XLPE insulated MV cables: 20 years of experience in a Swiss network

The authors present an evaluation of over 20 years of MV cable operation in a distribution network. Failures were recorded on the XLPE insulated cables installed since 1973; all of them occurred on cables manufactured before 1976. These are cables with a semi-conducting taped screen. Statistics on these failures are presented. Examinations (for faults, water trees) and electrical tests (impulse and 50 Hz) were performed in the laboratory on many cables which had failed in service. The results are compared to those obtained on new 1975 cables. Water has been detected in most of the failed cables. A campaign of sheath watertightness testing was carried out on about 100 cable lines. This revealed various sheath defects and damages. Finally, measurements of transient phenomena were recorded on the most severely affected part of the network. Results emerging from a first analysis are discussed. The conclusion summarizes the life expectancy of XLPE insulated MV cable as well as the criteria used when cables have to be changed