12.IJAEST Vol No 7 Issue No 2 Power Enhancement of Wind Energy Conversion System Using Hybrid Method...

8/6/2019 12.IJAEST Vol No 7 Issue No 2 Power Enhancement of Wind Energy Conversion System Using Hybrid Method 248 253

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Power Enhancement of Wind Energy Conversion

System Using Hybrid Method

Abstract- The wind energy conversion system can deliver

the maximum power when the load impedance matches

with the source impedance under a given wind speed.Since the load and wind speeds are varying dynamically,the maximum power point tracking (MPPT) becomes

more complex with the use of MPPT functions. The

advantages of the MPPT methods are that no need of the

Wind Generator (WG) optimal power characteristic or

measurement of the wind speed is not required and the

WG operates at a variable speed. Thus, the system features

higher reliability, lower complexity and less mechanical

stress on the Wind Generator.

The conventional methods for the maximum power

point tracking (MPPT) are perturbation and observation

(PAO) method and incremental conductance method. ThePAO method is slow in tracking the maximum power

operating point and may track the maximum power

operating point (MPOP) wrongly under rapidly changing

atmospheric or load conditions. In the incremental

conductance method, the incremental conductance and the

negative of present conductance value are compared with

this method it is not possible to sense the variation in the

environmental conditions. At steady state its tendency to

vary the system condition is very poor. To overcome thedisadvantages of above methods a hybrid method is used

for tracking the maximum power. In this method, on

power variation, the duty cycle are adjusted in according

to the variation in rectifier output voltage.

Keywords-Wind generator(WG), Wind energy conversion

system(WECS), maximum power operating point(MPOP),

maximum power point tracking(MPPT), perturb andobservation (PAO)method, incremental conductance

method(ICM).

I. INTRODUCTIONThe worldwide concern about the environment has led toincreasing interest in technologies for generation of renewable

electrical energy. The ever-increasing demand forconventional energy sources has driven society towards theneed for research and development of alternative energysources. Many such energy sources, such as wind energy andphotovoltaic are now well developed, cost effective and they

are widely used. These sources offer the advantages of loadshifting, customer demand, production of power inenvironmentally friendlier ways, and emergency backup

power.

Wind generators (WGs) have been widely used both in

autonomous systems for supplying power to remote loads andin grid-connected applications. Although WGs have a lowerinstallation cost compared to photovoltaic cells, the overallsystem cost can be further reduced using high-efficiency

power converters, controlled such that the optimal power isacquired according to the atmospheric conditions. Wind poweris used in large-scale wind farms for national electrical grid aswell as in small individual turbines for providing electricity to

rural residences or grid-isolated locations. The countries withthe highest total installed capacity are Germany (20,621 MW),Spain (11,615 MW), USA (11,603 MW), India (6,270 MW)

and Denmark (3,136 MW).The wind speed varies continuously and load also

changes continuously. In order to match the load with WECSthe maximum power point tracking is required. MPPT is not amechanical tracking system; it is a fully electronic system thatvaries the electrical operating point of the wind energy

conversion system. So that the wind energy conversion systemable to deliver maximum available power. MPPT can be usedin conjunction with a mechanical tracking system, but the twosystems are completely different. A power electronic

converter is connected in between WECS and load. Themaximum power tracking process is done by the powerelectronic converter.

S. Chandra Has1Associate Professor, Department of EEE

Regency institute of technologyYanam, India

Email: schandrahasa mail.com

Dr.P.Ajay-D-Vimalraj2

Associate Professor, EEE Dept

Pondicherry Engineering CollegePondicherry, India

Email: [email protected]

G.R.K.D.Satya Prasad3Associate Professor, Department of EEE

Regency institute of technologyYanam, India

Email: [email protected]

S. Chandra Has* et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES

Vol No. 7, Issue No. 2, 248 - 253

ISSN: 2230-7818 @ 2011 http://www.ijaest.iserp.org. All rights Reserved. Page 80
mailto:[email protected]:[email protected]


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II.WIND ENERGY CONVERSION SYSTEM

Wind energy is transformed into mechanical energy bymeans of a wind turbine that has one or several blades. Theturbine is coupled to the generator system by means of a

mechanical drive train. It usually includes a gearbox thatmatches the turbine low speed to the higher speed of thegenerator. New wind turbine designs use multi pole, low speedgenerators, usually synchronous with field winding or

permanent magnet excitation, in order to eliminate thegearbox. Some turbines include a blade pitch angle control forcontrolling the amount of power to be transformed. Stall

controlled turbines do not allow such control. Wind speed ismeasured by means of an anemometer. A general scheme ofWind energy conversion system is shown in Figure

Fig 1. Wind energy conversion system

The power obtained by the turbine is a function of wind

speed. This function may have a shape such as shown inFigure. For variable speed WECS the upper part of the curvebetween and can be kept linear, equal to the reference power.

The following notations are used:Pr: reference power, maximum power that the turbine canattain

Vr: reference power wind speed, wind speed for whichreference power is achievedVci: cut-in wind speed, wind speed at which the turbine starts

to produce powerVco: upper limit of the wind speed called cut-out wind speed,at which the turbine operates.

Fig.2 power versus speed curve

A. Mathematical Model of wind turbine:A wind energy conversion system is basically

comprised of two main components, the aerodynamic

component and the electrical component. The turbine forms amajor constituent of the aerodynamic system. The energy thatcould be captured from wind by a specific turbine depends onits design particulars and operating conditions. In this sectionall aspects related to the power conversion, from kinetic wind

energy to rotational energy, that are of relevance for thestability model are explained.

The kinetic energy Ek of a mass of air m having the speedVw is given by:

2

2 wVm

KE --- (1)

The power associated to this moving air mass is the derivativeof the kinetic energy with respect to time can be expressed as

follows:

2.

2

12.

2

1

wqVwV

t

m

t

KE

oP

-- (2)

Where q represents the mass flow given by the expression:

q =VW .AWhere : Air density;

A: Cross section of the air mass flow.Ek: kinetic energy of the air

This fraction of power (Pwind) depends on the wind speed,rotor speed and blade position (for pitch and active stallcontrol turbines) and on the turbine design. The aerodynamic

efficiency Cp is defined as follows:

o

wind

P

P

PC ---- (3)

For a specific turbine design, the values of Cp (, ) areusually presented as a function of the pitch angle () and thetip speed ratio (). The tip speed ratio is given by:

w

tur

V

R

---(4)

Where R: the radius of the turbine blades. tur: the turbine

angular speed.B. Speed Torque Characteristics Of Wind Turbine

Fig.3 Turbine speed power characteristics under differentwind speeds


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Fig.4: Torquespeed characteristics of the wind turbine and

the generator

III. Maximum Power Point Tracking

The wind speed varies continuously and the load alsochanges continuously. In order to match the load with thewind energy conversion system we require a power electronic

interface between the wind energy conversion system and theload. The basic block diagram of the wind energy conversion

system is shown in figure.

Fig. 5: Block Diagram of a WECS

This power-conditioning unit consists of AC-DCconverter and DC-DC buck converter. The frequency andvoltage at which power is producing are variable parameters.

If we are connecting directly to the load the load gets affected.To avoid this condition power conditioner is connected inbetween source and load by which we can change the voltage

and frequency of required level. The maximum power pointtracking is done by using DC-DC converter.A. Maximum Power Operating Point Tracking:

A maximum power point tracker is basically a converterconnected in between the WECS source and the load. Theduty cycle of is continuously changed and operated at a value

such that the maximum power is tracked from the source. Bymaximum power transfer theorem, a source will deliver itsmaximum power when the source impedance matches the loadimpedance.

Fig.6: Block Diagram for maximum power pointtracking

The duty cycle of the converter is maintained in such away that the effective impedance seen by the wind energyconversion system source will be equal to the internal sourceimpedance and hence maximum power is delivered.

Figure below represents the typical curve of wind powervariation according to the operating voltage and it shows thatthere are two operating zones: the first is located on the right

side of the MPP where dp/d < 0 and the second on the leftside of the MPP where dp/d > 0.

Fig. 6: Wind power variation

Table: 1 Operation modes of buck converter

B. Condition for Maximum Power Operating Point:MPPT process in wind energy conversion system is based ondirectly adjusting the dc/dc converter duty cycle according to

the result of the comparison of successive WG-output-powermeasurements. Thus, the problem of maximizing the WGoutput power using the converter duty cycle as a controlvariable can be effectively solved using the steepest ascent

method according to the following control law:

1

1

11.

k

k

kk

D

PCDD

Where Dk and Dk-1 are the duty-cycle values at

iterations k and k - 1, respectively (0 < D k < 1); Pk-1/Dk-1 isthe WG power gradient at step k - 1; and C1 is the step change.


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Thus, the function P (D) has a single extreme point, coincidingwith the WG MPOP, and the dc/dc converter duty-cycleadjustment according to the control law ensures convergence

to the WG MPOP under any wind-speed condition. The powermaximization process is shown in Figure below.

Fig.7 power maximization curve

IV. Methods of MPPTThe control of MPPT is done by using buck converter. By

varying the buck converter duty ratio we can achievemaximum power tracking. There are two popular methods ofmaximum power point tracking in use. They are Perturbationand Observation method and Incremental Conductancemethod equal to zero under power feedback control. A generalapproach to power feedback control is to measure andmaximize the power at the load terminal in this present work

power feedback

A. Perturbation and Observation method:

In this method the duty cycle of the converter iscontinuously varied for each and every switching period. Thevalue of power is found as multiplication of current andvoltage. This present power is compared with the previous

one. If both are equal, then no change in duty cycle is needed.i.e., the system is already operating in maximum poweroperating region. Thus the algorithm is very simple in

mathematical calculation but with proper logical decisions.Hence it is considered as one of the easiest algorithm availablefor maximum power point tracking.

B. Incremental conductance Method:In this method the value of conductance is calculated

and compared with the previous value and is taken as

incremental conductance. If this value is equal to the negationof the present conductance value, then the duty cycle is notvaried. If this value is more than the negation of presentconductance, then the duty cycle is increased, otherwisedecreased.

C. Hybrid method:

This method gives a new and simple control method formaximum power tracking in a variable speed wind turbine permanent magnet synchronous generator by using a buck

converter. A maximum power-tracking algorithm calculatesthe Voltage command that corresponds to maximum poweroutput of the turbine.

When the wind speed changes slowly the algorithmvaries the duty cycle with its own speed whether the presentoperating condition is at maximum power operating region or

at least nearer to it. So the value of power output will bealways less than that it could have yielded. Choose the initialreference rotor speed and measure the output power of thegenerator. According to the value of measured DC outputvoltage of generator the reference voltage of wind generator

increase or decrease by one step and measure the output poweragain. Calculate Sign (P) and Sign (V). Moreover thevariation of duty cycle at a particular instant might cause anincrease in power.

In order to overcome these disadvantages here, the

voltage generated at a particular wind speed is consideredinitially. The wind speeds will effects voltage at which thegenerator output is generated. So, wind speed is directly

proportional to the generator output voltage in permanentmagnet synchronous generators. The drawback of incrementalinductance method is the implementation of the algorithm isvery difficult since we require more accurate sensors, which

will calculate the effective impedance of the system at theparticular instant. So, in this hybrid algorithm we are takingthe voltage value as the control measure and calculating power

variations, which will be given to the duty cycle of theconverter. This method is always operating at MPPTalgorithm is easy to implement.

Fig.8: Flow chart for the hybrid method

This algorithm is hybrid to overcome the disadvantagesof the above-mentioned conventional methods of maximumpower tracking, and it also incorporates the advantages of both

the methods. The Perturbation and Observation method iserroneous because it does not consider whether the presentoperating value of duty cycle is correct or not. But it alwaysconcerns the next duty cycle to which it has to change. So in

this hybrid algorithm we taking the voltage value as that thevoltage of generator tracks the command voltage.


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Command to control the output power of the permanentmagnet generator, such that the voltage of generator tracks thecommand voltage. Control measure and calculating power

variations, which will be given to the duty cycle of theconverter. This method is always operating at MPPTalgorithm is easy to implement.

V. SIMULATION RESULTS

The wind energy conversion system is modelled inMATLAB simulink simulation software. The MPPTalgorithms were simulated in the graphical simulationsoftware Simulink for which the graphical representation has

been taken from below. The simulation is carried out usingMATLAB simulink for 24 hours duration.

Fig.9: Wind generator speed versus power characteristics

Fig.10: Power output With out MPPT under constant loadconditions

Fig.11: Power output using PAO method

Fig.12: Power o/p using incremental conductance method

Fig.13 Power output using hybridmethod

Fig.14: Power output using PAO method

Fig.15: Power output using incremental conductance method

Fig.16: Power output using hybrid method.


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VI. CONCLUSION

In this paper, a hybrid algorithm is presented to track the

maximum power from the wind energy conversion systemeliminating the drawbacks under rapidly changingatmospheric or load conditions. The PAO method is slow intracking the maximum power and may track the maximum power operating point (MPOP) wrongly under rapidly

changing atmospheric or load conditions. The average output power is increasing by 14% by using the conventionalmethods. But, from the above results it is clear that using thehybrid method the power output is further increased by 3.54%in case of constant load condition and 4.2 % in case of

variable load condition.

REFERENCES

[1] Tafticht, T.; Agbossou, K.; Cheriti, A, DC Bus Control ofVariable Speed Wind Turbine Using a Buck-Boost

Converter, IEEE Power Engineering Society Generalproceedings, pp.1-5, 18-22 June 2006.[2] Wang, Q., Chan, L.,An independent maximum powerextraction strategy for wind energy conversion systems IEEE

Canadian Conference on Electrical and ComputerEngineering, vol.2, pp.1142 1147, 9-12 May 1999.[3] Eftichios Koutroulis and Kostas Kalaitzakis, Design of a

Maximum Power Tracking System for Wind-Energy-Conversion Applications, IEEE Transactions On IndustrialElectronics, Vol. 53, No. 2,pp.486-494, April 2006.[4] Esmaili.R.,Xu.L.,Nichols.D.K, A new control method ofpermanent magnet generator for maximum power tracking inwind turbine application , IEEE Power Engineering SocietyGeneral Meeting, 2005. Vol. 3, pp.2090 2095, June 2005.

[5] E. Muljadi and C. P. Butterfield, Pitch-controlled

variable-speed wind turbine generation, IEEE Transactionson Industrial Applications, vol. 37, no. 1, pp. 240246,Jan.2001.[6] A. M. De Broe, S. Drouilhet, and V. Gevorgian, A peak power tracker for small wind turbines in battery charging

applications, IEEE Transactions on Energy Conversion, vol.14, no. 4, pp. 16301635, Dec. 1999.[7] Chihchiang Hua, Chihming Shen, Study of maximumpower tracking techniques and control ofDC/DC converters for photovoltaic power system IEEE

Power Electronics Specialists Conference. PESC 98,Volume1, pp.86 93, May 1998.[8] Mona N. Eskander, Neural network controller for a

permanent magnet generator applied in a wind energyconversion system, Renewable Energy, Volume 26, Issue3, Pp.463-477, July 2002.[9] Official Portal Malaysia Meteorological Department

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Inverter-Assisted Wind-Driven Induction Generators, IEEETrans. On Energy Conversion, vol. 19, no. 2, June 2004.[11] Fengge Zhang, Quanfu Shi, Yuxin Wang, FengXiang

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