Design of insulation test with one-minute step-up method for substation equipment

Reliability evaluation method with Weibull distribution function is used for development of test conditions for short-duration power frequency withstand voltage test. Although the evaluations by ldquoindependent methodrdquo and ldquocumulative methodrdquo are proposed, the handling of multiple voltage application still remains as one of the subjects. Therefore, insulation test with one-minute step-up method is studied as an insulation test with coexistence of multiple voltage level. As a result, the insulation characteristics by one-minute step-up method deviated from their true values. In this paper, it was calculated and evaluated by two methods with parameters of test condition. For the one-minute step-up method, the deviations from the true values were found to be influenced by test conditions: the larger the test voltage step width, or the lower the test starting voltage, or the greater the number of voltage steps, the smaller the deviations from the true values. Furthermore, the approaches to designing appropriate conditions for the test were summarized in order to design test conditions for oil-filled transformer and gas insulated switchgear. As a result, the reasonable test conditions were evaluated as follows: for oil-filled transformers, the test starting voltage was 70% of the one-minute breakdown voltage value and the voltage step was 7% and, for gas insulated switchgear, the test starting voltage was 80% and the voltage step was 5%. This result is approximately the same as the test conditions used for the actual measurement data in the past. These examinations will help to establish appropriate test conditions in the one-minute step-up method and to evaluate insulation characteristics obtained from the test.