Effect of oscillatory torques on the movement of generator rotors

The paper discusses the process whereby some of the kinetic energy stored in a generator rotor is converted to magnetic energy when a generator is short circuited. This gives rise to oscillatory electrical torques at fundamental frequency which, hitherto, have not been recognised as an important factor in the initial angular movement of a generator rotor and have usually been ignored; this is shown to be a serious omission which, when corrected, will influence future studies of generator stability. The effect of these oscillatory torques is to cause a back swing of the rotor when a generator is faulted. This phenomenon has been noticed during several system tests, and the results of such tests are used in the paper to support the theory which is presented. The practical system conditions necessary for the existence of these torques is discussed. Since these torques are of fundamental frequency, the exact calculation of them in digital-computer programs is time consuming, and thus a simple approximate method for simulating their effect on rotor movement is presented.