Experiment on multiple-times voltage application to evaluate insulation reliability of oil-immersed transformer

For a power-frequency withstand voltage test to be conducted on substation equipment, the conditions can be established using a technique based on insulation reliability evaluation using a Weibull distribution. This can be done by either the "independence model" or "accumulation model" which have been proposed in order to handle the coexistence of multiple voltage levels, but there remains a problem of how to use these models. This study attempted to solve this problem. In the author\´s preceding research, the V-t characteristics of oil gaps were obtained by focusing on the presence of oil flows in oilfilled transformers as a parameter that influences the voltage application history. This paper proposes a test method that clarifies the differences between the independence model and accumulation model. Furthermore, it shows the results of conducting the proposed test of applying the same voltage multiple times and evaluates the influences of voltage application history. These results present that when the oil flow velocity is sufficiently high, they mostly conform to the calculation with the independence model. When the oil flow velocity is slower, the test results are closer to the calculation with the accumulation model. It is also found that when a voltage is applied during intervals or the interval time is shortened, the test results are closer to the calculation with the accumulation model.