The effect of Transient Over Voltages on the Partial Discharges activity in HVDC joints

Rapid growth of HVDC cable line installations is expected in the coming years. Several research groups have investigated dielectric phenomena that occur in the insulation of HVDC cables and accessories that accelerate aging and reduce the reliability of this technology over time. Unlike in HVAC cables, under HVDC stress, the distribution of the electric field inside the insulation layer depends not only on the applied voltage but also on the temperature distribution and on other factors related to manufacturing such as the presence of impurities. Reaching local and transient electric field peaks can enhance the onset of Partial Discharges (PDs) which can lead to premature aging of the insulation. As demonstrated by several studies, the achievement of electric field peaks that can exceed the inception level for partial discharges is more likely in joints. This is due both to the greater probability that a defect is incorporated in two different dielectric layers, and to the greater thickness which determines a higher temperature drop. In this paper, the occurrence of PDs inception conditions in a void inside an HVDC joint during a Transient Over Voltage (TOV) event is investigated by means of FEM simulations. This type of event can be considered as one of the most severe from the point of view of the occurrence of partial discharges as the effect of the rapid variation of applied voltage is combined with the previously established electric field distribution due to the space charge accumulation.