Towards a smart replacement strategy of power cables based on the depolarization principle

In spite of having excellent short-term dielectric properties, polymeric insulated cables are subjected to serious degradation problems due to defects, water treeing, partial discharges, etc... Canada more than 20% of the installed power cables are older than their designed life-time. Therefore, utilities are challenged to make the right decisions in repairing, refurbishing or completely replacing older cables. The best option for utilities is to apply a so called “smart replacement strategy of power cables”, i.e. only replacing cables that could adversely affect the reliability of a network in the near future. This will be the best approach to cut operation costs as well as increase the reliability of a power system. The implementation of such a strategy can only be accomplished if and when the utilities have access to reliable and non-destructive diagnostic tools that can be applied on-site. At the National Research Council of Canada (NRC), an on-site testing technique for estimating the degree of degradation of power cables based on the polarization/depolarization current principle has been developed using a nearly noiseless high voltage solid state switch with a few ns rise time. The high frequency (HF) components of the depolarization current (I_DEP) can be measured and the area under the HF component of the I_DEP curve, Q_Dep, can be linked to the intensity of water treeing in cable insulations. This was observed from a series of tests on flat samples and miniature cables aged in a wet environment, and from measuring specimens removed from failed cables. Using Q_Dep as Diagnostic Indicator, the tested cables were divided into three categories: Good Condition, Fair Condition and Poor Condition. In 2011 this technique was applied to a large number of cables in the local utility network. The results of these tests will be described in this paper. It will be shown that the age was not the determining factor - f the cable condition. Also, it will be clarified that only -1 kV dc is sufficient to assess the condition of cables, therefore, testing with -3 kV will not change the condition determined with 1 k kV.