Tracking Failure At Interface Between XLPE And Silicon-rubber

XLPE power cables are increasingly used in power distribution system for their excellent mechanical and electric performance. However, interface between XLPE and silicon rubber in cable joint remains one of the weakest part of the cable system. Dielectric breakdown called tracking failure often occurs at the XLPE-Silicon-rubber interface, which is resulted by the formation of short-circuit carbonized conductive path at the interface. The failure often endangers the stability of power system and its equipments. Therefore, it is important to investigate the failure process and mechanism to improve the reliability of power system.This paper investigated the tracking failure process and mechanism by analyzing the distribution characteristics of the discharge light and carbonization at the XLPE-Silicon rubber interface from discharge to tracking failure. Test samples were made by pressing a piece of XLPE and silicon rubber together under different interfacial pressures. AC voltage was applied on a pair of needle-plate electrodes at the interface. Both the initial discharge voltage and the time to tracking failure were recorded. The discharge light and the carbonization were also measured with a high-speed camera. The methods of image processing and fractal dimension were employed to establish the relationship between the quantitative characteristics of tracking process and the interfacial factors. The factors include interfacial pressure, roughness, scratch marks /nick, mini-wire and silicon grease. Studying the distributing characteristics of discharge light and carbonization at the interface from initial discharge to tracking failure can assist in better understanding the failure mechanism because the discharge light and carbonization provide more direct and detailed information in two dimensions than initial discharge voltages alone. The main results are as follows:For the decrease of cable-joint interfacial pressure with aging influences the dielectric performance of cable joint, the effect of interfacial pressure on tracking failure of the interface was studied by investigating the distribution characteristics of discharge light and carbonization with imaging processing and fractal dimension technologies. Results provided the general failure process and the effect of interfacial pressure on the interface tracking failure.The installation of cable joint possibly introduces defects into the interface, such as roughness, scratch marks/nick, and mini-wire. Those defects decrease the interfacial dielectric performance. The effect of interfacial defects on the tracking failure of the interface was studied by analyzing the distribution characteristics of discharge light and tracking pattern. Results show that a rough interface easily leads to interfacial discharge and tracking failure. Scratch marks/nick strengthens the transportation of charge and easily leads to discharge and tracking failure. The distribution of discharge light and carbonization at the interface with mini-wire proved this.Because the silicon grease at the interface greatly influences the dielectric performance of the interface, the effects of interfacial silicon grease on the tracking failure of the interface was studied by investigating the distribution characteristics of discharge light and carbonization. Results show that the grease strengthens the interface dielectric performance and restrains the interfacial discharge and tracking failure. However, the path of discharge channel often remains unchanged, which results in repeated impact of discharge on the same discharge channel and easily leads to tracking failure once discharge occurs.