Investigation on partial discharges in HVDC cables after polarity reversal events

Due to the accumulation of space charge inside the insulating layer of HVDC cables, the electric field under load conditions may be altered compared to what is established in HVAC cables. For example, a high thermal gradient leads to the inversion of the electric field pattern until the maximum value is reached in proximity of the dielectric-semicon interfaces. These maximum values can be further increased due to transient overvoltages and polarity reversal events until reaching electric field values higher than the rated ones. The main goal of this research is to investigate the possibility that, during these transient phenomena, conditions are created that favor the occurrence of partial discharges (PDs). In order to calculate the effect of a polarity reversal on the occurrence of partial discharge in HVDC cables a 3D thermal and electric model of a typical HVDC cable is coupled with a conductance model for the evaluation of partial discharges. A void volume is inserted in the dielectric layer close to the outer surface. The main characteristics of partial discharges occurring during and immediately after a polarity reversal event are evaluated and discussed in this paper. The findings of this research offer in-depth view of the partial discharge phenomena in HVDC cables under load during a polarity reversal in terms of energy released around defects.