Reduction of voltage and torque fluctuations in DFIGs fed by multilevel inverters

This paper presents methods for eliminating flicker phenomenon and reducing torque and power fluctuations in doubly-fed induction generators whose rotors is fed by thyristor-based inverters. First, effects of rotor harmonics on interharmonic content of stator voltage and torque fluctuations are analyzed. Then, methods including selective harmonic elimination (SHE) in two- and multilevel inverters by controlling switching angles are applied. Two approaches for elimination of harmonics on rotor side are studied. In the first approach, interharmonics having frequencies lower than 40 Hz for the aim of flicker reduction are eliminated. In the second approach frequencies lower than 120 Hz are eliminated for the aim of improving voltage quality. Simulation of a DFIG in both stand-alone and grid-connected conditions validates the effectiveness of the proposed approaches.