The Applicability of Very Low Frequency Diagnostic Testing in the Evaluation of Thermally Aged Stator Coil

Abstract

In this study, stator coils were aged under thermal stress and changes in insulation condition with the aging cycles were evaluated using diagnostic tests. Diagnostic tests such as partial discharge measurement and tan delta and capacitance measurements were performed at 50 Hz, 1 Hz and 0.1 Hz (very low frequency). The results obtained from this study demonstrate the possibility of evaluating the signs of thermal aging in a stator insulation system with very low frequency tests. Partial discharge parameters like peak apparent charge magnitude and integrated charge per cycle along with tan delta and capacitance tip-up values were used to evaluate the stator insulation condition. Diagnostic test results showed an identical trend with aging cycles at all the chosen test frequency, i.e. 50 Hz, 1 Hz and 0.1 Hz, which indicates that testing at 0.1 Hz is suitable for insulation condition evaluation. Further, the tip-up values have shown distinct variations, especially at 0.1 Hz indicating significant thermal degradation in a stator coil.