Study of dielectric response characteristics for thermal aging of XLPE cable insulation

In urban power grids, the proportion of cross-linked polyethylene (XLPE) power cable continues to increase. Judging the insulation condition of XLPE cable precisely is of great significance. In this paper, a number of cable samples are thermally accelerated aged at different temperatures and during various periods. Polarization/depolarization current (PDC) and frequency-domain dielectric spectroscopy (FDS) tests are performed on these samples. Based on the results of PDC and FDS, combining with Debye model and dielectric theory, we analyzed their dielectric response characteristics and acquired some characteristic parameters representing the aging degree of XLPE insulation. From the aspect of PDC measurement, at a certain temperature, with the aging time increases, the attenuation time of depolarization current rises correspondingly and becomes slower to reach the steady state. What's more, an aging factors A could be calculated which rises with the aging time. As for the FDS test, with the aging time increases at one temperature, the curve of dissipation factor tanδ moves upward; curves of complex permittivity real component have a tendency of moving right. The results are alike when observing samples at different aging temperatures. With the increase of aging time and aging temperature, the pyrolysis reaction and oxidizing reaction will become more drastic. As a result, there will be more boundary between sphaero-crystal and amorphous region. Some polar oxides could also be created. These changes can produce more interfacial polarization and space charges which could make the dielectric relaxation phenomenon more obvious.