Wind energy conversion system based on Vienna rectifier with fuzzy logic control technique

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1 Wind energy conversion system based on Vienna rectifier with fuzzy logic control technique Meenakumari.S Department of Electrical and Electronics Engineering,Er.Perumal Manimekalai College of Engineering, Hosur, India Abstract In this project, a near-unity-power-factor front-end rectifier employing two current control methods, namely, average current control and hysteresis current control, is considered. This rectifier is interfaced with a fixed-pitch wind turbine driving a permanent-magnet synchronous generator. A traditional diode-bridge rectifier without any current control is used to compare the performance with the proposed converter. Two constant wind speed conditions and a varying wind speed profile are used to study the performance of this converter for a rated stand-alone load. The fuzzy logic control is used to maintain the constant voltage to the load. The parameters under study are the input power factor and total harmonic distortion of the input currents to the converter. The wind turbine generator power electronic converter is modeled in MATLAB and the simulation results verify the efficacy of the system in delivering satisfactory performance for the conditions discussed. The efficacy of the control techniques is validated with a MATLAB Simulink. Keywords- PMSG, Vienna rectifier, Hysteresis controller, Fuzzy logic controller, Pi controller. I. INTRODUCTION Wind is a highly stochastic energy source. There is also a strong interdependence between the aerodynamic characteristics of the wind turbine, the generator s rotor speed, and the amount of power that can be extracted from the wind. Hence, it becomes necessary to implement a control method that will enable the extraction of the maximum power from the system under all possible operating conditions. The following sections highlight the major components of a stand-alone WECS. Airflows can be used to run wind turbines. Modern utility-scale wind turbines range from around 600 kw to 5 MW of rated power, although turbines with rated output of MW have become the most common for commercial use; the power available from the wind is a function of the cube of the wind speed, so as wind speed increases, power output increases dramatically up to the maximum output for the particular turbine. Areas where winds are stronger and more constant, such as offshore and high altitude sites are preferred locations for wind farms. typical capacity factors are 20-40%, with values at the upper end of the range in particularly favorable sites. Globally, the long-term technical potential of wind energy is believed to be five times total current global energy production, or 40 times current electricity demand, assuming all practical barriers needed were overcome. This would require wind turbines to be installed over large areas, particularly in areas of higher wind resources, such as offshore. As offshore wind speeds average ~90% greater than that of land, so offshore resources can contribute substantially more energy than land stationed turbines. Front-End Power Converter In most existing small-scale wind turbine systems, the preferred choice for a front-end converter is a diode-bridge rectifier because of its inherent simplicity. However, due to their nonlinear nature, diode-bridge rectifiers inject harmonic components into the system, leading to an increased total harmonic distortion (THD) of input current, which is expressed as increased losses due to heating, malfunction of equipment, and reduced overall efficiency of the All rights Reserved 106

2 Fig 1. Front end power converter Along with input current THD, the PMSG operating power factor is another parameter of interest. If the PMSG operates at a lagging power factor, it leads to production of reactive power at the stator terminals, thereby reducing the maximum usable (real) power that the generator is capable of producing. Hence, it is desirable that the front-end converter is capable of maintaining high power factor and good quality currents, i.e. show good current control ability. Figure 2, Bidirectional assembly The converter employed in this work, based on a topology suggested by Mehl and Barbi, is shown in Figure. The selection of line inductance values (La, Lb, and Lc) plays a critical role in maintaining unity-power-factor (UPF) operation and is defined as the sum of source inductance and the transformer leakage reactance. In a transformer-less system, the value of line inductance is based on the PMSG stator inductance alone. It employs a diode-bridge rectifier with three currentcontrolled static bidirectional switches. If the bidirectional switch connected to a particular phase All rights Reserved 107

3 turned on, the corresponding phase is connected to the voltage central point, causing a rise of the associated phase current. Turning off the switch leads to conduction of the associated diode in the upper or lower half bridge (depending on the direction of current flow) and, therefore, to a reduction of the phase current. Thus, the switching of the bidirectional switches enables us to achieve the possibility of a sinusoidal line current control (improved power factor). Maswood and Fangrui proposed a number of modifications to this topology in, where actual variations in load level on the rectifier were taken into the dc link has two identical capacitors with a mid-point connection to the current control circuit, to maintain the voltage balance. The bidirectional switches are typically assembled using four diodes and an IGBT, as shown in Figure. 2. The voltage stress across the bidirectional switches in this topology is clamped to half of the dc output voltage by the dc-link capacitors, without the need for additional clamping circuits. This provides a significant advantage over a single-switch boost topology, which suffers from the problem of high-voltage and highcurrent stresses across the single switch. II. VIENNA RECTIFIER BASED WECS In this project, a WECS interfaced with a UPF converter feeding a stand-alone load has been investigated. The use of simple bidirectional switches in the three-phase converter results in near- UPF operation. Two current control methods, i.e., ACC and HCC, have been employed to perform active input line current shaping, and their performances have been compared for different wind speed conditions. Fig 3. Proposed block All rights Reserved 108

4 The quality of the line currents at the input of the converter is good, and the harmonic distortions are within the prescribed limits according to the IEEE 519 standard for a stand-alone system. A high power factor is achieved at the input of the converter, and the voltage maintained at the dc bus link shows excellent voltage balance. The proposed method yields better performance compared to a traditional uncontrolled diode bridge rectifier system typically employed in wind systems as the front-end converter. Finally, a laboratory prototype of the UPF converter driving a stand-alone load has been developed, and the ACC and HCC current control methods have been tested for comparison. The HCC current control technique was found to be superior and has better voltage balancing ability. It can thus be an excellent front-end converter in a WECS for stand-alone loads or grid connection. In the elaboration of the research, a harmonic analysis of source current distortion has been carried out. It has featured a nonlinear full-bridge diode rectifier with R-L load as a harmonic currents source. The time domain simulation is performed using MATLAB/Simulink simulation package. Basically the implementation of the control strategy will be done in three steps. In the first step, the required load current and source voltage signals are measured to know the exact information about the system studied. In the second step, by using instantaneous p-q theory the reference compensating currents are obtained. In the third step, by using hysteresis-based current control technique the required gating signals for the solid-state devices are generated. The performance of the Shunt Active Filter for mitigation of current harmonics in the source current was analyzed with the different combinations of Fixed, Adaptive Hysteresis and Fuzzy-adaptive hysteresis based current control techniques and PI, Fuzzy-Logic controller techniques for closed loop control of DC link capacitor voltage to get the reference current templates. The Vienna rectifier has three switches, and by choosing their (ON\OFF) state considering the polarity of the phase current in each phase, the voltage for each phase will be determined. So, the phase voltage is depending on the direction of phase current and switch position State of the switch (ON/OFF) and the polarity of the line current in each phase determine the rectifier pole voltages (VAM, VBM, VCM) at any instant of operation. In order to discuss operation principles of the rectifier, here Phase A is explained. Phase B and C have the same behavior. If the line current is positive, and the switch Ta is off, the current flows through diode D11, and the voltage between the converter pole A and the DC bus midpoint M (i.e. VAM) is DC/2. The conduction path for this case is illustrated in Figure 2(a). If the polarity of the line current is positive, and the switch Ta is on, the voltage VAM is 0. III. HYSTERESIS CURRENT CONTROL Hysteresis current control (HCC) method is used due to its better performance in obtaining a sinusoidal input current. Its advantages are no need of compensation ramp and low distorted input current waveforms. According to this control technique, the switch is turned on when the inductor current goes below the lower reference (IV,ref)and is turned off when the inductor current goes above the upper reference (IP,ref,) giving rise to a variable frequency control. The proposed control technique is adapted to the hysteresis current control technique. The model of the system is derived and simulated by Matlab/Simulink Program. In this method, the input current is made to switch within a reference current window called the hysteresis band. The controlled switch is turned `ON' when the inductor current becomes equal to the lower hysteresis limit. It is turned `OFF' when the inductor current becomes equal to the upper hysteresis limit. The switching frequency of the converter will not be a constant because the slope of the inductor current is deferent during each instant of the fundamental cycle. Hysteresis controllers have the advantage of simple implementation. The control in which two sinusoidal current references IP,ref, IV,ref are generated, one for the peak and the other for the valley of the inductor current. According to this control technique, the switch is turned on when the inductor current goes below the lower reference IV,ref and is turned off when the inductor current goes above the upper reference IP,ref, giving rise to a variable frequency control. Also with this control technique the converter works in All rights Reserved 109

5 IV. SIMULATION RESULTS Fig.4 Proposed system Matlab All rights Reserved 110

6 Fig. 5 Proposed system PMSG output voltage and current Fig.6 Proposed system load current Fig.7 Constant DC link voltage using fuzzy logic All rights Reserved 111

7 Fig.8 Load current waveform V. CONCLUSIONS In this project, a WECS interfaced with a UPF converter feeding a stand-alone load has been investigated. The use of simple bidirectional switches in the three-phase converter results in near- UPF operation. Two current control methods, i.e., ACC and HCC, have been employed to perform active input line current shaping, and their performances have been compared for different wind speed conditions. The HCC current control technique was found to be superior and has better voltage balancing ability. It can thus be an excellent front-end converter in a WECS for stand-alone loads or grid connection. The fuzzy logic controller makes constant voltage to the load. REFERENCES [1] C. E. A. Silva, D. S. Oliveira, L. H. S. C. Barreto, and R. P. T. Bascope, A novel three-phase rectifier with high power factor for wind energy conversion systems, in Proc. COBEP, Bonito-Mato Grosso do Sul, Brazil, 2009, pp [2] Online. Available: [3] M. Druga, C. Nichita, G. Barakat, B. Dakyo, and E. Ceanga, A peak power tracking wind system operating with a controlled load structure for stand-alone applications, in Proc. 13th EPE, 2009, pp [4] S. Kim, P. Enjeti, D. Rendusara, and I. J. Pitel, A new method to improve THD and reduce harmonics generated by a three phase diode rectifier type utility interface, in Conf. Rec. IEEE IAS Annu. Meeting, 1994, vol. 2, pp [5] A. I. Maswood and L. Fangrui, A novel unity power factor input stage for AC drive application, IEEE Trans. Power Electron., vol. 20, no. 4,pp , Jul [6] I. Maswood, A. K. Yusop, and M. A. Rahman, A novel suppressedlink rectifier inverter topology with near unity power factor, IEEE Trans. Power Electron., vol. 17, no. 5, pp , Sep [7] G. M. Masters, Renewable and Efficient Electric Power Systems. Hoboken, NJ, USA: Wiley, [8] V. Sheeja, P. Jayaprakash, B. Singh, and R. Uma, Stand alone wind power generating system employing permanent magnet synchronous generator, in Proc. IEEE ICSET, 2008, pp [9] M. Singh and A. Chandra, Control of PMSG based variable speed windbattery hybrid system in an isolated network, in Proc. IEEE PES, 2009, pp [10] J. G. Slootweg, S.W. H. de Haan, H. Polinder, andw. L. Kling, Modeling wind turbines in power system dynamics simulations, in Proc. IEEE Power Eng. Soc. Summer Meet., 2001, vol. 1, pp [10] P. C. Krause, O. Wasynczuk, and S. D. Sudhoff, Analysis of Electric Machinery. Piscataway, NJ, USA: IEEE Press, [11] N. Srighakollapu and P. S. Sensarma, Sensorless maximum power point tracking control in wind energy generation using permanent magnet synchronous generator, in Proc. 34th Annu. IEEE IECON, 2008, pp All rights Reserved 112

8 [12] N. A. Ahmed and M. Miyatake, A stand-alone hybrid generation system combining solar photovoltaic and wind turbine with simple maximum power point tracking control, in Proc. CES/IEEE 5th IPEMC, 2006, pp. [13] T. Tafticht, K. Agbossou, A. Cheriti, and M. L. Doumbia, Output power maximization of a permanent magnet synchronous generator based standalone wind turbine, in Proc. IEEE Int. Symp. Ind. Electron., Jul. 9 13, 2006, pp S.Meenakumari received the B.E. Degree in Electrical and Electronics Engineering from Golden Valley Institute of Technology, Bangalore University, India in 1992 and Post graduation in Power Electronics & Drives in VMKV Engineering College, Salem under Vinayaka Missions University, Chennai, India in Now he is working as a Assistant Professor in Er. Perumal Manimekalai College of Engineering, All rights Reserved 113