A Novel Control Scheme for DFIG-based Wind Energy Systems under Unbalanced Grid Conditions

This paper presents an analysis and a novel control scheme for a doubly-fed induction generator (DFIG) based wind energy generation under unbalanced grid conditions. The control objectives are: (i) to limit the rotor currents, (ii) to suppress ripples in the torque and (iii) to suppress the dc-link voltage fluctuation through converter controls. Negative sequence compensation techniques by one of the converters in a DFIG are discussed and their limitations are presented. A coordinated control scheme with a concise structure is proposed in this paper. The rotor side converter (RSC) is controlled to suppress ripples in the torque and the rotor currents while the grid side converter (GSC) is controlled to suppress ripples in the dc-link voltage by considering the rotor power effect. The major contributions of the paper include: (i) presentation of the limitation of negative sequence compensation using one converter; (ii) development of a much simpler control scheme compared with the widely used dual sequence control scheme. In the proposed control scheme, low pass filters are not necessary and only one reference frame for each converter is used. Matlab/Simulink tests for a 2MW DFIG demonstrate the effectiveness of the control scheme.