The electrostatic charging tendency of in-service oils and the investigation of application method to the flow electrification diagnosis of the transformer

The electrostatic charging tendency (ECT) was measured for oils from 75 in-service power transformers (voltage classes, 275 kV and 500 kV) and the influences of number of operation years, operation conditions, addition of BTA (1,2,3-benzotriazol) and repair work history in the field were studied. It was found that the ECT increased with operation years. For BTA-added oils the ECT also showed an increasing tendency with operation years, except at the beginning of transformer operation for which ECT was low. The ECT change with operation year had no significant relationship with oil types, transformer types and operation conditions. It was also found that the ECT had good correlation with power factor and volume resistivity of oil, but Cu content in oil had no positive correlation. As a method for flow electrification diagnosis of transformers, estimation of the leakage current from transformer windings by using the ECT of oil was investigated. It was found that the correlation between the leakage current and the value of (ECT)×x(flow rate of the windings) was good. This method was judged to be useful as a reliable estimation method for the leakage current