Impedance Modeling and SSR Analysis of DFIG Using Complex Vector Theory

Based on the complex vector theory, a more comprehensive doubly-fed induction generator (DFIG) impedance model under series compensated network, taking both outer loops and phase-locked loop (PLL) dynamic into consideration, is developed in stationary reference frame (αβ-frame). Then the complex vector-based impedance model is combined with the equivalent RLC circuit to intuitively and quantitatively explain the mechanism of sub-synchronous resonance (SSR) and its mitigation methods. The impact of the RSC controllers and series compensation level on the equivalent impedance and the SSR frequency of the DFIG system are analyzed. The mechanism of the two mitigating SSR methods, one is damping control and the other is virtual inductance control, is revealed based on the complex vector impedance model. Finally, the accuracy of the derived model and the theoretical analysis are verified by simulation.