Field failure analysis of medium voltage XLPE-insulated power cables

The following questions are addressed through statistical analysis of field failure data collected over the last 15 ye1rs (a975-90): (a) What is the life expectancy of cross-linked polyethylene (XLPE) cables? (b) What is the cause of the failure? (c) What is the remaining life of the installed cable circuits? (d) What criteria should one use to judge whether cable replacement is required? Weibull statistics were used to establish the lifetime expected of cables of a particular vintage, design, and voltage class. Cable history, postfailure electrical testing, and microscopic evaluation of the insulation were the parameters studied. The 63% probability lifetimes of 5-, 15-, and 25/28-kV cables of the 1970 vintage are, respectively, 5, 12, and 13 years. The corresponding tree growth rates in the 5-, 15- and 25/28-kV cables are, respectively, 42, 44, and 24% of the insulation wall. The tree growth rate in the jacketed 15-kV cable is 11% of the insulation wall versus 50% for the unjacketed cable. Distribution cables containing water trees with approximately 20% penetration through the insulation wall are likely to show a high incidence of failure