A Novel Approach for the Recognition of Partial Discharges Under DC

Unlike the insulation under AC, at the state of the art, the detection and recognition of PD under DC is still based on non-deterministic methods. This is because, without a phase reference, the measurement of signals cannot be related to the expected cause of the PD events. Moreover, for a given applied voltage, a lower PD activity under DC compared to the case under AC has been experimentally observed. For these reasons, PD detection and recognition under DC are still challenging and a method analogous the Phase Resolved PD (PRPD) pattern is missing. In this article, a novel method for the recognition of PD under DC is presented together with the experimental results of a case study. The method is based on the variation over time of the electric field distribution within the insulation of HVDC cables. In particular, the time trend of the electric field at several evaluation points is combined with the time trend of the repetition rate of the measured signals to obtain an Electric Field Resolved PD pattern. The recognition criterion is based on the expectation that the PD activity at a certain point should increase with the electric field that would be reached at the same point if no PD events were occurring. To validate the method an artificially defected cable has been subjected to varying voltage and current. The experimental results confirm that the method is useful for the recognition of internal PD under HVDC cables.