Effects of mechanical stretching on electrical treeing characteristics in EPDM

Abstract

Effects of mechanical stretching, produced after the expansion of cable accessories, on the electrical tree growth characteristics of EPDM (Ethylene Propylene Diene Monomer) are reported in the present work. FFV (fractional free volume) and CED (cohesion energy density), which were respectively related to free path of electrons and molecular chains splitting during electrical tree propagation process, were calculated through molecular dynamics simulations under different stretching ratios. A needle-plane electrode system with the stretching ratio ranging from 0 to 30% was used to investigate the electrical treeing characteristics in the experimental study. The simulation results show that the FFV becomes larger at higher stretching ratio, which enhances the collision between the hot electron and the molecular chain during the treeing process by extending the free path of the hot electron. While CED tends to become smaller at higher stretching ratio, reducing the cohesion strength for the tree channel crack to grow. Moreover, FFV and CED are investigated during the stretching recovery process, where FFV tends to decrease and CED turns to be larger. The experimental results indicate that mechanical stretching affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes shorter as stretching ratio increases from 0 to 30%. Tree length and accumulated damage become larger with increasing the stretching ratio. Electrical tree propagation mechanism considering chemical and physical traps, which are generated under mechanical stretching, are also discussed and analyzed.