Deterioration of cross-linked polyethylene due to water treeing

Because of superior electrical and mechanical properties, as well as favorable economics, today's demand in Japan for all types of solid dielectric insulated cables, especially cross-linked polyethylene (XLP), has greatly expanded. These rubber and plastic insulated cables are usually so simple to install and terminate that possible troubles due to adverse operating conditions (such as exposure to water) are sometimes ignored. Water in some form exists in practically all installation areas, especially underground. Therefore, it is essential to recognize and understand the possible deteriorating effects of moisture on these cables. The mechanism of the deterioration of XLP due to water will be discussed. Previously¹ we have reported that the deterioration of XLP insulated cables due to water can be illustrated as shown in Figure 1. Under an ac voltage, deterioration occurs in three stages: first, rapid drop in ac breakdown voltage occurs while the drop in insulation resistance is negligible. Then further deterioration is quite slow. Finally the cable fails abruptly or has a sudden drop in insulation resistance.