Assessment of long-term life expenditure for steam turbine shafts due to noncharacteristic subharmonic currents in asynchronous links

In this paper, the long-term effect of noncharacteristic subharmonic currents in a HVDC link on the fatigue life loss in turbine-generator shafts is analyzed. As soon as the asynchronous operation in HVDC links takes place, the disturbing rotor frequency distributions on both ac sides of the main harmonics are exactly subsynchronous. To completely investigate the long-term effects of inevitable asynchronous operation, a systematic scheme is motivated to examine the concerned fatigue life expenditure level. The simulation results show the influences under various operating conditions and prove the potential long-term failure of shafts due to such electrical disturbances. It is also justified that even though the shafts could withstand the most severe impact subject to a three-phase-to-ground fault, it still cannot guarantee long-term safety operations even under normal operating conditions.