Power Control in a Doubly Fed Induction Machine

This paper proposes a direct power control for doubly-fed induction machine for variable speed wind power generation. It provides decoupled regulation of the primary side active and reactive power and it is suitable for both electric energy generation and drive applications. In order to control the power flowing between the stator of the DFIG and the network, a decoupled control of active and reactive power is synthesized using PI controllers.The obtained simulation results show the feasibility and the effectiveness of the suggested method Keywords—Doubly fed induction machine , decoupled power control , vector control ,active and reactive power, PWM inverter I.INTRODUCTION HE wind energy systems using a doubly-fed induction generator (DFIG) have some advantages due to variable speed operation and four quadrant active and reactive power capabilities compared with fixed speed induction. Some investigations by using predictive functional controller [1] and internal mode controller [2], [3] have satisfactory power response when compared with the power response of PI but it is hardly to implement one due to the predictive functional controller and internal mode controller formulation. Another possibility to doubly-fed power control. can be made by using fuzzy logic [4], [5]. These strategies have satisfactory power response although it involves relatively complex transformation of voltages, currents and control outputs among the stationary, the rotor and the synchronous reference frames. The direct power control was applied to the DFIG power control and it has been presented in [6], [7], [8].This scheme calculates the required rotor controlling voltage directly based on the estimated stator flux, active and reactive power and their errors. In [6] the principles and the implementation of DPC is made with hysteresis controllers and variable switching frequency. In [7], [8] the principles of this method are described in detail and simulations results have been presented with variable and constant switching frequency respectively. Moreover, the conventional DPC complicates the AC filter design because of its variable switching frequency. An alternative to direct power control is the power error vectorcontrol [9]. This strategy is less complex and obtains similarresults to direct power control. To improve the power response, to eliminate the torque ripple and to protection of rotor-side converter under grid voltage sags a proportional control with anti-jamming control was proposed in [10]. This control has satisfactory power response and eliminate the rotor current overshoot in voltage sags when the A.Ourici is now with electrical engineering institute of Badji Mokhtar University of annaba Algeria; a_ourici@hotmail.com loop of torque control is applied, although power and rotorcurrents results were shown only in fixed speed operation.We present in this paper a direct power control , it results a good decoupling control between active and reactive power, it achieves high accuracy and fast dynamic power response. II.DESCRIPTION AND MODELING OF DFIM The proposed system is shown on figure 1, it is constituted by two pulse width modulation inverters supplying separately the stator and the rotor of the machine [11] Fig .1 DFIM supplied by two PWM inverters We choose three levels PWM for both stator and rotor inverters, it is constitute of three arms, every one has four switches formed by a transistor and a diode as shown in figure 2 Fig .2 General diagram of a three level PWM inverter The simple voltages are obtained starting from the following conditions: A. Ourici Power Control in a Doubly Fed Induction Machine