Propagation and Partial Discharge Characteristics of Electrical Trees in 110 kV XLPE Cable Insulation at High Temperature

Using the real-time microscopy digital imaging and partial discharge(PD) continuous measuring system,we studied the electrical tree shape properties,initiation,growth law,as well as PD characteristics in 110 kV cross-linked polyethylene(XLPE) power cable insulation under different applied voltages at high temperature by the typical needle-plane electrode geometry.The experimental results indicated that the effect of temperature on the shape,initiation and growth time of the typical electrical trees in XLPE cable insulation was dominant.The electrical tree shapes displayed the features of diversity under different stressing voltages at an elevated temperature.The typical tree shapes at 50°C are branch tree,branch-pine tree and bush tree.The trees at 70°C are branch trees,and the trees at 90°C are stagnation-type tree and branch tree.The electrical tree initial time declined with the increase of the applied voltages,and decreased with the increase of the temperature at the same applied voltage,which was due to the melting of the lamella in XLPE cable insulation at high temperatures,and the increase of the amorphous regions and free volumes in dielectrics.It was found that the electrical tree growth rate at low voltage(9 kV) declined due to the increase of the tree channel conductivity,which suppressed the PD activity in the tree structure.The tree growth rate decreased with higher fractal dimension.However,the electrical trees grew rapidly to the counter electrode in branch-type with the continuous PD activity under higher applied voltages(≥11 kV).Meanwhile,the elastic modulus of XLPE decreased at high temperatures,the dielectric strength declined,the effect of PD activity was fiercer,which accelerated greatly the electrical tree growth at high temperatures,and the fractal dimension of the tree was lower.