Different portions of stator insulation are subjected to multistresses with different strength during the operation of large generators. In order to study the aging extent of stator insulation in different portions, diagnostic tests are performed on the specimens from stator bars of a large turbogenerator, which has operated for 16 years. The tests include, polarization index test, dielectric loss measurement at various voltages and temperatures, ac current measurement, partial discharge test and acoustic detection. The test results are discussed and it is shown that the aging extent of stator insulation of endwindings is more serious than that of the insulation in the slots. Results also indicate that endwindings were subjected to stronger stresses than the slot portions of coils were. Finally, the suggestion could be made that during predictive maintenance testing of large turbogenerators, more attention should be drawn to the insulation of endwindings.
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