Experimental investigation on dielectric/electric properties of aged polymeric insulator: correlation of permittivity and dielectric loss tangent with surface hydrophobicity

Silicone rubber (SiR) insulators are widely used in transmission and distribution electrical power systems. Dielectrics properties give an important efficiency to SiR insulators that can be lost after ageing. The aim of this study is to investigate the effect of temperature, immersion and ultraviolet (UV) radiation on the ageing phenomenon of SiR insulator samples. The evolution of bulk condition of the samples after the ageing cycle was diagnosed by broadband dielectric spectroscopy measurement, a correlation between dielectric properties and the surface hydrophobicity of the samples has been discussed. DC surface resistivity was measured after ageing. A comparison was made between the characteristics of the samples in the new state and the aged one. Experimental results show that the low frequency dispersion dominates the dielectric response of the studied material. The thermal ageing causes a decrease of the dielectric loss tangent (DLT) in the low frequencies which improves the FOV of the aged samples. UV radiation can cause a change in the physicochemical structure of the samples, not only at the surface but also at the bulk level. The immersion ageing results in an increase in the DLT, especially for low conductivity solutions. The water can diffuse inside the samples during the immersion. In fact, a limited amount of water can still reside inside the samples even after a long period of drying. There is a good agreement between the surface hydrophobicity and the dielectric properties of the samples, the link with these properties is dependent to the ageing stress.