Analysis of Continuum and Reduced Shaft Models in Evaluating Turbine-Generator Shaft Torsional Response following Severe Disturbances on the System Supply

The paper examines use of continuum and reduced models of turbine-generator shafts in evaluating torque response following incidence and clearance of severe disturbances on the system supply. Using airgap torque which results from incidence and clearance of system disturbances at fault current zeros, torque excitation which acts along the active length of the generator rotor for each torsional vibration is determined by Fourier analysis. Amplitude of torsional vibration at each shaft cell for each modal vibration is determined from the torque prime mover and generator shaft sections of interest for each modal vibration is then calculated. These components, together with the steady-state component (and that due to rotor swing), are summed as absolute values to give approximate peak torque at critical shaft sections. Presented results illustrate torque excitation and shaft torque estimated from continuum, simple and stiffness-adjusted shaft models as a function of fault clearing time following L-L-L and L-L high voltage disturbances with clearance. Effect of employing continuum and reduced shaft models in estimating time constants for decay of predominant shaft torsional vibrations and on peak energy of the vibrations is illustrated. The effect saturation of the stator leakage reactance has on torque response following sustained stator-terminal L-L-L and L-L short-circuits at full-load and no-load and following worst-case synchronisation is also illustrated. 66O MW and 1000 MW two-pole machines are analysed.