Measuring the Torsional Modal Frequencies of a 900 MW Turbogenerator

Transient torques exerted on the rotors of turbine generators can produce high stresses. This paper describes an investigation into the torsional properties of the shaft of a 1500 rpm steam turbine generator rated 900 MW and includes 1. prediction, by theoretical model of the shaft and rotor structure, of its undamped natural vibrations and natural modes; 2. determination, by measurements and subsequent modal analysis, of the complex natural vibrations (free and damped) and natural modes. For the measurements the rotor was torqued by shock torques applied by the turning gear, as well as by harmonic sinusoidal torques of variable frequency applied by a shaker. Vibrations were measured by accelerometers on the shaft and on some selected turbine blades. Two modal analysis computer programs were used to process the signals. The results obtained by theoretical predictions and by measured modal analysis verify that the theoretical characterization of the rotor structure is indeed valid for calculating the values of the natural frequencies. However, the damping factors measured by the two modal programs differ significantly.