Electrical treeing in stress-whitened polypropylene thermoplastic elastomer cable insulation

Abstract

Polypropylene thermoplastic elastomer (PP-TPE) is emerging as an energy-saving and environmentally friendly alternative to cross-linked polyethylene (XLPE) for high-voltage cable insulation. However, the effect of mechanically induced stress whitening on electrical pre-breakdown of PP-TPE remains largely unexplored. This study investigates the influence of stress whitening generated by mechanical impact on electrical tree growth dynamics, morphology and associated partial discharge (PD) activity in an extruded PP-TPE insulation taken from a commercially available cable. Electrical trees are initiated using a needle-plane configuration under AC voltage in samples with and without prior mechanical impact and thermal treatment. Impact-generated stress whitening is found to promote increased branching and alters growth directions, likely due to mechanical stress lines and localized material weakening, although it is not observed to significantly accelerate tree growth. Heating to sub-melt temperatures visibly reduces stress whitening, but does not affect electrical treeing, suggesting an optical recovery does not reverse mechanical damage. Phase-resolved partial discharge analysis (PRPDA) reveals distinct PD patterns evolving through various phases of tree growth, with evidence of carbonization and gas pressure influencing PD dynamics.