Partial Discharges for Diagnosis of Cavity Growth in HVDC XLPE Model Cables

This paper presents results carried out on polymeric high voltage direct current (HVDC) model cable. Two aspects are considered: partial discharges (PD) and current measurements, with the aim of investigating possible growth of voids of micrometer size. PD can occur within defects already existing inside crosslinked polyethylene (XLPE) matrix under various conditions. These conditions are well known under alternative current (AC) but not so well under DC with harmonics. Currents are at the origin of charges accumulation at XLPE/void interfaces, which may in turn be responsible for local field enhancement, discharges and possibly into void growth. Model cables have been subjected to (70°C) only or to HVDC field with harmonics under 60 kV/mm DC and 8 kV/mm 1800 Hz AC at nominal temperature (70°C). PD measurements on unaged model cable suggest the existence of cavities whose diameters can be estimated around $10~\mu \mathrm {m}$. Current densities are compared between thermal and electro-thermal ageing, effect of ageing will be highlighted.