Characteristics of partial discharge and AC electrical tree in XLPE and MgO/XLPE nanocomposites

In order to investigate the effect of nano-doping on partial discharge (PD) and electrical tree growth in cross-linked polyethylene (XLPE), samples of 0.5 wt% magnesium oxide MgO/XLPE were prepared and an AC experiment was carried out. A color-coded diagram for PD signal was developed to describe the PD activity during the whole process of treeing. The results show that the PD intensity in positive half cycle of voltage (PHC) is larger than that in negative half cycle of voltage (NHC), and the PD magnitude in MgO/XLPE is lower than that in XLPE. Moreover, the tree length and morphology in MgO/XLPE are similar to those in XLPE. Furthermore, a finite element simulation based on the bipolar charge transport model was applied to investigate the effect of space charge on PD under AC voltage. Simulation results suggest that the characteristics of PD and tree growth may be associated with the accumulated charge around the needle tip. The distribution of negative charges in NHC would be wider than that of positive charges in PHC, which would weaken the electric field in NHC and lead to the less PD in NHC. Moreover, the range of charge shield in MgO/XLPE may be larger and is equivalent to increasing the effective radius of curvature of needle tip, which would lead to smaller electric field and lower PD magnitude in MgO/XLPE. However, the addition of nano-MgO reduces the crystallinity of material based on the differential scanning calorimetry (DSC) result. Hence, although the PD magnitude in MgO/XLPE is lower, the MgO/XLPE researched in this paper does not show significant resistance to AC tree growth.