Partial discharge behavior under HVDC superimposed with transients

Transient overvoltages are inevitable phenomena in power systems. They occur in high voltage direct current systems for much the same as the reasons they happen in high voltage alternating current systems and could be of either internal or external origin. Moreover, the switching operation of the power electronic valves in HVDC converters add to these transient sources. The effects of such transient overvoltages become more critical in the case of non-restoring insulation media like solid dielectrics that form the main insulation media of extruded HVDC power cables. In addition, the intrinsic behavior of solid dielectric under DC fields results in space charge accumulation due to inhomogeneity of the insulation, increasing the likelihood of a fault during overvoltage incidents or even during normal operations like polarity reversal. This paper presents the results of PD measurements that have been conducted on artificial test samples at HVDC superimposed with standard switching overvoltages and operational polarity reversal, which are common in modern converter schemes. The results show that under such transient conditions, PD occurs with higher repetition than the normal voltage level in the test sample. The results of this investigation should be of interest to network operators for asset management on fleets of HVDC cables that may typically be subject to the reported transients and polarity reversal effects.