Characteristics of partial discharges in an experimental setup reproducing HVDC cable insulation operating conditions

The occurrence of Partial Discharge (PD) inside a cavity subjected to HVDC voltage can be considered as one of the most likely sources of insulation ageing and failure. In practical applications, the recognition of PD under HVDC is more difficult than under AC stress because of the lower repetition rates and the impossibility to obtain a PD pattern synchronized with the waveform of the applied voltage. In this paper, the relationship between conduction phenomena in dielectrics and the occurrence of PD is investigated by means of a novel experimental setup. A couple of dielectric cells connected in series is subjected to various HVDC stress levels. The applied voltage is divided between the two cells by subjecting them to a temperature difference. The PD activity inside an air void defect existing within one of the two dielectric cell is measured by means of a PRY-CAM®. This setup reproduces, to the extreme, the effect of the distribution of the electric field in an HVDC cable on PD activity when it is exposed to a thermal gradient across the insulation. In addition, an equivalent circuit model has been developed to evaluate the PD Inception Voltage (PDIV) across the cavity. The comparison between the experimental and model results demonstrates the capability of the macroscopic conduction model to reproduce the inception of PD phenomena in solid dielectrics under HVDC stress and temperature gradient.