Induction Generator Using AC/DC/AC PWM Converters and Its Application To The

Wind-Energy SystemsAdel MEHDI#1, Abdelmalek BOULAHIA#2, Houssam MEDOUCE#3, Hocine BENALLA#4

#Electrotechnic Department University of Mentouri Ain el-bey 25000 Constantine, ALGERIA

1 adel_hdm@yahoo.com2 boulahiamalek@yahoo.fr3 b.houssam19@hotmail.fr

4 benalladz@yahoo.fr

Abstract— This paper proposes basic concepts of a fixed speed wind turbine (FSWT), as an introduction to a modern wind turbine concept; also, energy extraction from the wind turbine, in order to highlight the utility of the DPC in the field of the quality of the electric power. The VOC will be compared with structure known as DPC [1] applied to IG wind turbine connected to the network using back-to-back PWM converters. The theoretical principle of this method is discussed. From the simulation results, it is shown that DPC-2L displays several features, such as a simple algorithm and good dynamic response.

The turbine side converter is controlled by the structure known Indirect Field Oriented Control (IFOC) in tow case [2]. Both VOC and DPC are simulated using MATLAB software to illustrate the performance of each technic. Keywords: back-to-back PWM converters, Direct power control, Voltage oriented control, Wind power, power quality, squirrel cage induction generators (SCIG).

I. INTRODUCTION

uring the last years the low and medium voltage networks have been connected with new active systems such as

wind turbines, photovoltaic systems, storage devices, units of improvement the quality of power and other. Almost all of these new systems are connected to the grid by VSC (VSC Voltage Source Converter) with a filter. The rectifier at two levels based on a PWM control was typically used as robust and highly effective solution. Power electronics is becoming increasingly used by various systems related to power generation, and industrial equipment. Among the generating systems are cited, the wind power is growing rapidly around the world. This expansion is due to research efforts in this domain. So, wind turbines are widely developed as an alternative energy source in providing a cost-effective option in the energy market. Therefore, the wind sector shows great interest for direct connection of these units for medium voltage networks for future developments [3].

Furthermore, liberalization of the electricity market stimulates new ideas as in the case of power quality [3]. The distribution and transport are usually based on the airlines and

are very susceptible to the weather effect and other external actions.

To provide consumers with a good quality of electric power, even in the operating conditions of the most disturbed, several solutions are proposed [4,11], one of the innovative solutions proposed direct power control "DPC"[5]. This technique is based on the concept of direct torque control "DTC" applied to electrical machines. The objective is to directly control the active and reactive power in a PWM rectifier; the same principle has been applied previously to control the torque and flux in electrical machinery alternatives. In our case we have applied DPC to wind generation systems.

II. WIND ENERGY CONVERSION SYSTEMS

Figure 1. Induction machine (SCIG) based wind turbine [6].

A. Wind Turbine Induction Generator The wind turbine and the induction generator (WTIG) are

shown in Figure1. The stator winding is connected by back-to-back PWM converters (AC/DC/AC) to the grid and the rotor is driven by the wind turbine. The power captured by the wind turbine is converted into electrical power by the induction generator and is transmitted to the grid by the stator winding. The back-to-back PWM converters are controlled in order to limit the generator output power to its nominal value for variable wind speeds. In order to generate power the induction generator speed must be slightly above the synchronous speed. But the speed variation is typically so small that the WTIG is considered to be a fixed-speed wind generator. The reactive power absorbed by the induction generator is provided by the grid or by some devices like capacitor banks, SVC, STATCOM or synchronous condenser [6].

The fixed speed is related to the fact that an asynchronous machine coupled to a fixed frequency electrical network

D

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rotates at a quasi-fixed mechanical speed inwind speed.

B. Wind velocity The wind speed is a three-dimensional

the direction of the vector wind speed in the important in terms of the wing wind becausits active surface. For simplification, the moving in the horizontal plane The verticadevoid of any device orientation of the bsurface is always facing the wind), then the bof the wind is simplified considerably. Ththerefore be modelled as a scalar function time [3].

Figure 2. Wind velocity

�� � ����The wind speed will be modelled i

determined by a sum of several harmonics in

����� � ��� � ����������

���C. Wind Turbine

The model is based on the stecharacteristics of the turbine. The stiffness oinfinite and the friction factor and the inermust be combined with those of the generaturbine. The output power of the turbinefollowing equation [7].

�� � ����� �� �� � ��!"

Where

�� Mechanical output power o

�� Performance coefficient of

� Air density (kg/m3)

Turbine swept area (m2)

� ��! Wind speed (m/s)

� Tip speed ratio of the rotorwind speed

� Blade pitch angle (deg)

ndependent of the

vector. However, vertical axis is not e it is not seen by velocity vector is al axis blades are blades (the active behavioural pattern e wind speed can that changes over

�#� in this study, as the form

��� ���

eady-state power of the drive train is rtia of the turbine

ator coupled to the is given by the

�$�

of the turbine (W)

f the turbine

r blade tip speed to

Figure 3. Wind tu

A generic equation is used tbased on the modeling turbine c

����� �� � �� %�&�� 'With

#�� �

#� � (

The coefficients )� to )* ar0.4, )+ = 5, ), = 21 and )* = 0.

The ��� - �� characteristipitch angle�, are illustrated inof �� (��. / = 0.48) is achiev8.1. This particular value of � (��0�).

D. Asynchronous Machine The electrical part of the

fourth-order state-space modelsecond-order system. All elecare referred to the stator. All sthe arbitrary two-axis reference

Figure 4. IG elect

0 5

0

0.1

0.2

0.3

0.4

0.5

Cp

Cpmax

B=2

lamdano

12

urbine characteristics

to model����� ��. This equation, characteristics of [7], is

�"� ' �+3 45678 � �*� �9�

#(:(;� '

(:($<�" � # �<�

re: )� = 0.5176, )& = 116, )" = .0068.

ics, for different values of the n Figure 3. The maximum value ved for � = 0 degree and for � = is defined as the nominal value

e machine is represented by a l and the mechanical part by a ctrical variables and parameters stator and rotor quantities are in e frame (dq frame) [8].

tric equivalent model

10 15

X: 8.147Y: 0.48

lamda

B=0

B=5

B=10

B=1520

om

1=>�

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Electrical System

�?2 � @2A?2 � BB� 1?2 � C1!2 �D�

�!2 � @2A!2 � BB� 1!2 ' C1?2 �E�

�?= � @=A?= � BB� 1?= � �C'C=�1!= �;�

�!= � @=A!= � BB� 1!= ' �C'C=�1?= �F�

GH � #:<IJ1!2A?2 ' 1?2A!2K �#(� Mechanical System

BB� C� �

#L �GH ' MC� ' G�� �##�

BB� N� � C� �#��

Where

1?2 � >2A?2 � >�A?= �#$� 1!2 � >2A!2 � >�A!= �#9� 1?= � >=A?= � >�A?2 �#<� 1!= � >=A!= � >�A!2 �#D� >2 � >O2 � >� �#E� >= � >O= � >� �#;�

III. BACK-TO-BACK POWER ELECTRONIC CONVERTER

A. Vector Control of the Machine Side Converter The induction generator is fed by a current-controlled

PWM converter which is built using a forced-commutated device (MOSFET). The speed control loop uses a proportional-integral controller to produce the quadrature-axis current reference iq* which controls the motor torque. The motor flux is controlled by the direct-axis current reference id*. Block DQ-ABC is used to convert id* and iq* into current references ia*, ib*, and ic* for the PWM modulator. Current and Voltage Measurement blocks provide signals for visualization purpose. Revolution speed, current, and torque signals are available at the output of the 'Asynchronous Machine' block [2, 18].

A?2P � % �>=$I>�3 P %GHP1=3 �#F�

A!2P � 1=PQ>� ��(�

NH � R�C= � C2O� ��#�

C2O � �>� P A?2�Q�G= P 1=� ����

1= � >� P A!2 P ## � G= S T ��$�

G= � >=Q@= ��9� The speed controller is based on a PI regulator. The outputs

of this regulator are set points for the torque applied to the IFOC controller.

Figure 5. Speed Controller

Figure 6. Indirect Field Oriented Controller [9]

Simulations results

Figure 7. Induction machine current (IFOC)

Figure 8. Machine speed (IFOC)

4 4.02 4.04 4.06 4.08 4.1 4.12 4.14 4.16 4.18

-50

0

50

Time (s)

Mac

hine

cur

rent

(A)

0 2 4 6 8 10

200

400

600

g y y

Time (s)

Mac

hine

spe

ed(r

pm)

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Figure 9. Electromagnetic Torque Te (N.m)

B. Vector Control of the Grid Side Converter • Grid Voltage Oriented Vector Control

The grid side converter is in charge of controlling part of the power flow of the IG. This power flow that goes through the stator flows also through the DC link and finally is transmitted by the grid side converter to the grid [13]. The simplified block diagram of the grid side system, together with a schematic of its control block diagram, is given in Figure 10.

Figure 10. Grid voltage oriented vector control

The output voltage of the converter, are generated in order to control the voltage of the DC link and the reactive power exchanged with the grid. This is done, in general, according to a closed loop control law. Some typical controls are vector control or direct power control. However, in this section we are studies the grid voltage oriented vector control (GVOVC) [10].

On the other hand, control of DC link voltage is necessary since, the DC link is mainly formed by a capacitor. Thus, the active power flow through the stator must cross the DC link and then it must be transmitted to the grid. Therefore, by only controlling the Vdc variable to a constant value, this active power flow through the converters is ensured, together with a guarantee that both grid and machine side converters have available the required DC voltage to work properly.

In a similar way, one variable that can also be controlled with this schema is the reactive power exchange with the grid. In general, it can take different values depending on which current are to be minimized.

Since, for this control the magnitudes that must be measured are the grid side current and voltage, together with the DC link voltage [6].

Simulations results

Figure 11. DC link voltage (VOC)

Figure 12. Grid current and THD (VOC)

• Direct power control

Direct power control is based on the same control principles as the direct torque control technic. The unique difference is the directly controlled variables. In the case of DTC, the electromagnetic torque and the rotor flux are directly controlled, while in DPC, the stator active and reactive powers are directly controlled.

� � UVAV � UWAW � UXAX ��<�

Y � #Z$ [UV�AX ' AW� � UW�AV ' AX� � UX�AW ' AV�\��D�

N � ]^� _ %U`Ua3 ��E�Table I Switching Table

bc bI N� N& N" N+ N, N* Nd Ne Nf N�g N�� N�&0 0 U* U� U� U& U& U" U" U+ U+ U, U, U*0 1 U* Ud U� Ug U& Ud U" Ug U+ Ud U, Ug1 0 U� U& U& U" U" U+ U+ U, U, U* U* U� 1 1 Ud Ud Ug Ug Ud Ud Ug Ug Ud Ud Ug Ug

0 2 4 6 8 10-400

-200

0

200

Time (s)

0 2 4 6 8 10

200

400

600

800

1000

Time (s)

Vol

tage

Vdc

(v)

4 4.02 4.04 4.06 4.08 4.1 4.12 4.14 4.16 4.18-50

0

50

Time (s)

0 5 10 15 200

0.2

0.4

0.6

0.8

1

Harmonic order

Fundamental (50Hz) = 50.94 , THD= 2.93%

Mag

(%

of

Fun

dam

enta

l)

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Figure 13. Grid direct power control (DPC)

Figure 14. DC link voltage (DPC)

Figure 15. Grid current and THD (DPC)

Table II Parameters used in simulation

Power 37 kW Grid side inductor 2e-3H Grid voltage 250V Grid side resistor 3.87e-3� DC link voltage 900V DC link capacitor 1e-3F Current -35A Load resistor(DC link) 34.4� Switching frequency(voc) 2.5kHz Simple time (VOC) 1e-4s Grid frequency 50 Hz Simple time (DPC) 2e-5s

Figure 16. Active power (DPC)

Figure 7 shows the behaviour of the current generated by the induction machine (SCIG) with a root mean square value of 35(A) and a frequency of 16.66 (Hz), this value of frequency produce the mechanical speed of the rotor given under the Figure 8 this speed is approximately of 52 rad/s at t=4s (C= � I P C�)

A negative electromagnetic torque is applied to the shaft of the induction machine in Figure 9 that means The Asynchronous Machine operates in generator mode, we notes that the torque is around his nominal value 240 (N.m), however the machine acts as a motor mode (t=7 s) if the speed of wind is too low (t=7 s; wind velocity is 2 m/s), thus the mode of operation is dictated by the sign of the mechanical torque.

The voltage of the DC link capacitor is controlled using voltage oriented control, the response is showed in Figure 11 we can see the dynamic behaviour by the rise time which to reach (2 s) with a overshoot ratio 14%.

At (t=7 s), a big variation in the rotor speed is appeared. We can see that the dynamic response of the DC regulator to this sudden variation in the rotor speed (52 rad/s to 41 rad/s) is not satisfactory. The DC voltage is back to 900 V within 100 cycles (f=50Hz).

The current waveform can also be identified in the Figure.12; the FFT Analysis displays the frequency spectrum of the grid current. As expected, The Total Harmonic Distortion (THD) is displayed above the spectrum (THD=2.93%).

At (t=4 s to 4.2 s) the current to be injected to the grid gives a good waveform his frequency is f= 50 Hz and the RMS value is 35A.

Now the grid side converter is controlled using another algorithm known the direct power control the response voltage of the DC link capacitor, is showed in Figure 14 we can see also the dynamic behaviour by the rise time which to reach just one second (1s) with a overshoot ratio11%.

But the grid current waveform is not satisfactory witch deliver a bad total harmonic distortion (THD=13.63) however a small simple time Ts=2e-5s (Fs=50 KHz) is used [14, 15, 17].

The power active controlled is shown in the last Figure 16 we can see that in motoring mode the induction machine absorbed a great power due to the load torque and the inertia of the turbine.

0 2 4 6 8 10

200

400

600

800

1000g y ( ) y

Time (s)

DC

link

vol

tage

Vdc

(v)

4 4.02 4.04 4.06 4.08 4.1 4.12 4.14 4.16 4.18

-50

0

50

Time (s)

Grid

cur

rent

(A)

0 5 10 15 200

2

4

6

8

Harmonic order

Fundamental (50Hz) = 54.13 , THD= 13.63%

Mag

(%

of

Fun

dam

enta

l)

0 2 4 6 8 10

2

4

6

8

x 104

Time (s)

Act

ive

pow

er(w

)

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IV. COMPARISON BETWEEN DPC AND VOCTable III Advantages of DPC and VOC

Technique Advantages

VOC

*Fixed switching Freq. (easier design input filter) *Advanced PWM strategies can be used*Cheap A/D converters*Good steady state performance

DPC

*No separate PWM block *No current regulation loop *No coordinate transformation *Good dynamics performances *Simple algorithm *Decoupled active and reactive power control*Instantaneous variables with all harmonics componentsestimated (improve power factor and efficiency)

Table IV Disadvantages of DPC and VOC

Technique Disadvantages

VOC

*Coordinate transformation and decoupling betweenactive and reactive components is required *Complex algorithm*Input power factor lower than for DPC*Dynamic response is slower than that of DPC control

DPC

*High inductance and sample frequency needed*Power and voltage estimation should be avoid at the moment of switching *Variable switching frequency *Fast microprocessor and A/D converters required

V. CONCLUSION This paper has described two concepts to improve the total

power factor and efficiency of the PWM converter. The first key point of the method is direct instantaneous active and reactive power control of the converter, and the second one is an indirect technique voltages oriented control. The active and reactive power can be regulated directly by relay control of the power, which is implemented by using several comparators and a switching table. In this configuration, the errors between the power commands and the feedback signals are compared by the hysteresis elements, and the specific switching state of the converter is appropriately selected by the switching table, so that the errors can be restricted within the hysteresis bands

A good steady state performance could be obtained with VOC and also current control could be easier implemented compared with the other technic. On all accounts, it can be concluded that all the two control schemes can offer satisfactory performance while each has itself particular merits and drawbacks. The selection of the optimal scheme should therefore depend on the specific application.

The renewable energy based on the wind Conversion Systems need a robust algorithms and a good dynamic response witch can always supported the sudden variation of the wind speed. The DPC become more suitable if we combined it with other techniques to give DPC-SVM [12, 16].

REFERENCES

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[2] Marian P. Kamierkowski “Control Strategies for PWM Rectifier/inverter-Fed Induction Motors” ISIE'2000, Cholula, Puebla, Mexico

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[4] A. López, de Heredia. «Commandes Avancees Des Systemes Dedies A L’amelioration De La Qualite De L’energie :De La Basse Tension A La Montee En Tension.» thèse doctorat. GRENOBLE, Génie Electrique. ,14 novembre 2006.

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[7] Siegfried Heier, "Grid Integration of Wind Energy Conversion Systems," John Wiley & Sons Ltd, 1998, ISBN 0-471-97143-X

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[9] Bose, B. K., Modern Power Electronics and AC Drives, Prentice-Hall, N.J., 2002.

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[13] J.Hu, L.Shang, Y.He, Z.Q.Zhu,. «Direct Active and Reactive Power Regulation of Grid-Connected DC/AC Converters Using Sliding Mode Control Approach.» IEEE TRANS, POWER ELECTRONICS. Vol. 26. n° 1. JANUARY 2011.

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