Use of spectral analysis and phase resolved partial discharge measurements for generator testing

A water cooled 291.5 MVA steam turbine generator was monitored online for partial discharge activity by means of electromagnetic interference measurements (EMI). The results presented an increase of activity by a factor of 10 in the frequency ranges 10 to 60 kHz and 30 to 50 MHz over 7 months of operation. After this period the generator was taken out of service for a planned stator rewind. Off-line measurements were then carried out on the three individual phases, both from the neutral and line end of the phase windings. For comparison with EMI measurements a combination of spectral analysis and phase resolved partial discharge analysis (PRPDA) was used for the off-line tests in the frequency range of 100 kHz to 100 MHz. In the low frequency range, activity could be observed on all phases whereas high frequency discharges in the range of 50 MHz to 100 MHz were only present on phase 1. Moreover, it was determined by the off-line spectral analysis that the discharge source was in the end arm region on the line side of phase 1. The PRPDA results added one dimension to this information, revealing that the high frequency discharge pattern was asymmetrical with regard to the applied 50 Hz with rated line-to-ground voltage applied. At the end of these tests each phase was subjected to overpotential testing