Nyquist-Stability-Criterion-Based SSR Explanation for Type-3 Wind Generators

Type-3 wind generators in series-compensated networks could lead to subsynchronous resonance (SSR) oscillations- a phenomenon observed recently. In this letter, impedance-based Nyquist stability criterion is applied to explain the phenomenon. The resonance is not due to the shaft's torsional characteristics, but rather due to its slip related impedance, which can give rise to equivalent negative resistance. The explanation corroborates the authors' previous explanation of self-excitation using net negative resistance concept. Nyquist map is also able to demonstrate the impact of wind speed on SSR.