Effects of silicone oil coating on interfacial space charge behavior in XLPE-EPDM composite insulation structures

As one of the weakest points in high-voltage direct current cables and accessories, the accumulation of space charges at the crosslinked polyethylene (XLPE)/ethylene propylene diene monomer (EPDM) interface coated with silicone oil is crucial to insulating properties. The physical mechanisms underlying this charge accumulation and dissipation phenomenon remain unclear, particularly at the molecular level. Thus, the interfacial space charge accumulation and dissipation behavior at EPDM/XLPE, EPDM/non-polar dimethyl silicone oil (PDMS)/XLPE, and EPDM/polar fluorinated silicone oil (PMTFS)/XLPE interfaces was measured using pulsed electroacoustic (PEA) method, and molecular simulation techniques were employed to calculate the electronic properties across those interfaces. It was found that the transformation law of the interfacial charge polarity does not completely align with the Maxwell-Wagner (MW) model, which is related to the contact type of the interface (with or without silicone oil and the type of silicone oil) and the voltage polarity. The presence of a high interfacial potential barrier is an important factor behind the fact that the transformation law of the interfacial charge polarity does not align with the MW model. The high hole potential barrier (greater than 1 eV) of EPDM/XLPE and EPDM/PMTFS is the reason why the interfacial charges of EPDM/XLPE and EPDM/PMTFS/XLPE remain always positive as the applied negative voltage and temperature increases. Due to the low potential barrier of the EPDM/PDMS/XLPE interface, the polarity of the interfacial charge is always consistent with the polarity of the voltage applied to the medium with a greater conductivity. At 40 °C and 60 °C, EPDM/XLPE and EPDM/PMTFS/XLPE positive interface charge accumulation is significantly reduced compared to that observed at room temperature under a negative voltage, which is attributed to the enhanced charge injection and migration of XLPE with rising temperature. This study provides theoretical insights for finding an effective coating material to reduce charge accumulation at the cable accessory interface.