Virtual Capacitance Control for Improving Dynamic Stability of the DFIG-Based Wind Turbines During a Symmetrical Fault in a Weak AC Grid

As the share of wind power in the power system increases, the power grid exhibits the characteristics of a weak grid. The dynamic stability issues of the grid-connected doubly fed induction generator (DFIG)-based wind turbines caused by the interaction between the phase-looked loop (PLL) and rotor current controller (RCC) by high grid impedance have become acute, especially during low-voltage ride-through. In this article, the output impedance model of the DFIG is derived, where the dynamics of the PLL have been first taken into account. In addition, the interaction between the PLL and RCC is analyzed. Furthermore, a novel virtual capacitance control strategy that directly controls the input voltage of the PLL and does not need to change the internal controller parameters and structure is proposed. The proposed control strategy can reduce the effect of the high line impedance by compensating for the PLL output angle deviation. Subsequently, the interaction between the PLL and RCC becomes weaker, and the phase margin of the DFIG system during the severe grid fault increases. Meanwhile, the applicability of the proposed control strategy is presented. Finally, the effectiveness of simulation and experimental results is presented.