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Abstract In this paper, we present a variable speed windinduction generator associated to a flywheel energy storage system.

Direct torque control strategy for an induction generator used in the

flywheel energy storage system, is applied. Both rotor flux and DC

bus voltage are regulated by the applied of the standard switching

table for an operation in the 4 quadrants. This system is used for

improving the quality of the electric power delivered by the windgenerator. The proposed system with DTC control strategy is

validated through simulations. Theobtained results are presented anddiscussed.

Keywords Direct Torque control; Flywheel energy storage

systemInduction generator; Variable speed wind turbine;

I. INTRODUCTIONor a standalone operating, the squirrel induction machine

is preferred because it is robust, needs little maintenance

and does not need a supply to magnetize it. The simple

way to use it as an autonomous generator consists in

connecting its stator windings to a capacitor bank in parallel tothe load. The remaining magnetic flux, added to the

magnetizing current through the capacitor bank yields the

built up of the electromotive force and its increase to a useful

value. This approach is very cheap and is well adapted to

convert the wind energy into electrical one for isolated or

faraway areas from the grid distribution [1-3]. However, the

magnitude of the stator voltage and frequency are very

sensitive to both speed and load values. Another way to

achieve an autonomous operating is to connect the stator

windings to a rectifier/inverter. In this case, the device has to

be controlled in order to maintain the DC voltage at a constant

value whatever the speed and the load values as long as the

wind power is sufficient to satisfy the electric needs. The rotorflux oriented control is used to maintain the terminal voltage

constant [4-6]. Besides, different others solutions have been

suggested to control the voltage [6-9].

D.Rekioua, Pr is with the Electrical Engineering department, University of

Bejaia 06000 Algeria (e-mail: dja_rekioua@yahoo.fr).

T. Rekioua, Pr is with the Electrical Engineering department, University of

Bejaia 06000 Algeria (phone: +213 34215006; fax: +213215105; e-mail:

to_reki@ yahoo.fr).

K.Idjdarene, Jr is with the Electrical Engineering Department, University

of Bejaia 06000 Algeria (e-mail: idjdarene@yahoo.fr).

A.M Tounzi, Pr is with L2EP Laboratory, USTL of Lille France (e-mail:

mounaim.tounzi@univ-lille1.fr).

Comparing with the vector control, DTC is a very simple

control scheme with low computational time. Current

regulator, rotor speed sensor, and co-ordinate transformation

is not required with DTC. In generator, the speed of the

machine is already determined by the wind turbine. However,

the speed is essential for coordinate transformation in vector

control.Therefore, DTC scheme is better than vector controlscheme for generator application [5], [11].

For standalone applications storage cost still represents the

major economic restraint. In our application we choose to use

an inertial storage system. Energy storage in wind systems can

be achieved in different ways [12-14]. However the inertial

energy storage adapts well to sudden changes of the power

from the wind generator. Moreover it allows obtaining high

power to weight and number of charge cycles and discharge

very high.

In this paper, we propose to study DTC strategy to control the

DC voltage of an autonomous induction generator connected

to a rectifier when the input speed varies. Due to the important

fluctuations of the wind, a flywheel energy storage system isassociated for improving the quality of the electric power

delivered by the wind generator. To control the flux and the

DC voltage at the rectifier output, we are applied in this work

a standard switching table for an operation in the 4 quadrants.

It is elaborate according to the exits of the hysteresis regulator

of the flux, the hysteresis regulator of the torque and the zone

of position N. The proposed control system is then simulated

using MATLAB-SIMULINK package[15]. The obtained

results are presented and discussed.

II. PROPOSED CONTROL STRUCTUREThe system studied is constituted of a wind turbine, an

induction generator, a rectifier/inverter, and flywheel energystorage system as shown in the Figure 1. The goal of the

device is to provide a constant power and voltage to the load

connected to the rectifier/inverter even if the speed varies.

This can be achieved mainly by the control of the DC bus

voltage at a constant value and the flywheel energy storage

system participate to maintain the power of the load constant

as long as the wind power is sufficient. To control the speed

of the flywheel energy storage system we must find reference

speed which with the system must turn to ensure the energy

transfer required at each time. The reference speed can be

determinate by the reference energy. The power assessmentof

the overall system is given by:

D. Rekioua, T. Rekioua, K. Idjdarene, and A. Tounzi

Control of a wind energy conversion system

associated to a flywheel energy storage system

F

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PPPlP windoadref = (1) Where:Prefis the reference power,Ploadthe load power;Pwindthe wind power and P is the power required to control theDC voltage Vdcat constant value.

Fig. 1. The system studied

III MODEL OF INDUCTION MACHINE

The linear model of the induction machine is widely knownand used. It yields results relatively accurate when the

operating point studied is not so far from the conditions of the

model parameter identification.

This is often the case when the motor operating, at rated

voltage, is studied. In our approach, we adopt the d-q model

of the induction machine expressed in the stator frame noted

by (). The electrical equations are then written as follows:

=

r

r

s

s

rr

rr

s

s

s

s

i

i

i

i

.

RL..p0M..p

L..pRM..p0

00R0

000R

0

0

V

V

+

dt

di

dt

di

dt

di

dt

di

.

L0M0

0L0M

M0L0

0M0L

r

r

s

s

r

r

s

s

(2)

Where Rs, Rr, Ls and Lr are the stator and rotor phase

resistances and leakage inductance respectively, M is the

mutual inductance and the speed.

Besides, Vs, is, Vs and is are the -stator voltages andcurrents respectively along the and axis. irand irare the

-rotor currents along the and axis.

IV.CONTROL STRATEGY

The conditions of dynamic control on the torque of the

induction machine can be highlighted, by the vector model of

the machine. For that one we present the electrical equations

of the machine in the spatial vector:

dt

dIRV ssss

+= (3)

rr

rr jdt

dIR0

+= (4)

The voltage vector Vsis delivered by a three-phase voltage

inverter, whose state of the switches are supposed perfect, is

represented in theory by three (3) Boolean sizes of control Sj

(j=a,b,c) such as :

Sj =1 high switch is closed and low switch is open. Sj =0 high switch is open and low switch is closed.

Thus the voltage vector Vscan be written in the form:

++= 3

4

3

2

2

3

j

c

j

badcs eSeSSVV (5)

The combinations of the three (3) sizes (Sa, Sb, Sc) make it

possible to generate eight (8) positions of the voltage vector

Vs whose two (2) positions correspond to the zero vector :

(SaSbSc) = (111) or (000).

We use Concordia transformation [11]:

( )

=

=

scsbs

sas

iii

ii

2

1

23

(6)

( )

( )

=

=

cbdcs

cbadcs

SSVV

SSSVV

2

1

2

1

2

3

(7)

With: Sj(j=a,b,c) are the Boolean sizes of control.

The magnitude of the stator flux is estimated from its

components along the axes and [10];

( ) ( )

( ) ( )

=

=

t

0

ssss

t

0

ssss

dtiRVt

dtiRVt

(8)

2

s

2

ss += (9)

The electromagnetic torque can be estimated starting from

the estimated sizes of flux (s and s ) and the calculatedsizes of current (isand is) (Fig.2).

ssssme iipT = (10)

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Fig. 2 Block diagram

The switching table applied in this work is a standard table for

an operation in the 4 quadrants. It is elaborate according to the

exits of the hysteresis regulator of the flux, the hysteresis

regulator of the torque and the zone of position N as shown in

table 1. [5] .The vectors V0and V7are alternatively selected

so as to minimize the number of commutation in the arms of

the rectifier/inverter.

V MODELING OF THE FLYWHEEL ENERGY STORAGE SYSTEM

The reference energy for the SISE as follows [14]:

+=2t

1t

ref

1t

crefc dt.PEE (11)

Where: 1tcE is the flywheel initial energy.We determinate the reference speed as follows:

t

refc

refJ

E.2= (12)

With: FlywheelIGt JJJ += (13)

The reference speed is limited in order to maintain the IG in

the area of operation at constant power and not exceed the

maximum speed of the flywheel [14].

Figure.3 represents the torque and power as a function of

speed. We notice that:

Forrated0 The torque may be maximal giving

up a power proportional to the speed = kPIG . For

rated the power is maximum and corresponds to

the rated power of the machine; the electromagnetic torque

is inversely proportional to the speed

kTem= .

So, if we want to have the machine rated power, it is

necessary to use it beyond its rated speed, which lets us to

consider the speed as the lower limit storage and the dual

value of speed as the upper limit storage

(rd/s)

P (W)

Prated

rated 2.rated

(rd/s)

Tem(N.m)

Tem max

rated 2.rated

Fig. 3: Power and torque as a function of speed

Thus a field weakening operation will be necessary to obtain a

constant power in the speed range 1500 to 3000 rpm.

The reference flux is then determinate by:

=rated

rated

rated

ratedrated

efrif

if

(14)

With : Flywheel speed, rated : rated speed, rated :

rated flux and ref : Reference flux.

Simulation is made with a wind power profile which provides

power continuously required by the load through the SISE.

The proposed structure is given in Figure 4.

Figure 5 shows the development of wind power which varies

between 1000W and 2400W. We note that the power supplied

to the load is kept constant through the flywheel energy

storage system. The parameters of the machine are listed in

the table 2.

Figure 6 correspond to the storage power flywheel energy

storage system. This power can be positive or negative. It

depends on the wind power and the power load required. We

note that it is positive when the wind power produced is

greater than the load power required by the load and is

negative when there is a less power produced compared with

that of the load.

TABLEI

SWITCHING TABLE

N 1 2 3 4 5 6

Cflx=1 Ctrq = 1 V2 V3 V4 V5 V6 V1

Ctrq = 0 V7 V0 V7 V0 V7 V0

Ctrq = -1 V6 V1 V2 V3 V4 V5

Cflx=0 Ctrq = 1 V3 V4 V5 V6 V1 V2

Ctrq = 0 V0 V7 V0 V7 V0 V7

Ctrq= -1 V5 V6 V1 V2 V3 V4

TABLE2

PARAMETERS INDUCTION MACHINE

Parameter Value Parameter Value

UN

220/380 V M 0.07767H

NN 3000 rpm J 0.2271 kg.m2

Rr= Rr 0.76 f 0.0022 Nm/rd.s-1

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Fig.4. Proposed Control System

0 5 10 15 20 25 30-2500

-2000

-1500

-1000

-500

0

Fig. 5. Wind and load power

0 5 10 15 20 25 30-1000

-800

-600

-400

-200

0

200

400

600

800

1000

Fig.6.Power storage SISE.

To regulate the bus voltage, we need a required power

represented in Figure 7

0 5 10 15 20 25 30-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

Fig.7. Power of the DC bus

Flywheel speed and the reference speed are represented in

Figure 8. The rotational speed increases when the energy is

transferred to the flywheel, and decreases when the flywheel

is unloaded..

0 5 10 15 20 25 30120

140

160

180

200

220

240

260

280

Fig.8. Flywheel and reference speed

t(s)

t(s)

PFlw

(W)

P(W)

P(W)

t(s)

windP

loadP

t(s)

ref

Flyw

(rd/s)

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The electromagnetic torque follows the evolution of the speed

(Figure 9)

0 5 10 15 20 25 30-5

-4

-3

-2

-1

0

1

2

3

4

5

Fig.9. Electromagnetic torque

The voltage DC bus is kept constant around 465 V, with an

overshoot at startup by about 10% (figure 10).

0 5 10 15 20 25 300

50

100

150

200

250

300

350

400

450

500

550

Fig.10. DC voltage Vdc.

Variations of stator flux are represented in Figure 11. The

fluxs

, and s follows the variations of the speed and

does not exceed the nominal flux.

0 5 10 15 20 25 300

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Fig.11.Statorique flux

The simulation stator flux trajectory is represented in Figure

102

Fig.12. Stator flux trajectory.

VI CONCLUSION

In this paper, a variable speed wind system with a flywheel

energy storage system has been presented and studied. A

direct torque control is applied to the induction machine of the

flywheel energy storage system generator. From the Park

model of the machine, we defined a direct torque control forthe machine operating. The developed control answers the laid

down objectives well. The results of simulation clearly show

the good operation of the flywheel energy storage system

storage. Simulation calculations show the well controlled of

theDC voltage.REFERENCES

[1] D. Rekioua., T. Rekioua, K. Idjdarene and A. M. Tounzi, An approachfor the modelling of an autonomous induction generator taking into

account the saturation effect, International Journal of Emerging Electric

Power Systems (IJEEPS) Vol 04 Iss 1, Nov 2005, pp. 1-23.

[2] A. M. Alsalloum, A. I. Alolah and R. M. Hamouda, Operation of three-phase self-excited induction generator under unbalanced load, in the

Proceeding of Electrimacs 2002, August 18-21, pp. 1-5.

[3] [3] L.Wang and C. H. Lee,A Novel Analysis on the Performance of anIsolated Self-Excited Induction Generator, IEEE Trans. on Energy

Conversion, Vol 12 No 2, June 1997, pp. 109-117.

[4] E. Levi, Y. W. Liao. Rotor flux oriented induction machine as a DCpower generator. In: CDROM of the 8th European Conference on

Power Electronics and Applications EPE99, Lausanne, Switzerland;

1999, p. 1-8.

[5] K. Idjdarene, D. Rekioua, T. Rekioua. and A. M. Tounzi, Controlstrategies for an autonomous induction generator taking the saturation

effect into account In: CDROM of the 12th European Conference on

Power Electronics and Applications EPE07, Aalborg, Denmark, 02-05

September 2007.p. 1-10.

[6] D. Seyoum, C. Grantham, Terminal voltage control of a wind turbinedriven isolated induction generator using stator oriented field control.

In: IEEE transactions on industrial Applications; September 2003, p.

846-852.[7] S.S. Murthy, C. Parabhu, A. K .Tandon., M. O.Vaishya, Analysis of

Series Compensated Self-Excited Induction Generators for Autonomous

Power Generation. In IEEE Conference on Power Electronics, Drives

and Energy Systems for Industrial Growth; 1996, p. 687-693.

[8] V. N. Nandakumar, K. Yadukumar, T. Sureshkumar, S. Ragupathi, R. K.Hegde, A Wind Driven Self-Excited Induction Generator with Terminal

Voltage Controller and Protection Circuits. In IEEE Power Conversion

Conference; 1993, p. 484-489.

[9] R. Bonert, R. Rajakaruna, Self-Excited Induction Generator withExcellent Voltage and Frequency Control In IEE Proc.-Gener. Transm.

Distrib.1998; 145 (1): 33-39.

[10] D. Rekioua, T. Rekioua, S. Alloune, Switching Strategies in DirectTorque Control of Induction Machine: Modelling and simulation,

International Conference Modelling And Simulation (MS2004)-Lyon

France, 4-7 Juillet 2004, pp : 3-183-21.

[11] H. Ziane, J.M. Retif, T.Rekioua, Fixed-switching-frequency DTCcontrol for PM synchronous machine with minimum torque ripples.Canadian Journal of Electrical and Computer Engineering, 33, (3), 2008;

183-189

[12] C. Carrilloa, , A. Feijo, a, and J. Cidrsa, Comparative study offlywheel systems in an isolated wind plant; Renewable Energy Journal ,

Volume 34, Issue 3, March 2009, pp: 890-898.

[13] L. Jerbi, L. Krichen, A. Ouali;A fuzzy logic supervisor for active andreactive power control of a variable speed wind energy conversion

system associated to a flywheel storage system, Electric Power Systems

Research Journal, Volume 79, Issue 6, June 2009, pp: 919-925.

[14] G.O.Cimuca, Systme inertiels de stockage dnergie associe desgnrateurs oliens, Thesis Doctorat Ecole Nationale Suprieure dArts

et Mtiers, Lille (France), 2005.

[15] Simulink: Dynamic System Simulation for Matlab. Using Simulink. TheMathWorks, Inc. Copyright 1984-2002. Release 13.

emT

refT

t(s)

Tem(N.m

)

t(s)

vdc

(V)

t(s)

s(Wb)

s(Wb)

s

(Wb)

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Djamila Rekioua (ZIANI), (1963), studied Electrical

Engineering at the Ecole Nationale Polytechnique of

Algiers, and received her engineer degree in 1987, the

Master Degree (MS93) in 1993 in the same school and

the Doctorate (PHD) in 2002 at the University UFAS Setif

(Algeria).e-mail: dja_rekioua@yahoo.fr

URL:works.bepress.com/djamila_rekioua/Since 1989, she is teaching and pursuing research, firstly

at the University of Sciences and technology, Bab ezzouar (Algiers) and now

she is Professor at the University of Bejaia (Algeria). Some previous

publications:

-D. Rekioua, K. Idjdarene, T. Rekioua Et A. M. Tounzi , An approach for the

modeling of an autonomous induction generator taking into account the

saturation effect , International Journal of Emerging Electric Power

Systems, Volume 4, Issue 1, article 1052, pp 1-25; 2005.

-K. Idjdarene, D. Rekioua, T. Rekioua, A. Tounzi, Contrle dune olienne

en fonctionnement autonome base sur une gnratrice asynchrone Energy

Conversion and Management, Elsevier, Volume 49, Issue 10, October 2008,

Pages 2609-2617.

-S. Lalouni, D. Rekioua, T. Rekioua, E. Matagne, Fuzzy logic control of

stand-alone photovoltaic system with battery storage, Journal of Power

Sources, Elsevier(2009) Volume 193, Issue 2, 5 September 2009, Pages 899-

907

Prof Rekioua Djamila is reviewer in Conferences (CNESOL06, ICRE07,

MS07, WIERA09, SMEE10) and in different Research Journals and her

research interests are in the modeling, control of A.C machines and Electric

drives, Renewable Energy (photovoltaic, wind turbine, and hybrid systems).

Toufik Rekioua(1962), received his Dipl.

d'Ingenieur from the Ecole Nationale Polytechnique

and earned the Diplome of Doctorat d'I.N.P.L of

Nancy (France) in 1991. e-mail: to_reki@yahoo.fr

He is professor at the Electrical department-

University of Bejaia (Algeria). His research

activities have been devoted to several topics:

control of electrical drives, modeling, wind turbine

and control in A.C machines.

Some previous publications:

-M. Hadef, T. Rekioua, M.R. Mekideche And D. Rekioua, A New Direct

Torque Control Method for Switched Reluctance Motor Drives Compared

with Vector Control, Asian Journal of Information Technology 6 (4), pp

462-467, 2007

-Hocine Ziane, Jean-Marie Rtif, Toufik Rekioua

Contrle DTC frquence fixe appliqu une MSAP avec minimisation des

oscillations du couple ; Canadian Journal of Electrical and ComputerEngineering33, 3/4 (2008) 183-189.

K. Idjdarene, D. Rekioua, T. Rekioua, A. Tounzi, Contrle dune olienne

en fonctionnement autonome base sur une gnratrice asynchrone Energy

Conversion and Management, Elsevier, Volume 49, Issue 10, October 2008,

Pages 2609-2617.

Prof Rekioua Toufik is reviewer in different Research Journals and

conferences (ICRE07, WIERA09)

Kassa Idjdarene (1975) studied Electrical Engineering at

the University of Bejaia and received his engineer degree

in 2002, the Master Degree (MS05) in 2005 in the same

University where he is teaching and pursuing his

researches in wind turbine systems.

e-mail: idjdarene@yahoo.fr

Some previous publications:

-D. Rekioua, T. Rekioua, K. Idjdareneet A. M. Tounzi

An Approach for the Modelling of an Autonomous Induction Generator

Taking into Account the Saturation Effect , International Journal of

Emerging Electric Power Systems (IJEEPS), Vol. 04, Iss. 1, Nov 2005, pp: 1-

23. Manuscript 1052, Produced by the Berkeley Electronic Press.

- K.Idjdarene, D. Rekioua, T. Rekioua Et A. M. Tounzi, A Control Strategy

for an Autonomous Induction Generator Taking the Saturation Effect into

Account, European Power Electronics and drives EPE07, Aolborg,

September 2007.

-K. Idjdarene,D. Rekioua, T. Rekioua, et A. M. Tounzi

Vector control of autonomous induction generator taking saturation effect

into account, Energy Conversion and Management (Elsevier science), Vol.

49, Iss. 10, Oct 2008, pp: 2609-2617.

Abdel Mounaim Tounzi (1965) received the Doctorate

(PHD) in 1993 at the Institute polytechnic of Loraine

(INPL)-University of NancyFrence and the HDR in 2004

at the university of Science and Technology of Lille

(Frence). Actually he is teaching and pursuing research in

l2EP Laboratory (USTL).

e-mail:mounaim.tounzi@univ-lille1.fr

Some previous publications:

-Ghislain Remy, Julien GOMAND, Abdelmounam TOUNZI, Pierre-Jean

BARRE "Analysis of the force ripples of a current loaded PMLSM"

COMPEL: The International Journal for Computation and Mathematics in

Electrical and Electronic Engineering, Vol. 28, N. 3, pages. 750 - 761, ISBN.

DOI: 10.1108/03321640910941007, 6-2009.

- Kassa IDJDARENE, Djamila Rkioua, Toufik Rkioua, AbdelmounamTOUNZI

"Vector control of autonomous induction generator taking saturation effect

into account", Energy Conversion and Management, 11-2008

-Mathieu AUBERTIN, Abdelmounam TOUNZI,Yvonnick LE MENACH

"Study of an electromagnetic gearbox involving two permanent magnet

synchronous machines using 3D-FEM", IEEE Trans Mag, Vol. 44, N. 11,

pages. 4381 - 4384, 10-2008

Prof Tounzi is reviewer in conferences (JCGE2001, ICRE07, IECON2009,)

and in different Research Journals (IEEE trans.mag, IEEE trans Industrial

Electronics, EPJ-AP, RIGE, IJEEPS) and his research interests are in the 3D

finite elements, magnetic materials and wind turbine.