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MPPT CONTROL FOR WECS C.Manoj Kumar 1,N.Beaulah 2 Student 1,Assistant Professor 2 1, 2 Department of Electrical and Electronics Engineering, RVS Padhmavathy College Of Engineering & Technology, Chennai [email protected] [email protected] Abstract- A PMSG based variable speed WECS for rectifier topology using MPPT consisting of two three phase diode bridges and three thyristors is proposed in the paper for variable-speed high-power permanent-magnet synchronous generator (PMSG) wind energy conversion systems WECSs). The proposed rectifier has several prominent features such as low cost, low power loss, and simple control. Its ability to cascade the input voltages allows it to properly regulate generator speed even when the wind velocity drops to half of the rated value. Consequently Maximum power-point-tracking algorithms can be applied to optimize power capture in a wide range of wind velocities. The operating principle of the rectifier is elaborated. Its use and control in the WECS is presented. The converter and control are verified by simulation and experimental results. Index Terms—Maximum power-point tracking (MPPT), permanent-magnet synchronous generator (PMSG), rectifier, wind-energy-conversion system (WECS).Impedance source inverter (ZSI) I. INTRODUCTION Wind turbine usage as sources of energy has increased significantly in the world. With growing application of wind energy conversion systems (WECSs), various technologies are developed for them. With numerous advantages, permanent-magnet synchronous generator (PMSG) generation system represents an important trend in development of wind power applications. Extracting maximum power from wind and feeding the grid with highquality electricity are two main objectives or WECSs. To realize these objectives, the ac–dc–ac converter is one of the best topology for WECS. Variable speed operation yields 20 to 30 percent more energy than the fixed speed operation, providing benefits in reducing power fluctuations and improving variable supply. Falling prices of the power electronics have made the variable speed technology more economical and common. Such a wind turbine system as other types of dispersed generation is mostly connected to distribution feeders and the generation system cannot be easily connected to the electric power network without conducting comprehensive evaluations of control performance and load impacts. This requires a reliable tool for simulating and assessing dynamics of a load connected Variable speed wind turbine. The purpose of the work is to provide the capability of design, modelling, simulating and analyzing the dynamic performance of a variable speed wind energy conversion system using MATLAB / SIMULINK. The modelled system includes a fixed-pitch type wind blades, a direct-drive Permanent Magnet Synchronous Generator without a gear-box, and a controllable power electronics system, which consists of a two-three phase rectifier and three phase inverter. Models of the elements and the system control scheme are proposed in the form of mathematical equations and graphical control blocks and implemented in MATLAB/SIMULINK. The simulation results demonstrate the modelling work provide a reliable and useful simulation tool for evaluating the dynamic performance of a variable speed wind turbine integrated into load. 1. 2-3 Phase Rectifier Topology PMSG based variable speed WECS for rectifier topology using MPPT consisting of two three phase diode bridges and three thyristors is proposed in the paper for variablespeed high-power permanent-magnet synchronous generator (PMSG) wind energy conversion systems (WECSs). The proposed rectifier has several prominent features such as low cost, low power loss, and simple control. Its ability to cascade the input voltages allows it to properly regulate generator speed even when the wind velocity drops to half of the rated value. Consequently Maximum power-point-tracking algorithms can be applied to optimize power capture in a wide range of wind velocities. The operating principle of the rectifier is elaborated. Its use and control in the WECS is presented. The converter and control are verified by simulation and experimental results. 2. Wind Energy Conversion System The impedance source inverter has been reported recently as a competitive alternative to existing inverter topologies with many inherent advantages such as voltage boost. This inverter facilitates voltage boost capability with the turning ON of both switches in the same inverter phase leg (shootthrough state). system where a PWM-VSI is employed as the grid-side converter, the rectifier provides boosted dc voltage when the wind speed is lower than rated,such that the PWMVSI’s voltage rating can be well-matched with the generators while its current rating does not need to be resized Operating principle of the rectifier and its use in the WECS will be explained in detail in the following sections. II. MODES OF OPERATION Fig.2. Two-Three phase rectifier topology A. Current Paths and Output Voltages Of The Rectifier The proposed topology is shown in Fig.1 with this topology; boost converter is omitted without any change in the objectives of WECS. Moreover, reliability of the system is greatly improved 3. Two-Three Phase Rectifier Topology Fig.2 shows the basic structure of the proposed rectifier topology. The main circuit is made up of two three-phase diode rectifier bridges and three individual thyristors .Each diode ridge is fed by a three-phase power supply and their outputs are connected in parallel. The three thyristors are used to connect the corresponding input phases of the two diode bridges. Fig.3(a) Current paths in Section I. (a) Thyristors off. Fig.3 (a), the two diode bridges have separate current paths and maintain their own rectification when the thyristors are off. Assuming that we have an inductive dc load that behaves like an ideal current source, the rectifier’s output voltage vd is equal to the input line-to-line voltage vab. However if the thyristors (T2 ) connecting the minimum voltage phase in the upper supply (phase b) and the maximum voltage phase in the lower supply (phase y) is turned on at any time within Section I, the two diode bridges are forced into a series connection through the thyristors. The dc voltage vd then becomes the sum of vab and vyx, equalling to 2vab in magnitude. Fig.2 Simplified circuit diagram of the two-three phase rectifier topology In normal operation, when the thyristors are off, the rectifier’s dc output voltage is equal to the output of a single diode bridge; when the thyristors are controlled and turned on at the right instants ,the outputs of the two diode bridges can be cascaded and the resultant dc voltage is doubled. For the rectifier to operate properly, the two threephase input power supplies should be equal in magnitude and have 180◦phase displacement with each other. This constraint guarantees that doubled output voltage is obtained when the thyristors are turned on . The voltage-doubling feature of the proposed rectifier adapts it to be used in variable-speed WECSs to interface the generators .Either a PMSG with dual stator windings or two PMSGs,each having one set of three-phase winding, can be used as the ac power supplies for the rectifier. In a Fig.3 (b). Current paths in Section I. (b) Thyristors on. Fig.3 (b) The other five sections have situations similar to Section I and are not described further for the sake of brevity. The illustration of all the current paths in all sections can be found. III. MPPT CONTROL METHODS FOR PMSG BASED WECS Permanent Magnet Synchronous Generator is favoured more and more in developing new designs because of higher efficiency, high power density, availability of highenergy permanent magnet material at reasonable price, and possibility of smaller turbine diameter in direct drive applications. Presently, a lot of research efforts are directed towards designing of WECS which is reliable, having low wear and tear, compact, efficient, having low noise and maintenance cost; such a WECS is realisable in the form of a direct drive PMSG wind energy conversion system. There are three commonly used configurations for WECS with these machines for converting variable voltage and variable frequency power to a fixed frequency and fixed voltage power. The power electronics converter configurations most commonly used for PMSG WECS are shown in Fig.4 (a),(b),(c) Depending upon the power electronics converter configuration used with a particular PMSG WECS a suitable MPPT controller is developed for its control. All the three methods of MPPT control algorithm are found to be in use for the control of PMSG WECS. IV. IMPEDANCE SOURCE INVERTER (ZSI) Several types of power electronics interfaces have been investigated. In this study, system is interfaced with a six diode rectifier and three phase Impedance source inverter which is less expensive than others and commonly put into industrial use, has been modelled for AC-DC–AC conversion a rectifier model and inverter model. The six diodes rectifier converts ac power generated by the wind generator into dc power in an uncontrollable way and it is given to Impedance source inverter it convert into ac and inverter output is given Voltage grid system. The ZSI is a voltage harmonic source in the point view of ac system and a harmonic filter need be placed appropriately to reduce the voltage harmonics it generates. AL-C harmonic filter consisting of a series interconnection inductor and a parallel capacitor is located at the ZSI terminal. Fig.5.shows a rectifier and ZSI system model that has been implemented in Matlab/Simulink. (a) Fig.5 Two-Three Rectifier and inverter model. (b) (c) The six diodes rectifier converts ac power generated by the wind generator into dc power in an uncontrollable way and so control has to be implemented by the power electronics inverter. Current-controlled ZSI can generate an AC current which follows a desired reference waveform so can transfer the captured real power along with controllable reactive power. For the modelling study, DQ control method that is widely used for ZSI current control is employed. Variables in the ABC three phase coordinates may be transformed into those in the d-q reference frame rotating at synchronous speed by the rotational d-q transformation matrix. In the three-phase balanced system, the instantaneous active and reactive power outputs, P and Q, of the wind turbine are described by equation, 3 3 2 2 𝑃 = (𝑉𝐷 𝐼𝐷 + 𝑉𝑄 𝐼𝑄 ), 𝑄 = (𝑉𝐷 𝐼𝑄 − 𝑉𝑄 𝐼𝐷 ) Fig.4 PMSG wind energy conversion systems with MPPT Where, VD = d-axis voltage at the wind turbine VQ = q-axis voltage at the wind turbine ID = d-axis current at the wind turbine IQ = q-axis current at the wind turbine. Here, VQ is identical to the magnitude of the instantaneous voltage at the wind generation system and VD is zero in the rotating d-q coordinates, by equation 3 3 2 2 𝑃 = |𝑉𝑜 |𝐼𝑄 ,𝑄 = |𝑉𝑜 |𝐼𝐷 ---(1) where |VO| is the instantaneous voltage magnitude of the wind turbine system. Since the voltage remains at a level of the grid AC voltage and the voltage variation is very small compared to changes in the magnitude of IQ and ID, P and Q are mainly subject to the d-axis current and q-axis current respectively. DQ control decouples real and reactive components and enables real power and reactive power to be separately controlled by specifying the respective reference values of P ref and Q ref for the both power outputs and independently adjusting the magnitude of the d-axis current IQ and that of the q-axis current ID. The reference values P ref and Q ref of the wind generation are specified by what ZSI’s control strategies are taken for real and reactive power output. The firing signals are generated by the sine pulse width modulation (SPWM) technique. The desired current vector IABC_REF and the actual output current vector IABC_WT of the wind system are compared and the error signal vector IERR is compared with a triangle waveform vector to create the switching signals. Where, CP max = the maximum power coefficient opt = value of where CP max = CP ( opt) = electrical loss in generator and inverter (I) Reactive Power Control Various control modes can be used for determining the amount of reactive compensation to provide. Possible control modes include power factor, current and voltage. Constant power factor mode and voltage regulation mode are implemented in this analysis. In constant power factor control (PFC) mode, the reference value of the reactive power of the wind turbine, QREF, may be specified by equation (3). 𝑄𝑅𝐸𝐹 = 𝑃𝑅𝐸𝐹 √1 − 𝑃𝑓 2 /𝑃𝑓 --- (3) Where Pf is power factor and P ref is the reference value of real power output of the VSWT. In voltage regulation (VR) mode, reactive power compensation is controlled in such a manner that the voltage magnitude of the VSWT-connected bus being kept constant at a specified level. The reference magnitude of the voltage to be regulated must be set as the nominal voltage of the AC grid where the wind turbine is considered as being interconnected. Whether the mode controls constant power factor or voltage, the reactive power capability of a VSWT is limited. Such a limitation is required to be considered in the modelling study. The reactive capability limits of the wind turbine used in this study are determined by MVA rating of the inverter which may be described by equation (4) 2 2 𝑄𝑙𝑖𝑚𝑖𝑡𝑠 = ±√𝑆𝐼𝑁𝑉 − 𝑃𝐼𝑁𝑉 ----- (4) Where Q limits, PINV and SINV are the reactive power limits, the real power output and MVA rating of the inverter respectively. V. SIMULATION RESULT ANALYSIS Fig.6 Current control scheme of an Impedance source inverter (I)Capturing the Maximum Power from Wind The maximum aerodynamic power available from wind energy can be described by equation (1). This simply means that the maximum power may be achieved by varying the turbine speed with varying wind speed such that at all times it is on the track of the maximum power curve. One way of enabling the maximum power capture is to specify the reference value of real power for the inverter control as the available maximum power multiplied by the inverter efficiency, as shown in equation (2). 𝑃𝑀𝑀𝑎𝑥 = 1 2 𝑀𝑎𝑥 𝜋𝜌𝑅5 𝐶𝑃 3 /𝜆3𝑜𝑝𝑡 𝜔𝑀 ---------- (2) 𝑃𝑅𝐸𝐹 = ɳ𝑃𝑀𝑀𝑎𝑥 The rating capacity is chosen to be 1.5 MVA and real power 1.5 MW. The rated speed of the rotor is chosen to be 40 rpm. The rated wind speed is 8 m/s. the cut-in and cutout speeds are 4 m/s and 16 m/s respectively. The switching frequency of the grid interface inverter is 1.040 kHz. The capacitor value of grid interface rectifier is 2500uF and dc link voltage is 2.5kv. The generated voltage of Permanent Magnet Synchronous Generator is 0.6kv. The transformer rating of grid connected side is 2.5k/130kv. The p.u voltage magnitude of primary of the transformer is 0.99p.u. The maximum value of Cp is 1.2. The proposed system operate the unity power factor control Simulation waveform real power of step change of wind velocity from 8 to 11m/s in variable speed wind turbine. It shows that there is a light overshoot at variation of wind velocity Fig.7. is observed from the simulation waveform reactive power of step change of wind velocity from 8 to 11m/s in variable speed wind turbine. Fig.7.shows that simulation waveform pitch angle in variable speed wind turbine coupled Permanent Magnet Synchronous Generator at different velocity. Fig.9. Simulation waveform two-three phase rectifier step change of wind velocity from 8 to 11m/s in variable speed wind turbine coupled Permanent Magnet Synchronous Generator. Fig.7. Simulation waveform generated speed of step change of wind velocity from 8 to 11m/s in variable speed wind turbine coupled Permanent Magnet Synchronous Generator in 1.01p.u. Show that simulation waveform grid voltage in p.u of step change of wind velocity from 8 to 11m/s in variable speed wind turbine coupled Permanent Magnet Synchronous Generator. Maximum rate change = 2 degree Controller = PI controller Fig 7 PMSG Simulation Fig.11. is observed that Simulation waveform mechanical torque in p.u of step change of wind velocity from 8 to 11m/s in variable speed wind turbine coupled Permanent Magnet Synchronous Generator. Fig.12. Simulation waveform torque in p.u of step change of wind velocity from 8 to 11m/s in variable speed wind turbine coupled Permanent Magnet Synchronous Generator. Fig.4.6.Simulation waveform of Vf in p.u of step change of wind velocity from 8 to 11m/s in variable speed wind turbine coupled Permanent Magnet Synchronous Generator. Fig.7 Simulation waveform of boosted voltage in shoot through state occur in leg 1. Fig.13. Simulation waveform of boosted voltage in shoot through state occurs in leg 2. Fig.7.7. Simulation waveforms of boosted voltage in shoot through state occur in leg 3. Fig.Simulation output of speed vs time in m/s. Fig.14Simulation output of power vs time in m/s. Fig.14.Simulation output of power vs time inm/s..Simulation output o f rated power vs wind speed in m/s..Simulation output of real and reactive power in m/s. Fig 8.MPPT control using PMSG Simulation Model for 2-3phase Rectifier Topology SIMULATION PARAMETERS Rating capacity = 1.5MVA Real power = 1.5MW Rated speed = 40rpm Rated wind speed = 16 m/s Cut-in speed = 4 m/s Cut-out speed = 25 m/s DC link voltage = 1800 V Fig 9.2-3phase rectifier topology Maximum pitch angle = 45 degree Fig.14 Output Speed of VSI Fig 10 Dc output voltage from 2-3phase rectifier topology Fig. 15 Output torque of ZSI Fig 11.PMSG Voltage SIMULATION OF ZSI Fig 12. PMSG current Fig 16 Simulation of ZSI SIMULATION OF VSI Fig 13 VSI Output Voltage Fig 17 Output voltage of ZSI turbines in networks must ensure their systems meet the requirements for grid connection. Therefore, the work done in this study provides a reliable tool for evaluating the performance of variable speed wind turbines and their impacts on power networks in terms of dynamic behaviours as a preliminary analysis for their actual integrations and operations. REFERENCE 1.S. Bhowmik, R. Spee, and J. H. R. Enslin, “Performance optimization Fig 18 Output speed of ZSI for doubly fed wind power generation systems,” IEEE Trans. Ind. Appl.,vol. 35, no. 4, pp. 949–958, Jul./Aug. 1999. 2. Z. Chen and E. Spooner, “Voltage source inverters for highpower,variable-voltage DC power sources,” Inst. Electr. Eng Proc. Gener.,Transm. Distrib., vol. 148, no. 5, pp. 439–447, Sep. 2001. 3. H. Geng and G. Yang, “A novel control strategy of MPPT taking dynamics of wind turbine into account,” Proc. IEEE PESC, pp. 1–6, 2006. 4. K. E. Johnson, L. J. Fingersh, M. J. Balas, and L. Y. Pao, “Methods for increasing region 2 power capture on a variable-speed wind turbine,” J.Solar Energy Eng., vol. 126, p. 1092, 2004. Fig19.Output torque ZSI 5.S. Morimoto,H.Nakayama, M. Sanada, andY. Takeda, “Sensorless output maximization control for variable-speed wind generation system Comparison of ZSI &VSI using IPMSG,” IEEE Trans. Ind. Appl., vol. 41, no. 1, pp. 60–67, Jan./Feb.2005. ZSI VSI 6. J. D. S. Oliveira, M. M. Reis, C. E. A. Silva, L. H. S. Colado Barreto,F. Voltage =1800 Voltage =1500 Speed =1800 Speed = 1500 L. M. Antunes, and B. L. Soares, “A three-phase high-frequency semicontrolled rectifier for PM WECS,” in IEEE Trans. Power Electron.,no. 3. vol. 25, Mar. 2010, pp. 677–685. 7. W. Quincy and C. Liuchen, “An intelligent maximum power extraction Torque = 100 Torque =100 algorithm for inverter-based variable speed wind turbine systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1242–1249, Sep. 2004. Harmonics is reduced Harmonics is more 8. K. Tan and S. Islam, “Optimum control strategies in energy conversion of PMSG wind turbine system without mechanical sensors,” IEEE Trans.Energy Conv., vol. 19, no. 2, pp. 392–399, Jun. 2004. VI.CONCLUSION 9. D. Xu and Z. Luo, “A novel AC–DC converter for PMSG variable speed wind energy conversion systems,” in Proc. IEEE IPEMC, 2009, pp. A dynamic model of a variable speed wind generation with power electronic interface was proposed for computer software tool simulation study and implemented in Matlab/Simulink. Component models of a VSWT and its control scheme have been built by using Matlab function block and control block provided in the software. A wind model was integrated into the modelling to see the wind impact. Dynamic responses of the wind turbine to varying wind speeds and under different reactive control schemes were simulated and analyzed based on the modelled system. In the view point of electric utilities, grid interface of intermittent generation sources such as wind turbines has been a challenge that can cause lower power quality in power systems. So comprehensive impact studies are absolutely necessary before wind turbines being added to real networks. Also, users who intend to install wind 1117–1122. 10. A. Yazdani and R. Iravani, “A neutral-point clamped converter system for direct-drive variable-speed wind power unit,” IEEE Trans. Energy Conv.,vol. 21, no. 2, pp. 596–607, Jun. 2006 Authors Biography N.Beaulah received her B.E degree in Electrical & Electronics Engineering from Karpaga Vinayaga College of Engineering & Technology (Affiliated to Anna University-Chennai) and M.E (Power Electronics & Drives) from Sri Sai Ram Engineering College (Affiliated to Anna University),Chennai. At present she is working with RVS Padhmavathy College Of Engineering & Technology, Chennai as Assistant Professor. Her area of interest includes Renewable Energy and Power Electronics C.Manoj Kumar currently pursuing B.E degree in Electrical & Electronics Engineering from RVS Padhmavathy College Of Engineering & Technology (Affiliated to Anna University-Chennai) .His area of interest includes Power Electronics, Renewable Energy & Drives.