Negative sequence compensation techniques of DFIG-based wind energy systems under unbalanced grid conditions

This paper presents analysis and control techniques for a doubly-fed induction generator (DFIG) used in wind energy systems under unbalanced grid conditions. The rotor currents and electromagnetic torque of the DFIG under unbalanced conditions are first analyzed. Negative sequence circuit models are investigated. Control strategies implementing negative sequence compensation from either the grid-side converter (GSC) or the rotor-side converter (RSC) are developed. The control objective of the GSC compensation is to provide the negative sequence currents for the grid to keep the stator currents free of negative sequence component. The control objective of the RSC compensation is to eliminate the rotor current harmonics. The main contribution of the paper is i) the analysis of the root cause of the high frequency harmonics in rotor currents and the electromagnetic torque and, ii) the comparison of the two control techniques frequently used in research. Matlab/Simulink tests for a 2 MW DFIG are performed to confirm the analysis.