Condition assessment of 12- and 24-kV XLPE cables installed during the 80s. Results from a joint Norwegian/Swedish research project

This paper reports the results from the condition assessment of 12- and 24-kV cross-linked polyethylene (XPLE) cables using a technique based on dielectric spectroscopy initially developed at KTH in Sweden. The work aims to examine whether the method could detect water tree degradation for the second generation medium voltage (MV) cables with long, but not bridging, water trees. While the overall cable condition was better than expected for second generation XPLE cables, water trees were found in most of the selected cables. The diagnostic method based on the measurement of the dielectric response could only detect water tree degradation in the examined second generation cables when the water trees bridged the insulation wall. Condition assessment above service stress may, in some cases, be required to detect bridging water trees. The results indicate that there is a correlation between the voltage level and the breakdown voltage of the cable. This can be used as a diagnostic criterion for this group of cables.