Detection and localisation of water treeing for condition based replacement of medium voltage cables

Extruded polyethylene cables were first introduced on the Swedish market in the late 1960s. A specific degradation of the insulation, called “water treeing” was soon discovered. This type of ageing substantially reduces the electric breakdown strength of the insulation and is the cause for cable failures. In Sweden a significant amount of the cross-linked polyethylene (XLPE) cables from the 1970s are still in service and these cables represent a valuable asset to the utilities. Some cables are still “as good as new”, and to replace them would be an unnecessary and costly option. There is hence a need for a non-destructive diagnostic technique to assess the quality of the cables.