Insulation Diagnosis Characteristics According to the Stator Winding Maintenance Procedure of a 20 kV Class Steam Turbine Generator

Although the AC current and dissipation factor in phases A, B, and C were good, the magnitude of partial discharge (PD) was measured to be 191,525 pC, 11,430 pC, and 10,809 pC for phases A, B, and C, respectively, at the phase voltage of 11.5 kV, in the insulation diagnostic test of a steam turbine generator (374 MVA, 20 kV) stator winding. Root cause analysis and maintenance work were performed over the 2nd–4th tests since the magnitude of the PD of the generator was determined to be defective in phase A. It was found that the largest discharge pulse was generated in Turbine End (TE) 54 on the endwinding side of the generator in phase A. The groundwall insulation of TE 54 on the endwinding side of the generator was removed in the 2nd test and the PD magnitude that was measured after separating the temperature signal line from the copper bar was 177,649 pC, which was not significantly different from the 191,525 pC in the 1st test. Therefore, the magnitude of the PD decreased significantly to 6057 pC after connecting the temperature signal line to the copper bar. The PD magnitude was 6121 pC as a result of recovering the removed main insulation material in the 3rd test, and the PD magnitude was good at 8123 pC as a result of recovering the groundwall insulation and finally finishing the external paintwork in the 4th test.