A control strategy for DFIG-based systems operating under unbalanced grid voltage conditions

Abstract

An integral control strategy for DFIG-based systems operating under unbalanced grid-voltage conditions is presented in this article. First, dynamic models of the DFIG and the back-to-back converter needed to design the system's current control-loops are presented. Then, a strategy to control the generator's electromagnetic torque and reactive power is introduced. The strategy is implemented by using a stator-flux aligned reference frame, where the stator-flux signals are obtained from an observer. Next, a control strategy for sinusoidal current injection under unbalanced voltage conditions is presented. This strategy generates the current references for the grid side converter from the desired mean values of the output active and reactive powers and the positive-sequence component of the grid voltage. Also, a DC-Link voltage control that works in combination with the sinusoidal current injection strategy is proposed. The design of a non-linear passivity-based controller to track the rotor- and grid-side converters current reference signals is presented. The controller allows the implementation of the integral control strategy without negative-sequence detectors which increases robustness and simplicity. Finally, simulation results are presented to validate the proposal and a stability analysis is performed.