An Improved DC Voltage Control Method for Standalone DFIG-DC System Based on Direct Torque Resonant Control

In the standalone doubly fed induction generator connected to direct current (DFIG-DC) system, the dc link is not interfaced with a constant dc source. The dc voltage needs to be controlled by the DFIG. Since the stator of DFIG is connected to dc link through the diode bridge, the dc voltage contains a lot of low frequency ripples which might be harmful for the lifetime of dc capacitor. This paper presents an improved dc voltage ripple suppression strategy for standalone DFIG-DC system based on direct torque resonant control. Through establishing the relationship between dc voltage and rotor current, the dc voltage can be directly controlled by the magnitude of the rotor current. Furthermore, the relationship between the torque ripple and dc voltage ripple is also revealed, which indicates that the dc voltage ripple and torque ripple can be reduced simultaneously. Finally, the simulation results are presented to verify the proposed control strategy, which is effective in reducing the dc voltage ripple and torque ripple simultaneously.