For establishing a method to diagnose the deterioration of ZnO surge arresters with a series gap of power distribution Systems, changes in leakage current of the arrester against the number of operation times were measured when the arrester was operated repeatedly at a large current of 10kA, 4/10μs. To clarify the cause for the changes, the surface of the spacer was observed using an electron probe micro analyzer and a x-ray photoelectron spectroscope after the arrester was operated repeatedly by a large current. Then, the optical emission of the impulse discharge plasma was investigated using a spectroscope with an optical multichannel analyzer. As the results show, it was found that the change in the leakage current of the arrester against the number of operation times was dominantly affected by change in contamination of the surface of the gap spacer which was caused by the metallic vapor emitted from the gap electrodes and transported by shock wave. This means that the deterioration of arresters can be diagnosed by measuring the leakage current and taking into consideration the tendencies in changes of leakage-current and sparkover-voltage changes of the arresters with respect to the number of operation times.
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