A new direct torque control of doubly-fed induction generator under unbalanced grid voltage

In this paper a new direct torque control method for doubly-fed induction generator (DFIG) based wind power generation system is presented. The angle and magnitude of rotor voltage are controlled to achieve independent control of electromagnetic torque and reactive power respectively. Space vector modulation is used to address the limitations, like variable switching frequency and torque ripple, of hysteresis based schemes. Further, this paper proposes the compensation of effects of unbalanced grid voltage. When stator of DFIG is connected to unbalanced grid, the stator flux consists of positive sequence component and negative sequence component. The scheme compensates the negative sequence stator flux by negative sequence rotor flux using rotor side converter. When negative sequence component of stator flux is compensated, all effects of stator unbalanced voltages such as second harmonic pulsation in torque, stator active power, stator reactive power, stator current and rotor current are reduced. Simulation results for a 3 kW DFIG system demonstrates the effectiveness of the proposed direct control method and scheme for compensation of effects of unbalanced grid voltage.