Investigation on effect of semiconducting screen on space charge behaviour of polypropylene-based polymers for HVDC cables

High voltage direct current (HVDC) power transmission cable is critical for realising sustainability through renewable energy revolution. Eco-friendly thermoplastic polypropylene (PP)-based polymers/nanocomposites are regarded as promising candidates for replacing current thermoset crosslinked polyethylene (XLPE) cables. As an essential component of the extruded HVDC cable for improving conductor/insulation interface and suppressing charge injection to insulation at high DC electrical stresses, developing semiconducting (SC) screens that are compatible with PP-based insulation is of similar importance but has not been well studied yet. This work aims at designing PP-based semiconducting screens and investigating space charge behaviours of SC/PP/SC sandwich specimen to unfold the effect of semiconducting materials, bonding methods, applied DC electric field, and temperature on charge injection, accumulation, transportation, and dissipation in PP-based insulation. Although conventional thermal, mechanical, and low field electrical characterisations demonstrated that all of the developed semiconducting materials meet the performance criteria of commercial semiconducting materials, their space charge and local electric field distribution varied significantly at high DC fields. Compared with the traditional non-bonded configuration used at lab-scale, charge injection was enhanced in hot-pressed SC/PP/SC samples with tightly bonded interfaces, which better reflects the real situation in extruded cables. High temperature further intensified charge injections. Besides, our results also revealed that high temperature and electric field strongly influence charge mobilities and consequently their distribution and local electric field in PP-based insulations.