New direct torque control of DFIG under balanced and unbalanced grid voltage

This paper presents a direct torque control method for doubly-fed induction generator (DFIG) based wind power generation systems. 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 presents a technique to reduce the torque pulsations of DFIG under unbalanced grid voltage condition. Under unbalanced grid voltage condition, the torque angle (δ) is controlled so that electromagnetic torque pulsations are reduced. To achieve this, a compensation method based on proportional-integral and resonant (PI+R) controller is explored. The proposed control method does not require rotating frame transformations and it maintains the simplicity of DTC. It has fast dynamic response, which is comparable to vector control. Simulation results for a 2 MW DFIG system demonstrates the effectiveness of the proposed control strategy with various loading conditions under both balanced and unbalanced grid voltage.