Case History: Generator Rotor Shorted Turn Induced Vibration Excursion

The diagnosis of a generator rotor vibration in a 1,000 MW nuclear plant unit is presented. Several vibration excursions were recorded between July and August 2002. The excursions were somewhat repetitive in shape, but erratic and presented a clearly worsening trend, which threatened the continuous operation of the unit. In addition to high bearing vibration levels, the vibration force was transmitted to the Generator’s Current Transformers showing vibration levels in excess of 18 mils. The diagnosis of the rotor vibration excursions was simultaneously conducted by the Original Equipment Manufacturer (OEM) and a third party. An existing rubbing condition in the exciter and the generator hydrogen seals is presumed to have contributed to the elevated vibration levels reached during the excursion. A flat orbit in the generator side of the low pressure rotor LP-C was also recorded during the investigation. The existing rubbing condition caused discrepancy in the diagnostic between both parties. The OEM attributed the excursions to that rubbing condition while the third party attributed it to Generator Rotor Shorted Turns. Later inspection of the generator rotor revealed that the second diagnostic was correct. The characteristics of the vibration condition, the trouble shooting approach and the solution are described in this article.