Criteria for diagnostic evaluation of water treed XLPE cables

To detect cables with water trees bridging the insulation, it is necessary to perform measurements during application of test voltage. This is automatically obtained by the AC frequency domain methods. When using the DC time domain methods the polarisation current should be measured at a sufficient high DC test voltage. The good correlation between the degree of non-linearity and the cable condition, indicate that the test voltage can be limited to U0. To base the diagnostic evaluation on the S values facilitates comparison of results from different diagnostic methods. In addition such transformation makes it easier to compare results from time domain measurements performed at several voltages, different types and lengths of cables.