Design and control of doubly-fed induction generators with series grid-side converter

This paper proposes and describes the controllers for doubly-fed induction generators (DFIG) with series grid side converter. It is demonstrated that with series grid side converter instead of a conventional shunt grid side converter the DFIG stator voltage can be kept balanced even under heavy unbalance grid voltages. Therefore, in this way it is avoided the undesired effects such as torque oscillations and high stator and rotor currents presents under unbalance voltages. In addition, it is demonstrated that it is possible with a conventional back-to-back three phase PWM converter to satisfy the grid code active and reactive power requirements. The design and control of the series grid-side converter with a LC-filter will also be presented. An H_infin controller is designed to achieve robustness as well as good tracking and disturbance rejection performance for the series voltage converter. Furthermore, the DC-link, rotor currents and active and reactive power controllers are as well described, where the controllers transfer functions are selected to ensure good performance even in the presence of grid voltage unbalance. Finally, simulation results of a 2.27 MVA wind turbine generator are presented to demonstrate the good performance of the DFIG with the proposed controllers.