Compressive stress dependence of electrical tree growth characteristics in EPDM

To ensure a certain interface electrical strength resistance, the cable accessories are densely coated onto the main cable insulation. Compressive stress is generated in the normal direction of high voltage cable by the spring clamp device or the banding force after the expansion of EPDM (Ethylene Propylene Diene Monomer) stress cone. This paper presented a compressive stress experimental study considering electrical treeing propagation in EPDM by employing a needle-plane electrode system. The results indicate that compressive stress affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes longer as compressive rate increases from 0 to 30%. Tree length and accumulated damage become smaller with increasing the compressive rate. Electrical tree structure is also depended on compressive stress, and branch tree structure becomes the main electrical tree morphology at higher compressive rate. For better understanding the effects of compressive stress on electrical tree propagation characteristics, FFV (fractional free volume) and CED (cohesion energy density) were calculated and analyzed through molecular dynamics simulations under different compressive rates. Results show that the FFV becomes smaller at higher compressive rate, which shortens the free path of the hot electron and weakens the collision with the molecular chain during the treeing process. While CED tends to be lager under higher compressive rate, which improves the cohesion strength for the crack to grow in the tree channel tips and restrains the propagation of electrical tree.