Temperature Gradient Affecting Electrical Trees in EPDM With Combined DC and AC Voltage

In this article, the electrical tree growth characteristics of ethylene propylene diene monomer (EPDM) are studied under combined dc and ac voltage at different temperature gradients ( $\Delta {T}\text{s}$ ). The temperature of the needle tip of the high voltage (HV) electrode is fixed at 110 °C, while the ground electrode (GD) temperature varies from 110 °C to −40 °C, thus forming a $\Delta {T}$ within the EPDM. An 8-kV ac voltage, with frequencies from 50 to 250 Hz, superimposed −10-kV dc voltage is applied to the EPDM samples. It is found that $\Delta {T}$ promotes the growth of the electrical tree and makes the tree tend to grow more around the needle tip. The application of a high-order ac voltage will accelerate the growth of the electrical tree. However, when $\Delta {T}$ reaches 150 °C, electrical tree growth is inhibited, and the influence of the ac voltage frequency is limited by $\Delta {T}$ . The electric field distribution, which is dependent on the dc conductivity, and the free volume simulation are employed to explain the treeing process. This article proposes that $\Delta {T}$ could promote charge accumulation, which increases the probability of partial discharge and promotes the growth of electrical trees. The lower temperature of the GD leads to a weaker electric field strength near the needle tip and decreases the free volume near the GD side, resulting in the electrical tree growing more easily in the high temperature and high electric field area around the needle tip.