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A Stability Assessment and Estimation of Equivalent Damping Gain for SSR Stability by Nyquist stability criterion in DFIG-based windfarms

Sub-synchronous resonances (SSR) arise in a doubly fed induction generator (DFIG) based wind farm due to the rotor circuit's negative resistance characteristic at low wind speeds and a high compensation level in a transmission line. This paper presents the Nyquist criterion-based stability assessment approach to investigate SSR in the induction generator-based series compensated network and estimate for damping gain for the open-loop transfer function of the study system. The frequency-domain-based impedance model of the induction generator and transmission line impedance model is used to develop the study system's overall open-loop transfer function. The stability analysis shows that low wind speed and high compensation levels induce the SSR phenomenon in DFIG based wind farms. The relative stability shown by phase margin and gain margin is used to calculate the approximate damping gain for the open-loop transfer function of the study system. Using the proposed approach, the results show that the estimation for the damping gain makes the phase margin and gain margin of the study system positive at low wind speed and high compensation level, reducing the risk of SSR.