12 and 24 kV XLPE cables in Norway. Faults related to water treeing

In the end of 1990 approximately 13000 km three core XLPE cables were installed in the Norwegian 12 and 24 kV cable network. The fault rate of these cables has always been very low compared to other published results. Only 83 cables are reported to have failed in service. This corresponds to a fault rate for these cables as low as 0.1 faults/100 km three core cable in the year. 85% of the water treeing failures have occurred on the old construction with insulation screen made of semiconducting paint and tapes. However, five cables with an early type of strippable insulation screen have also failed due to long vented water trees from this screen after 10 years of service. Only limited lengths of this cable are installed in the Norwegian network. In spite of this favourable fault statistics the Norwegian cable construction has been changed several times. One of the purposes for these changes has been to further improve the cable with respect to water ingress and subsequently the risk of water tree initiation and growth. >