Research on Mechanism and Damping Control Strategy of DFIG-Based Wind Farm Grid-Connected System SSR Based on the Complex Torque Method

The subsynchronous resonance (SSR) of a double-fed induction generator (DFIG) and its suppression method are studied in this paper. The SSR may be aroused by the interaction between the double-fed induction generator and the series-compensated transmission lines. This paper proposes an expression of the electrical damping for assessing the SSR stability based on the complex torque method. The expression is derived by linearizing the DFIG model at the operating point. When the mechanical damping is neglected, the expression can be used to calculate whether the electrical damping is positive or negative to judge the SSR stability. The expression can quantitatively analyze the impact of the wind speed, the compensation degree, and the parameters of the rotor speed controller and the rotor-side converter controller on the SSR stability. Furthermore, a subsynchronous damping control (SDC) strategy is designed to suppress the SSR. The parameters of the SDC are optimized by particle swarm optimization (PSO) based on the electrical damping. Finally, the above research is verified by the PSCAD/EMTDC time-domain simulations. The results show that the stability of SSR decreases with the decrease of wind speed, the increase of series compensation degree, the increase of proportional coefficient, and the decrease of integral coefficient in rotor speed controller and rotor-side converter. The designed subsynchronous oscillation controller can suppress the SSR of a DFIG