Water migration in degraded XLPE cables

During dielectric response measurements on samples of XLPE 22 kV cable that were known to have been severely-degraded in service by water trees, it was observed that the insulation of the cable appeared to degrade considerably after water-immersed cable sample were connected to the rated voltage AC supply. This paper describes results of systematic experiments made to investigate this factor further using polarisation and depolarisation current (PDC) equipment to measure the condition of cable samples. It is found that the cable condition appears to improve from poor to good condition within a few days after short circuiting of the previously-energised cable commenced. Conversely it is found that the condition of the cable appears to degenerate over time following re-application of the rated voltage to the cable samples. It is hypothesised that electrostatic forces maintain moisture in the trees during energisation and that the moisture retreats from the trees when the supply voltage is removed. The implication of this phenomenon on dielectric response measurements is discussed in the paper.