*2.3. Pitch Control*

To maintain the extracted wind power at rated power in Region 3, it requires that the corresponding pitch angle should be achieved, which in turn requires both the mechanical rotation speed *ω*m and the mechanical torque *T*m should be kept at their rated values, respectively. The rated mechanical torque *T*mr is achieved when the electromagnetic torque *T*e can track its rated value *T*er and the *ω*m is kept at it rated value. According to Equation (7), the electromagnetic torque *T*e can be maintained at *T*er if the q-axis stator current *<sup>i</sup>*mq can track its rated value *<sup>i</sup>*mqr and *i*md is kept at 0.

The brief overall control approach is shown in Figure 3. The control approach consists three controllers: two stator current controllers and a rotation speed controller.

**Figure 3.** Brief overall control structure of the PMSG-WT.

### **3. Perturbation Observer-Based Nonlinear Adaptive Controller Design**

In this section, the design of NAC for the PMSG-WT based on the feedback linearization will be presented. The NAC based on perturbation estimation proposed in [29] will be used. First, a nonlinear system is transformed as interacted subsystems through input/output linearization. Secondly, in each subsystem, uncertainties, nonlinearities and interaction among subsystems are contained in a defined perturbation term. The perturbation term is estimated via a designed observer. The estimated perturbation is used for compensating the real perturbation and obtaining adaptive linearizing control of the original nonlinear system.
