The variation of the electrical properties of 225 MW stator bars of a hydrogenerator which has run for about 15 years and the remaining service life of the bars with different deterioration in electrical properties have been investigated. Methods of diagnosing deterioration which are convenient to use on site are evaluated. The characteristic curves of insulation in AC or DC conditions which have been obtained experimentally may be divided into three regions: the working region, the testing region, and the losing-effectiveness region. The deterioration degrees of the insulation could be judged by using these regions.
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