Grid-Forming Control of DFIG-Based Wind Turbine Generator by Using Internal Voltage Vectors for Asymmetrical Fault Ride-Through

Grid-forming (GFM) controls exhibit robust frequency and voltage support capabilities for inverter-based resources (IBRs). This also shows promise for doubly fed induction generator-based wind turbine generators (DFIG-based WTGs). However, during asymmetrical faults, the DFIG-based WTG that employs GFM controls (GFM-DFIG) might suffer from overcurrent and overmodulation of the rotor-side converter (RSC). Therefore, from the perspective of positive-sequence, negative-sequence, and transient components, this paper proposes asymmetrical fault ride-through (FRT) controls for the GFM-DFIG based on the mechanism for forming the grid voltage. Firstly, internal voltage vectors are designed for the assessment of asymmetrical FRT capabilities. Then a positive- and negative-sequence control (PNSC) is proposed to support the sequence components of internal voltage vectors for the GFM-DFIG. On this basis, an asymmetrical FRT control structure is proposed, incorporating negative-sequence reactive current injection and two types of positive-sequence control schemes: the current saturation-based method and virtual impedance-based method. Additionally, a simplified calculation method for transient voltages is utilized to eliminate the impacts of transient flux leakage. Finally, the proposed FRT controls for the GFM-DFIG are validated by using the EPRI benchmark system. The results indicate that, with the proposed control, GFM-DFIG can maintain stable voltages and achieve required negative-sequence behaviors.