14 Maximum Power Point Tracker for Wind Turbines

8/13/2019 14 Maximum Power Point Tracker for Wind Turbines

1/6

2004 35th Annua l IEEE Power Electronics Specialisls Conference Aachen. Germany 20

Maximum Power Point Trackers for Wind Turbines

G.D. MoorUniversity of Stellenbosch,Dept. Electrical EngineeringStellenbosch 7600, So uthAfricaTel./ Fax +27 (0) 21 808 3220Email: [email protected]

AbslracI- Maximum power point tracking methods arepresented whereby the loading on the wind turbine iscontrolled to ensure that the maximum available energy fromthe wind is captured. The wind turbine system is modelled andused in simulations to evaluate two proposed maximum powerpoint trackers, named anemometer control and calculationcontrol for the purpose of this paper. A n additional analogsystem is also created whereby the complete wind turbinesystem can be simulated. A n inverter is used to replicate thegenerator and the loading is controlled using an activerectifier. The results from the simulations and analog systemare presented whereby the two trackers are shown to be closeto ideal. The appeal of the calculation method is i n theredundancy of an anemometer making it attractive to lessexpensive, small-scale systems.1. INTRODUCTION

Much attention has been paid in recent times to thegeneration of clean energy. These natural and cleansources of energy need to have no by-products associatedwith their operation [ I]. W ind energy is gaining momentumin this field of clean energ y due to its relatively low co st.In South Africa, more attention is being paid to small-scale wind turbines fo r areas where th e national grid supplyis too far away to utilize [2]. Small stand-alone systems arenow being used in remote areas as an alternative to solarpanels. This is due to the huge losses of panels as a result oftheft [3]. Wind turbines are unattractive to th e common thiefdue to their electrical complexity and the difficulty inconcealment during operation.

In order for small-scale wind turbines to be viable, themaximum available wind energy must be captured. Thecharacteristic of a wind turbine is such that there is amaximum available power that the rotor can produce fordifferent wind speeds. This maximum pow er point occurs atdifferent rotor speed s for different wind speeds. The way inwhich the rotor speed can be controlled, is by adjusting theelectrical loading on the wind turbine. A wind turbine has aunique maximum power for a particular wind speed, and anattempt to try and a cquire more power by deviating from theoptimal rotor speed will result in an inefficient transfer ofpower from the wind.The most common method of energy capture on smallscale wind turbine systems in operation is the use of passive

H.J. BeukesUniversity of Stellenbosch,Dept. Electrical EngineeringStellenbosch 7600, SouthAfricaTel./ Fax 27 (0) 21 808 2290Email: [email protected] of the A ll sign als from the wind turbine, to aDC bus. A diode bridge is connected directly between thewind turbine and DC bus, and thus generation to the bus isonly possible when the wind turbine is able to generate avoltage higher than the bus voltage. As the rotor speed isdirectly proportional to the voltage produced by the windturbine, the rotor spei:d is held constant over varying windspeeds by t he voltage (equal to the DC bus voltage). Th is isan inefficient method of energy capture, as the maximumavailable power will not be produced for all but one windspeed.

Power point trackers are algorithms that control theloading on a particular source so that the required powertransfer can be obtained. This can be achieved un der variousexternal load conditions by using energy storage for excessor shortage of power.

The large wind turbine system in the 100s of kilowattsrange, use pitch control of their blades to extract themaximum available power from th e wind [4][5]. Th e largestwind turbine systems also incorporate a regenerative slipcontrol system on their induction machines for a moreefficient system [4][6]. The additional price of thesemethods on small-scale systems far outweighs the costsavings and so a cheap alternative is required for thesecheaper wind turbiries. The two power point trackermethod s proposed in this paper require that the wind turbinecharacteristics are known prior to implementation. Theinitial method requires an anemometer t o measure the windspeed striking the roior. This is feasible with larger scaleturbines since the cost of the additional an emom eter is smallas compared to the overall system. A cheaper alternativeand improved method was created resulting in the removalof the anemometer. This method was found to be similar tomethod s proposed by[7], [8] and [9].

I I . MAXIMUMOWER OINT TRACKINGIn order for a wind turbine to harvest the maximum

amount of energy available from the wind at any giveninstant, the electrical loading on the genera tor needs to becorrect for that moment. If the load is too large or too smallfor a particular wind speed, the oper ating point of the windturbine deviates from optimal power point and the systemefficiency is lower.A method to determine the optimal loading on the

0-7803-8399-0/04/ 20.0002004 IEEE. 2044


2/6

2wQ 35rh Annual IEEE Power Electronics SpecialisfsConference Aac hen Germany, 2004

generator at any specific time is needed to be able to transferthe maximum amount of available energy from the wind.Recall that the maximum available power is determined:byithe wind speed and thus this method must be able toconstantly track the maximum available power throughoutwind speed variations.A. Anemometer Method

A method is proposed to make use of a predeterminedlook-up table or an equation describing the loading requiredto achieve the maximum power point.The maximum power point of a wind turbine is a functionof the wind speed. Thus the wind speed must be known at

any given instant to be able to calculate the optimal loadrequired for that particular wind speed in this proposedopen-loop-type system. The wind speed is measured usingan anemometer and the load value required for optimalefficiency is determined and applied.The optimal loading for various wind speeds needs to bepre-programmed into the control system. These optimalpoints can either be calculated from the expected powerprofiles or physically measured during preliminary practicaltesting of the wind turbine. Once the optimal loading forvarious wind speeds is determined, an equation describingthe relationship is deduced.During operation in the field, the wind speed is obtainedvia the anemometer and the optimal load corresponding tothat wind speed is calculated from the fitted equation (alook-up table can also be used). The loading is then adjustedfor maximum pow er transfer.

IYEmrr Rapion

R ot or S p e d rpn)Figure 1. I reduced power region

Due to the small derivation of power near the maximumavailable power points on the power profiles, there is arelatively large room for error in the algorithm's accuracywhere the power transfer efficiency of the system will not

represents the deviation region where the reduction inpower will be less than 1%. It can be seen i?om this f igurethat a large room for maximum power point tracking errorexists.B Calculation MethodAnemometers are relatively expensive instruments andtheir inclusion for power point tracking is not financiallyviable on small-scale wind turbines. A method is proposedwhereby the anemometer is removed and the wind speedsare calculated from the available electrical parameters at thegenerator's output terminals.

Due to the generator on the wind turbine being asynchronous machine, the electrical frequency produced bythe generator is directly proportional to the angularacceleration of the rotor. Thus, by measuring the electricalfrequency, the rotor speed can be calculated. The powerversus rotor speed profiles are unique for each wind speed.This means that for one particular rotor speed, there is aunique power that the rotor blades will deliver for eachwind speed.

Because the power delivered by the wind turbine ismeasurable and the electrical frequency can also bedetermined, it is possible to calculate the instantaneous windspeed responsible for the system's power generation at anytime. Because the power profiles are only available for afew of the wind speeds, interpolation is used to determinethe wind speeds where the measured power do es not exactlycorrespond to a given power profile.Once the wind speed is known, the anemometer controlmethod described in section A is used to calculate theoptimal load. This method effectively removes the need foran anemometer.

111. SIMULATIONTUDYThe wind turbine system was modelled in SimplorePusing the blade profiles obtained from the manufacturer o fthe actual rotor blades and the electrical characteristics of

the electrical machine chosen for this system. Figure 2represents the model used.

Figure 2. Block diagram of a wind hlrbinee great& affected. In other words, if the algorithm isslightly inaccurate in the optimal loading calculation, thereduction in the power produced is minimal ~i~~~ 1 The following equation, which describes the relationship

2045


3/6

2 4 ?5thAnnua l IEEE Power Electronics SiJecial ists Conference

of the torques on an electrical machine, was used tocalculate the output frequency and thus the voltageamplitude at the inverters output (the generator under test isa synchronous machine).

J is the complete machines inertia, p is the number ofpoles in the machine, T is the electromagnetic torque, T sthe torque developed by the rotor blades and me is theelectrical rotor speed. Th e electro magn etic torqu e iscalculated by dividing the electrical power entering theinverter by the rotor speed (proportional to electricalfrequency) at that instant. The rotor blades torque profilesare obtained by dividing the blades power profiles by therotor speed. The rotor blades torque at any instant is thenobtained from these profiles where the current rotor andwind speed is also required.In order for the modelling of the generator to besimplified, the electrical char acteristic s describ ed in th ecommon abc plane, were transformed to the dq0 rotatingreference frame.The loading on the system was modelled as a variableresistor over which the two maximum power point trackershad control.A. Wind Step Simularions

too0

800

zoo

Figure 3 Comparison of anemometer and calculation control methodsFigure 3 shows that the calculation method, where the

wind speed is calculated from the system parameters, has animproved response time when compared to the anemometermethod, where the w ind speed is measured using an externalanemometer. In order to explain this, the systems must beanalysed from an instantaneous perspective. The effect ofinertia prohibits the mechanical system from changinginstantaneously. Time is needed for the rotor speed toincrease or decrease to the new maximum power pointsrespective rotor speed. The anemometer method is not aseffective, as it attempts to load the wind turbine at the

~

2 46

Aochen. Germany. 2004

optimal load for the new wind speed, but this new loading isnot optimal for the transition to the new operating point.The calculation method is more in tune with the actualsystem and loads the system in accordance with the windspeed that the system is experiencing at that particularmoment.Th e results of the anemometer method could b e improvedby using a running merage of the measured wind speedvalue from the anemclmeter. This would low-pass filter theanemo meter measurement; artificially introducing inertiainto the control loop.B. Random Wind Sir.rulations

Th e absence of real, high resolution wind speed data hasmeant that the dynamics of wind cou ld not b e investigatedfor this paper. A random wind profile was generated froman image of measured wind speed over 180 s in [IO]. Thesame random wind profile was then used on the differentalgorithms to ascertain which algorithm performed best.

Figure 4. Simulated result for power supplied vs. reference power foranemometer methodFigure 4is the tract: of the response from th e anemometermethod. It can be seen that the output power follows themaximum available power reference quite accurately. This

ideal maximum available power reference trace is obtainedfrom instantaneous evaluation of the wind speed and themaximum available power deliverable by the wind turbine,and thus do es not take the effect of inertia into account.


4/6

2004 35th Annual IEEE Pow er Electronic s Specialists Conference Aachen Gemany, 2004

2500 - . :4 0.

OD 2 4 6 i o 100 120 1 i o 160 1 8T i n e (6)Figure 5 Smulated result for power supplied v s reference power forcalculation method

The result of the calculation method for the same randomwind speed is shown in Figure 5 The result looks verysimilar to the anemometer method but comparison of thetwo responses shows them to be slightly different. This canbe contributed to the reasoning given in Section A above.

Z i1:00 20 40 60 80 100 120 140 160 180Tine 5)

Figure 6. Calculated wind speed vs. real wind speedFigure 6 represents the values calculated by thecalculation method against the actual wind speed that wasbeing inputted to the simulator. It can be seen that thecalculated wind speed is very similar to the actual windspeed.

IV. ANALCX SYSTEMThe algorithms proposed and computer simulated inSimplorerTMwere then tested on an analog model in orderfor the system that was simulated to be verified. This systemwas modelled around the actual wind turbine and itprovided a realistic model of w hat w as to be expected froma wind turbine and rectifier unit. The generation process,ko m the different wind sp eeds striking the blades to the DCcurrent produced, was simulated. Figure 7below shows the

setup used. The analog model that was used consisted oftwo sections- a wind turbine simulator (inverter) sectio n anda loading regulator (ac tive rectifier) section.

Wind SpeedReferenceI

Power Point Tracker

Inverter Active RectifierFigure 7 Analog system setup

A . Wind turbine simulatorAn inverter, which switches the inputted DC to a 3-phaseoutput, was used to electrically simulate the wind turbine sothat it provided the exact electrical characteristics at its

output, as was to be expected at th e terminals of the windturbine. The onboard DSP was pre-programmed with thewind turbines power profiles, which is used to calculate theresponse to loading and wind. This meant that the inverterwas programmed to react to the loading on itself andrespond to different wind speed s provided to it, as the windturbine would in the field.

The wind speed was predetermined and loaded into alook-up table in the DSP. Th e electrical loading on the windturbine was obtained by measuring the DC voltage andcurrent into the inverter. This was d one as measurement ofthe power flow out of the inverters three phases is costlyand complex, but it also meant that the switching losseswere not accounted for.B Loading regulator

The converter is the system by which the loading can beregulated. Th is converter was operated as an active rectifierthat switches the 3-phase waveforms inputted, to a DCoutput. The waveforms inputted to the active rectifier areevaluated and the present power point tracking method isused to alter the power flow to the load.The electrical power delivered by the wind turbine andthe rotational speed of the blades is calculated k o m themeasurement of the input current and voltage to theconverter. These results are used in the algorithms o f thetwo max imum power point trackers.In the case of the anemometer method, the wind speedwas fed to the converters DSP by means of acommunication cable from the inverters DSP. This in effectacted as an anemometer measurement for the algorithm. The

2 47


5/6

2004 35th Annual I EEE Power Electronics Specialisis Conference Aachen. German y 2004

calculation method used the electrical information acquiredto perform it's algorithm as explained in Section 1I.B.

V. EXPERIMENTALESULTSThe result from the anemometer method is shown inFigure 8 below. The instantaneous result of the theoreticalmaximum available power is superimposed onto themeasured result to show the similarities.

Tine s)Figure 8. Experimental result far power supplied VS. reference power for

The result of the calculation method using the sam e windspeed profile is shown in Figure 9below. Once again, thepower produced by the system is very similar to themaximum available power except for the slight offset,which can be attributed mostly to switching losses. Theeffects of th e losses in the switches in the inverter and theactive rectifier in the practical system were not accountedfor.

anemometer method

T h e s)Figure 9.Experimental result for power supplied vs reference power forcalculation methodThe anemometer and calculation methods are compared

in Figure IO. As in the simulations, the results of the twomethod s are very similar.

6M1-400-200

o 20 40 Go i o i b o i i o i i o i b o oT h e 8)Figure IO Cornparism ofpower from anemometer and calculation

Figure I I shows a comparison between the wind speedgenerated by the SF and the wind speed calculated in thecalculation method. Th e pink trace displays the actual windspeed generated by the DSP and the green trace representsthe calculated wind ;peed. As can be seen in the figure,there appears to be a constant offset between t he tw results.Te

methods

. . .. . . . . . . I:

I I Sep 20031 34841Figurr: 1 1 . Wind speed comparisons

This can be accounted to slight inaccuracies in themeasurement system an dlor slightly incorrect modelling ofthe wind turbine power profiles. Field systems couldincorporate a calibration system in order to reduce apossib le offset.The offset error between the actual and calculated windspeeds was adjusted in the calculation method procedureand the analog system was re-run. The power produced w asincreased due to this ;adjustment and th e results o f all testedmethod s are tabulated in Table 1below.

2048


6/6

2004 35th Annua l IEEE Pow er Electro nics Specialists Conference Aachen Germany. 2 w 4

TABLE 1AVERAGEOMR PRODUCTION N HE ANALOG SYSTEM

Adjusted calculation 805.81

VI. CONCLUSIONMaximum power point trackers are m ethods whereby theelectrical loading on the system is optimized to ensure a

maximum power transfer from the available wind energy.Two methods were presented in this paper wherebymaximum power point tracking for small-scale windturbines can be achieved.An anem ometer. method was discussed. The presen twind speed is measured using an anemometer, and apredetermined optimal loading equation is used to adjust theloading accordingly.

A variation on the anemometer method entitled thecalculation method was proposed, where the wind speed iscalculated from the present system parameters. Theanemometer method is then used to calculate the optimalloading for that calculated wind speed. The attraction of thismethod is that it removes the need for an expensive externalanemometer, but the disadvantage is that it introduces moreopportunity for error in the system measurements. Initialsetup of the system parameters is needed in the wind speedcalculation and error here results in a permanentinefficiency. This disadvantage is not of great concern as thenature of the rotors power profiles allows for a broadregion of inaccuracy where the reduction of output power isminimal.

The results from the analog system confirmed the resultsobtained in the computer simulations, though a slightlyinferior result of the calculation method confirmed thatincorrect modelling of the wind turbine system would resultin a slightly lower efficiency. An adjustment made in thecalculated wind speed improved the average output powermeasured and it was once again superior to the anemometermethod. This is under exact modelling of the system and soa slightly less efficient system can be expected in practice.The anemometer and calculation methods have shown t oproduce an output power close to the maximum availablepower. This leaves very little opportunity for improvementand so it can be assumed from the simulated results, thatthese methods oftrackin g are very good.

ACKNOWLEDGMENTThe authors would like to thank Eskom for the funding ofthis project and Telkom SA Ltd. for the personal support ofthe principal author.

REFERENCESThe Potential of Wind Energy to Reduce CO missions, A vailable:hnp://~w.garradhassan.wukRenewable Energyfor Rural Electrification in the Emtern CapAvailable:hnp://www.ganadhassa.ca.ukM. Rycroft, Wind Energy as an Alternalive to Solar Power in aSouth African Telecommunications Netw ork Eskom Standby PowerSyslem Conference, 2002.The Danish Wind Industry Association, Available:hnp://www.windpower.argE. Muljadi, C.P. Bunerfield, Pitch-Controlled Variable-Speed WindTurbine Generation, IEEE ronsoctionson ndusnyApplicationsVol. 37, No. I , Januaryffebruary 2001 pp. 240 246.S Bhowmik, R. Spee, J.H.R. Enslin, Performance optimization fordoubly fed wind pow er generation systems, IEEE Tronsmrlonr onI ndushy Applications Vol. 35, No. 4 JulyIAugust 1999, pp. 949-958.T. Thiringer, J. Lind en, Control by Variable Rotor Speed of a Fixed-Pitch Wind Turbine Operating in a Wide Speed Range,lEEETmnractions on Energy Conversion Vol. 8,No. 3, pp. 520-526,September 1993.T. Nakamura, S. Morimoto, M. Sanada, Y. akeda, OptimumControl of IPMSG for Wind Generation System, PCC Osaka, April2002.A. Miller. E. Muliadi. A Variable Soeed Wind Turbine PowerControl IEEE Trmsactions on Energy Conversion Vol. 12, No. 2,June 1997,pp. 181-186.1101 B.S. Borowy,Z.M. Salameh, Dynam ic Response of a Stand-AloneWind Energy Conversion System with Banery Energy Storage t o aWind Gust IEEE Trmsoctions on E m r g y Conversion Val. 12, No,1 March 1997, pp. 73-78.

2049
http://hnp//www.ganadhassa.ca.ukhttp://hnp//www.ganadhassa.ca.ukhttp://hnp//www.ganadhassa.ca.uk

Documents

14 Maximum Power Point Tracker for Wind Turbines