a review on maximum power point control ...the mppt controllers can be classi ed as tip speed ratio...

A REVIEW ON MAXIMUM POWERPOINT CONTROL TECHNIQUESAPPLIED IN WIND ENERGY

CONVERSION SYTEM

S.Vijayalakshmi1, C.Anuradha2,V.Vinitha3, V. Ganapathy4

1,2,3Departement of Electrical and Electronics Engineering,4Departement of Information and Technology,

SRM Institute of Science and Technology, Chennai, [email protected],

[email protected]

April 9, 2018

Abstract

This paper presents the review on Maximum Power PointTracking (MPPT) algorithms applied in wind energy con-version system (WECS). The MPPT algorithms are used formaximizing the power from the wind turbine generator aswind energy is nonlinear system. Therefore, it is necessaryto track maximum power irrespective of the varying natureof the wind. The different MPPT techniques suggested bythe researchers are discussed in this review paper.

Key Words:WECS, Hill Climb Search, Tip Speed ra-tio, MPPT.

1 Introduction

Among available alternative sources, the wind energy systems havebecome more popular as it is available abundantly and renewable.

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International Journal of Pure and Applied MathematicsVolume 118 No. 24 2018ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

It converts the kinetic energy from wind to the utilizable power.Wind turbine, Generator, and control system are the major appara-tus of Wind Energy Conversion System (WECS).Wind turbine con-verts kinetic energy to mechanical power. This mechanical powerrotates the generator which yields electrical electricity. The windturbines are of types, the horizontal axis and Vertical axis.Fig.1.shows generated wind power output for every year from 1990 to2017, each year wind power generation is increased. This surveydescribes the consumption of wind power in global level [23].

Fig.1. Global Wind power Consumption

Maximum energy can be obtained when the tip speed ratio isheld constant. The maximum power taken out from the MPPT isrealized only through dc link power. The MPPT control algorithmsteps with the DC-DC converters are simulated using matlab simu-lation/ simulink. The information of the wind turbine characteris-tics is not necessary for the control algorithm to work [1].The stan-dard methods of MPPT in wind energy conversion system includesTip speed Ratio (TSR) , Power Signal Feedback (PSF) and favouredHCS method [1],[3],[5]. The MPPT algorithm has been consideredwhich captured maximum power for alternating wind condition Toattain better convergence of the control, the proposed algorithmuses a modified HCS algorithm [2].In TSR control scheme, the tur-bine should constantly operate at optimum tip speed ratio. It ispossible by controlling the rotational speed to be estimated by re-quiring the knowledge of optimum TSR to enable the maximumpossible power. The main purpose of vector control strategy is tomaintain the quadrature current component as zero [5].

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International Journal of Pure and Applied Mathematics Special Issue

Fig.2. Wind energy conversion system:

The Fig.1. Shows the wind generator system connected to thegrid through back to back converter and a boost converter, the windpower is estimated by [1].The wind power is estimated by,

Pwind =1

2ρaV 3 (1)

The tip speed ratio of a wind turbine is defined as

λ =VωR

v(2)

The output power of the wind turbine it is calculated as:

Pt =1

2Cp(λ)aV 3 (3)

The generated mechanical power is given by

Pmech = Tmech(t)ωR(t) (4)

Fig. 3. Mechanical Power versus Wind speed

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The MPPT controllers can be classified as Tip Speed Ratio(TSR), Hill-Climbing Search (HCS), and Power Signal Feedback(PSF). The standard HCS MPPT method is applied in WECS.The variable wind speed of the turbine is given as,

TSR =Linearspeedofbladeoutermosttip

Freeupstreamwindvelocity=ωR

v(5)

The maximum power is extracted from this TSR method reg-ulates the rotational speed of the generator in order to maintainthe TSR to an optimum value. This met-hod needs the measuredvalue of the turbine speed. In PSF control, it needs the knowledgeof the power which is obtained by using recorded datas in lookup ta-ble. TSR and PSF are mostly used for generating maximum power.These two methods required field test first and then assembled tothe controllers. In HCS method, it does not need any field test andit is not dependent of wind turbine characteristics. An Intelligentcontrol approach [3] is usedto operate the variable wind turbine atmaximum power point. It utilizes the generator speed and activepower of the squirrel cage induction generator (SCIG). Here theFuzzy Logic which used the gradient detection method to operatethe system at maximum power point (MPP).The turbine is startedfrom stopped mode in the region 1. Region 2 is an operationalmode where control strategies were applied to maximize the windcaption. In the Region 3 the turbine is operated with in the region.The power and torque obtained in the region 2 are presented as,

Fig. 4. Fredkin Gate

Pwt =1

2ρCpπR

2V 3 (6)

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Twt =1

2ρCr(λ)πR3V 2 (7)

The following equation provides the expression of optimum powerof the wind turbine.

Pwtopt =ρCpmax(λopt)πR

2Ωt3

2λ3opt(8)

Nonlinear control is applied for wind energy conversion systemwith PMSG [4] to generate the maximum power from wind tur-bine. The control scheme combines the capability of MPPT andSliding Mode control (SMC); it increased the performance of thesystem. The adopted MPPT a controller generatesωm opt, the max-imum power is produced by applying the reference speed to thespeed control loop. In addition, SMC scheme is used, it gathersfrom the equation of,

Te =3

2PnIqψf (9)

Jdωm

dt= Te − Tm − Fωm (10)

Fig. 5. Pitch angle controller

By regulating the q-axis stator current, the wind turbine speedis controlled with sliding mode control.

Sω = ωm−opt − ωm (11)

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Fig. 6.Schematic of Control strategy for WECS based on thePMSG

The control schemes at generator and grid side converter arerealized using SMC, where the q-axis current is controlled by usinggenerator speed which varies according to the variation of windspeed. In the ref [5] the TSR method is applied to achieve themaximum power, hence the turbine is operated at the optimumTip Speed ratioλopt , where

λopt =Ωopt

VωR (12)

It is found that tip speed ratio and power coefficient is main-tained constant. But the control algorithm requires the wind speeddata and rotational speed of the generator.

Fig. 7. Overall WECS studied scheme

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The rectifier model make easier and reduce the simulation timeby modelling the rectifier with ideal switches. In the ref [9], sen-sor less control method is adopted. By controlling the current con-trolled output voltage of the inverter, the maximum power has beenobtained. This method involves two tracking loops, (i) Power Map-ping loop (ii) Alternative frequency derivative loop. This tracingsignal requires output power from the WECS and PMSG statorfrequency. In this power mapping loop, the power (Pdc) versus Vdccharacteristics are plotted. From that characteristic, the operatingdc voltage and operating power are calculated in alternative fre-quency loop, the gain is maintained zero when the dc operatingvoltage leads the maximum power [10].

Fig. 8. WECS Controlled Block Diagram

The active power is used as the generator input. The sensor lessmode is combined with Extremum Seeking (ESC) method which isused for generating the reference speed. The Fig.8. Shows thesliding mode ESC algorithm for generating the MPPT.

Fig. 9. Block diagram of Sliding Mode ECS algorithm

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The performance of SM-ESC is verified with doublyfed induc-tion generator. Wind speed, Air density or Turbine parameters in-formation are not needed in sensor less MPPT controller [11].Thisalgorithm used the instant power for producing the optimum ref-erence speed at the machine side converter controller. It presentedthe enhanced tracking ability under dynamic and steady state con-ditions. The voltage reference is provided by the current controllers.

Fig.10. Machine side converter controller

The capability of controlling fast dynamic COMPgdandCOMPgq

are added to the direct and quadrature axis as shown in Fig.9. Forhigh and low wind speeds the controller have the capability of goodtracking, the value of C should vary simultaneously with the windspeed. Generated power is regulated using Field Oriented control[14] as depicted in the Fig.9. The maximum power is obtainedwith power control algorithm. The control algorithm generates thepower at the optimum torque. When PMPPT > Pg, Pgis providedand hence the optimum torque is obtained from the optimum power,

Tg =Pg

ωopt

(13)

Pg= rated power, so that Pg is produced and is less than PMPPT .IfPMPPT ≤ Pg, Pgis not obtained from the turbine thus the maximumpower is extracted and the torque is given by,

Tg =PMPPT

ωopt

(14)

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Also pitch control is incorporated in order to control the gen-erator speed. When the wind velocities are larger than the rated,thus the maximum energy is obtained by using pitch control.

Fig.11.Schematic diagram of control strategy for generator andgrid side converter

The Radial Basis Function Neural Network (RBFNN) and Par-ticle Swarm Optimization (PSO) were applied to achieve maximumpower [15]. The RBFNN approach has increased the maximumpower output of the wind generator at varying wind velocity. Theexperimental tests were also done to validate the results. The PSO-Recurrent Neural controller has been implemented for generatingMPPT, the hybrid method have shown better tracking of maxi-mum power [22]. The combination of two modules ANN and PSOare used to improve the power output for different conditions. Atransfer control strategy which used the modified (P&O) algorithm[18] for the MPPT control. The system dynamic can be derivedin that stall region is used to help in designing a robust controlsystem. When the maximum powers obtained are beyond the sys-tem rating, the stall control is activated to limit the power. TheP&O algorithm is used in this reference because of simplicity. Themain focus of this reference was in implementing the stall controlstrategy which is depicted in the Fig.12.

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Fig.12. Proposed overall control strategy schematic diagram

2 CONCLUSION

This paper summarizes the various MPPT techniques applied tothe wind energy conversions system. The standard MPPT meth-ods TSR, PSF and HCS were frequently used. Then P&O methodis found to be flexible and simple method for implementation. Butthen again speed of the convergence is less, which lessens the ef-ficiency of the system. By using the combination of MPPT andSliding Mode Nonlinear control, it shows the good performanceunder various wind speed. With the artificial neural network themaximum power is generated, But it find difficult in implementing.In near future with the new technique in the ANN combined withevolutionary algorithm control can be applied for better operationof the wind energy conversion system to regulate the power of thewind turbine generators.

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a review on maximum power point control ...the mppt controllers can be classi ed as tip speed ratio...

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