Active and Reactive Power Control of a Doubly Fed Induction Generator

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 5, No. 2, October 2014, pp. 244~251
ISSN: 2088-8694  244
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Active and Reactive Power Control of a Doubly Fed Induction
Generator
Zerzouri Nora, Labar Hocine
Department of Electrical Engineering, Badji Mokthar University Annaba, Algeria
Article Info ABSTRACT
Article history:
Received Jul 13, 2014
Revised Sep 14, 2014
Accepted Sep 30, 2014
Wind energy has many advantages, it does not pollute and it is an
inexhaustible source. However, the cost of this energy is still too high to
compete with traditional fossil fuels, especially on sites less windy. The
performance of a wind turbine depends on three parameters: the power of
wind, the power curve of the turbine and the generator's ability to respond to
wind fluctuations. This paper presents a control chain conversion based on a
double-fed asynchronous machine (D.F.I.G). To improve the transient and
steady state performance and the power factor of generation, a stator flux
oriented vector control scheme is used in this work. The vector control
structure employs conventional PI controllers for the decoupled control of
the stator side active and reactive power. The whole system is modeled and
simulated using Matlab/Simulink and the results are analyzed.
Keyword:
Doubly Fed Induction
Generator (DFIG)
Wind Turbine
Active and Reactive Power
Control Copyright © 2014 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Zerzouri Nora
Departement of Electrical Engineering,
Badji Mokthar University Annaba,
Université Badji Mokhtar -Annaba- B.P.12, Annaba, 23000 Algeria.
Email: Zerzouri_karima@yahoo.fr
1. INTRODUCTION
Wind energy is one of the most important and promising source of renewable energy all over the
world, mainly because it reduces the environmental pollution caused by traditional power plants as well as
the dependence on fossil fuel, which have limited reserves. Electric energy, generated by wind power plants
is the fastest developing and most promising renewable energy source [1]. Off-shore wind power plants
provide higher yields because of better conditions. With increased penetration of wind power into electrical
grids, wind turbines are largely deployed due to their variable speed feature and hence influencing system
dynamics. But unbalances in wind energy are highly impacting the energy conversion and this problem can
be overcome by using a Doubly Fed Induction Generator (DFIG) [2]. Doubly fed wound rotor induction
machine with vector control is very attractive to the high performance variable speed drive and generating
applications. In variable speed drive application, the so called slip power recovery scheme is a common
practice here the power due to the rotor slip below or above synchronous speed is recovered to or supplied
from the power source resulting in a highly efficient variable speed system. Slip power control can be
obtained by using popular Static Scherbius drive for bi directional power flow. Advantage of the DFIG is that
the power electronic equipment used a back to back converter that handles a fraction of (20-30%) total
system power. The back to back converter consists of two converters. Grid Side Converter (GSC) and Rotor
Side Converter (RSC) connected back to back through a dc link capacitor for energy storage purpose [2].

IJPEDS ISSN: 2088-8694 
Active and Reactive Power Control of a Doubly Fed Induction Generator (Zerzouri Nora)
245
Figure 1. Wind energy conversion chain
2. WIND TURBINE MODEL RESEARCH
Wind turbines produce electricity by using the power of the wind to drive an electrical generator.
Wind passes over the blades, generating lift and exerting a turning force. The rotating blades turn a shaft
inside the nacelle, which goes into a gearbox. The gearbox increases the rotational speed to that which is
appropriate for the generator, which uses magnetic fields to convert the rotational energy into electrical
energy.
The power contained in the wind is given by the kinetic energy of the flowing air mass per unit time
[3], [4].
P ρSv (1)
Where Pair the power contained in wind (in watts) , ρ is the air density (1.225 kg/m3 at 15°C and normal
pressure), S is the swept area in (square meter), and v is the wind velocity without rotor interference, ideally
at infinite distance from the rotor (in meter per second). Although (1) gives the power available in the wind,
the power transferred to the wind turbine rotor is reduced by the power coefficient Cp

(2)
A maximum value of Cp is defined by the Betz limit, which states that a turbine can never extract
more than 59.3% of the power from an air stream. In reality, wind turbine rotors have maximum Cp values in
the range 25-45%. It is also conventional to define a tip speed ratio as [5], [6]:
(3)
Where ω is rotational speed of rotor (in rpm), R is the radius of the swept area (in meter).The tip speed ratio
and the power coefficient Cp are the dimensionless and so can be used to describe the performance of any
size of wind turbine rotor.
Figure 2. The typical curves of Cp versus for various values of the pitch angle β
0 5 10 15 20 25
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Vitesse de Vent(m/s)
coefficientCp
1beta=0
2beta=2
3beta=4
4beta=6
5beta=8
6beta=10
7beta=12
8beta=14
9beta=16

 ISSN: 2088-8694
IJPEDS Vol. 5, No. 2, October 2014 : 244 – 251
246
3. DFIG MODELING AND POWER CONTROL
3.1. Principe of Operation
The machine stator winding is directly connected to the grid and the rotor winding is connected to
the rotor-side VSC by slip rings and brushes. A wide range of variable speed operating mode can be achieved
by applying a controllable voltage across the rotor terminals. This is done through the rotor-side VSC. The
applied rotor voltage can be varied in both magnitude and phase by the converter controller, which controls
the rotor currents. The rotor side VSC changes the magnitude and angle of the applied voltages and hence
decoupled control of real and reactive power can be achieved.
3.2. Mathematical Model of DFIG
For a doubly fed induction machine, the Concordia and Park transformation's application to the
traditional a,b,c model allows to write a dynamic model in a d-q reference frame as follows [7]:

(4)
The flux équations are:
(5)
Where
ωs: synchronous angular frequency
ωr: rotor angular frequency
Rs, Rr: equivalent resistances of stator and rotor windings, respectively
Ls, Lr, M: self and mutual inductances of stator and rotor windings, respectively
The motion equations are given as follows:
(6)
(7)
(8)
Where
g: slip angular frequency
s: slip
Cm: mechanical torque provided to the wind turbine
Ce: electromagnetic torque
J: moment of inertia
3.3. Establishment of the Control Strategy
Neglecting the resistance of the generator stator winding, the phase difference between stator flux
and stator voltage vector is just 90°. Therefore, utilizing the stator flux-oriented to align the stator flux vector
position with d-axis, the flux equation is:
0 (9)

247
To keep the stator flux ϕs constant, the voltage equations can be expressed as:
0
(10)
Where Vs is the space vector amplitude of stator voltage. The active and reactive powers of stator can be
derived as:

(11)
According to (10), while DFIG is connected to an infinite grid, the stator voltage is considered a
constant. The stator current is the only controlled quantity. Therefore, the DFIG output power to grid can be
controlled by the stator current, which achieves the goal of independent control for the DFIG active and
reactive power output. Due to the stator windings are directly connected to the power systems and the effect
of the stator resistance is very small.
Substituting (9) into (5), d-q axis stator current can be calculated as:
(12)
Substituting “(12)” into “(4)”, the rotor voltage can be express as:
(13)
Where 1 is the leakage factor.
The control variables Vdr and Vqr of the rotor voltage can be obtained from “(13)”. The influence of
the cross-coupling between the d-q axis components of rotor current on system performance is small, which
can be eliminated by adopting some control law. The model of the vector control of the rotor-side converter
obtained from the above analysis is shown in Figure 3.
Figure 3. Power control of the DFIG
4. SIMULATION RESULTS
The structure of the DFIG wind energy system is illustrated in Figure 1. The DFIG connected
directly to the grid through the stator, and its speed is controlled via a back-to-back PWM converter. The
parameters of the DFIG are given in Table 1. A speed wind profile is applied to the system Figure 4.

 ISSN: 2088-8694
248
Table 1. 3MW WTG Induction Machine Parameters
Parameter Value
Rotor resistor per phase 2,97mΩ
Rotor resistor per phase 3,82mΩ
Inductance of the stator winding 121 mH
Inductance of the retor winding 57,3 mH
Mutual Inductance 12,12 mH
Number of pole pairs 2
inertia 114 kg.m2
Rated power 3MW
Rated voltage 690V
Figure 4. Wind speed profile
Figure 5. Mechanical speed of the DFIG
Figure 6. Rotor slip
0 1 2 3 4 5 6
8
8.5
9
9.5
10
10.5
11
Time(s)
Vt(m/s)
0 1 2 3 4 5 6
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Time(s)
Wr(tr/min)
0 1 2 3 4 5 6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time(s)
s

249

Figure 7. Stator current and voltage
Figure 8. Zoom stator current and voltage
Figure 9. Rotor current and voltage
Figure 10. Zoom rotor current and voltage
1.8 1.9 2 2.1 2.2 2.3
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
Time(s)
Vas(V)andias(A)
Vas
ias
2.28 2.3 2.32 2.34 2.36 2.38 2.4
-3000
-2000
-1000
0
1000
2000
3000
Time(s)
Vas(V)andias(A)
Vas
ias
1 1.5 2 2.5 3 3.5
-3000
-2000
-1000
0
1000
2000
3000
Time(s)
Var(V)andiar(A)
Var
iar
3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5
-3000
-2000
-1000
0
1000
2000
3000
Time(s)
Var(V)andiar(A)
Var
iar

 ISSN: 2088-8694
250
Figure 11. Stator active power
Figure 12. Stator reactive power
Figure 7 shows the zoom of the waveform of the stator voltage and current are in phase opposition.
This confirms that the DFIG is sending active power to the grid. We can see that the current and voltage are
in phase when the machine acts the motor. Figure 6 shows the generator slip, below synchronous speed the
slip is positive and the machine acts as motor, above synchronous speed the slip is negative and machine acts
as generator. Figures 11 and 12 illustrate respectively the stator active power and reactive power. We can see
the robustness of the power control of the DFIG. Figures 9 and 10 show the rotor voltage and current
waveforms. The frequency of these voltage and current, vary according to the slip s.
The active power of DFIG increase from 1MW to the power 2.5MW and the reactive power remains
0Mvar, which signified the reactive power output is not affected. The simulation result indicates that the
active and reactive power decoupled control is achieved and the performance is good.
5. CONCLUSION
This paper presents the doubly fed induction generator used in variable-speed wind power
generation. And a control structure using standard proportional integral PI controller and a field-oriented
control strategy based on a reference frame rotating synchronously with the rotor flux for variable speed wind
turbines using doubly fed induction generator and for obtaining injected rotor voltages is described and
simulated. Hence results are determined sub-synchronous and super synchronous speeds and the active and
reactive power control is achieved by the RSC and GSC. For the purpose of future extension instead of
standard PI controllers fuzzy controllers etc. can be used.
REFERENCES
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Grid Voltage Drop. ETASR - Engineering, Technology & Applied Science Research. 2011; 1(5): 121-125.
[2] MA Mossa. Field Orientation Control of a Wind Driven DFIG Connected to the Grid. Wseas Transactions On
Power Systems. 2012; 4(7).
[3] Hachemi Glaoui, Harrouz Abdelkader, Ismail Messaoudi, Hamid Saab. Modeling of Wind Energy on Isolated Area”
International Journal of Power Electronics and Drive System (IJPEDS). 2014; 4(2): 274~280.
0 1 2 3 4 5 6
-4
-2
0
2
4
6
8
10
12
Time(s)
Ps(MW)
0 1 2 3 4 5 6
-6
-4
-2
0
2
4
6
8
Time(s)
Qs(MVAR)

251
[4] Yu Ling, Xu Cai. Rotor current dynamics of doubly fed induction generators during grid voltage dip and rise.
Electrical Power and Energy Systems. 2013; 44: 17–24.
[5] Srinath Vanukuru, Sateesh Sukhavasi. Active & Reactive Power Control Of A Doubly Fed Induction Generator
Driven By A Wind Turbine. International Journal of Power System Operation and Energy Management. ISSN
(PRINT): 2011; 1(2): 2231–4407.
[6] Sai Sindhura K, G Srinivas Rao. Control And Modeling Of Doubly Fed Induction Machine For Wind Turbines.Int.
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