The effect of water ingress on dielectric properties of polyethylene cable insulation

This paper reports on dielectric loss diagnostic features (absolute values, voltage dependence, etc.) measurements and results of residual breakdown voltages obtained on miniature (RG-58) cables as a function of water absorption and electrical aging by water trees (WT) under a 5 kV/mm voltage stress (AC). The dielectric losses were measured at voltages up to 5 kV (i) in polarization and depolarization using Time Domain Spectroscopy (TDS) and (ii) with a classical very low frequency (VLF) system using a 0.1 Hz sine wave. The corresponding residual AC breakdown voltages were analyzed using the standard procedure. After ~3000 h of water and voltage exposure, aged cable samples showed a relatively low level of degradation, mainly bow tie water trees, with longest being ~19% of insulation thickness. TDS and VLF dielectric loss results show that for such an aging condition, the dielectric response is mainly dominated by the presence of water in the insulation.