Interfacial Breakdown Behavior of XLPE/EPDM Bilayer Insulation in DC Cable Terminations Under Varying Treatment Conditions

To examine the influence of interface conditions on DC breakdown behavior at submarine cable terminations, double-layer samples composed of cross-linked polyethylene (XLPE) and ethylene-propylene-diene terpolymer (EPDM) were prepared with two distinct interface states: direct contact and coating with standard silicone oil. Controlled variations in temperature and pressure were applied to replicate real operating environments. The study systematically analyzed how these factors affect the DC interfacial breakdown characteristics. Results show that the interface DC breakdown voltage is directly proportional to pressure and inversely proportional to temperature. Additionally, when a vertical preloading DC electric field is applied, the breakdown voltage is significantly affected by the polarity of the electrodes. The presence of silicone oil coating significantly improves the interface insulation performance. Specifically, at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10{ }^{\circ} \mathrm{C}$</tex> and 0.05 MPa, the breakdown voltage change under positive and negative voltage without silicone oil coating is 25.9 % and 34.14 %, respectively, while the change in breakdown voltage after silicone oil coating is reduced to 16.22 % and 9.07 %. The study demonstrates that the DC breakdown voltage at the interface is affected by both temperature and pressure, and silicone oil coating significantly enhances the insulation performance, providing valuable insights for improving the reliability of cable terminations under real operating conditions.