Polarity Effect on Electric Tree Inception in HVDC Cable Insulation

This paper studies the influence of polarity of the applied voltage on DC tree inception in XLPE by analyzing dissipated and stored electric energies. Space charge dynamics, electric field distributions and resulting energies in the vicinity of the needle tip in the needle-plane type sample are calculated using the bipolar charge transport model. Simulations are performed for three modes of DC tree initiation: by applying continuous DC voltage, by grounding pre-stressed sample and by polarity reversal. In case of continuous DC, the estimated dissipated energy is found to be higher for positive applied voltage compared to negative that results in a lower inception voltage level. It is observed that the energy dissipation is controlled by the amount of mobile charge carriers and their mobility. In the case of trees due to grounding of pre-charged sample, differences in the electrostatic energy stored in the vicinity of the needle electrode right before grounding leads to tree inception at lower magnitude for the negative polarity compared to positive. Here, the amount of stored energy is dependent on the electric field controlled by the distribution of space charge in the material and the mobility of charge carriers plays a major role by defining locations of space charge clouds. When trees are initiated by polarity reversal, both stored energy before reversal and dissipated energy after, are in favor of tree inception during negative-to-positive transition than for the opposite. The obtained energy densities are compared with the threshold values required for tree inception by electromechanically induced cracks in the material. It is concluded that stored and/or dissipated energy are determining the polarity effect in DC tree inception.