Analysis and Reshaping on Impedance Characteristic of DFIG System based on Symmetrical PLL

With the increasing penetration of renewable energy, wind turbines based on doubly fed induction generators (DFIG) are becoming more and more important in the power system. The impedance reshaping control strategy based on the virtual impedance method can be utilized to solve the stability problem of the DFIG-grid interconnected system under the weak grid. However, the stability issues caused by phase-locked loop (PLL) is more complicated due to the frequency coupling characteristic. This article presents an improved control strategy of the DFIG system based on a symmetrical PLL, in which the frequency coupling characteristic can be avoided and the system can be simplified as a single-input single-output system that facilitates the design of virtual impedance. The influence of symmetrical PLL on the operation of the DFIG system is studied, and an improved impedance reshaping control strategy is proposed. Simulation and experimental results are conducted to verify the effectiveness of the proposed impedance reshaping control strategy.