Analysis of torques in large steam turbine driven induction generator shafts following disturbances on the system supply

Abstract

The paper first summarises the advantages of steam turbine-driven induction generators over conventional generators such as low cost, less maintenance, rugged and brushless rotors (squirrel cage type, no need for synchronisation, etc.), together with problems concerning excitation (VAr compensation at loads etc). A mathematical model of the induction generator simulated in direct-phase quantities where saturation of the magnetising reactances is simulated and saturation of stator and rotor leakage reactances is ignored is developed and employed for detailed simulation of the machine. Discrete-mass models of the machine shaft where both steam and electrical viscous damping is simulated are employed in comparing transient shaft torsional response evaluated by time domain simulation and frequency domain analysis following incidence and clearance of severe system faults. The paper then investigates torsional response following incidence and clearance of severe supply system disturbances, when the rotor is stationary and when running at close to synchronous speed unexcited, and following malsynchronisation when excited by a controlled VAr source, together with torsional response following bolted stator-terminal short-circuits at full-load and no-load following switching in of the induction generator onto the system supply. It examines precision of predicting torque in turbine-generator shafts by frequency domain analysis not analyzed for induction generators in the literature heretofore following incidence and clearance of worst-case disturbances on the supply. Effect of steam and electrical damping on maximum shaft torques predicted by frequency domain analysis is also illustrated. The results illustrate there is no tendency for shaft torques to become more onerous as the fault clearing time is increased as is the case for shaft torques in large synchronous machines. Three large two-pole machines of rating of up to a few hundred MWs are analysed.