direct power control

442 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 2, FEBRUARY 2010 Direct Power Control of Doubly-Fed-Induction- Generator-Based Wind Turbines Under Unbalanced Grid Voltage Gonzalo Abad , Member , IEEE , Miguel ´ Angel Rodr´ ıguez , Member , IEEE , Grzegorz Iwanski, and Javier Poza , Member , IEEE Abstract—In this paper, the behavior of a doubly fed induction generator (DFIG) is studied under unbalanced grid voltage conditions. It is shown that if no special control efforts are em- ployed, the behavior of the generator is deteriorated, basically due to two reasons: electr omagn etic torqu e oscil lation s and nonsi nu- soidal curre nt exchange with the grid. These phenome na are first analyz ed theore tic all y as a functi on of the sta tor act ive and rea ctive instantaneous power exchange by the stator of the DFIG and the grid-s ide con verter (GSC). This analys is provides the main ideas for generation of the active and reactive power references for the rotor-side converter (RSC) and the GSC, controlled by means of direct power control techniques. Therefore, this paper proposes a new algorithm that generates the RSC power references, with- out the necessity of a sequence component extraction, in order to elimin ate torqu e oscil lation s and achie ve sinusoidal stato r currents exch ange. On the contrary , the GSC power ref erencesare pro vid ed by means of voltage and current sequence extraction. Finally, sim- ulatio n and experimental resu lts successfull y valid ate the propo sed power reference generation methods. Index T erms—Doubly fed induction generator (DFIG), direct power control (DPC), unbalanced voltage. NOMENCLATURE L h Mutual inductance. L s , L r Stator and rotor self-inductances. P, Q, S Active, reactive, and apparent powers. R s , R r Stator and rotor resistances. T em Electromagnetic torque. V bu s V olt age of the dc sid e of the vo lta ge- sourcecon- verter (VSC). Ψ, v, i Flux, voltage, and current space vectors. ω s , ω m , ω r Synchronous, rotor , and slip speed s. Superscripts and Subscripts s, r , g Stator, rotor, and grid reference frame for space vectors. Manuscript received November 26, 2008; revised May 27, 2009. Current version published February 12, 2010. This work was supported in part by the Depar tamen to de Educa ci´ on, Uni vers idad es e Inv estig aci´ on del GobiernoVasco. Recommended for publication by Associate Editor J. M. Guerrero. G. Aba d and J. Poz a are with the Depar tme nt of Ele ctr ica l Engine er- ing, University of Mondragon, Mondragon 20500, Spain (e-mail: gabad@eps. mondragon.ed u; [email protected] dragon.edu). M. A. Rodr´ ıgue z is with Ingeteam Transmi ssion and Distribu tion S.A., Basauri 48970, Spain (e-mail: miguelangel.ro driguez@ingeteam.com). G. Iwans ki is withWarsaw Univ ersit y of Techno logy , Warsa w 00-6 62, Polan d (e-mail: iwanskig@isep .pw.ed u.pl). Color versions of one or more of the figures in this paper are available online at http://ieeexplo re.ieee.org. Digital Object Identifier 10.1109/TPEL.2009.20 27438 αβ Stator reference frame axes (stationary). ∗ Complex conjugate. I. INTRODUCTION I N RECENT years, the development of wind energy generation has been associated with wind farms located onshore and offshore. The wind farms are connected to strong transmission grids and their power range from some tens to a hundred megawatts. More and more modern wind turbines are being installed in distribution and rural grids, with low X/R ratios, and also in developing countries where the distribution grid is very weak. This application of modern power-electronics-based wind turbines has led to the development of new functionalities for the wind turbines, such as voltage or frequency control or island- ing operation, or the requirement of operating with unbalanced voltages. Nowadays, wind turbines based on a doubly fed induction generator (DFIG) [1] have arisen as a commonly used solution for many variable-speed wind turbine manufacturers. Several aut hor s [2] –[8 ] havesho wn tha t an unbalanced vo ltageope rat ion requires additional control efforts in order to avoid electromagnetic torque oscillations and nonsinusoidal currents exchange. Traditionally, the techniques to control the rotor-side converter (RSC) and the grid-side converter (GSC) of DFIG-based wind turbines include the vector control (VC) and the direct torque control or the direct power control (DTC-DPC). The first vector control proposal for the DFIG, which is associated with a back-to-back converter, was presented in [1]; many authors then proposed improvements such as sensorless operation or new current regulators. After this, the direct control techniques we re pr opos edfor thismachine:the DPC in [9] an d the DTC [10] from ABB. Until recently, the problems associated with the DFIG unbalanced operation have not been a main research topic. A new speed regulator is proposed in [11] for compensating small asymmetries in weak transmission lines, and the first doubly vector control (DVC) for the positive and negative sequences is presented in [12]. More recen tly , sev eral autho rs hav e analy zed the gener ator behavior in this situation [5]–[13] and others have proposed mod- ifications to the classical vector control. In [4], two controllers are added to the standard stator flux VC current controllers in order to eliminate the 100 Hz power oscillation. In [2], the 0885-8993/$ 26.00 © 2010 IEEE

direct power control

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