Accelerated laboratory ageing of model insulator samples with semiconducting glazes

Semiconducting surfaces on outdoor insulators can improve performance under contaminated conditions. Numerous attempts have been undertaken to find suitable glaze compositions and today a range of reliable products is available. On the other hand, there are still concerns on the long-term stability of the semiconducting glazes and investigations aiming to find out the main ageing mechanisms, especially under DC voltage, are of great importance. In recent investigations, the authors found on insulator samples under natural field conditions, that similar surface changes took place under AC and DC voltage after several months of exposure. These changes were more severe for samples under DC stress. The aim of the investigations reported in this paper, was to model the ageing of samples with semiconducting glaze in the laboratory using the rotating-wheel-dip test. Besides the measurement of the voltage-current characteristics (I-V), dielectric spectroscopy was applied, in order to characterise the changes observed and to extend the knowledge concerning the conduction mechanism in the glaze before and after ageing