Based on wind speed, the operating regions of a wind turbine are divided into two parts: below‐rated wind speed (region II) and above‐rated wind speed (region III). This article presents a new control strategy for a variable speed wind turbine for the below‐rated wind speed category (region II). In this operating region, maximization of the conversion efficiency and capturing the maximum energy available in the wind are desirable. The controller is designed based on sliding mode control techniques and fractional order calculations. The sliding‐mode approach has been widely used in wind turbine systems due to the existence of nonlinearity and uncertainty in wind turbine dynamics. The fractional order sliding mode controller has the capacity to produce better performance in comparison to integer order sliding mode controllers due to higher degrees of freedom for tuning. The generator torque is used as the control input to vary the rotor speed in order to maximize energy conversion. To achieve this goal, the rotor angular speed must track the time variable reference value and, hence, fast variations in wind speed. This requires fast reactions on the part of design control. The fractional‐order sliding mode controller can have a good transition response, a low tracking error, and a very fast reaction against rapid variations in wind speed. The stability analysis of the suggested controller was provided using Lyapunov stability theory. Finally, the numerical simulation results are presented and compared with integer order sliding mode control to illustrate the effectiveness of the proposed method. The results show the better performance of the fractional‐order sliding mode controller in comparison to the integer order sliding mode controllers. © 2018 American Institute of Chemical Engineers Environ Prog, 37: 1901–1907, 2018

At below‐rated wind speeds, the main goal of a controller design is to maximize power coefficient and consequently captured power. Due to nonlinearities and uncertainties in wind turbine systems and rapid variations in the wind speed profiles, the need for more effective and robust controllers is inevitable. To optimize the captured power, the rotor angular speed must track the time variable reference value that requires fast reactions of the designed controller. In this article, an advanced controller called a back‐stepping sliding‐mode controller was proposed to improve the turbine efficiency at a below‐rated wind speed. The controller was designed based on the combination of the back‐stepping sliding mode approach and fractional order calculus. The stability of the proposed controller is analyzed using the Lyapunov stability criterion. Finally, numerical simulation results are presented and compared with integer order, back‐stepping sliding mode control to illustrate the effectiveness of the proposed method. The simulation results demonstrated that the proposed controller had a good transition response, a low tracking error, and a very fast reaction against rapid variations in the wind speed in comparison to the back‐stepping sliding mode controller. © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13110, 2019