Humidity and Temperature Dependences of Resistivities of Various Formulations of Silicone Rubber For HVDC Applications

Silicone rubber (SiR) is extensively used as housing material for outdoor HV insulators thanks to its excellent long-term stability combined with high hydrophobicity. For HVDC applications, the resistivity of SiR is a key design parameter and knowledge about its changes under various conditions is crucial. In the present work, the influences of relative humidity in air and temperature on the resistivities of five different HTV SiR formulations are studied. The resistivity is calculated from the results of the leakage current measurements conducted on 1.3 mm thick SiR samples under DC voltage. It is found that both the relative humidity and temperature have a significant impact on the resistivity of the studied materials the magnitudes of which varied over several orders. Furthermore, it is observed that SiR formulations with similar composition may have significantly different resistivities under the same temperature and humidity. Accordingly, it is concluded that the resistive properties of each SiR formulation used for HVDC applications need to be carefully evaluated to secure a reliable design and operation.