This paper reports on experimental investigations in which both HVDC and HVAC voltages were applied to solid dielectric insulation samples containing known defects. The insulation samples were subjected to HVAC and HVDC containing superimposed harmonic ‘ripple’, similar to that produced by the converter switching operation. Of particular interest were the effects of the applied voltage on partial discharge (PD) amplitude, repetition rate and pulse-spacing, which showed a correlation with harmonic amplitude and phase position. Under HVAC, the phase of the power frequency cycle is the dominant driver of PD activity and forms the basis of most diagnostic techniques for PD. However, under HVDC, PD activity can be influenced by comparatively minor features of the applied voltage. Established knowledge rules for defect identification based on repetitive PD characteristics at HVAC are therefore unlikely to be applicable directly to HVDC systems. Given the rapidly expanding HVDC infrastructure, accruing new knowledge rules through laboratory tests and online measurement trials will be essential in assisting network operators to identify incipient defects with a greater degree of confidence on these systems in the futur
