Direct power control of DFIG based on discrete space vector modulation Mohammad Verij Kazemi, Ahmad Sadeghi Yazdankhah * , Hossein Madadi Kojabadi Electrical Engineering Department, Sahand University of Technology, Tabriz, Iran article info Article history: Received 15 February 2009 Accepted 13 September 2009 Available online 28 October 2009 Keywords: Constant switching frequency Direct power control Discrete space vector modulation Doubly fed induction generator Wind turbine abstract This paper presents a new direct power control (DPC) strategy for a double fed induction generator (DFIG) based wind energy generation system. Switching vectors for rotor side converter were selected from the optimal switching table using the estimated stator flux position and the errors of the active and reactive power. A few number of voltage vectors may cause undesired power and stator current ripple. In this paper the increased number of voltage vectors with application of the Discrete Space Vector Modulation (DSVM) will be presented. Then a new switching table in supersynchronous and sub- synchronous frames will be proposed. Simulation results of a 2 MW DFIG system demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, machine parameters, and wind speed. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The most important advantages of the variable speed wind turbines as compared with conventional constant speed system are the improved dynamic behavior, resulting in the reduction of the drive train mechanical stress and electrical power fluctuation, and also the increase of power capture [1]. One of the generation systems commercially available in the wind energy market currently is the doubly fed induction generator (DFIG) with its stator winding directly connected to the grid and with its rotor winding connected to the grid through a variable frequency converter as shown in Fig. 1 . One of the most advantages of this system is that the rating of the power converter is one third of that of the generator. Direct torque control (DTC) of induction machine drives was developed in the mid 1980s [2,3]. DTC is one of the actively researched control schemes which is based on the decoupled control of flux and torque. DTC provides a very fast and precise torque response without the complex field-orientation block and the inner current regulation loop. There have been some DTC based strategies, e.g., voltage vector selection using switching table [2], direct self control [3], and space vector modulation [4]. Based on the principles of DTC strategy, direct power control (DPC) was developed for three-phase pulse width modulation (PWM) rectifiers [5–7]. DPC method is based on the instantaneous space vector theory. More recently, DPC control of DFIG-based wind turbine systems has been proposed [8–11]. DPC directly controls the stator active and reactive powers, it is possible to control the rotor flux space vector amplitude and its relative distance to the stator flux space vector, i.e., a necessary condition for the control of the DFIG. Several authors have already developed DPC techniques that operate at a variable switching frequency [8,9]. The variable switching frequency makes the power converter and the ac harmonic filter complicated and expensive. More recently in [10,11] DPC at constant switching frequency have been developed for the DFIG. Constant converter switching frequency eases the design of the power converter and ac harmonic filter. The proposed strategy in [10,11] needs to calculate rotor flux in each sampling period. The implementation of these methods requires the knowledge of the rotor and stator parameters. The calculation of stator flux angle requires PLL circuit that makes the real time implementation of the proposed algorithm in [10] more expensive. In [11], using three non equal time intervals in each switching period causes the calculation and selection of suitable voltage vector in each time interval to become more complicated. Meanwhile, this method is suitable for low switching frequency applications. In this paper a new control technique is introduced which allows the performance of DPC scheme in terms of active and reactive power ripple and current distortion to be improved. A DPC with constant switching frequency was proposed. These improve- ments could be achieved with simple control circuit like the one in [9] without increasing the inverter switching frequency. Practical implementation of this strategy is much easier than that of other DPC with constant switching frequency. The new control algorithm is based on a discrete space vector modulation (DSVM) technique which uses prefixed time intervals within a cycle period. In this way * Corresponding author. Tel.: þ98 412 3459321; fax: þ98 0412 3444322. E-mail address: sadeghi@sut.ac.ir (A. Sadeghi Yazdankhah). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2009.09.008 Renewable Energy 35 (2010) 1033–1042