Power Control of DFIG-generators for Wind Turbines Variable-speed

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 8, No. 1, March 2017, pp. 444~453
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v8i1.pp444-453  444
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Power Control of DFIG-generators for Wind Turbines
Variable-speed
Ihedrane Yasmine1
, El Bekkali Chakib2
, Bossoufi Badre3
1,2
LISTA Laboratory, Faculty of Sciences Dhar El Mahraz, University Sidi Mohammed Ben Abdellah, Fez, Morocco
3
Laboratory LGEM, Higher School of Technology, EST-Oujda, University of Mohammed I, Oujda, Morocco
Article Info ABSTRACT
Article history:
Received Nov 01, 2016
Revised Jan 07, 2017
Accepted Jan 17, 2017
In this paper, we focus on the modeling and control of a wind power system
based on a double-fed induction generator DFIG. We proposed a technique
of active and reactive power control to improve the performance and
dynamics of variable speed wind system. The objective of the modeling is to
apply the direct and indirect vector control stator flux orientation to control
independently, the active and reactive power generated doubly-fed induction
generator (DFIG). The simulation results are tested and compared in order to
evaluate the performance of the proposed system.
Keyword:
DFIG
Direct field oriented control
(DFOC)
Indirect field oriented control
(IDFOC)
Wind
Copyright © 2017 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Ihedrane Yasmine,
LISTA Laboratory, Faculty of Sciences Dhar El Mahraz,
University Sidi Mohammed Ben Abdellah,
Fez, Morocco.
Email: yasmine.ihedrane1991@gmail.com
1. INTRODUCTION
By reason of the fight against the green house effect and CO2 emissions into the atmosphere,
renewables have experienced strong growth in recent years. Among these sources of energy are found '' wind
generators '' that occupy a particular place [1],[2].
The wind power system using doubly fed induction generator composed by stator circuit connected
directly to the network. A second rotor circuit is also connected to the network but via power converters
[1],[3]. Since the rotor power transited is lower, the cost of the converters is reduced in comparison with a
variable speed wind with a stator circuit connected to the network by power converters.This is the main
reason why we find this generator for the production of high power. A second reason is the ability to adjust
the voltage at the connection point of this generator [2]-[5].
The first part of this article will be devoted to modeling various parts of the wind power system
based on the DFIG. The second part of the field oriented control specifically the stator field oriented control.
In the third part with the use of MATLAB / Simulink we will present and analyze the simulation results to
validate our theoretical study.
2. MODELING THE WIND CONVERSION CHAIN
A wind turbine is a device that converts a portion of the kinetic energy of wind into mechanical
energy and then into electrical energy via a generator through a speed gain multiplier G [6], as the Figure 1
shows below [2]:

IJPEDS ISSN: 2088-8694 
Power Control of DFIG-generators for Wind Turbines Variable Speed (Ihedrane Yasmine)
445
Figure 1. Wind turbine system
2.1. Wind Turbine Model
The wind power is given by [7]:
(1)
ρ: density of the area which equal to 1,225 Kg m3
s: the swept surface area of the turbine (π .R²)
v: wind speed
The wind turbine can only convert a percentage of the power of wind [6],[8].The latter is
represented by C p (λ, β) which is a function of speed ratio λ and the angle of the blade orientation β. The
aerodynamic power of the turbine [6],[8],[9] is given by:
( )
(2)
The power coefficient Cp represents the aerodynamic efficiency of the wind turbine [6]. It depends
on the characteristic of the turbine [6],[8]. It can be described as follows:
( ) ( ) ( ) (3)
With : ( )
and C1=0.5179 ; C2=116 ; C3=0.4 ; C4=5 ; C5=21 ; C6=0.0068. The speed ratio
λ, which is defined as the ratio between the linear speed of the turbine Ωt and the wind speed v [6],[8]:
(4)
Knowing the speed of the turbine the aerodynamic torque is given by:
( )
(5)
2.2. Multiplier Model
The speed multiplier [10]-[13] represents the connection between the turbine and the generator. It
aims at adapting the slow speed of the turbine Ωt with the Ωmec high speed of the generator .
{ (6)

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IJPEDS Vol. 8, No. 1, March 2017 : 444 – 453
446
2.3. Mechanical Shaft Model
The fundamental equation of dynamics permits the determination of the evolution of the mechanical
speed from the mechanical torque (Cem) applied to the rotor [14]:
(7)
With: J: the total inertia that appears on the generator rotor
This mechanical torque takes into account; the electromagnetic torque Cem produced by the
generator, the torque of the viscous friction torque Cf and that of multiplier Cg.
With: . When replacing the mechanical torque and torque friction by their expressions, we
obtain the following relation [5],[2]:
(8)
The wind speed is given as a sum of several harmonics:
∑ ( ) (9)
Figure 2. Wind speed reference
2.4. DFIG-Generator Model
Before doing the modeling of an asynchronous machine, it is necessary to represent a two-phase
model (d, q) given by Park transformation [15].
The equations for the stator voltages Vs (d, q) and the rotor Vr (d, q) of the dynamic model of the
DFIG are expressed by [16]:
{
(10)
The field equations are given by [16],[2]:
{ (11)

447
The mechanical equation is given by [17]:
(12)
The expression of electromagnetic torque based on stator field and currents is given, by:
( ) (13)
With :
Φs (d, q), Φr (d, q): stator and rotor field in the reference of PARK.
Is (d, q), Ir (d, q) stator and rotor currents in the reference of PARK.
Rs, Rr: stator and rotor resistances.
Ls, Lr: cyclical Inductors own stator and rotor.
M: mutual inductance cycle.
p: Number of pole pairs of the machine.
ωs: Pulse of the stator electrical quantities.
ωr: Pulse of the rotor electrical quantities.
3. FIELD ORIENTED CONTROL
3.1. PRINCIPLE OF CONTROL
The principle of control by stator field direction is to orient the stator field along the axis 'd' [18],
that is to say : Φsd = Φs and Φsq = 0. We know that for medium and large power equipment used in wind
turbines, the stator resistance is negligible, and it is assumed that the grid is stable, the field is constant.
The previous equations becomes as follows:
Expressions of the field [4]:
{ (14)
We can obtain from the equation (14) the relation between the stator and rotor currents, we get the
following equation [11]:
{
( )
(15)
The expression of electromagnetic torque [11]:
(16)
Based on the assumptions:
{ (17)
The stator active and reactive powers are written according to following expressions [11]:
{ (18)
By replacing the equation (18) by (14), (15) and (17), we get following expression of power [11]:
{ (19)
The expression of the rotor field becomes:

 ISSN: 2088-8694
448
{
( )
( )
(20)
From these equations we can deduce the relation between the rotor voltages (d, q) and the rotor currents
(d, q):
{
( ( )) ( )
( ( )) ( )
(21)
These equations allow establishing a block diagram of the electrical system that can be regulated given by
the figure below:
Figure 3. Block diagram of the DFIG
In this diagram we can notice that the powers and the voltages are linked by transfer functions with
first order. Because the value of the slip „‟ g „‟ is low and the influences of the coupling is weak, the d and q
axes can be controlled separately. This will allow us to easily establish vector control [3].
3.2. Field Oriented Control
There are two ways to perform the control power of this machine. The first is to ignore the terms
of coupling and to establish an independent regulator in each axis to independently control the active and
reactive power. This method is called Direct Method [2]-[4],[7],[8] because the power regulators directly
control the rotor voltage of the machine as shown in the following Figure:

449
Figure 4. Direct vector control
The second method is to consider the coupling terms and compensate them performing a
system with two loops to control the powers and rotor currents. This method which is called Indirect
Method [3] is generated directly from equations (19) and (21).
Figure 5. Indirect Field Oriented Control
4. SIMULATIONS RESULTS
We subjected the wind system to a speed of variable wind. We chose a reference active power Psref
and reference reactive power Qsref in the form of steps. The following figures represent the overall pattern of
the wind system in the MATLAB / SIMULINK. The simulation results are summarized in Figure 6:

 ISSN: 2088-8694
450
Figure 6. Wind System and direct field oriented control with MATLAB /SIMULINK
Figure 7. Wind System and indirect field oriented control with MATLAB /SIMULINK
According to (Figure 8), we see that the power steps are followed by the generator both for the
active and reactive powers. We also see that the stator active power Ps depends on the quadrature rotor
current Iqr and the stator reactive power Qs depends on the direct rotor current Idr and there the effect of
coupling is also observed between the two control axes d and q. The active and reactive powers of the stator
side are adjustable depending on network requirements. In our case it is negative, which means that the
network is a receiver of the energy supplied by the DFIG.
The (Figure 9) shows that our system has satisfactory dynamics which react rapidly, without
overtaking and the static error is almost zero for both the active or reactive powers. The coupling between the
two powers is very low and barely perceptible.

451
Figure 8. Direct Field Oriented Control Figure 9. Indirect Field Oriented Control
0 2 4 6 8 10
-12
-10
-8
-6
-4
-2
0
x 10
5
Puissance active DFOC
Ps
Psref
0 2 4 6 8 10
-12
-10
-8
-6
-4
-2
0
x 10
5
Puissance active IFOC
Ps
Psref
0 2 4 6 8 10
0
500
1000
1500
Iqr - DFOC
0 2 4 6 8 10
0
1000
2000
3000
4000
5000
Iqr - IFOC
0 2 4 6 8 10
-5
-4
-3
-2
-1
0
1
x 10
5
Puissance réactive DFOC
Qs
Qsref
0 2 4 6 8 10
-5
-4
-3
-2
-1
0
1
x 10
5
Puissance réactive IFOC
Qs
Qsref
0 2 4 6 8 10
0
200
400
600
800
1000
Idr - DFOC
0 2 4 6 8 10
-500
0
500
1000
1500
2000
Idr - IFOC
2.07 2.075 2.08 2.085 2.09 2.095
-1000
-500
0
500
1000
is abc - DFOC
2.07 2.075 2.08 2.085 2.09 2.095
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
is abc - IFOC

 ISSN: 2088-8694
452
5. CONCLUSION
This article presents a comparative study between the direct and indirect field oriented control of
the Doubly Fed Induction Generator DFIG integrated in a wind system. First, we started by modeling both
the turbine, then Doubly Fed Induction Generator DFIG in order to apply the flux orientation control (FOC).
According to this study, we found that direct control is the simplest to implement, but not the most
efficient. However, the indirect method allows us, with the loop cascade, to have an inefficient system. It is
certainly more complex to implement compared to direct control, but will have optimal operation of electrical
generation system by minimizing potential problems related to parametric variations of the DFIG. As
perspective, this work can be continued and completed by the implementation of this command in an FPGA.
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BIOGRAPHIES OF AUTHORS
Yasmine IHEDRANE born December 26, 1991 at FES, Morocco, she obtained her master's
degree in Engineering of Industrial Automated Systems at the Faculty of Sciences Dhar el
Mahrez -FES- where he currently works, PhD. graduate student in the same university and is a
member laboratory LISTA. Her interests in machine control.

453
Badre BOSSOUFI was born in Fez city, Morocco, on May 21, 1985. He received the Ph.D.
degree in Electrical Engineering from University Sidi Mohammed Ben Abdellah, Faculty of
Sciences, Morocco and PhD. degree from University of Pitesti, Faculty of Electronics and
Computer, Romanie and Montefiore Institute of electrical engineering, Luik, Belgium, in 2013.
He was an Assistant Professor of Electrical Engineering, at the Higher School of technologie,
Oujda Morocco. His research interests include static converters, electrical motor drives, power
electronics, Smart Grid, Renewable Energy and Artificial Intelligent.
Chakib EL BEKKALI was born in Fez city, Morocco, on March 19, 1965. He received the
Ph.D degree in Signal processing University Sidi Mohammed Ben Abdellah, Faculty of
Sciences. Professor of signal processing at faculty of science FES Morocco. director adjoint of
laboratory LISTA. His research interests signal processing and automatic

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