COMPARISON OF TURBINE-GENERATOR SHAFT TORSIONAL RESPONSE PREDICTED BYFREQUENCY-DOMAIN AND TIME-DOMAIN METHODS FOLLOWING WORST-CASE SUPPLY-SYSTEM EVENTS

The paper examines precision of predicting time response for torque in turbine-generator -exciter shafts by frequency-domain analysis follow ing incidence and clearance of short-circuits etc on the electrical su pply system. The analysis is based on Fourier analysis of generator ai r gap torque following incidence and clearance of a supply network dis turbance or following worst-case mal-synchronisation to obtain torque excitation which acts on the generator rotor corresponding to each mod al vibration. Amplitude and phase of each vibration is thereby determi ned. Using appropriate damping, time responses for shaft torque at eac h shaft cell is constructed by summing components which correspond to each frequency of modal vibration of the shaft. These time responses a re compared with those obtained by solution of more than 50 differenti al equations which simulate the shaft train, turbine, generator, excit er, electrical supply system, the fault clearing process, the turbine governor, and the generator excitation system. It is shown that time r esponses for transient turbine-generator-exciter shaft torques can be predicted faithfully by frequency domain analysis taking due account o f magnitude and phase of each modal vibration, and damping, following (i) worst-case Line-Line-Line, Line-Line; and Line-Ground disturbances from full-load to no-load, with clearance, and (ii) mal-synchronisata ion. Rotor swing has a significant influence on transient turbine shaf t torque at shaft locations in proximity to the generator, but the eff ect decreases almost in direct proportion to turbine inertia which is accelerated by rotor swing. Simulation of damping of rotor swing, toge ther turbine governor action, is important in making precise assessmen ts of transient turbine shaft torque at shaft locations which are clos e to the generator. Predominance of a particular modal vibration at di fferent shaft locations is dependant on the fault clearing time, and v aries cyclically as the fault clearing time is raised. 660MW and 1000M W two-pole Machines are analysed.