Space Charge and Breakdown Properties of PP-Based Insulation Interface in Extrusion Molded Joint for HVDC Submarine Cables

This paper focuses on the space charge and breakdown characteristics of polypropylene (PP)-based insulation interface in extrusion moulded joint (EMJ) for high-voltage direct current (HVDC) submarine cables. The double-layered flat samples and cylindrical samples are prepared to imitate the interface in the PP-insulated EMJ. The DC conductivity, space charge, and breakdown strength are tested. The results demonstrate that in the EMJ manufacturing process, the lower wielding temperature leads to microdefects at the insulation interface. As shallow traps, the microdefects exacerbate hetero charge accumulation, thereby intensifying the electric field distortion and increasing the conductivity. Meanwhile, the interfacial microdefects lead to a reduction in the insulation breakdown strength. At 90°C, the normal and tangential breakdown strengths decrease by a maximum of 20.6% and 54.5%, respectively. Notably, the space charges and microdefects lead to a rapid decline in the breakdown strength after hetero polarity pre-stressing. Especially for the tangential breakdown strength, the maximum decrease rate reaches 22.9%. Therefore, the interfacial microdefects caused by the drop in the welding temperature are the primary factors leading to a serious decrease in the electrical properties of EMJ insulation, making the EMJ insulation weaker than PP cable insulation.