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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

WIND ENERGY CONVERSION SYSTEM WITH ADUAL INVERTER BASED ULTRA-CAPACITOR

DIRECT INTEGRATION SCHEME

K Raju1* and S Mallikarjunaiah1

*Corresponding Author: K Raju, rajueps712@gmail.com

Interfacing converters utilized in connecting energy storage systems like ultra capacitors andbattery banks to wind generation systems introduce additional cost and power losses. Thispaper thus presents an immediate integration theme for ultra capacitors utilized in mitigatingshort-run power fluctuations in wind generation systems. This theme uses a dual electricalconverter topology for each grid affiliation and interfacing a ultra capacitor bank. The most electricalconverter of the dual electrical converter system is hopped-up by the corrected output of a windturbine-coupled permanent-magnet synchronous generator. The auxiliary electrical converter isdirectly connected to the ultra capacitor bank. With this associate interfacing converter is notneeded, continuity of load maintenance is possible and there are not any associated cost andpower losses incurred. The performance of the proposed system is evaluated in MATLAB/SIMULINK software. Finally, the Simulation results shows that the short-run fluctuations issuppressed and posses the desired characteristics in wind energy systems.

Keywords: Direct integration of ultra-capacitors, Dual inverter, Energy storage interfacing,Non-integer voltage ratio

INTRODUCTIONGlobal warming, one of the most criticalenvironmental problems arising in the world,must be taken into consideration. In an attemptto prevent the realization of these fears, weknow that utilization of renewable energy hasbecome very useful in the emissions ofgreenhouse effect gases (CO2, NOx, SOx,

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Int. J. Elec&Electr.Eng&Telecoms. 2014

1 Department of EEE, Chadalawada Ramanamma Engineering College, Tirupati, AP, India.

etc.). However, the generated power is alwaysfluctuated due to an environmental status. Inorder to promote renewable energy andcompensate the fluctuating power, though anenergy storage system is effective, one mustresolve a number of issues, including thoserelated to eff iciency, economy andenvironment. In recent years, Short-Term

Research Paper

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

power exchange exploitation ultra-capacitorsis actively pursued in wind generation systemsas evident from the massive variety oftechniques according within the literature.These techniques may be divided into 2 majorclasses, looking on the means that ultra-capacitors square measure connected to thewind generation system. within the initial class,the ultra-capacitors square measureconnected to the intermediate dc link of theconsecutive convertor system as shown inFigures 1a and 1b. Despite the fact that thedirect affiliation shown in Figure 1a is that thesimplest, it might be troublesome to urge themost use of the ultra-capacitor bank thanks todc-link voltage limitations obligatory by thegrid-side electrical converter. The effects of thisissue will otherwise be reduced if associate

degree intermediate dc–dc convertor is placedbetween the ultra capacitor and also the dclink as shown in Figure 1bs (Abedini andNasiri, 2008; and Allegre et al., 2009). Thisdc–dc converter has to possess two-waypower flow capability and therefore needs aminimum of 2 quick change devices rated tothe peak power. Moreover, the Low-Pass Filter(LPF), comprising an inductor and a capacitor,degrades the dynamic response (Jin et al.,2010). Therefore, the interfacing dc–dcconverter increases the system cost, powerlosses, and complexity. In the second category,the common ac bus is used for powerexchange, as shown in Figure 1c, and itrequires an additional dc–dc converter, a dc–ac inverter, and a coupling transformer (Weiand Joos, 2007). Therefore, in both cases,

Figure 1: (a) Ultra Capacitor Directly Connected to the dc Link, (b) Connection to the dcLink Through a dc–dc Converter, (c) Connection to the Common ac Bus, (d) Proposed

Grid-Side Inverter with Direct Connection

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

these additional converters essentiallyincrease the overall cost and power losses,which would be absent if a direct integrationscheme with full controllability is available.

This paper therefore presents a directintegration scheme for ultra capacitors withthe use of the grid-side inverter. Theproposed inverter system is shown in Figure1d, and it is formed by cascading two two-level inverters through a couplingtransformer. The two inverters are named asthe main inverter and the auxiliary inverter.The ultra capacitor bank is direct lyconnected across the dc link of the auxiliaryinverter. The dynamic behavior of the ultracapacitor voltage is handled by the invertercontroller, eliminating the need for anadditional boost converter. The main inverteris a high-power low-speed inverter whichoperates at fundamental f requency,producing square wave outputs. Harmonicsproduced by the square wave output are

compensated by the low-power high-speedauxiliary inverter. As the high-power maininverter operates at the fundamentalfrequency, it can be constructed usingdevices like Gate Turn-Off thyristors (GTOs),Emitter Turn-Off thyristors (ETOs), orIntegrated Gate-Commutated Thyristors(IGCTs). On the other hand, the auxiliaryinverter can be constructed using morecommonly available devices like InsulatedGate Bipolar Transistors (IGBTs). Thisparticular power and frequency splittingarrangement helps to reduce power lossesin the inverter system (Corzine et al., 2004;and Lu and Corzine, 2007).

ANALYSIS ON ULTRACAPACITOR CHARGINGANDDISCHARGING PROCESSIn the proposed system, the dc-link voltagesof both the main inverter and the auxiliaryinverter are allowed to vary independently anddynamically.

Figure 1 (Cont.)

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

As a result, the space vector diagram of thecombined inverter takes different shapes atdifferent dc-link voltage ratios as shown inFigure 2. Here in Figure 2, hexagons formedby the voltage vectors of the main inverter arenamed as main hexagons while the hexagonsformed by auxiliary inverter vectors are namedas sub hexagons. This vector representationexplains the operations of the main inverter andauxiliary inverter.

Figure 2: Space Vector Diagramsat Different dc-Link Voltage Ratios

Note: (a)-(e) Varying Vdc while Vdcx is constant. (f)-(i) VaryingVdcx while Vdc is constant.

Figure 3: (a) Output Voltage Vas, and(b) Average Super Capacitor Current Idcx

for Diagrams Shown in Figure 2(a)–(e)

Figure 4: (a) Output Voltage Vas, and(b) Fundamental Component of the

Output Voltage Vasf

Figure 5: Per-Phase Equivalent Circuitof the Dual Inverter

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

POWER SHARING MPPT ANDGENERATOR-SIDECONTROLLERThe output voltage vector Vr is combination ofmain inverter voltage and auxiliary invertervoltage. The following mathematical equations

AMr vvv ...(1)

ivP rgrid .23 ...(2)

ivvP AMgrid ).(23

...(3)

3

2

33

2

2132 j

M

j

MMdcM eSeSSVv ...(4)

3

2

33

2

2132 j

A

j

AAdcxA eSeSSVv ...(5)

ieSeSSVvPj

M

j

MMdcrA .32

23 3

2

33

2

21

...(6)

gives the power, voltage equations of the grid,main inverter and auxiliary inverter. Theequivalent circuit of the proposed system isshown in Figure 5. The circuit describes theoutput voltage vector (vr), main inverter voltagevector (VM), auxiliary inverter voltage vector(VA), and the current vector (i). The outputvoltage vector is combined output of the maininverter voltage vector and the auxiliary invertervoltage vector as in Equation (1). The realpower delivered to the load can be expressedas the dot product in Equation (2).Furthermore, the output power is equivalent tothe sum of the main inverter power and theauxiliary inverter power as expressed inEquations (3), (4) and (5) show therelationships between inverter voltage vectorsand corresponding switching states. With thehelp of these five equations, an expression canbe derived for the power of the auxiliaryinverter, i.e., the supercapacitor power, as inEquation (6) where VM, PM, and SiM (i = 1, 2,3) represent the main inverter voltage, power,

Figure 6: Control Circuit of Proposed System

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

and switching function while those of theauxiliary inverter are given by vr, PM, and SiA(i= 1, 2, 3), respectively. vr represents the outputvoltage, and Pgrid represents the real power flowto the grid.

According to (6), it can be deduced that,for a given output power, supercapacitorpower varies with the main inverter dc-linkvoltage Vdc. Therefore, super capacitor powercan be controlled by controlling the maininverter dc-link voltage. Furthermore,according to (3), for a given output power, themain inverter power solely depends on theauxiliary inverter power.

Therefore, the maximum power point of thewind turbine can indirectly be tracked bychanging the main inverter dc-link voltage. Theusual practice is to maintain this voltage at aconstant level with the help of a controlledrectifier or a boost rectifier placed betweenthe main inverter and the Wind TurbineGenerator (WTG) system. However, in theproposed system, the boost rectifier is usedto vary the main inverter dc-link voltage andthus indirectly track the maximum power pointof the WTG. The controller block diagram forthis indirect Maximum Power Point Tracking(MPPT) is shown in Figure 6. In this controller,the measured wind speed and the parametersof the turbine model are used to derive apower reference for the generator-sideconverter. The actual generator power iscompared with the reference, and the error isfed into a Proportional-Integral (PI) controllerwhich generates a voltage reference for theboost rectifier. This voltage reference isnormalized to produce the modulation indexfor the boost rectifier.

DESIGN OF THE GRID-SIDEINVERTERIn the proposed system, the main inverter isoperated in the six-step mode, which movesslowly from one vector to the next. Thisgenerates square wave outputs from the maininverter which get smoothened by the auxiliaryinverter. Owing to this low-frequencyoperation, the main inverter switching lossesget reduced (Corzine et al., 2004; and Lu andCorzine, 2007). The auxiliary inverter, whichacts as an active filter, is operated at highswitching frequency by utilizing a SpaceVector Modulation (SVM) method. Therefore,the overall modulation is a combination of thesix-step mode and SVM (Jayasinghe et al.,2011). The grid-side inverter controlleremploys an inner current controller and anouter power controller as shown in Figure 6.The grid voltage and current injected into thegrid are converted into the synchronousreference frame. The direct component of theinverter output current id controls the activepower exchange with the grid while thequadrature component iq controls the reactivepower. Therefore, to generate a reference forthe direct current component, theinstantaneous active power of the generator-side converter is passed through an LPF(Muyeen et al., 2009). The quadrature currentreference I*q is set to zero to keep the powerfactor at the grid connection point at unity.These active and reactive current references(I*d and I*q) are then compared with actualcurrent components, and the errors arepassed through PI controllers to producevoltage references v*d and v*q, respectively.Equations (7) and (8) are then used tocalculate the amplitude and angle of the

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

reference voltage vector. In Equation (8), isthe initial phase angle of the reference vector.

qdM vvA22 ** ...(7)

d

qM v

v*

*1tan ...(8)

DESIGN OF ULTRACAPACITOR ANDGENERATOR-SIDECONTROLLERThe main cause of power fluctuations is thechange of wind speed. Therefore, the capacityof the energy storage system is also a functionof the wind speed variation. In order to analyzethis relationship, the wind is modelled as thesum of a dc quantity and a series of harmonicsas in Equation (9). In the following simulation,the wind speed fluctuation is assumed to be20% of the mean value as in Equation (10).The power captured from the wind can beexpressed as in Equation (11).

)sin()( twVvtv iwiwow ...(9)

tT

vtv wow2sin2.01)( ...(10)

)(5.0)( 3 tvACtP wp ...(11)

where Vw is the instantaneous wind speed, Vw0

is the mean wind speed, Vwi is the harmonicamplitude, i is the angular frequency (f = 1/T= /2 = 0.1 <“ 10 Hz), is the density of air, Ais the swept area of wind turbine blades, andCp is the coefficient of power conversion.Power f luctuations caused by theaforementioned wind speed change have tobe compensated by the energy storagesystem.

dttPtPE grid

t

t

edischsc ))()((2

1

arg, ...(12)

)(

2

,22

,

arg,

LscHsc

edischscsc VV

EC

...(13)

Therefore, the required capacity of thesupercapacitor bank is determined accordingto where Esc,discharge is the amount of energytaken out from the supercapacitor during thedischarging period, t1 and t2 are the startingtime and end time of the discharging period,respectively, and Vsc,H and Vsc,L are the startingand end values of the super capacitor voltage.Super capacitors behave as resistors at highfrequencies (typically beyond few tens of hertz).Therefore, the proposed system requires anelectrolytic capacitor to assistsupercapacitors at high frequencies.

SIMULATION RESULTSThe proposed direct integration scheme hasbeen tested using computer simulations onMATLAB/SIMULINK digital simulationplatform. The schematic diagram of thesimulation setup is shown in Figure 1d. Thewind speed profile shown in Figure 7a with anaverage speed of 10 m/s, 20% harmonicamplitude, and 1-Hz harmonic frequency isused to test the performance of the proposedsystem and control strategy. The proposedsystem is evaluated in matlab Simulinksoftware using the Table 1 parameter values.

The corresponding variations of generatoroutput is shown in Figure 7a. The input powershows a large variation ranging to 200 kW. Theaverage value of the output power is 200 kWwith a small variation of kW. A simplecalculation would reveal that the corresponding

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

Fundamental Frequency 50 Hz

Resistance of generator 0.2 ohm

Inductance of generator 1 mH

Capacitance of dc link capacitor C1, C2 = 2.2 mF

Capacitance of ultra capacitor 60 mF

Rated voltage of the ultra capacitorbank Vdcx = 1000 v

Grid voltage 400 V

Rated power of the generator Ppmsg = 100 kw

Wind turbine Cut-in speed 4 m/s

Rated speed of the wind turbine 12 m/s

Turns ratio of the couplingTransformer 1:1

Table 1: System Parametersfor Simulation

Figure 7a: Generator Output

Figure 7b: Wind Profile

Figure 7c: DC Rectifier Output

Figure 7d: Boost Converter Input/Output

Figure 7e: Capacitor Voltage

Figure 7f: Capacitor Current (Charging/Discharging)

input power fluctuation is about 66% while theoutput power fluctuation is less than 8.4%. Thisproves the efficiency of the proposed systemin mitigating power fluctuations caused by

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Int. J. Elec&Electr.Eng&Telecoms. 2014 K Raju and S Mallikarjunaiah, 2014

wind changes. As mentioned in section, themain inverter dc-link voltage is changed tocontrol the ultra capacitor power and thus toindirectly track the MPPT of the WTG. Thecorresponding main inverter dc-link voltageand the supercapacitor voltage variations areshown in Figure 7e. According to this diagram,the main inverter dc-link voltage shows a largevariation ranging from 400 to 1200 V which isfar below the upper limit of modern IGCTdevices, and therefore, the proposed systemis feasible. Similarly, the supercapacitorvoltage varies with a maximum of Vsc, H = 800V and a minimum of Vsc, L = 400 V. Therefore,according to (12) and (13), the requiredminimum capacitance of the supercapacitorbank is found to be 43.75 mF which is arealistic value. The boost converter input andoutputs are shown in Figure 7d. the maininverter dc-link current is nearly constant dueto the six-step operation of the main inverter.However, the supercapacitor current varieswith the main inverter dc-link voltage owing tothe pulse width modulation (PWM) operationof the auxiliary inverter. The change of themodulation index of the generator-sideconverter (boost rectif ier) for theaforementioned wind speed profile is shownin Figure 7b. The corresponding generator

Figure 7g: Gridside Inverter Output currents are shown in Figure 7a. Similarly, thevariations of the amplitude and power angleof the grid-side inverter output voltage areshown in Figures 7c and 7d, respectively.These two values are nearly constant owing tothe smooth power delivery to the grid.Consequently, the d-q-axis currents of the gridside inverter are nearly constant as shown inFigure 7e. And the capacitor current chargingand discharging is shown in Figure 7f. Thecorresponding three phase currents of the grid-side inverter and their zoomed-in view areshown in Figure 7g.

CONCLUSIONThe direct integration of energy storagedevices has a number of advantages such asreduction in power losses, cost, andcomplexity. Therefore, in this paper, thepopular dual inverter topology was customizedto connect a supercapacitor bank directly intothe dc link of the auxiliary inverter. The operationof the proposed system was discussed indetail. Simulation and experimental resultswere presented to verify the efficiency of theproposed system in suppressing short-termwind power fluctuations.

REFERENCES1. Abbey C and Joos G (2007),

“Supercapacitor Energy Storage for WindEnergy Applications”, IEEE Trans. Ind.Appl., Vol. 43, No. 3, pp. 769-776.

2. Abedini and Nasiri A (2008),“Applications of Super Capacitors forPMSG Wind Turbine Power Smoothing”,in Proc. IEEE Ind. Electron. Conf.,November, pp. 3347-3351.

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3. Allegre L, Bouscayrol A and Trigui R(2009), “Influence of Control Strategies onBattery/Supercapacitor Hybrid EnergyStorage Systems for TractionApplications”, in Proc. IEEE Veh. PowerPropulsion Conf., Septeber, pp. 213-220.

4. Bingchang N and Sourkounis C (2011),“Energy Yield and Power Fluctuation ofDifferent Control Methods for WindEnergy Converters”, IEEE Trans. Ind.Appl., Vol. 47, No. 3, pp. 1480-1486.

5. Corzine K A, Wielebsk W, Peng F Z andWang J (2004), “Control of CascadedMultilevel Inverters”, IEEE Trans. PowerElectron., Vol. 19, No. 3, pp. 732-738.

6. Guidi G, Undeland T M and Hori Y (2007),“An Optimized Converter for Battery-Supercapacitor Interface”, in Proc. IEEEPower Electron. Spec. Conf., June,pp. 2976-2981.

7. Jayasinghe S D G, Vilathgamuwa D Mand Madawala U K (2011), “DirectIntegration of Battery Energy StorageSystems in Distributed PowerGeneration”, IEEE Trans. EnergyConvers., Vol. 26, No. 2, pp. 677-685.

8. Jin K, Yang M, Ruan X-B and Xu M (2010),“Three-Level Bidirectional Converter forFuel-Cell/Battery Hybrid Power System”,IEEE Trans. Ind. Electron., Vol. 57,No. 6, pp. 1976-1986.

9. Kinjo T, Senjyu T, Urasaki N and Fujita H(2006), “Output Leveling of RenewableEnergy by Electric Double-LayerCapacitor Applied for Energy StorageSystem”, IEEE Trans. Energy Convers.,Vol. 21, No. 1, pp. 221-227.

10. Li Wei, Joos G and Belanger J (2010),“Real-Time Simulation of a Wind TurbineGenerator Coupled with a BatterySupercapacitor Energy Storage System”,IEEE Trans. Ind. Electron., Vol. 57,No. 4, pp. 1137-1145.

11. Lu M S, Chang C L, Lee W J and Wang L(2009), “Combining the Wind PowerGeneration System with Energy StorageEquipment”, IEEE Trans. Ind. Appl.,Vol. 45, No. 6, pp. 2109-2115.

12. Lu S and Corzine K A (2007), “AdvancedControl and Analysis of CascadedMultilevel Converters Based on P-QCompensation”, IEEE Trans. PowerElectron., Vol. 22, No. 4, pp. 1242-1252.

13. Muyeen S M, Takahashi R, Murata T andTamura J (2009), “Integration of an EnergyCapacitor System with a Variable-SpeedWind Generator”, IEEE Trans. EnergyConvers., Vol. 24, No. 3, pp. 740-749.

14. Naswali E, Alexander C, Han H Y, NaviauxD, Bistrika A, Jouanne A, Yokochi A andBrekken T K A (2011), “SupercapacitorEnergy Storage for Wind EnergyIntegration”, in Proc. ECCE, Sep. 17-21,pp. 298-305.

15. Qu L and Qiao W (2011), “ConstantPower Control of DFIG Wind Turbineswith Supercapacitor Energy Storage”,IEEE Trans. Ind. Appl., Vol. 47, No. 1,pp. 359-367.

16. Wei L and Joos G (2007), “Comparisonof Energy Storage System Technologiesand Configurations in a Wind Farm”, inProc. IEEE Power Electron. Spec. Conf.,June, pp. 1280-1285.