ADVANCED SIGNAL PROCESSING METHODS FOR EVALUATION OF HARMONIC DISTORTION CAUSED BY DFIG WIND GENERATOR Przemyslaw Janik, Zbigniew Leonowicz, Jacek Rezmer Wroclaw University of Technology Wroclaw, Poland przemyslaw.janik@pwr.wroc.pl zbigniew.leonowicz@pwr.wroc.pl jacek.rezmer@pwr.wroc.pl Steffen Schostan Detlef Schulz Helmut-Schmidt-University Hamburg, Germany steffen.schostan@hsu-hh.de detlef.schulz@hsu-hh.de Abstract – Intensified growth of wind power generation emphasizes the need for detailed evaluation of disturbances caused by wind generators. The paper presents a physical model of doubly fed induction generator (DFIG) connected to low voltage distribution grid. Harmonic distortions caused by wind generator under various operational conditions regarding wind speed, active and reactive power have been investigated. Fourier transform, ESPRIT and Prony analysis have been applied simultaneously to evaluate distortion levels. Keywords: wind turbine, doubly feed induction generator, power quality, Prony method, ESPRIT 1 INTRODUCTION Wind generation is one of the most mature and cost effective resources among different renewably energy technologies [1]. Further intensification of wind farms proliferation into electric systems is supported by ecologists and governments. The target of European Community is to increase the share of renewable energy resources to 22% by 2010 [2]. A forward-looking energy supply policy implies the use of wind turbines with good grid compatibility and measures for grid reinforcement as well [3]. The quality of energy supply and compatibility issues underlay standardization. Power quality criteria defined in various standards [4], [5] are not only important from the technical point of view. Also the deregulated energy market forces high quality of electricity supply and reliability [6], [7]. Utilities have developed interconnection requirements to which new wind generators must abide [9]. Stricter requirements demanding accurate reactive power control and voltage regulation capability has led to the integration of power electronic converter in many of wind generator designs [8]. This allows better control of the generator in terms of reactive power control and also limited control of real power characteristics. There are various design concepts which allow fulfillment of those requirements [3]. Examples are conversion systems with synchronous generator (with exciter and permanent magnet) and converter-inverter units. Additionally, there are short circuit rotor asynchronous machines also with converter-inverter units or asynchronous machines with static compensators [10]. One of the popular and efficient designs is the doubly fed induction generator DFIG [3]. The DFIG allows the regulation of reactive power and the adjustment of angular velocity to maximize the output power by given wind speeds. The generator can also stay connected to the grid during voltage sags. However, one of the drawbacks is current with harmonic content introduced into the system [11]. The purpose of the paper is the evaluation of harmonic distortion in various operation conditions of the DFIG. Prony method and ESPRIT algorithm were applied to get information on harmonics parameters. It was assumed that those advanced methods could give more detailed information, then using traditional FFT. The laboratory model of DFIG allowed quite free regulation of active and reactive power and angular velocity of the shaft. The interest was focused on the harmonics generated by DFIG. Due to restrictions imposed by utilities on wind park operators it is difficult to run such research activity on real object within widespread range of parameters changes in acceptably short time. Firstly, the physical, grid connected model consisting of induction generator, direct converter, DC machine and control unit is introduced. Secondly, the Prony method and ESPRIT algorithms are presented. Then, the measurement procedure and assessment of grid parameters follow. Current measurements and harmonics estimation results are finally presented and discussed. 2 MODEL OF DOUBLY FED INDUCTION GENERATOR Simplified model of DFIG is shown in Fig.1. The rotor of induction machine is connected to the grid with a back-to-back voltage source converter which controls the excitation system. This most significant feature enables sub synchronous and super synchronous operation speeds in generator mode and adjustable reactive power generation. Model components are shortly characterized below. 2.1 Grid The generator was connected to a three phase low voltage distribution grid. In order to assess the system