Vector control optimization of DFIGs under unbalanced conditions

To efficiently address the unbalanced grid voltage requirement in the converter controller of a doubly-fed induction generator (DFIG), an additional function must be implemented in the control software. This function is added to the main control algorithm in order to reduce the side effects of negative sequence voltages that result in negative sequence components in rotor current, negative sequence components in stator current and oscillatory components in torque, and active/reactive power. This paper proposes various strategies for the design of this additional controller and compares between them. Furthermore, an optimal control algorithm is presented for the design of the auxiliary controller to achieve several desired operational characteristics of DFIG for rotor current, stator current, torque, and active/reactive power simultaneously. This is achieved by solving a multi-objective optimization problem through a weighted-sum approach that is validated using simulation studies. Therein, various methods are evaluated on a test-case wind farm under 6% of steady-state voltage unbalance. It is demonstrated that the proposed optimal control algorithm improves all generator operational characteristics simultaneously. It is verified that the operational limits of DFIG will be preserved only when the proposed optimal controller is applied.