مطالعه مقایسه ای درباره سه نوع کنترل کننده DFIG در سیستم مبدل انرژی باد Comparative study of three types of controllers for DFIG in wind energy conversion system

1 Introduction Over the last decade, wind energy has taken an increasingly important place in the field of electric energy generation. This kind of energy source is developed due to the global growing of electricity demand and the trend towards renewable and non-polluting energy sources in the world [1]. Indeed, in wind energy conversion system (WECS), the maximum wind power could be extracted when the tip-speed-ratio of the turbine is maintained at its optimum value for different wind speed patterns [2]. Thus, it is necessary to develop more advanced control strategies for WECS. To this end, several control methods have been designed and implemented for wind energy generation such as, vector control which is based on voltage and flux oriented vector using the d-q rotating frame to decouple the active and reactive power, [3, 4]. In fact, this strategy is sensible to parameters variations of the system such as resistance and inductance variations. To overcome this problem, direct torque control (DTC) has been introduced by [5, 6] to directly control generator torque and stator flux using a predefined lookup table based on the estimation of the stator flux and electromagnetic torque. Direct power control (DPC) proposed in [7], has used the same concept of the DTC method. DPC control strategy is based on decoupling and direct control of reactive and active power [8]. In fact, the non-linear behaviors of mechanical and electrical parts of WECS as well as variations of electromechanical parameters represent crucial problems [9]. In addition, wind turbine (WT) works under high wind speed variations, which makes its control a serious challenge [10]. As result, several nonlinear control techniques have been developed in the literature for WT, such as fuzzy logic [11], neural networks [12], and high-order sliding mode control [13]. Currently, most WECS use Double feed induction generator (DFIG). This is due to many advantages such as variable speed operation of the generator (± 30% around the synchronous speed), decoupling between active and reactive powers, maximization of energy generation and competitive price [1]. But, DFIG is subjected to many constraints, such as the effects of parametric variations and the disturbance of the wind speed, which could deviate the system from its optimal operation point. Many control techniques of the DFIG have been presented with different control schemes. The conventional Proportional-Integral (PI) controller, although widely used in many control applications [14], requires adjustment for every change in reference patterns. Another main disadvantage of this controller is its sensibility to external disturbances and parameters variations. Because of the frequent uncertainty in wind speed variations, this type of conventional controller fails to give quality power generation and tracking references given by an MPPT. Predictive control based on rotor voltage and stator power equations for direct power control, is proposed in [15, 16]. Nevertheless, the calculated output reactive and active power which depend on the generator parameters as well as the time calculation, are the main drawbacks of this method [17]. In addition, internal uncertainties and external disturbances produce serious oscillations of the WECS. To ensure the robustness of the system against parametric variations and external disturbances, the authors in [18, 19] have introduced sliding mode control (SMC). In fact, SMC could achieve active and reactive power tracking and improve the dynamic.