Non-linear effects on modal estimates obtained from power system ringdowns

In this paper the effect on ringdown signals of three power system nonlinearities namely the swing equation, the field winding magnetic saturation and excitation limits are examined in terms of variations of instantaneous frequency and damping. All three non-linearities are shown to generate time varying modal frequencies and damping. It is found that for the system studied that the excitation system non-linearity is the most significant and can generate the greatest error in modal estimates.