Electrical ageing of polymers in a humid environment

The present work reports the results of a study of the ageing behaviour of cross-linked polyethylene (XLPE) cables under AC stress in a humid environment by (i) dielectric spectroscopy, (ii) thermally stimulated discharge current (TSDC) and thermal transient current (TTC). XLPE cables (15 kV rating) were aged in water at room temperature at a field of 6 kV/mm at 50 Hz for a maximum time period of 6000 hours. The dielectric properties of the unaged and aged cables were determined in the frequency range of 10^-5 Hz to 10⁵ Hz. Subsequently TSDC and TTC measurements were made for these cables. For the dielectric studies, the cluster model of Dissado and Hill (1983) was employed to analyse the experimental data. This model explains two independent processes, i.e. dipolar responses and the quasi-DC (QDC) conduction process in terms of intra-cluster and inter-cluster exchange mechanisms. The dielectric data of the aged cables show that as the ageing progresses a QDC process develops at the expense of a dipolar response. Further evidence of the change of the nature of the dielectric response from dipolar to a space charge origin was provided by the TTC and TSDC measurements