Evaluation of a class of improved DTC method applied in DFIG for wind energy applications

Large torque ripple and variable switching frequency are the two most notable drawbacks of conventional switching table based direct torque control (DTC) for doubly fed induction generator (DFIG). By using one active vector and one null vector during one control cycle, the torque ripple can be significantly reduced while achieving almost constant switching frequency. This paper propose a very simple but effective method to obtain the duty ratio of the active vector, which is able to reduce the complexity and improve the system robustness while reducing both torque and flux ripples. Furthermore, this paper points that by appropriately arranging the sequence of active vector and null vector, the switching frequency can be further reduced and the performance is only slightly affected. This fact is useful for high power wind energy applications with restricted switching frequency. The developed method is compared with one of the prior analytical methods based on torque ripple RMS minimization and exhibits lower rotor flux ripple and better harmonic performance of stator and rotor currents. The presented simulation results obtained from a 15 kW DFIG validates its effectiveness.