DC conduction of polymeric insulation of medium voltage cables

The transient leakage current of XLPE (cross-linked polyethylene) cables to which a DC voltage was applied was studied under different temperatures and applied electric fields. The cables used were commercial XLPE cables with inner and outer semiconducting layers which were extruded in one operation together with the insulation layer and cured in a water vapor cross-linking unit. The thickness of the XLPE insulation was 2.5 mm and the inner diameter of the outer semiconducting layer was 17.8 mm. The ranges for the electric fields were from 3 to about 30 MV/m and for the temperatures from room temperature to about 90°C. The results demonstrate the complexity of the electric conduction in XLPE and give further evidence that the conduction is space charge limited, injection controlled, and very strongly dependent on the externally applied field and, especially, the cable temperature. The degree of complexity in the electric conduction of realistic cables increases when the cable is not isothermally heated and when there is a temperature gradient between the conductor and sheath. An argument between existing theories and experiments is found