Characteristics of Indicators for Insulation Deterioration in 26 kV Generator Stator Windings

This study conducted and analyzed the results of non-destructive tests (AC current, dissipation factor, and partial discharge tests) and a destructive test (overvoltage test) on 26 kV-generator stator windings with a ground fault. The target generator was a steam turbine generator in operation for over 30 years, and ground fault in windings occurred because of the rapid and instantaneous temperature rise in the copper conductor owing to the partial loss of generator cooling water. By comparing the non-destructive test data measured during the planned preventive maintenance period two years before the ground fault and the data gathered just after the ground fault, the study conducted an in-depth analysis of the effect of moisture on insulation diagnosis factors. If both the dissipation factor and capacitance data increased when compared to those of previously estimated values at same applied AC voltage level, it represented that the insulation materials absorbed moisture. Moreover, it was further developed that both of the dissipation factor and capacitance surge voltage were detected when the discharge started whereas the concerned surge voltage for AC current was detected when the discharge was proceeded in some extent. It is expected that a wider understanding of insulation diagnosis factors developed from this study will contribute not only to a more reliable diagnosis data analysis but also a stable power supply by preventing accidents in advance.