Due to a major “motoring” incident involving a 60MW generator, a fitness for service study was initiated to investigate the effect that arcing had had on the life expectancy of the generator rotor.
The BS7910 Code was used to evaluate the fracture resistance and fatigue life on the rotor, specifically the wedge retaining lugs and body. Two loading scenarios were assumed, namely centrifugal loading on the retaining wedges as a result of the copper bars rotating at 3000 rpm (Low cycle loading); and the stress fluctuations in the rotor body was a result of self weight (high cycle loading).
The low cycle loading scenario cycles every start/stop or major load change of the generator, whilst the high cycle loading scenario cycles at the operating speed of the rotor (3000 rpm). In terms of the calculations relating to the induced arc pitting defects, the rotor life was limited by the number of start/stops. However it was not limited by the number of operating revolutions. Ultimately in close conjunction with the client technical team it was recommended that the unit be operated for a limited period (six months of critical service) before being removed from service and modified to remove defects.