Dynamic of partial discharges in cables with inner cavities during thermal and DC stresses

Over last decades, electrical networks have evolved towards high-voltage direct current systems, ensuring connection of offshore renewable energy production sites and to shore-based networks. The constant growth of HVDC networks brings about improvements in several fields. In particular, the use of cables with synthetic polymeric insulation instead of traditional cables with oil-impregnated insulation reduces costs. However, despite continuous upgrade of production lines, it is not possible to totally avoid the formation of microvoids inside crosslinked polyethylene insulation that appear at the junctions of spherulites [1]. Considering applied field in DC cables, cavities in the micrometre range are considered below a critical value and should not generate partial discharges. Thus, they cannot be detected by conventional partial discharges (PD) tests. The possible impact of these small cavities on the cable lifetime and behaviour is unknown. A question arises whether those microscale cavities may increase in size inside the polymer matrix, due to various applied constraints, until reaching a critical size above which discharges can occur. More generally, can shape and dimensions of cavities evolve under the combined effects of electric field, including locally enhanced field due to space charges, and temperature?