Effect of DC Prestressing on Periodic Grounded DC Tree in Cross-Linked Polyethylene at Different Temperatures

Periodic grounded dc trees in cross-linked polyethylene under various dc prestressing times are investigated in the temperature range of 20 °C-80 °C. Space charge behaviors in the samples during dc prestressing are simulated based on the bipolar charge transport model. The results reveal that the dc prestressing time has different effects on the tree growth at different temperatures, which is because that the space charge behavior in the sample during dc prestressing is closely related to the dc prestressing time and the temperature, and it has both promotion effect and impedance effect on the tree growth. The moderate promotion effect and impedance effect under 160 s dc prestressing at 20 °C, and the strongest promotion effect and the weakest impedance effect under 240 s dc prestressing at 40 °C result in the largest tree lengths, widths, and accumulated damages under each corresponding condition. At 60 °C and 80 °C, the lengths, widths, and accumulated damages are the largest under 240 s dc prestressing, smaller under 80 s dc prestressing, and the smallest under 160 s dc prestressing, and the reason is believed to be the growth rates reversal during the tree growth. Also it is found that the dc prestressing time has significant effects on the tree shape at 60 °C and 80 °C, which is believed to be related to the wider charge diffusion range under longer dc prestressing time. The electrical tree characteristics are discussed in detail combined with the space charge behaviors.