Improved control of DFIG using stator-voltage oriented frame under unbalanced grid voltage conditions

SUMMARY

This paper analyzes two control strategies of doubly fed induction generator (DFIG)-based wind turbine under unbalanced voltage which were introduced in the previous works and then proposes a new control strategy to achieve more improved performance of DFIG. Under unbalanced conditions, the rotor side converter (RSC) and grid side converter (GSC) are controlled in two positive and negative ((dq)⁺ and (dq)^–) reference frames. The control of these converters in the (dq)⁺ frame is the same as the conventional control under balanced grid conditions. In the proposed control strategy, the control of the RSC in the (dq)^– frame is done to balance the stator currents, and also the total active power oscillations are removed by the control of the GSC in this frame. In this strategy, the oscillations of both the torque and stator active power are properly reduced even when the voltage unbalance is somewhat severe. Consequently, the elimination of the stator active power oscillation by the active power output from the GSC is accomplished with fewer constraints. To provide the accurate coordinated control of the RSC and GSC, this proposed control strategy is implemented in the stator-voltage oriented frame. The simulation results obtained for a 7.5-kW DFIG-based wind turbine validate the enhancement of system operation using the proposed control strategy during unbalanced voltage network. Copyright © 2012 John Wiley & Sons, Ltd.