Improved Grid Synchronization Control of Doubly Fed Induction Generator Under Unbalanced Grid Voltage

An improved grid synchronization control scheme of the wind energy conversion system based on the doubly fed induction generator (DFIG) under unbalanced grid voltage is proposed in this paper. In both the positive and negative sequence models, respectively, the degrees of relevancy among rotor voltages, rotor currents, and stator voltages are calculated using the relative gain array (RGA) methodology. According to the analysis results of RGA, the main controller is designed to control positive sequence stator voltages directly with positive sequence rotor voltages, and the auxiliary controller is designed to control negative sequence stator voltages directly with negative sequence rotor voltages. Hence, the rotor current control loops are eliminated, which simplifies the structure of the controller. Simulation and hardware experimental results validate that the improved control scheme effectively controls stator voltages of the DFIG to accurately follow unbalanced grid voltage and, hence, avoids current, torque, and power impacts to both the DFIG and the grid at the time of connecting.