Dielectric response characterization of in-service aged sheds of (U) HVDC silicone rubber composite insulators

Investigations of bulk dielectric properties of specimens extracted from in-service aged and aged UHVDC composite insulator sheds are reported in this paper. The program of the study included measurements of DC conductivity and complex permittivity at frequency range 10
^-4
-10
³
Hz at various temperatures in dry and wet (water immersion) conditions. The obtained results are analyzed by considering a combination of relations describing various dielectric relaxation modes in dielectrics, included three Cole-Cole types of responses and a hopping process. The hopping has the strongest impact on the responses in the whole frequency range. Two of the Cole-Cole responses appearing in the low frequency range are attributed to Maxwell-Wagner-Sillars relaxations at macroscopic and mesoscopic levels; first one representing interfacial polarization at the border between specimen and air gap in the electrode arrangement, while the second one is attributed to the charge relaxation at filler boundaries and internal cavities in the polymeric materials. The third Cole-Cole process, active at higher frequencies, is considered as a sign of polymer degradation, as it shifts with the ageing to higher frequencies. Based on the results of this study, it is claimed that dielectric response measurements on specimens of in-service aged composite insulator sheds can provide useful information on the degree of material degradation. In addition, a water immersion test in combination with such measurements may be a good way to enhancing the observed changes.