Temperature dependent space charge and breakdown strength of PP/ULDPE/graphene nanocomposites for HVDC extruded cable insulation

The space charge behavior, conductivity and breakdown strength are critical issues in the design of insulation for high voltage direct current (HVDC) extruded cables, which is greatly affected by the operating temperature at different load conditions. The space charge, DC conductivity and breakdown characteristics of polypropylene (PP) based insulation at various temperatures are investigated in this paper. The space charge, DC conductivity and the breakdown strength of pure PP, PP/ultralow density polyethylene (ULDPE) blends and its graphene nanocomposites were examined at 30, 60 and 90 °C. The results show that, at elevated temperature, the homo-charge injection in PP from both electrodes is enhanced. Significant charge accumulates in PP/ULDPE blends in the vicinity of the anode at 90 °C. A small amount (0.01 wt%) of graphene nanoplatelets in PP/ULDPE blends inhibit the injection of homo-charge at elevated temperatures. No dramatic change in the breakdown strength with increasing temperature is observed. The DC conductivity increases with increasing temperature, and the conductivity significantly increases from 60 to 90 °C compared to from 30 to 60 °C. The PP/ULDPE blends with 0.01 wt% graphene nanoplatelets exhibit good properties for potential HVDC cable insulation.