Abstract
We have been studying the diagnostics (5)(7)(9)(11) and the degradation(3)(8)(10) of ZnO elements which were installed in 275kV surge arresters operated for 20 years. I have examined the leakage currents of ZnO elements as I have accomplished ageing tests. As the results of examinations, I found that migrated oxygen ions are evaporated by supplied voltages(2)(6), and again this oxygen gas evaporated is absorbed into ZnO elements in no voltage period(1)(4). The evaporation and the absorption are concerned with the Gibbs free energy and the diffusion energy of oxygen. When the Schottky barriers of ZnO elements are higher than 0.5RT; R is the gas constant and T is the absolute temperature, the evaporation and the absorption are stopped earlier. This reason is that the diffusion energy of oxygen becomes zero before the Gibbs free energy is reached to the balance.
