To increase output and efficiency of steam turbine, long or ultra-long blades are used for last stage blades of low pressure rotors. The application of long blades enhance the coupled effect between the shaft torsional vibration and nodal diameter zero umbrella vibration mode of shrouded blade. In order to precisely calculate the shaft-blade coupled vibration characteristics for large steam turbine generator sets, a reduced method consisted of the three dimensional finite element method and the component mode synthesis method is studied. The study shows that the precision of the reduction method can be guaranteed if the maximum frequency of high order mode used in the coordinate transformation matrix of substructure is higher than 5 times of the maximum frequency of the whole system that one hopes to calculate. The last-stage, the next to last stage blades, and the whole shaft of a 1000MW steam turbine generator set are described by the three dimensional finite element method. The degrees of freedom of the whole system are reduced by using the component mode synthesis method. The coupled vibration of the steam turbine generator set is computed. The vibration of the shaft neglecting the coupled effect is also calculated, and only the moment of inertia of disk-blades is considered. The results by two models are compared with each other. The results show that there exist 4 extra torsional coupled vibration modes when the structural flexibility influence of last-stage and the next to last stage blades is considered.

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