Torsional vibration analysis and fatigue life estimation of turbo-generator coupled shaft under electrical disturbances (MS)

Abstract

In a power plant, electrical disturbances induce electromagnetic torque in the generator. This electromagnetic torque generates torsional vibration in whole turbo-generator shaft system. Sometimes these vibrations cross the fatigue limit causing partial or complete failure of a turbo-generator shaft that finally leads to an unexpected shutdown of the power plant. In this thesis, investigation of shaft damage due to torsional vibrations initiated by three phase, two phase, single phase short circuit and synchronization fault on the power system has been carried out for the unloaded and loaded generator. A mathematical model of a generator has been developed to predict the electromagnetic torque under various electrical disturbances. Developed model has been simulated using Adams-Predictor corrector scheme, the startup being tackled by the fourth order Runge-Kutta method. Predicted electromagnetic torque has been applied to the turbo-generator shaft. Turbo-generator shaft has been modelled using the finite element method. Based on the shape of individual section two nodded solid cylinder, hollow cylinder or tapered, finite elements have been considered. Torsional vibration response under electromagnetic torque has been predicted using Duhamel integral. The Min-max algorithm has been used to convert the number of cycles of corresponding amplitude of stress. Finally, shaft damage has been calculated using Palmgen-Minor rule. Numerical results predict that stator current, field current, angular velocity and electromagnetic torque dominate under three phase short circuit fault over two phase short circuit and single phase short circuit fault. Torsional vibrations induced during three phase short circuit are higher than two phase short circuit and single phase short circuit. Further,maximum value torsional moment not only depends on the location at shaft but also on the duration of the short circuit. Fatigue damage during short circuit is different at different locations of the rotor system and highest for three phase short circuit.