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.