Fatigue damage assessment method of turbine shafts' torsional vibrations under SSO incidents

Sub-Synchronous Oscillation (SSO) can pose severe threat to the steam turbine shafting system, which in particular is the torsional vibrations leading to the hazard or shut down of the power units. A method is developed to evaluate fatigue damage under SSO incidents, which utilizes field data and modal decomposition and superposition concepts, and combines the structural full-size FEA (finite element analysis) modal analysis with refined finite element structural analysis of specific parts. Applying such method to a 330 MW steam turbine shafting system, our study illustrates its torsional vibration characteristics under different modes of SSO incidents. The results indicate that the proposed method is faster than conventional full-size transient analysis, decreasing the computational cost. We find that coupling shrinkage structure is the most vulnerable part, which is consistent with engineering practice and thus perfects our traditional understanding that connecting parts in journals are the only vulnerable parts. At the end of this paper, we also point out that with the presence of SSO incident, decreasing the load on shafts in time would help alleviate the damage.