Study of dielectric properties of composite insulators for use in medium voltage overhead lines

•The flashover voltage decrease after thermal and ultraviolet ageing.•The leakage current increases after ageing.•The dielectrics loss becomes more important after ageing.•The correlation between the flashover voltage (macroscopic parameter) and the dielectric losses (microscopic parameter).•The influence of rate charge of trihydrate alumina ATH on the dielectric properties.

The dielectric properties of composite insulator materials based on silicone are studied as a function of ageing factors (temperature, UV). Our study includes composite insulators used in the Tunisian Electricity and Gas Company STEG networks (medium voltage overhead lines). Macroscopic characterization using conventional normalized test was performed at the Center of Measurement Tests of El Omrane (Tunis). In the other hand, samples taken from treated insulator are investigated by dielectric spectroscopy technique. The measurement was carried out according to the same ageing factors. So the correlation between the results of standardized tests and those obtained by laboratory characterization is possible. The results of our measurements show the existence of significant dielectric losses in the case of samples that have been aged under temperature and UV radiation. We attribute these losses to the rate of added filler during material elaboration. The dielectric losses increase respectively with time and ageing factors. This increase is remarkable at the vicinity of industrial frequencies.