Decrease of voltage-current characteristics of varistor has been discussed as a serious problem in surge arresters. Prolonged exposure of varistor to electrical field increases the leakage current and power loss of varistor which consequently leads to the degradation of varistor. Increase of leakage current and aging of varistor is related to the structural defects and distribution of intergranular region. Varistor regeneration phenomenon leads to the return of varistor character to the initial mode and consequently the aged varistor is regenerated. Immersion of degraded varistor in nano bismuth nitrate solution under a special heat-treatment leads to the regeneration of varistor. In this method, the sintered varistor is immersed in a solution of nano bismuth nitrate under a specific thermal curve for 12 hours. Results show this process improves the nonlinearity of varistor. Also all samples indicate the stability of nonlinear coefficients and decrease of leakage current. Optimum concentration of this solution is in the range of 2.5-3 mol%. In this paper, microstructure and electrical properties of immersed varistor are investigated. This process shows that nano bismuth oxide penetrates from the surface into the varistor to overcome bismuth loss. Consequently after the special heat-treatment, bismuth phase is formed and the intergranular layer is reconstructed and the grain boundaries are filled with γ-Bi2O3.
Fahimeh Sadat abedsaeedi- Ehsan Kousha
R&D unit
Barghgir Toos Company
Mashhad
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