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- 1. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623 PI & Fuzzy Logic Based Controllers STATCOM for Grid Connected Wind Generator N. Sreekanth1, N. Pavan Kumar Reddy2 1 Asst. Professor (SL), 2 Asst. Professor Dept. of EEE, Sree Vidyanikethan Engineering College, Tirupathi, Andhra Pradesh, IndiaAbstract- When integrated to the power system, because of mismatch of their characteristics gridlarge wind farms pose stability and control performance and stability is affected. Therefore,issues. A thorough study is needed to identify transmission system operators (TSO) were forced tothe potential problems and to develop impose New requirements for the connection ofmeasures to mitigate them. Although Wind Turbine Generators to the power network.integration of high levels of wind power into an This way, TSOs try to that all regulatory actions,existing transmission system does not require a which are needed for maintaining grid stability,major redesign, it necessitates additional control are still performed on a satisfactory level, whenand compensating equipment to enable recovery renewable energy is introduced into the system. Onfrom severe system disturbances. In this paper the other hand certain devices like Flexible ACthe STATCOM control scheme for the grid Transmission Systems (FACTS) were developedconnected wind energy generation system for in order to dynamically control and enhancepower quality improvement is simulated using power system performance. Stability is the keyMATLAB/SIMULINK in power systems block aspect for introducing FACTS devices. Thereforeset. Fuzzy based controller is designed to one of the present day concerns is employment ofimprove the source current in STATCOM. A FACTS devices for enhancing wind farmmarked reduction in the Total Harmonic performance with respect to the grid codes andDistortion is observed in source current of Wind power system stability.Power Generation System (WPGS) with theincorporation of Fuzzy controller. In this paper, we are presenting the work carried out in designing the Fuzzy logic controllerIndex Terms- Fuzzy Logic controller (FLC), Static for switching operation of STATCOM, a member ofSynchronous Compensator (STATCOM), Total FACTS family. A simple control strategy ofHarmonic Distortion (THD), Wind Power STATCOM is adopted where the measurementGeneration System (WPGS). of rms current at the source terminal is needed. Then the performance of conventional PII. INTRODUCTION controller and fuzzy logic controller (FLC) are Growing concern for limited fossil fuels investigated. Simulation results show that Totalreserves and CO2 emission reduction stimulated Harmonic Distortion (THD) in source current isdevelopment of the renewable energy sector. drastically reduced fuzzy controller is included inEspecially, wind energy sector experienced huge the STATCOM control circuit. Simulation work hasthrust in recent years due to clean and been done using MATLAB/SIMULINK software.economical energy generation. As an example,in European Union, in 2008 one third of the II. Static Synchronous Compensator23.85GW power generation was from wind turbine (Statcom)generators (WTG). As induction generator, which The STATCOM is a shunt-connectedis the major source of reactive power, is connected reactive-power compensation device that is capablewith a wind turbine to generate electricity, the of generating and/ or absorbing reactive powercompensation of reactive power is necessary in and in which the output can be varied to controlorder to maintain rated voltage in the network. the specific parameters of an electric power system.Integration of wind energy into power systems It is in general a solid-state switching converteron such a large scale is not straightforward. capable of generating or absorbing independentlyPower system and its operation was concerned controllable real and reactive power at its outputwith conventional power plants where terminals when it is fed from an energy sourcesynchronous generators were directly coupled to or energy-storage device at its input terminals.the grid. Wind power plants have different Specifically, the STATCOM, which is acharacteristics from the conventional ones. Thus, voltage-source converter which when fed from a 617 | P a g e
- 2. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623given input of dc voltage, produces a set of 3-phase Compensator (SVC). The maximumac-output voltages, each in phase with and compensation current in a STATCOM iscoupled to the corresponding ac system independent of the system voltage. A STATCOMvoltage through a relatively small reactance (which provides dynamic voltage control and poweris provided by either an interface reactor or the oscillation damping and improves the system‘sleakage inductance of a coupling transformer). The transient stability. By controlling the phase angle,dc voltage is provided by an energy-storage the flow of current between the converter and the accapacitor. system are controlled. A STATCOM was chosen as a source for reactive power support because A STATCOM can improve power-system it has the ability to continuously vary itsPerformance like: susceptance while reacting fast and providing1. The dynamic voltage control in voltage support at a local node. Fig. 1 shows thetransmission and distribution systems, block diagram of the STATCOM controller.2. The power-oscillation damping in power-transmission systems,3. The transient stability;4. The voltage flicker control; and5. The control of not only reactive power but also(if needed) active power in the connected line,requiring a dc energy source. Furthermore, a STATCOM does thefollowing:1. It occupies a small footprint, for it replaces Fig. 1 Basic STATCOM control schemepassive banks of circuit elements by compactelectronic converters; A STATCOM injects almost a2. It offers modular, factory-built equipment, sinusoidal current Io of variable magnitude at athereby reducing site work and Commissioning point of connection. The injected current is almosttime; and in quadrature with the line voltage V, thereby3. It uses encapsulated electronic converters, emulating an inductive or a capacitivethereby minimizing its environmental impact. reactance at the point of connection with the transmission line. The functionality of the A STATCOM is analogous to an ideal STATCOM model is verified by regulating thesynchronous machine, which generates a reactive current flow through it this is useful tobalanced set of three sinusoidal voltages at the generate or absorb reactive power for regulatingfundamental frequency with controllable the line voltage of the bus where theamplitude and phase angle. This ideal STATCOM is connected.machine has no inertia, is practicallyinstantaneous, does not significantly alter the Similarly when the system voltage isexisting system impedance, and can internally higher than the converter voltage, the system ―sees‖generate reactive (both an inductive reactance connected at its terminal.Capacitive and inductive) power. Hence, the STATCOM ―sees‖ the system as the capacitive reactance and the STATCOM isControl Scheme: operating in an inductive mode. The current flows The STATCOM is a static var generator from the ac system to the STATCOM, resulting inwhose output can be varied so as to maintain or the device absorbing reactive power. For ancontrol certain specific parameters of the electric inductive operation the current lags the ac voltagepower system. The STATCOM is a power by an angle of 90 degrees, by assuming thatelectronic component that can be applied to the converter losses are neglected.dynamic control of the reactive power and the gridvoltage. The reactive output power of thecompensator is varied to control the voltage atgiven transmission network terminals, thusmaintaining the desired power flows duringpossible system disturbances and contingencies. STATCOMs have the ability to addresstransient events at a faster rate and with betterperformance at lower voltages than a Static Var 618 | P a g e
- 3. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623 power electronics, as a converter is connected to the Fig. 2 A STATCOM operated in inductive and stator winding and another converter is connected to capacitive modes. the grid. Thus the total power delivered by the wind power is transmitted by an HVDC link. If the amplitude of the STATCOMoutput voltage and the ac system voltage are In this paper, the configuration of windequal, the reactive current is zero and the generator is based on constant speed topologies withSTATCOM does not generate or absorb reactive pitch control turbine. The induction generator ispower. Since the STATCOM is generating or used in the proposed scheme because of itsabsorbing only reactive power the output voltage of simplicity, it does not require a separate field circuit,the converter, and the ac system voltage V are in it can accept constant and variable loads, and hasphase when neglecting circuit losses. The ac natural protection against short circuit. The availablecurrent magnitude can be calculated by using the power of wind energy system is given by thefollowing equation equation, Pwind = ρA (3) = (1) Where ρ (kg/m3) is the air density, A (m2)By assuming current flows from converter to the ac is the area swept out by turbine blade, is thesystem. X is represented as the coupling transformer wind speed in m/s. It is not possible to extract allleakage reactance. The corresponding reactive kinetic energy of wind, thus it extracts a fraction ofpower exchanged can be expressed as power in wind, called power coefficient Cp of the wind turbine, and is given by the following equation Q= (2) = (4) where Cp is the power coefficient, dependsWhere angle α is the angle between the ac system on type and operating condition of wind turbine.bus voltage and the converter output voltage. This coefficient can be expressed as a function of tip speed ratio and pitch angle. The mechanical powerIii. Wind Energy Generating System produced by wind turbine is given by the following The working principle of the wind equationturbine includes the following conversionprocesses: the rotor extracts the kinetic energyfrom the wind creating generator torque and =π (5)the generator converts this torque into electricity andfeeds it into the grid. Presently there are three main Iv. Test System Modelturbine types available. They are The STATCOM based current control voltage source inverter injects the current into the Squirrel-cage induction generator grid in such a way that the harmonics are reduced in Doubly fed induction generator. source current and their phase-angle with respect to Direct-drive synchronous generator. source voltage has a desired value. The injected current will cancel out the reactive part and The first one which is the simplest and harmonic part of the load and induction generatoroldest system consists of a conventional directly current, thus it improves the power factor and thegrid-coupled squirrel cage induction generator. The power quality. To accomplish these goals, the gridslip, and the resultant rotor speed of the Generator voltages are sensed and are synchronized invaries with the amount of power generated . generating the current command for the inverter.The rotor speed variation is small, approximately The proposed grid connected system is implemented1% to 2%, and hence this is normally referred to as for power quality improvement at point of commona constant speed turbine. The other two generating coupling (PCC), as shown in Fig. 3.systems are variable –speed systems. In the doublyfed induction generator, a back to back voltagesource converter feeds the three phase rotorwinding, resulting that the mechanical andelectrical rotor frequency are decoupled and theelectrical stator and rotor frequency can matchindependently of the mechanical rotor speed. Inthe direct-drive synchronous generator, thegenerator is completely decoupled from the grid by 619 | P a g e
- 4. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623. controller‖. The controller uses a hysteresis current controlled technique. Using such technique, the controller keeps the control system variable between boundaries of hysteresis area and gives correct switching signals for STATCOM operation. The control system scheme for generating the switching signals to the STATCOM is shown in Fig. 4 and Fig. 5. The control algorithm needs the measurements of several variables such as three- phase source current, DC voltage, inverter current with the help of sensor. The current control block receives an input of reference current and actual current are subtracted so as to activate the operation of STATCOM in current control mode. Fig. 3 Grid connected system for power quality In three-phase balance system, the RMS improvement voltage source amplitude is calculated at the sampling frequency from the source phase voltageV. Modelling Of Statcom (Vsa ,Vsb ,Vsc) and is expressed, as sample template(a) Using conventional PI Voltage Regulator Vsm, sampled peak voltage, as in (6). 1/2 Vsm= { (6) The STATCOM control block diagram is The in-phase unit vectors are obtained fromshown in Fig. 4. The voltage regulator is of AC source—phase voltage and the RMS value ofproportional plus integral type. unit vector , , as shown in (7). = ; = ; = (7) The in-phase generated reference currents are derived using in-phase unit voltage template as, in (8) =I. ; =I. ; =I. (8) Where I is proportional to magnitude of filtered source voltage for respective phases. This ensures that the source current is controlled to be Fig. 4 STATCOM model with PI – Voltage sinusoidal. The unit vectors implement the Regulator block diagram important function in the grid connection for the synchronization for STATCOM. This method is(b) Using FLC Voltage Regulator with FLC Bang- simple, robust and favorable as compared with otherBang controller methods. FLC voltage regulator is fed by one inputthat is voltage error (Ve). This gives the Bang-Bang Current Controller:appropriate Reactive source current (Is), which is Bang-Bang current controller isrequired to the system. It is shown in Fig. 5. implemented in the current control scheme. The reference current is generated as in (5) and actual current are detected by current sensors and are subtracted for obtaining a current error for a hysteresis based bang-bang controller. Thus the ON/OFF switching signals for IGBT of STATCOM are derived from hysteresis controller. The switching function for phase ‗a‘ is expressed as equation (9). < ( - HB) =0Fig. 5 STATCOM model with FLC- Voltage > ( - HB) =1 (9)Regulator block diagram Where HB is a hysteresis current-band, similarly theVI. Control Circuit switching function can be derived for phases ―b‖ The control scheme approach is based on and ―c‖.injecting the currents into the grid using ―bang-bang The control scheme is simulated using 620 | P a g e
- 5. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623SIMULINK in power system block set. The system This paper focuses on fuzzy logic controlparameter for given system is given Table I. The based on mamdanis system. This system has foursystem performance under dynamic condition is main parts. First, using input membershipalso presented. functions, inputs are fuzzified, then based on rule bases and inference system, outputs are produced TABLE I and finally the fuzzy outputs are defuzzified and SYSTEM PARAMETERS applied to the main control system. Error ofS. Parameters Ratings inputs from is chosen as input.N Fig. 6 shows input and outputo membership functions. To avoid miscalculations1. Grid Voltage 3-Phase, 415V,50Hz due to fluctuations in wind speed and the effects of2. Induction 3.5KVA, 415V,50Hz,P=4, noise on data, triangular membership functions motor/generator Speed=1440rpm,Rr=0.1Ω, are chosen to have smooth and constant region Rs=0.15Ω,Ls=Lr=0.06H in the main points.3. Line series 0.05mH Inductance4. Inverter DC Link Parameters Voltage=800V,DC Link Capacitance=100µF, Switching Frequency=2kHz5. IGBT rating Collector Voltage=1000V, Forward Current=50A,Gate Input membership function Voltage=20V,Power Dissipation=310w6. Load Parameter Non-Linear Load=25kwVii. Fuzzy Control Fuzzy logic controller, approaching thehuman reasoning that makes use of the tolerance,uncertainty, imprecision and fuzziness in thedecision-making process, manages to offer a verysatisfactory performance, without the need of a Output membership functiondetailed mathematical model of the system, just byincorporating the expert‘s knowledge into fuzzy Fig. 6 Input and output membership functions ofrules. In addition, it has inherent abilities to FLC voltage regulatordeal with imprecise or noisy data; thus, it isable to extend its control capability even to VIII Simulation Resultsthose operating conditions where linear control The wind energy generating system istechniques fail (i.e., large parameter connected with grid having the nonlinear load. It isvariations). observed that the source current on the grid is affected due to the effects of nonlinear load and FLC voltage regulator is fed by one wind generator, thus purity of waveform may be lostinput that is voltage error (Ve). The rules for the on both sides in the system. The three phase injectedproposed FLC voltage controller are: current into the grid from STATCOM will canceli) If ‗Ve‘ is ‗ENVVH‘ Then ‗I‘ is ‗INVVH‘ out the distortion caused by the non-linear load andii) If ‗Ve‘ is ‗ENVH‘ Then ‗I‘ is ‗INVH‘ wind generator. Fig. 7 shows the source currentiii) If ‗Ve‘ is ‗ENH‘ Then ‗I‘ is ‗INH‘ waveform of the test system without STATCOMiv) If ‗Ve‘ is ‗ENM‘ Then ‗I‘ is ‗INM‘ and the Fig. 8 shows the corresponding FFT analysisv) If ‗Ve‘ is ‗ENL‘ Then ‗I‘ is ‗INL‘ waveform. From FFT analysis, it is observed thatvi) If ‗Ve‘ is ‗EZ‘ Then ‗I‘ is ‗IZ‘ the Total Harmonic Distortion (THD) of the sourcevii) If ‗Ve‘ is ‗EPL‘ Then ‗I‘ is ‗IPL‘ current waveform of the test system withoutviii) If ‗Ve‘ is ‗EPM‘ Then ‗I‘ is ‗IPM‘ STATCOM is 26.92%.ix) If ‗Ve‘ is ‗EPH‘ Then ‗I‘ is ‗IPH‘x) If ‗Ve‘ is ‗EPVH‘ Then ‗I‘ is ‗IPVH‘xi) If ‗Ve‘ is ‗EPVVH‘ Then ‗I‘ is ‗IPVVH‘ 621 | P a g e
- 6. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623 Fig. 11 Source current waveform of the test system Fig. 7 Source current waveform of the test system with FLC based STATCOM (without STATCOM) Fig. 12 FFT analysis of source current waveform of Fig. 8 FFT analysis of source current waveform of the test system with FLC based STATCOM the test system (without STATCOM) Fig. 9 shows the source current waveform of the test system with PI controller based STATCOM and the Fig. 10 shows the corresponding FFT analysis waveform. From FFT analysis, it is observed that the THD of the source current waveform of the test system with PI controller based STATCOM is 2.92 %. Fig. 11 shows the source current waveform of the test system with FLC based STATCOM and the Fig. 12 shows the corresponding FFT analysis waveform. From FFT analysis, it is observed that Fig. 9 Source current waveform of the test system the THD of the source current waveform of the test with PI controller based STATCOM system with FLC based STATCOM is 1.42 %. Thus, it is observed that there is a further reduction in the THD value of the source current waveform. IX Conclusion In this paper fuzzy logic controller based STATCOM is presented for grid connected Wind Energy Generating System. The proposed FLC based STATCOM have improved the power quality of source current significantly by reducing the THD from 26.92% to1.42 %. It is clearly presented that STATCOM with FLC gives better performance than STATCOM with conventional PI controller.Fig. 10 FFT analysis source current waveform of thetest system with PI controller based STATCOM 622 | P a g e
- 7. N. Sreekanth, N. Pavan Kumar Reddy / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.617-623REFERENCES N. PAVAN KUMAR REDDY[1] Sharad W. Mohod, Mohan V. Aware,‖ A received B.Tech degree and M.Tech STATCOM-Control Scheme for Grid degree from JNTU Hyderabad. Connected Wind Energy System for Power Presently working as Assistant Quality Improvement, IEEE SYSTEMS Professor (SL) in Sree Vidyanikethan JOURNAL, VOL. 4, NO. 3, SEPTEMBER Engineering College, Tirupathi. 2010,[2] EWEA,‖ Wind Energy Statistics‖, 2008.[3] Claudio L.Souza et. al.,: "Power System Transient Stability Analysis Including Synchronous and Induction Generator", IEEE porto Power Tech Proceedings, Vol.2, pp.6, 2001.[4] I. Erlich, F. Shewarega, J.L. Rueda ―Impact of Large Offshore Wind Farms on Power System Transient Stability‖, PSCE, 2009.[5] Mrs. M. Mohammadha Hussaini & Dr. R. Anita, ―The Study of Dynamic Performance of Wind Farms with the Application of a STATCOM and its Effect‖ in International Journal of Recent Trends in Engineering, Vol 2, No. 6,November 2009, page no.158-160.[6] M.Mohammadha Hussaini, Dr. R. Anita, ― Dynamic Response of Wind Power Generators using STATCOM ― in International Journal of Engineering and Technology Vol.2 (4), 2010, 297-304.[7] H. Karimi-Davijani, A. Sheikholeslami, H. Livani and M. Karimi-Davijani, ―Fuzzy Logic Control of Doubly Fed Induction Generator Wind Turbine‖, in World Applied Sciences Journal 6 (4): 499- 508, 2009.[8] I. Prabhakar Reddy, B.V. Sanker Ram, ―STATCOM with FLC for Damping of Oscillations of Multi machine Power System‖ in Journal of Theoretical and Applied Information Technology. AUTHORS N. SREEKANTH received B.Tech degree and M.Tech degree from JNTU Hyderabad. Presently working as Assistant Professor (SL) in Sree Vidyanikethan Engineering College, Tirupathi, since 2006. 623 | P a g e

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