Integrated power characteristic study of DFIG and its frequency converter in wind power generation Shuhui Li * , Timothy A. Haskew, Jeff Jackson Department of Electrical and Computer Engineering, The University of Alabama, 317 Houser Hall, Tuscaloosa, AL 35487, USA article info Article history: Received 26 April 2008 Accepted 22 July 2009 Available online 13 August 2009 Keywords: Wind power Doubly fed inductor generator PWM converter d–q vector control Modeling Simulation abstract A doubly fed induction generator (DFIG) is a variable speed induction machine. It is a standard, wound rotor induction machine with its stator windings directly connected to the grid and its rotor windings connected to the grid through a back-to-back AC/DC/AC PWM converter. The power generation of a DFIG includes power delivered from two paths, one from the stator to the grid and the other from the rotor, through the frequency converter, to the grid. The power production characteristics, therefore, depend not only on the induction machine but also on the two PWM converters as well as how they are controlled. This paper investigates power generation characteristics of a DFIG system through computer simulation. The specific features of the study are (1) a steady-state model of a DFIG system in d–q reference frame, (2) a simulation mechanism that reflects decoupled d–q control strategies, (3) power characteristic simu- lation for both generator and converter, and (4) an integrative study combining stator, rotor and converter together. An extensive analysis is conducted to examine integrated power generation char- acteristics of DFIG and its frequency converter under different wind and d–q control conditions so as to benefit the development of advanced DFIG control technology. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction A doubly fed induction generator (DFIG) is an adjustable-speed induction machine widely used in modern wind power industry [1,2]. Wind turbine manufacturers are moving to variable speed concepts because of the following reasons: (1) a higher energy yield, (2) a reduction of mechanical loads and a simpler pitch control, (3) an extensive controllability of both active and reactive powers, and (4) less fluctuation in output power [2,3]. However, the performance of a DFIG depends not only on the induction machine but also on the two back-to-back AC/DC PWM converters as well as how they are controlled. In order to comprehend DFIG power generation characteristics under different control conditions, various techniques have been devel- oped. This can be divided into two categories: (1) transient approaches [4–6], and (2) steady-state techniques [7–9]. Transient approaches are essential to study DFIG dynamic performance in a short time period. But, steady-state techniques are important to examine DFIG characteristics in a broader spectrum. Unlike a conventional fixed-speed induction machine, a DFIG delivers power to the grid from both the stator and rotor paths, and its characteristics depend strongly on the d–q control approaches applied to the rotor- and grid-side converters. Those specific regularities must be considered in the steady-state study of a DFIG system. The purpose of this paper is to investigate steady-state power characteristics of integrated DFIG and its frequency converter under general d–q control strategies so as to benefit the develop- ment of advanced control technology. Different from conventional steady-state studies [7–9], the main features of this paper are (1) steady-state models of a DFIG system in d–q reference frame, (2) a steady-state simulation mechanism that reflects general decou- pled d–q control strategies, (3) power simulation for both the generator and the converter, and (4) integrative power character- istic study of DFIG stator, rotor and converter together under different wind and d–q control conditions. In the sections that follow, the paper first introduces the oper- ation of a DFIG, its back-to-back PWM converter, and the funda- mental converter control principles. Then, steady-state models in d–q reference frame are developed. Simulation studies are per- formed to investigate the power generation regularities of a DFIG and its frequency converter under different d–q control conditions. Then, the models of the two parts are combined together for an integrative study by considering wind power extraction charac- teristics. Finally, the paper concludes with the summary of the main points. * Corresponding author. Tel.: +205 348 9085; fax: +205 348 6959. E-mail address: sli@eng.ua.edu (S. Li). 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.2009.07.016 Renewable Energy 35 (2010) 42–51