Coordinated Derived Current Control of DFIG's RSC and GSC Without PLL Under Unbalanced Grid Voltage Conditions

This paper proposes a coordinated derived current control strategy of doubly fed induction generator (DFIG) system under unbalanced grid voltage conditions. The rotor-side converter (RSC) and the grid-side converter (GSC) are synchronized to the grid with a virtual phase angle of nominal frequency, which can be simply set as theta = omega(n)t (omega(n) = 100 pi). Consequently, the proposed control strategy can implement in an arbitrary reference frame without phase-locked loop (PLL). The derived d-q axis currents are obtained from the DFIG stator and GSC currents with virtual angle-based Park Transformations. For a simple implementation, a reduced-order vector integrator with a voltage vector multiplier is used to directly regulate the measured powers/currents containing both the dc part and the oscillating part without sequence extractions. By this means, RSC achieves torque ripple reduction and then GSC can rebalance the currents and smooth the active/reactive powers. Finally, the experimental results on a small laboratory setup are presented to validate the effectiveness of the proposed control strategy.