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Abstract A microcontroller based system is proposed for low cost and high efficiency control of small wind turbine. The

perturbation and observation method is employed to achieve maximum peak power tracking (MPPT). The algorithm, controller functions,and driving signals are implemented on ATMEGA16 microcontroller. The prototype of MPPT controller was tested with real wind turbine

at the laboratory in Germany. The actual operating points of the wind turbine with MPPT controller were compared with power versus

rotational speed curve in order to verify the tracking performance. The result confirms that small wind turbine with proposed MPPT

controller can operate in the maximum power region for the whole range of assigned wind speeds.

Keywords small wind turbine, vertical twisted H-Rotor, wind energy, maximum peak power tracking, dc/dc converter,

microcontrollers.

1NOMENCLATURE

Aturb = cross-section area of the turbine perpendicularto the wind direction.

cT = torque coefficient

cP = power coefficientib = battery current [A]ig = generator current [A]iref = reference current [A]

ibref = battery current reference [A]

Jt t& = rotational torque[N.m],

M = step size

R = radius of blade of blade [m]

Rg = generator resistance []

Ta = aerodynamic torque [N]Tf = friction torque [N]

Tg = generator torque [N]Ts = update time [s]Pe = electrical power [W]

Plosses

= power lossse [W]Pt = turbine power [W]

Pwind = wind power [W]v = voltage [V]

vt = wind speed [m/s]VB = battery voltage [V]

Vb = voltage drop at generator brush [V]Vg = generator voltage [V]

t = rotational angular speed of blade [rad/s]

cutin = cut in speed [rad/s]

= tip speed ratio TSR

= pitch angle [radian]

= the air density [kg/m3]

1. INTRODUCTION

Nowadays, there are more and more concerns over the

fossil fuel exhaustion and the environment problems caused byfossil fuel based conventional power generation. To solve those

problems, renewable energy is widely accepted as one of

B. Neumanee is with Department of Electrical Engineering, Faculty of

Engineering, King Mongkuts Institute of Technology North Bangkok,

Thailand, Pibulsongkram Rd., Bangsue, Bangkok 10800, Thailand, E-mail: bln@kmitnb.ac.th

S. Chatratana is the Deputy Director of Technology Management Center,

National Science and Technology Development Agency (NSTDA), 111Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang,

Phathumthani 12120,Thailand, E-mail : somchaich@nstda.or.th

appropriate alternatives. If renewable energy is available in

abundance, such as wind and solar energy, it is widely exploited.Wind turbines are now mainly used for the electric energy

production. Large wind turbine systems can be directlyconnected to the grid for transmission of electrical power

whereas small wind turbine systems are normally equipped withbattery systems to store the electrical power.The basic structure for the H- and Twisted H-rotors are

shown in Fig. 1. The vertical H-Rotor wind turbine (Fig. 1 a) hasmany advantages, for example: 1) it can work very well in very

bad condition, 2) the construction is simple and it needs littlemaintenance, 3) it has long life time, 4) it is easy to install and

produces low noise, 5) it does not need yaw system, 6)mechanical, electrical components, and the generator can be

equipped on or near the ground. However, the main disadvantageis that it requires high starting torque [1]. To solve this problem,

a normal H-Rotor turbine was redesigned to a new Twisted-H-Rotor (THR) as shown in Fig. 1 b). The new THR retains all the

advantages of H-Rotor but with low starting torque and reducedpeak of tangential and thrust force with same energy capture. Thenew design results in a turbine with very low noise, low

vibration, and low stress on the blade.A 10 kW normal vertical H-Rotor installed at Alfred-

Wegener-Institute for Polar and Marine Research station inAntarctica is shown in Fig. 2. This wind turbine has beenoperated in very bad conditions (snowing, windy, stormy, and

very cold) in Antarctica since 1989 (more than 16 years). It needsonly single maintenance for eddy current break [2].

Fig. 1 Comparison of normal H- Rotor and Twist-H-Rotor

wind turbine

Bunlung Neammanee and Somchai Chatratana

Maximum Peak Power Tracking Control for the new Small Twisted

H-Rotor Wind Turbine

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Fig. 2 A 10kW H-Rotor at Alfred-Wegener-Institute station in

Antarctica 1989.

2. AERODYNAMIC OF WIND TURINE

Wind power is a function of wind speed, v t given in (1)

Pwind =2

Aturb

3tv

(1)

According to Betz law, the amount of power can be

captured by an ideal wind turbine is 16/27th (0.5925) of P windcalculated from Equation (1). It is shown in [3] that if v tdecelerates by 2/3, the optimum extraction is achieved.

Tip speed ratio (TSR), , is defined as the ratio of the tip

speed to the wind speed, given by

t

t

vR= (2)

The power produced by the wind turbine is given by

Pturb = Pwind cP(,) =2

Aturb 3tv cP(,) (3)

The aerodynamic torque acting on turbine blades is

obtained respectively:

Ta =2

Aturb

2tv cT(,) (4)

It can be seen in (3) and (4) that power and torque

coefficient are the function of TSR and . In a fixed pitch wind

turbine (e.g., twisted H-rotor), is constant; thus, cP and cTdepend only on TSR. It can be found from Equations (2), (3) and

(4) that the relation between , cP and cT is

cP = cT (5)

For the aerodynamic of the vertical axis twisted H rotor,reference [2] gives full account of the derivation of tangential

force, trust force and total force on the blade of the turbine withrespect to attack angles and wind velocity. However, for small

wind turbine and for practical purpose, there is always analternative of describing the characteristic of the wind turbine. In

this paper the characteristics of wind turbine was obtained from a

laboratory test. The wind turbine was blown with a fan whosespeed can be controlled . Output power from the turbine is thenmeasured for each value of rotation speed. The typical outputcharacteristic of power and torque versus rotational frequency

curves are shown in Fig. 3 (a) and (b) [4], [5].

Fig. 3 Typical characteristic curves of wind turbine at

various wind speeds. (a) Power versus rotational frequency.

(b) Torque versus rotational frequency.

0 0.5 1 1.5 2 2.50

0.05

0.1

0.15

0.2

0.25

vt= 8.1 m/s

vt= 7.1 m/s

vt= 6.7 m/s

vt= 6.0 m/s

vt= 4.8 m/s

vt= 4.3 m/s

Tip speed ratio

Powercoefficientspeedratio

Fig. 4 Characteristic of power coefficient and tip speed ratio

at wind speed from 4.3 m/s to 8.1 m/s

From test results, cP and TSR can be calculated and theirrelationship is shown in Fig. 4 for six wind speed values (from

4.3 m/s to 8.1 m/s). For an ideal wind turbine, there should beonly one curve (the solid line) for any wind speed. These

variations are due to air turbulence and non laminar flow aroundthe blades and errors in measurement of wind speed.

To derive a mathematical model for the twisted H rotorshown in Fig. 2 b), we assume that there is only single

representation of cT versus TSR (for example the solid line inFig. 4) and that the blades and the whole rotor are considered a

single lumped mass. The relationship among aerodynamic,friction and generator torque, wind turbine power, moment ofinertia and angular speed can be expressed by

Ta = Jt t&

+ Tf+Tg (6)

Pt = (Jt t& + Tf+ Tg)t (7)

From Equations (2), (3), (6) and (7), a block diagram of the

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simplified wind turbine system can be established as shown inFig. 5.

Fig. 5 Block diagram of the simplified wind turbine plant.

3. MPPT PROCESS AND CONTROL

ALGORITHM

The MPPT algorithm commonly used for solar panel

control can be applied to maximize the wind turbine outputpower. Fig. 6 shows the characteristic of power and torque versus

speed of wind turbine that needs to be controlled [6], [7]. The

main purpose of the MPPT controller is to maintain operatingpoint on Pm_max for any wind speeds. The operating point can beon the positive slope (the left side of the Pm_max), zero slope (thepoint where Pm_max occurs), and negative slope (the right side of

the Pm_max). If the operating point is in the positive slope region,the controller must move the operating point to the right to get

closer to the maximum point. This can be achieved by decreasingload (battery) current, which results in an increase in rotationalspeed. Conversely, if the operating point lies on the right hand

side of the peak, the load current has to be increased, resulting ina decrease in the rotational speed. The step to increase/decrease

load current references is

ibref[(k+1)Ts]= ibref[(k)Ts] + M

]T)k[(

]T)k[(p

st

s

(8)

The instantaneous power slope is calculated by

slope =]T)k[(

]T)k[(p

st

s

(9)

Fig. 6 Wind turbine output power and torque characteristics

with MPP tracking process.

The reference current is updated by the MPPT controller at every

time step Ts. The above mentioned actions bring the operating point toward Pm_max by increasing or decreasing the rotational

speed step-by-step. This tracking methodology in the controlscheme is called the perturbation and observation method (P&O)

method. [1]The control flowchart of the maximum power tracking

system in Fig. 7 illustrates the details of decision processes basedon the tracking procedure in Fig. 6. If the rotational speed is

higher than the cut-in speed, cutin, the MPPT controller will start

the procedure. If a given perturbation leads to a positive ornegative slope, the next perturbation increases or decreases

rotational speed until the slope becomes zero (i.e., maximumpower point is reached).

Fig. 7 The MPPT control program flowchart.

To implement the MPPT control in the ATMEGA16

microcontroller unit for wind turbine control, Equation (7)should be rewritten in more detail as given (10). The right handside of Equation (10) is the sum of rotational power, electrical

power, friction power, and brush and copper power losses,respectively.

Pt = J t& t + Pe + Tft+ Vbig + ig2Rg (10)

Ta = Pt/t (11)

Fig. 8 Block diagram of MPPT control system for wind

turbine for battery charger.

The overall block diagram of control system is shown in Fig. 8.

The dash line covers the MPPT controller built from Equations

(2), (8), (10) and (11). It is connected to the plant consisting ofthe wind turbine coupled with a generator, a dc/dc converter, a

battery and sensors. The MPPT controller receives generatorvoltage and current signals from the sensors. The controller

calculates turbine speed (i.e., dc generator speed) from the

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product of generator voltage, electromotive force (EMF) andgenerator constant. The output power, the slope of power with

respect to speed, and battery current references are updated usingEquation (8). This reference will be sent to a battery regulation

loop (lower right conner of the figure), where a proportional-integral (PI) compensator is used to control the load current as

desired and improve the system stability.

4. WIND TURBINE CHARATERISTIC

Since the Twisted H-Rotor is a new vertical wind turbine,it is therefore important to know its characteristics before theMPPT controller is implemented. The test system, in Fig. 9,

comprises a twisted H-Rotor directly coupled to an encoder and adc generator. The output of the generator is connected to a

variable resistive load with a voltage sensor and a dc currentsensor. This system employs an anemometer to measure wind

speed. All sensors are connected to the signal conditioner, PLC

and a computer to data monitoring and recording (i.e., vt, ig, ,vt). A wind speed simulator consists of a fan driven by a 15 kW

induction motor whose speed is controlled by a frequencyconverter.

Fig. 9 Diagram of the Twisted H-Rotor with testedequipments for find the characteristic of wind turbine.

Figure 10 shows the power coefficient versus TSR (tip

speed ratio) with 6 values of wind speed. The graphs in the figurehave similar characteristics; namely, one curve has only one peak

where the optimum TSR locates and the positive and negativeslopes lie on the left and right of the optimum TSR. This

characteristic is an important property for each wind turbine. Theoptimum TSR of a wind turbine is useful to determine the

maximum power the turbine can supply. For example, twoturbines can have the same sweep areas and same power

coefficients but one of them may have higher optimum TSR thanthe other. The turbine with higher TSR can operate at higherspeed and supplies more power. For this reason, a power

coefficient-TSR curve can be used to compare the efficiency ofdifferent wind turbines.

0 0.5 1 1.5 2 2.50

0.05

0.1

0.15

0.2

0.25

vt= 8.1 m/s

vt= 7.1 m/s

vt= 6.7 m/s

vt= 6.0 m/s

vt= 4.8 m/s

vt= 4.3 m/s

Tip speed ratio

Powercoefficientspeedratio

Fig. 10 Power coefficients versus tip speed ratio

Figure 11 shows 6 curves of the torque coefficient versus TSRfor the same wind speeds as those in Figure 10. As can be seen

from the two figures, power coefficient-TSR and torquecoefficient-TSR curves have a similar trend. It is, however,

important to note that the maximum cP does not coincide withmaximum cT at the same TRS and wind speed.

0 0.5 1 1.5 2 2.50

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

vt=8.1 m/s

vt=7.1 m/s

vt=6.7 m/svt=6.0 m/s

vt=4.8 m/s

vt=4.3 m/s

Tip speed ratio

Torquecoefficientspeed

ratio

Fig. 11 Torque coefficients versus tip speed ratio at various

wind speed.

Fig. 12 Power versus rotational frequency

Figure 12 shows the output power of the new Twisted H-Rotor starts from zero at zero rotational frequency and increases

with the rotational frequency until maximum power is reachedthen the power drops with increasing rotational frequency. These

curves in Fig. 13 is used to validate the MPPT controller whetherit can operate with maximum power for a given wind speed.

Fig. 13 Torque versus rotational frequency

Figure 13 illustrates the relationship between torque androtational frequency for the 6 wind speed values. Such arelationship represents an important characteristic of a wind

turbine, particularly for its starting torque. The torque gives

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guidance for the selection of a generator to be connected with thewind turbine as the system will function efficiently if one is

suitably chosen. Also, the starting torque indicates a minimumwind speed that starts the turbine or the speed which turbine

torque overcomes static friction of the system.

5. WIND TURBINE WITH MPPT CONROL

SYSTEM

In figure 14, the wind turbine generator is connected to a

buck converter with MPPT controller. The buck dc/dc converteris used to interface the wind turbine and the battery. The

converter power switch consists of two power-MOSFETs: Q1and Q2. The Q1 is used as a main switch to control the load

current in the normal operating region. The load current dependson the output power of the wind turbine. The Q2 is used as a

switch to brake the system.

A/D A/D

ATMEGA16

PWM1PWM2

interface circuit

L

D

Q1

Q2

C1

Rg

VG

VB

ig

ib

Emf

C2

Fig. 14 Twisted H-Rotor, buck converter and the control

block diagram.

The lower part of Fig. 14 is a control system implemented on an

Atmels ATMEGA16 high-performance, a low-power AVR 8-bitmicrocontroller, and an interface circuit. The interface circuit

comprises current filter and signal conditioner circuit connectedto voltage, current and A/D converter in the microcontroller. Theoutput of the microcontroller is PWM signals sent to the gate

driver circuit of the power MOSFETs. Monitoring andcalculation of control variables (e.g. power slope, the update step

of the controller, angular speed, and acceleration) are carried outby the microcontroller. The parameters of Twisted H-Rotor wind

turbine and DC generator are shown in Tables 1 and 2 of theappendix.

Fig. 15 The recorded data of generator current, voltage,

rotational speed, and wind speed from the plotter.

The record of wind speed, generator voltage, generatorcurrent and rotational speed experiment by the PLC and plotter

are shown in Fig. 15. Note that the results in Fig. 15 started fromright hand side and progressed to the left hand side. It can be

seen that the experiment started at the wind speed of 6 m/sec andthe speed was increased to 9 m/sec. The rotational speed of the

generator started at approximately 5 Hz and increased to 7.2 Hz.The generator voltage was kept around 36 volts throughout the

experiment. The load current started at 0.4 A. and increased withthe wind speed. The fluctuation of current was due to the action

of the controller and the step increase of fan speed.The output torque and power with various wind speed are

recorded every 0.08 second by serial communication betweenATMEGA 16 microcontroller and a computer. The controller

update command of the outer loop every 1.04 second to make thesystem stable due to the system time constant. If the update timeis faster, it will make the system unstable. The output power and

torque versus the rotational speed are illustrated in Fig. 15 andFig. 16 respectively. Fig. 15 also shows the optimum controlled

power represented by plus signs and ideal power represented bybold line. The comparison of the optimum controlled torque andthe ideal torque is given in Fig. 16 with the same representation.

In both cases, the control trajectories are close to the ideal powerand torque curve. Even with the uncertainty of power coefficient,

air turbulence, and controller update command; it can beobserved in both figures that the MPPT controller is able to track

the optimum power and torque.

Fig. 16 The experimental result of real power curve and

controller tracking performance.

Fig. 17 The experimental result of real torque curve and

controller tracking performance.

6. CONCLUSIONS

The maximum peak power tracking algorithm is proposed

in this paper to capture the maximum power from wind by aMPPT controller. The control algorithm is based on the

perturbation and observation method. The advantages of theproposed algorithm are:

The controller will be able to maximize the output power of a

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generator for any wind speeds,

It requires only the current and voltage signals from sensors toperform the control law,

With self tuning, step size of perturbation and peak currentcan be reduced and the system delivers better response thanthe control with fixed step of perturbation,

The algorithm can be applied to any wind turbines without theknowledge of their operating characteristics, and

It can be implemented in a low cost controller.7. ACKNOWLEDGMENT

The authors would like to express deepest gratitude to Mr.Heiko Schier, Prof. Dr. rer. nat. Bernd Stephan, Prof. Dr. Ing

Friedrich Zastrow and Prof. Dr.-Ing Kai Mller for their

kindness, helpful suggestion and valuable guidance during theconstruction period of the prototype in Hochschule Bremerhaven,

Germany.

8. REFERENCES

[1] Chihchiang Hua; Jongrong Lin; Chihming Shen; 1998 Implementation of a DSP-controlled photovoltaic systemwith peak power tracking Industrial Electronics, IEEE

Trans. Vol. 45, Issue 1, Feb. Page(s):99 107.[2] Sullivan, C.R.; and Powers, M. J. ; 1993 A High-

Efficiency Maximum PowerPoint Tracker for PhotovoltaicArrays in a Solar-Powered Race Vehicle Power

Electronics Specialists Conf.,. PESC '93 Record, 24thAnnual IEEE, 20-24 June, Page(s):574 - 580

[3] Koutroulis, E. ; Kalaitzakis, K.; Voulgaris, N.C.; Development of a Microcontroller-Based, Photovoltaic

Maximum power Point Tracking Control System. PowerElectronics,IEEE trans. vol. 16, Jan. 2001 page (s) 46-54.

[4] Erich Hau; 2000 Windturbines , Springer-Verlag,.[5] F. Zastrow, 2003 Tangential force on an H-Rotor-Blade,

Sep. Hochschule Bremerhaven, Germany.[6] H. Vihrial; 2002. Control of Variable Speed Wind

Turbines, Ph.D Thesis, Tampere University ofTechnology,

[7] Huynh, P.; Cho, B.H.; 1996 Design and analysis of a

Microprocessor-Controlled Peak-power-TrackingSystem, Aerospace and Electronic Systems, IEEE Trans.Vol. 32, Issue 1, Jan. Page(s):182 190.

[8] Quincy Wang; Liuchen Chang; 2004 An IntelligentMaximum Power Extraction Algorithm for Inverter-Based

Variable Speed Wind Turbine Systems, Power Electronics, IEEETrans. Vol. 19, Issue 5, Sept.

Page(s):1242 - 1249.[9] Morimoto, S.; Nakayama, H.; Sanada, M.; Takeda, Y.;

2005 Sensorless Output Maximization Control for

Variable-Speed Wind Generatio System Using IPMSG, Ind. Appl., IEEETrans.s Vol. 41, Issue 1, Jan.-Feb.

Page(s):60 67.

9. APPENDIX

Table 1. Twisted H-Rotor wind turbine specification under

test condition.

Twisted H-Rotor

Rotor radius: 0.5m

Blade height: 1m with 30 twist

Angular position between each blade: 120

Blade width: 0.235m

Blades number : 3

Blade profile: NAC 0021

Rotor areas: 1m2

Table 2. DC generator parameters under test condition.

Parameterss Units MTP-

1 Rated Speed rpm 1000

2 Rated Voltage 5% V 233

3 Rated Current A 3.85

4 Rated Output Power (1) W 780.00

5 Rated Torque Nm 7.45

6 Max Torque Nm 47.60

7 Max Speed rpm 1200

8 EMF Constant 5% V/1000rpm 207

9 Torque Constant 5% Nm/A 1.98

10 Friction Torque Nm 0.060

11 Damping Constant Nm/1000rpm 0.113

12 Terminal resistance 4.760

13 Rotor Inertia Kgm210-3 2.400

14 Mechanical Time Constant ms 2.924

15 Electrical Time Constant ms 0.588