Journal of Multidisciplinary Engineering Science and Technology (JMEST) ISSN: 2458-9403 Vol. 7 Issue 12, December - 2020 www.jmest.org JMESTN42353624 13130 Power Quality Analysis of Grid-Connected Wind Turbine System Mohamed A. Shaban Electric & Electronic, College of Science And Technology, Surman, LIBYA Moh_tub@scst.edu.ly Nuri M.Trika Industrial Engineering, Faculty of Engineering Regdalen, Sabratha University, LIBYA Nuri.triki@sabu.edu.ly Adel Yahya Electrical Engineering, Faculty of Engineering, Regdalen, Sabratha University, LIBYA Adel.yahya@sabu.edu.ly Abstract—this paper develops time-frequency methods for complex investigation of transient states in wind power plants two algorithms were proposed: short-time Fourier transform (STFT) and Choi-williams (CWD). In order to explore advantages of the methods several experiments were performed using model of squirrel-cage induction machine connected directly to the grid. Investigated phenomena concerned power distortion caused by fault in accordance with the influence of wind speed on instantaneous character of appeared power distortions. Keywords—Transient states, Choi -Williams, Induction machine, power distribution, power plants. I. INTRODUCTION . In the era of technological development power quality issues have more and more crucial meaning. In spite of achieved experience in specifications of distribution, including IEC norms, some cases and accompany phenomena require individual approach. As examples can use wide researchers on influence of dispersed energy sources on power quality, especially occluding wind power plants. Wind turbines become nowadays regular element of power systems with all its desirable as well as undesirable influence .Behind the undisputed significance of wind power plants for searching the renewable energy sources there are some aspects, which have impact on power quality. One of them is natural result of variable weather conditions. Another comes from mechanical construction of power plant and power electrical equipment. Recognizing sources and symptoms of mentioned impacts it can be detailed [2], [3], [10]. Influence of stochastic wind variation on output torque, power, voltage and current fluctuation, periodical drop of output torque when the mill blade passes the tower (shadow effect), complex, nonlinear oscillation of the tower and wind turbine which can be transferred to turbine shaft (the frequency of generated oscillation can attain value from tenth to few HZ), and finally wide spectrum of harmonica in current and voltage caused by present of power converters. Selection of proper method for analysis of power distortion in wind turbine systems is still actual and crucial. In [10] we can find an idea. Which apply classical Fourier spectrum in order to investigate and classify power distortion? In this paper the propose to apply two-dimensional time-frequency analysis in order to obtain comprehensive analysis of power distortion. One of the contributions of this paper developing a new qualitative method for analysis of transient phenomena in wind turbine systems. Particularly, the investigation is aimed at revealing the instantaneous character of power distortion under transient conditions. In order to explore the effects, grid connected wind turbine system was modeled using Mata lab simpowersystem toolbox. II. METHODS OF ANALYSIS The standard method for study time-varying signals is the short-time Fourier transform (STFT), which utilizes short-time window h ( ). Then, the Fourier transform of this windowed signal is calculated to obtain energy distribution along the frequency direction at the time corresponding to the window [1], [5], and [7]:      h x w t STFT x ) ( ) , (  ( ) t     d e jw  (1) This method belongs to non-parametric liner transformation the crucial drawback of this method is that the length of the window is related to the frequency resolution. This inherent relationship between time and frequency resolution becomes more important when one is dealing with signals whose frequency content is changing rapidly. A time frequency characterization that would envelopment based on non-parametric, bilinear transformation. Performing the transformation brings two dimensional planes which represent the changes of frequency component, here called auto-terms (a-t).unfortunately, bilinear nature of discussed transformation manifests itself in existing of undesirable components, called cross-terms (c-t). Cross-terms are located between the auto-terms and have an oscillating nature. It makes interpretation of the distribution difficult. Development in discussed family was concentrated on selection of suitable kernel functions, which would