In a traditional turbine-generator set, rotor shaft designers and blade designers have their own models and process of design by neglecting the coupled torsional effect. Recent longer blades system for higher output and efficiency, torsional coupled umbrella mode of blade system tends to be nearby the double frequency electric disturbance. (i.e. 100Hz or 120Hz) For estimating the rotor-blade coupled vibration of rotating shafts in high accuracy, the analysis must include the identification process of mathematical model's parameters by real model. In this paper we propose to use a quasimodal technique based on the similar concept of modal synthesis method, but it is a unique method to provide a visual reduced model. An equivalent mass-spring system is produced for uncoupled umbrella mode and modal parameters are measured in one turbine rotor real system. These parameters are used to estimate the rotor-blade coupled torsional frequencies of a 700MW turbinegenerator set and the accuracy of the estimation is verified through a field test.
