Abstract
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.
Highlights
Generator stator windings that are operational over a long time suffer from progressive deterioration and undergo insulation failure owing to continuous discharge between phase and phase caused by dirt on the surface of endwinding and that in voids inside insulation materials caused from the windings’ thermal, electric, mechanical, and environmental effects [1, 2]
An insulation diagnosis test includes insulation resistance, polarization index, AC current, dissipation factor, and partial discharge, and the measured data are comprehensively analyzed to determine the level of insulation deterioration
Considering that the partial discharge pattern two years ago was an internal discharge pattern, it is believed that as dramatic rise in Joule’s heat attributed to the loss of cooling water, heat expansion occurred in the copper conductor, and the insulation material showed delamination as its heat expansion was lower than the copper conductor
Summary
Generator stator windings that are operational over a long time suffer from progressive deterioration and undergo insulation failure owing to continuous discharge between phase and phase caused by dirt on the surface of endwinding and that in voids inside insulation materials caused from the windings’ thermal, electric, mechanical, and environmental effects [1, 2]. This study targets a steam turbine generator (800 MVA, 26 kV) that is operational for over 30 years to perform the offline insulation diagnosis test and evaluate dielectric strength status of stator windings, and toward this end, a overvoltage test was conducted and its results were analyzed while permitting the voltage up to 1.25 times of the normal voltage at 26 kV, i.e., up to 32.5 kV. While part of the defect types and discharge pattern analysis have been examined in the reference [6], it mainly focused on the conductor delamination discharge attributed to the rise of the windings temperature and its pattern analysis In another previous study, insulation diagnosis was performed to evaluate the insulation quality of a 500 MW, 22 kV generator stator bars and a 350 MW, 24 kV generator stator windings. In the case of the overvoltage test, the HV supply (50kV) and the bridge (Type 2819) were used to increase the voltage by 1 kV and permit 32.5 kV, 125% of the line-to-line voltage, for 1 min
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