Partial discharge diagnostics of defective medium voltage three-phase PILC cables

There are a number of medium voltage (MV) power distribution cable networks worldwide that are constructed predominantly of mass impregnated paper cables - London being one of these. Paper insulated lead covered (PILC) cables were extensively laid in the 50s and 60s before the introduction of cheaper polymeric alternatives that were sufficiently reliable. The current operational state of these networks has shown a gradual increase in failure rates of the previously reliable paper cables that are drawing to the end of their expected design life. Utilities are faced with the prospect of the impending failure of large sections of their prized asset and are keen to develop tools to better understand the health of their hardware. The analysis of partial discharge (PD) signals produced by the cables has been identified as a economically viable option to provide continuous condition monitoring of PILC cable circuits. Clearly, a comprehensive understanding of how PD activity relates to the various failure mechanisms exhibited by cable circuits in the field is required. In order to generate representative and repeatable PD signals from a number of PILC cable samples under rated conditions, an experiment was designed and commissioned. The scope of this paper is to introduce two cable degradation mechanisms that were applied to test cable and show the associated PD activity that was produced. A recently published technique for PD source discrimination was coupled with an understanding of the system and applied to the experiment data to isolate the signals specific to each degradation mechanism.