Control of back-to-back PWM converters for DFIG wind turbine systems under unbalanced grid voltage

This paper presents a new control scheme for minimizing the torque ripple of the generator under unbalanced grid voltage for wind turbine systems using doubly-fed induction generators, where the negative sequence component of the rotor current is utilized. It is found that the generator torque ripples are related with the reactive power component. Increase of stator active power ripples caused by reducing the torque ripple is compensated by controlling-the active power ripples of grid-side converter. Simulation results using PSCAD and experimental results show that the conventional vector control of DFIG without considering grid voltage unbalance results in excessive oscillations on the stator active and reactive power, and electromagnetic torque. On the other hand, with the proposed control strategy, improved system control and operation such as reducing oscillations of active power and generator torque can be achieved.