Signal separation and identification of partial discharge in XLPE insulation under DC voltage

Partial discharge (PD) analysis is the most effective method to assess the quality of HVAC cable systems. Regarding HVDC cables, due to the lack of experience and testing tools available, PD tests under DC voltage are performed rarely, and quality control is carried out using AC. However, doubts arise concerning the feasibility of this approach in that PD tests under AC voltage might be significant due to the different electric field distribution between AC (controlled by permittivity) and DC (controlled by resistivity). In this paper, HVDC XLPE flat specimens with knife cut defect are used as cable insulation models and are tested for PD under DC voltage. To carry out this investigation, a technique for measuring PD with high sensitivity under DC voltage has been developed, focusing first on methods to separate noises from PD. The results show that noise rejection can be performed successfully taking profit of the TF map algorithm, developed for AC PD measurements but proving to be effective also under DC.