Additional salt deposit density of polluted insulators in salt fog

The surface pollution layer of polluted insulator absorbs fog water and gets damp in salt fog. Meanwhile, the salts in the salt fog deposit on insulator surface, which increases the surface conductivity and degrades the flashover performance of insulators. At present, the effects of fog water conductivity (γ 20) and equivalent salt deposit density (ESDD) are analysed separately, which does not reveal the essence of the flashover in salt fog. In this study, a plenty of experiments on porcelain, glass and composite insulators are carried out in salt fog conditions, and the 50% flashover voltage stress, leakage current and variation of ESDD during the wetting process are studied. The concept of additional salt deposit density (ASDD) is proposed to quantitatively analyse the additional ESDD caused by salt fog. The test results indicate that the ac flashover voltage stress decreases with the increase of both ESDD and fog-water conductivity. Both the ESDD and leakage current become larger in salt fog compared with those in clean fog. The ASDD can be applied to analyse the combined effect of ESDD and fog-water conductivity and be expressed as ASDD = k × γ 20 × ESDD. Then the flashover voltage stress is E = A × (ESDD + ASDD)−a . The values of k for the porcelain, glass and composite insulators are 0.179, 0.191 and 0.230, respectively. The flashover in salt fog environment is a special kind of pollution flashover in essence.