Generally, a doubly fed induction generator (DFIG) has been used popularly in wind turbines, especially in large-scale wind turbine and large wind farms. The main reason is that the DFIG wind turbine can work in variable speed to extract maximum power from wind energy; it is interfaced to the connected grid through a partial scale back-to-back converter, approximate 30% of DFIG capacity 1, 2. However, since DFIG has two output sides including the stator side and the rotor side, the control of DFIG wind turbine is quite complicated.
Until now, many control laws have been proposed for DFIG wind turbine with different control purposes. A conventional controller using PI control has been well-known in literature 2, 3. This controller normally consists of an internal loop and an external loop; the external loop is to adjust power/speed/torque by determining the desired rotor current from errors between the actual power/speed/torque and their references; the internal loop is to adjust rotor current to the desired rotor current. However, determining parameters for PI controller is not easy, in some case, these need to be adapted 4. Sliding mode control was also introduced to replace PI controllers and it can track reference value quite good; however, for the use of the sliding mode control, chattering problem must be considered carefully; moreover, it is hard to be implemented in practice because of multiplying measurement noises by differential equations. Intelligent controller using Fuzzy, neural network… were also suggested 5, 6, 7. For the Fuzzy based controller, propose fuzzy sets and fuzzy laws must be defined carefully while the neural network-based controller requires a training process. A simple controller which is based Linear Quadratic Regulator (LQR) was proposed for DFIG 8, 9, 10, 11. In 8, LQR was implemented to design the pitch controller of DFIG wind turbine; LQR or advanced LQR-based controllers were also designed to adjust the rotor flux 9 and rotor current 10, 11 of DFIG In 10 and 11, two control loops were used, the outerloop is to calculate directly the reference rotor current from the reference power and the interloop adjusts the rotor current following the output of the outer loop by LQR.