A Unity Power Factor, Maximum Power PointTracking Battery Charger for Low Power Wind

TurbinesGustavo Gamboa, John Elmes, Christopher Hamilton, Jonathan Baker, Michael Pepper, and Issa Batarseh

School of Electrical andComputer Engineering

University of Central FloridaOrlando, Florida 32816

Email: john.elmes@knights.ucf.edu

Abstract— This paper proposes a unique implementation ofpower factor correction (PFC) and maximum power pointtracking (MPPT) for low power wind turbines. For a givenwind condition, there is a unique electrical load which willharvest the maximum power from a wind turbine, the proposedcontrol algorithm actively tracks this electrical loading conditionfor maximum power. An active 3-phase rectifier (VIENNA)converter is used to rectify the 3-phase AC voltage with nearunity power factor which is critical in this application where theseries resistance of the turbine is very high. An experimental300W prototype was designed and tested to verify the design.Experimental results showed a significant increase in powerextracted from the low power wind turbine when PFC and MPPTwere implemented.

I. INTRODUCTION

Low power wind turbines (100 W - 1000 W) are typicallyfixed blade 3-phase permanent magnet machines. Unlike largerturbine systems, the pitch angle of the blades are fixed, sothe only way to maximize the harvested energy is to varythe electrical loading on the turbine. By varying the electricalloading, the power delivered by the turbine can be maximizedwithout any knowledge of the wind conditions or the turbinecharacteristics. The proposed maximum power point tracking(MPPT) battery charger for low power wind turbines alsofeatures unity power factor control so as to reduce the ohmiclosses in the relatively high series resistance of the turbine.In [1], [2], and [3], different PFC techniques have beenproposed. Also, [4] [5] explains methods for maximum powertransfer. However, these papers only describe implementationfor high power wind turbines. MPPT has been implementedfor low power wind turbines but with the requirement of ananemometer as described in [6]. However, the introduction ofan anemometer can significantly increase the overall cost ofthe low power turbine. An alternative approach was proposedin [7] where a six diode rectifier at the input stage and adummy load was used.

As shown in Fig. 1, there exists a specific load resistancewhich will result in the maximum amount of harvested power.The implemented MPPT algorithm implements the perturb-and-observe algorithm, which is a popular choice for the

comparable photovoltaic system. All algorithms will be imple-mented without the need of an anemometer or a dummy load.Also, an active rectifier is used which achieves unity powerfactor while also reducing the voltage drop of the rectificationstage in comparison to the typical 6-diode rectifier.

Fig. 1: Power curve of experimental wind turbine underdifferent simulated wind conditions

Depending on the load and wind speed, the wind turbinemay have trouble keeping a clean sinusoidal waveform. Theproposed control algorithm also features a three phase digitalphase locked loop (DPLL) control to further improve theimplementation of PFC. This paper presents one approach forfrequency and phase detection and produces a clean sinusoidalwave without the need of a filter.

II. REALIZING PFC FOR LOW POWER WIND TURBINEAPPLICATIONS

Due to the high impedance of a wind turbine, to notutilize any PFC techniques would introduce high I2R losses.Therefore, if the power converter is designed to behave like apurely resistive load, reducing the high peak currents, minimiz-ing the I2R losses which will maximize the power extracted

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from the wind turbine. The battery charger implements aunique approach of controlling the conductance (G) of theunity power factor 3-phase AC-DC stage using the VIENNAtopology. With PFC, the VIENNA converter allowed morepower extraction, by increasing the efficiency of the converteras well as the wind turbine.

The sensed battery voltage and current determines thecontrolled conductance value. Over-voltage battery protectionis implemented using an output voltage and output currentregulation (OVR and OCR, respectively). When the batteryvoltage reaches the desired reference value, it limits thereference current going into the OCR controller. The OCR thentries to reduce the maximum current allowed to the battery byincreasing the conductance value. On the other hand, whenthe battery voltage is below the reference value, the MPPTalgorithm tracks the optimum conductance value. Therefore,the rotor speed is optimized in order to allow maximum powertransfer from a given wind speed. The maximum of thesetwo conductance values is selected so that when the batteryrequires current limiting, the converter will command a highconductance value, G, which corresponds to a slower rotorspeed. This approach is illustrated in Fig. 2.

Fig. 2: Conductance (G) controller

After the proper conductance is calculated, it is received bythe individual controllers for each phase as shown in Fig. 3.The duty cycle of the individual phase is controlled and shapesthe input current as a scalar of the input sinusoidal waveform.This scalar is the conductance value. Therefore, as the batterygets charged, the converter limits the amount of power thatis pushed to the bus. When the maximum power is no longerneeded, the increased conductance value causes the turbine toslow the rotor speed.

Fig. 3: PFC control block diagram

III. PROPOSED CONTROL STRUCTURE

With the implementation of PFC in a low power wind tur-bine (300W), it can be challenging to sense a clean sinusoidalwaveform. Sometimes, noise can affect the stability of anycontrol strategy that is implemented in the converter. Due tothe high ESR of the turbine, the input voltage waveform ishighly affected by the amount of current extracted from theturbine. The sinusoidal current reference is a factor of the inputvoltage and it is used to shape the input current. Noise inthe sensed signal can cause the controller to become unstable.Another important issue is stability at low RMS voltage due toeither low wind speeds or heavy loading. It becomes difficult toscale a small RMS value to create the input current controllerreference to shape the current when PFC is desired under theseconditions. The system needs to have more reliable operationduring the heavy load condition because this operating pointis used to keep the wind turbine in the current limit mode thusslowing the wind turbine down.

A new control structure is proposed for low power windturbine battery chargers where it introduces the implemen-tation of a digital phase-locked-loop (PLL) generates threeseparate sinusoidal waveforms, each with a 120 degrees phaseshift. These waveforms are used as the current references forthe input current controllers on the active rectifier stage. Adigital approach is preferred in this research instead of analogbecause noise can be cancelled without any extra hardware.Therefore, heavy loading and low wind speeds will not affectthe stability of the controller thus making it more reliable whilemaintaining a close to unity power factor.

A three phase DSP based PLL implementation is discussedin [8]. However, based on analog PLL structures, a simulationwas formulated in Simulink that confirms the successfuloperation of the PLL and the general structure is shown inFig. 4.

A lookup table approach for the digital oscillator was se-lected because of the high quality sine wave at particularly lowfrequencies as well as it’s ease of increasing accuracy alongthe sin wave by adding more sample points. A disadvantage

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Fig. 4: Phase Locked Loop control diagram

to this method is the non-linearity that exists in a lookup tablebased oscillator. However, the oscillator can be approximatedto be a linear device along the lower frequency band.

The implementation of the PLL system is a hybrid ap-proach, using a timer based zero crossing detection algorithmfor coarse frequency tracking, and a phase-error detectingcontroller, to minimize the phase shift between the sensedinput voltage and the digital oscillator reference. The coarsefrequency detection is excecuted using a hysteresis windowedzero crossing detection algorithm to detect the frequency ofboth the input current as well as the digital oscillator. Fromthis, we can directly compute the proper frequency for thedigital oscillator to operate at such that the two frequenciesare matched. This frequency is changed slightly using a phaseerror detection controller to minimize the measured phasedifference. The phase difference measurement is done usinga zero crossing detection algorithm as well. By introducingthe control algorithm shown in Fig. 4, the controller can usea fine tune adjustment method to make small changes to thefrequency so that the two waveforms are perfectly locked onwithout completely relying on zero crossing detection.

The new PLL control structure implemented into the exist-ing PFC control structure as shown in Fig. 5. Note how thePLL controller is fed into the PFC controller just as the originalalgorithm does allowing us to use the same PFC algorithmwithout any modifications.

Fig. 5: PLL system interaction with PFC system

IV. TOPOLOGY

To reduce the voltage drop of the rectifying stage and toactively control the power factor, a controllable rectifier (VI-

ENNA rectifier) is used for the first stage of the system. TheVIENNA operates in boost mode and assures a bus voltagethat is always higher than the battery voltage. The secondstage is a synchronous buck DC/DC converter controlled bythe IVR loop. The topology realized for this research alongwith a block diagram describing the controls for one of thephase (Phase C), is shown in Fig. 6.

Fig. 6: VIENNA rectifier with buck topology showing acontroller for one of the phases

As explained previously, each phase has its individualcontroller that shapes the phase current to be a scalar of thedigital PLL referenced locked in phase to the input voltage.The proper conductance value is determined by the sensedinformation from the load. As the battery current demanddecreases, higher values in the conductance value will forcethe wind turbine to rotate at a slower speed while stillsupplying the maximum power permitted by the load battery.This topology also allows the end user to increase the desiredbattery voltage. Because it uses an intermediate bus, the loadvoltage could be as high as 72V as long as the bus has avoltage value greater than the load. This gives flexibility inthe application for the topology and the proposed algorithm.

V. EXPERIMENTAL RESULTS

A. Power Factor Correction in low power wind turbines

In a system which utilizes the typical 6-diode rectifier togenerate the DC voltage which charges the battery, there arehigh current spikes resulting in low overall power factor. Thisis due to the clamping of the input voltage by the battery. Evenin cases where there is a second DC-DC stage to charge thebattery from the rectified bus voltage, the current will spikeas the large bus capacitance is quickly charged with the risingAC voltage. With high current spikes in the input current,the high ESR of a wind turbine can significantly affect theoverall efficiency of the AC generator. In Fig. 7, high currentspikes were observed when the converter was operated as aconventional diode bridge rectifier with a 470uF bus capacitor.These high current spikes cause significant wire loss.

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Fig. 7: Conventional 6-diode rectifier

With the implementation of PFC, the input current is inphase with the voltage thus achieving near unity power factoras seen in Fig. 8. The high power factor eliminates the currentspikes and allows the turbine to operate more efficiently.

Fig. 8: Active PFC rectification

The effect of PFC in a low power wind turbine is shownin Table I. With a power factor of 0.98 with PFC versus 0.94with constant duty cycle control, a significant increase in inputand output power was observed. Also, due to the eliminationof the current spikes, the efficiency of the converter increasedas well.

TABLE I: Overall system efficiency w/active PFC vs.constant duty cycle for the AC/DC stage

Constant Duty Active % IncreaseCycle Control PFC

PF 0.94 0.98 4.08 %Pin 187.90 W 191.59 W 1.93 %η 91.10% 92.89 % 1.93 %

Pout 171.18 W 177.97 W 1.93 %

B. Phase Locked Loop

The PLL implemented in this research is able to lock ontoa frequency as low as 8Hz and can go as high as 35Hz. Fig.9 shows how it successfully locks to an 8Hz signal. On the

other end of the spectrum, Fig. 10 shows the phase lockingat 35Hz. These results were taken from the PLL system thatuses zero crossing detection strictly as it’s frequency and phasecontroller.

Fig. 9: PLL results at 8 Hz: (blue) digital oscillator, (orange)input voltage

Fig. 10: PLL results at 35 Hz: (blue) digital oscillator,(orange) input voltage

The simulation results for the simulation diagram shown inFig. 4 yield the results shown below in Fig. 11. Note howthe initial sine wave starts out with high distortion but quicklylocks on with a low THD sinusoidal wave.

Fig. 11: PLL simulation results: (red) input voltage, (black)digital oscillator

C. Topology Prototype

The top and bottom view of a 300W experimental prototype,including the VIENNA rectifier with the buck converter isshown in Fig. 12 and 13, respectively.

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Fig. 12: Experimental 300W prototype (top view)

Fig. 13: Experimental 300W prototype (bottom view)

This design utilizes three isolated MOSFET drivers, eachpowered by a bootstrapping diode and capacitor, which takesthe energy from the rectified turbine input voltage. Thisdesign removes the need for floating isolate power suppliesto drive the bidirectional blocking MOSFETs of the VIENNAconverter. The enclosed prototype is shown in Fig. 14

Fig. 14: Experimental 300W prototype (enclosed)

D. MPPT

A conventional perturb-and-observe MPPT algorithm wasexperimentally verified, and is shown in Fig. 15 . When

the load voltage is below the desired reference, the digitalcontroller continues to make changes to the PFC constant.For a conventional hill climbing algorithm, the old power iscompared with the new power. If the new power is greaterthan the old power, the controller will keep making changesin the same direction. However, if the new power is less, itswitches the direction of change in the PFC constant.

Fig. 15: Hill climbing MPPT steps

At a given wind speed of 11m/s, the MPPT was capable ofextracting 325W of power from a 300W rated wind turbine.For every step in the PCF constant there is a change in theoutput battery current. The rate of change in this PFC constantmust be slow enough for the wind turbine to react to the newvalue. If these changes occur too quickly, the controller mightbecome unstable.

E. Efficiency and reliability

It was important to test the efficiency of the topology wherethe proposed algorithm was tested on. Results show that allthe control loops were stable and the energy transfer was doneat high efficiency. The test conditions are as follows:

• Constant Bus Voltage of 40 V• Open loop switching on the rectifier so that test conditions

can be kept constant at varying wind speeds• 12 V battery outputTable II shows the efficiency results for this test case. Note

that these results do not reflect the effectiveness of the MPPTalgorithm or the PFC algorithm as these two algorithms workto increase the input power to the converter, not the overallefficiency of the power electronics.

TABLE II: Overall system power efficiency testingPower In (W) Power Out (W) Efficiency (%)

103.38 94.06 91114.09 104.09 91134.39 123.1 92172.17 158.76 92210.18 195.16 93243.69 223.36 92247.82 228.78 92

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Reliability was also tested on this prototype when it wastested with a two hour test of sourcing 200W to a 12V battery.The prototype performed at efficiencies greater than 90%.The proposed algorithm is also able to increase the allowablebattery voltage.

F. Test bench

These test results were implemented in a test bench where a300W turbine was coupled to a DC motor. The rotational speedproperly controlled the rotational speed of a 90V DC motorin order to simulate real characteristics of a wind turbine. Thetest bench is shown in Fig. 16.

Fig. 16: 300W wind turbine test bench

VI. SUMMARY AND CONCLUSION

Through theoretical and experimental verification, it wasconfirmed that an actively controlled AC-DC PFC rectifiercan significantly improve the overall power transfer of a lowpower wind turbine converter in contrast to the traditional6-diode rectifier. Furthermore, an optimal converter topologyand control structure has been presented which demonstratehigh efficiency, near unity power factor, and the ability toactively track the maximum power point of the wind turbine.These benefits are in addition the elimination of the need fora dummy load or an anemometer with the implementation ofthe proposed algorithm, as well as it allows flexibility withbatteries of various voltages and chemistries.

ACKNOWLEDGMENT

This work is partially funded by the National ScienceFoundation - IRES Award # 0652048

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