Transient damping torques in synchronous machines during disturbances

During power-system disturbances, the relative speed between the resultant flux and the rotor causes eddy currents to be induced in the rotor iron structure and an induction-motor or induction-generator torque to be developed owing to the amortisseur winding; both these effects damp out the synchronousmachine oscillations. Damping has been considered previously as directly proportional to the slip of the rotor with respect to synchronous speed for a single-machine study, and with respect to instantaneous frequency of Thévénin´s equivalent e.m.f. for a multimachine study; this is erroneous and can lead to a substantially different assessment of power-system transient stability, as is shown by illustrative examples. It is indicated that the conclusions drawn from a single-machine study for damping purposes (as is sometimes done in the literature) are not really applicable to a multimachine study; and, moreover, it cannot be said with certainty that the damping will be positive or negative. The damping torque depends on the instantaneous slip between the rotor and the resultant flux, and requires knowledge of synchronous-machine torque/slip characteristics.