breakdown field strength of the XLPE/EPDM interface. Two types of commonly used non-polar dimethyl silicone oil (SO) and polar methyl fluorosilicone oil (FSO) were applied to the XLPE/EPDM interface. Experiments were conducted to measure the interfacial conductivity and breakdown of the XLPE/EPDM interface under varying pressures and different interfacial contact types (with or without silicone oil and the type of silicone oil). The interfacial conductivity of the XLPE/EPDM interface decreases with increasing interface pressure (0-0.2 MPa) across different interfacial contact types, and the breakdown field strength increases with increasing interfacial pressure. The interfacial conductivity of XLPE/FSO/EPDM is approximately two orders of magnitude higher than that of XLPE/SO/EPDM, and its breakdown field strength is comparatively lower. An inverse relationship was observed between interfacial conductivity and breakdown under different interfacial pressures, whereby lower interfacial conductivity corresponds to higher breakdown field strength. Regardless of whether SO or FSO is applied, coating the interface significantly enhances the breakdown voltage of the XLPE/EPDM interface.