Influence of ageing on depolarization currents in polymer-insulated medium-voltage cables

Depolarization currents in a larger number of virgin and aged polymer-insulated medium-voltage cables have been measured as a function of the magnitude of the applied electric field. In comparison with new cables some service-aged cables revealed unusual dielectric properties. As expected, unaged cables show a lineal relation between the depolarization current and the polarizing field at moderate electric field strengths. In contrast, for many service-aged cables a more than proportional increase of the depolarization current with the applied electric field was observed (indicating some nonlinearity). Visual inspection of the cables showed a very high density of bow-tie trees in the 'non-linear' cables. Furthermore, it was observed that the nonlinearity vanished when the cable was dried and reappeared again after wetting the cable. As a conclusion from these observations, it is believed that the non-linear dielectric behaviour is a consequence of the presence of bow-tie trees in the cables. In our opinion it may be worthwhile considering the measurement of depolarization currents as a diagnostic measurement technique to detect water trees in buried cables.