Partial Discharges in HVDC Insulation with Superimposed AC Harmonics

This paper describes the influence of AC harmonics in HVDC systems on one of the main indicators of high voltage insulation deterioration, partial discharge. HVDC systems may be subject to various disturbances including harmonics and ripples in the form of pulsating voltages. Experiments, performed on model void specimen at DC voltage with selected superimposed harmonics, are described. A novel approach for recording partial discharges at DC, based on synchronization with selected superimposed harmonics, is proposed. In this way individual partial discharge pulses, occurring during a given measurement time, are accumulated with respect to the phase angles of certain harmonics. The partial discharge inception threshold is analyzed for cases where the DC level is held constant and the AC magnitude and harmonic frequency are varied and for cases where the DC level is varied and the AC magnitude is held constant. In addition, a long term aging test where insulating paper and resin impregnated paper was subjected to pure DC voltage and DC containing 12th harmonic 600 Hz, is described. The aging test shows that the superimposed harmonic content resulted in an amplification of partial discharge presence. By mapping the discharge spots occurring on the void surface, it is possible to quantify the level of erosion in both cases. It was observed that in case of harmonic crest voltage being below partial discharge PD inception level the DC component is crucial, however when harmonics magnitude is above the inception voltage the role of AC is dominant relative to DC. Thus, the presence of harmonics may result in a strong acceleration of the deterioration of insulating materials subjected to DC voltage.