Evaluation of on-site dielectric response methods for nondestructive testing of water treed MV XLPE cables

This paper presents results from a joint Nordic project for condition assessment of water tree degraded 12 and 24 kV XLPE cables. The results show that that the condition of old cables with a high density of vented water trees, are correctly assessed by all the test methods. To detect bad cables with water trees bridging the insulation, it is necessary to also measure the polarisation currents when using the time domain methods. In case of cables with very few but long vented water trees, the evaluation criteria need to be further developed. During diagnostic testing of cables in service, the response from the installed accessories and the air humidity should be known in order to avoid wrong interpretation of the cable condition