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IJESR/August 2013/ Vol-3/Issue-8/413-418 e-ISSN 2277-2685, p-ISSN 2320-9763
International Journal of Engineering & Science Research
SIMULATION AND CONTROL OF DFIG WIND ENERGY CONVERSION SYSTEM
WITH PI-R CONTROLLER
Pooja Dewangan*1, SD Bharti
2
1M.E. Student, Department of Electrical Engineering, Rungta College of Engineering & Technology, Bhilai, India.
2Assoc Prof, Department of Electrical Engineering, Rungta College of Engineering & Technology, Bhilai, India.
ABSTRACT
A doubly-fed induction generator (DFIG) applied to wind power generation system, driven by wind turbine is under
study for low voltage ride-through application during system unbalance. Use of DFIG in wind turbine is widely
spreading due to its control over DC voltage and active and reactive power. An improved control and operation of DFIG
system under unbalanced grid voltage conditions is done in this paper by coordinating the control of both the rotor side
converter (RSC) and the grid side converter (GSC). Conventional dq axis current control using voltage source
converters for both the grid side and the rotor side of the DFIG are analyzed and simulated. Current control scheme
consisting of a proportional integral (PI) controller and a resonant (R) compensator. The PI plus R current regulator is
implemented in the positive synchronous reference frame without the need to decompose the positive and negative-
sequence components. The MATLAB software is used to simulate all the components of grid connected DFIG-based
wind energy conversion system (WECS). DFIG consists of a common wound rotor induction generator with slip ring and
a back-to-back voltage source convertor.
Keywords: DC-link voltage, Doubly Fed Induction Generator (DFIG), Grid side convertor (GSC), PI-R controllers.
1. INTRODUCTION
The conventional energy sources are limited and have pollution to the environment, thus more attention and interest have
been paid to the utilization of renewable energy sources. In recent years, wind energy has become one of the most
important and promising sources of renewable energy and that is not harmful for the environment. One of the most
significant developments of the late 20th century was the re-emergence of the wind as a potential source of energy
generation [1]. Wind energy conversion is a fast-growing interdisciplinary field that encompasses many different
branches of engineering and science [2]. Wind energy is one of the most available and exploitable forms of renewable
energy. The presented system is a variable speed wind generation system based on DFIG. DFIG is one of the most
popular wind turbines which include an induction generator, a back-to-back voltage source converter and a common DC-
link capacitor. The stator of the generator is directly connected to the grid while the rotor is connected through a back-to-
back converter. The back-to- back converter has two main parts; grid side converter (GSC) and rotor side converter
(RSC). The back-to-back power convertor has full controllability over the system [3,4].
The great advantage of the DFIG is that it only requires a ‘partial’ roughly 35% of the generator’s rated capacity
because only 25%-30% of the input mechanical energy is fed to the grid through the converter from the rotor, the rest
going directly to the grid from the stator. The efficiency of the DFIG is very good for the same reason; little power is lost
via the converter. The back-to-back power convertor has full controllability over the system [4].
The control system is an important issue for the WECS. It maximizes the extracted power from the wind through all the
components and also makes sure that the delivered power to the grid complies with the interconnection requirements [3].
This paper deals with the control of dc-link voltage, active and reactive power. The controlling schemes are applied on
both RSC and GSC. The common DC-link voltage is controlled by grid side converter and control of DFIG’s stator
output active and reactive power is controlled by rotor side converter. In this paper dq axis current control scheme is
used. Current control schemes with PI–R controllers in the positive synchronous reference frame for the two converters
are implemented [5]. PI-R controllers are applied to current regulation loop of both convertors (RSC and GSC), RSC and
GSC are designed in the dq reference frame. For current regulation the positive and negative sequence currents must be
controlled, it requires decomposition of positive and negative –sequence components of the current. The positive and
IJESR/August 2013/ Vol-3/Issue-8/413-418 e-ISSN 2277-2685, p-ISSN 2320-9763
Copyright © 2013 Published by IJESR. All rights reserved 414
negative –sequence components of the current have to be decomposed from measured signals involving time delay and
resulting in amplitude and phase errors. During transients the systems cannot be fully decomposed. The PI plus R current
regulator is implemented without the need to decompose the positive and negative-sequence components.
2. MODELLING OF DFIG SYSTEM
Fig.1 represents the operation of DFIG and the way connected to the grid. DFIG is basically a standard rotor-wounded
induction machine in which stator is directly connected to the grid, and the Connection of the rotor to the grid is via a
back-to-back convertor [3]. The back-to-back converter is divided into two components: the rotor-side converter RSC
and the grid-side converter GSC. RSC and GSC are Voltage-Sourced Converters that use forced-commutated power
electronic devices (IGBTs) to synthesize an AC voltage from a DC voltage source.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Fig 1:The wind turbine and the doubly-fed induction generator system
A capacitor connected on the DC side acts as the DC voltage source. A coupling inductor L is used to connect GSC to the
grid. The three-phase rotor winding is connected to RSC by slip rings and brushes and the three-phase stator winding is
directly connected to the grid. The power captured by the wind turbine is converted into electrical power by the induction
generator and it is transmitted to the grid by the stator and the rotor windings. The control system generates the pitch
angle command and the voltage command signals Vr and Vgc for RSC and GSC respectively in order to control the power
of the wind turbine, the DC bus voltage and the reactive power or the voltage at the grid terminals.
2.1 Induction generator model
The equivalent circuit of the induction generator is shown in Fig.2 [5-7,9,10] and the electric and magnetic equations of
the model are described by equations (1.1)-(1.4).
Rs Ls Lr Rr
+ Lm +
Vsdq V+
rdq
- -
- + - +
Jωs ᴪ+
rdq J (ωs-ωr)ᴪ+
rdq
Fig 2: Equivalent circuit of the induction generator
Stator Voltage is given by:
Rotor
Control
AC/DC/AC Converter
Wind
Turbine
Drive
Train Stator
AC AC DC
RSC GSC
Three –
Phase Grid Vr Vgc
Induction
Generator
L
IJESR/August 2013/ Vol-3/Issue-8/413-418 e-ISSN 2277-2685, p-ISSN 2320-9763
Copyright © 2013 Published by IJESR. All rights reserved 415
��� = ����� − ��Ψ� + �Ψ� ��
�� = ���� + ���� +�� �� (1.1)
Rotor Voltage is given by:
��� = ����� − ���Ψ� + �Ψ����
�� = ���� + ����� + ����� (1.2)
Flux Linkage is given by:
Ψ�� = ����� − ������
Ψ� = ���� − �����
Ψ�� = −����� − ������ (1.3)
Ψ� = ���� − �����
Electromagnetic Toque is:
��� = Ψ���� − Ψ���� (1.4)
where vs, is and Ψs are stator voltage, current and flux respectively; vr, ir and Ψr are rotor voltage, current and flux
respectively; ωs is the angular velocity of the chosen frame of reference; d and q represent d and q axis, respectively. Lm
is the mutual inductance; Lsl and Lrl are the stator and rotor leakage inductances, respectively.
2.2 Converter model
With the assumption that the converters are lossless, the equations of converters are as follows:
The power at the rotor side (also called slip power) is given by:
�� = ������ + ������ = ����� − ����� (1.5)
And the power at the stator side is given by:�� = ������ + ����
�� = ����� − ����� (1.6)
So the total output power is:
� = �� + �� = ������ + �� �� + ������ + ����
� = �� + �� = ����� − ����� + ����� − ����� (1.7)
3. PI-R CONTROLLER
For current regulation the positive and negative sequence currents must be controlled precisely. To get the accuracy in
current controlling the accuracy of the d-q components decoupling and the removal of network voltage disturbance is
must. The positive and negative –sequence components of the current have to be decomposed from measured signals
involving time delay and resulting in amplitude and phase errors. During transients the systems cannot be fully
decomposed. Therefore here a strategy is adopted to have a PI-R current controller to overcome from these problems and
this regulator consists of a proportional and integrator term, which contains an R pole.
IJESR/August 2013/ Vol-3/Issue-8/413-418 e-ISSN 2277-2685, p-ISSN 2320-9763
Copyright © 2013 Published by IJESR. All rights reserved 416
R
����
���� --- --- --- --- --- ���� ����
+
- PI ����
--- --- --- --- ---
Fig 3: Rotor current control scheme based on PI-R controller in the dq+ reference frame
In order to reduce the sensitivity towards possible grid frequency variation, a component with a cut-off frequency of ωc1
can be inserted into the R part to widen its frequency bandwidth as shown in Fig.3 [5,9].
With a PI-R controller, the DC components are mainly regulated by the PI controller while the double-frequency ac
signals are fully controlled by the R regulator. Hence, a PI-R current controller in the positive synchronous reference
frame can directly regulate both positive and negative sequence components without involving sequential decomposition.
3.1 The modeling of PI-R controller RSC
The modeling of PI-R controller for RSC During network imbalance, a DFIG system can be represented in the dq+
reference frame as
�� ���
� = !"#$
���� − !"#$
������ − !"#$
% &#'#(
)���� − ������ − *��+��� , + *���-. + +��
� / (2.1)
The rotor control voltage produced by the PI-R controller without any decomposition of positive and negative-sequence
components, V+rdq is given as.
���� = 0������ + ���
� (2.2)
Where U2�3� is the output from the PI-R controller and σL2 is stator transitory inductance.
���� = 6
67 ����
= &8-9! + :;<=� + �:;>=
�?�@AB=��C@A(D?/%C����∗ − ���� D (2.3)
Where E+
rdq is the equivalent rotor back electromagnetic force acting as a disturbance to the PI-R controller and is given
by,
���� = #'
#()���
� − ������ − *��+��� , + *���-. ++��
� + ������ (2.4)
Where KiP1, KiI1, and KiR1, are the proportional integral and R parameters respectively.
3.2 The modeling of PI-R controller
The modeling of PI-R controller for GSC .The GSC under unbalanced supply voltage can be represented in the dq+
frame as
�� �F�� = G(HI
J KLMNMHIJ KOA(#MNMHI
J KGMHIJ
#M (2.5)
Where V+
gdq presents the control voltage produced by the GSC PI-R controller, and is designed as
�F�� = −�F�F�
� + �F�� (2.6)
Where
�8-L!�@ + 2�QB=� + C2��D@
8-�!R
8-.
1R0�� + ��
IJESR/August 2013/ Vol-3/Issue-8/413
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�F�� = 6
67 �F��
= &8-9@ + :;<?� + �:;>?
�?�@AB?��C@A(D?/
�F�� = ���
� − �F�F�� − *���F�F�
�
Where KiP2, KiI2, and KiR2, are the proportional, integral and R parameters for the GSC,
4. SIMULATION ANALYSIS
A wind energy generation system based on doubly fed induction generator connected to grid system with PI
on both side is simulated using MATLAB. The DFIG is rated at 1.5 MW and frequency is set to 60Hz. The
voltage is regulated at1200V. A three-phase RLC load at the primary side of the coupling transformer is used to generate
the voltage unbalance. Under voltage unbalance condition in the system, grid supplies unbalance current. In induction
machine negative sequence current produce pulsation in torque, which causes the total output active power and the dc
link voltage, both contain oscillations. With PI
dc-link voltage, active and reactive power oscillations decrease.
In Fig.4 to Fig.6, Shows different simulation results of DFIG wind turbine conversion system under voltage unbalance
with PI-R controller.
Fig 4: Simulation results for DC
Fig 5: Simulation results for Active Power (P) with P
Fig 6: Simulation results for Reactive Power (Q) with P
8/413-418 e-ISSN 2277-2685, p
Copyright © 2013 Published by IJESR. All rights reserved
D /%C�F��∗ � �F�
� D
, are the proportional, integral and R parameters for the GSC, respectively.
A wind energy generation system based on doubly fed induction generator connected to grid system with PI
on both side is simulated using MATLAB. The DFIG is rated at 1.5 MW and frequency is set to 60Hz. The
phase RLC load at the primary side of the coupling transformer is used to generate
the voltage unbalance. Under voltage unbalance condition in the system, grid supplies unbalance current. In induction
negative sequence current produce pulsation in torque, which causes the total output active power and the dc
link voltage, both contain oscillations. With PI-R controller negative sequence current is quickly regulated and as a result,
ve and reactive power oscillations decrease.
In Fig.4 to Fig.6, Shows different simulation results of DFIG wind turbine conversion system under voltage unbalance
Simulation results for DC-link Voltage (Vdc) with P-IR controller
Simulation results for Active Power (P) with P-IR controller
Simulation results for Reactive Power (Q) with P-IR controller
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417
(2.7)
(2.8)
respectively.
A wind energy generation system based on doubly fed induction generator connected to grid system with PI-R controller
on both side is simulated using MATLAB. The DFIG is rated at 1.5 MW and frequency is set to 60Hz. The dc link
phase RLC load at the primary side of the coupling transformer is used to generate
the voltage unbalance. Under voltage unbalance condition in the system, grid supplies unbalance current. In induction
negative sequence current produce pulsation in torque, which causes the total output active power and the dc-
R controller negative sequence current is quickly regulated and as a result,
In Fig.4 to Fig.6, Shows different simulation results of DFIG wind turbine conversion system under voltage unbalance
controller
IR controller
IJESR/August 2013/ Vol-3/Issue-8/413-418 e-ISSN 2277-2685, p-ISSN 2320-9763
Copyright © 2013 Published by IJESR. All rights reserved 418
From the fig.4 to fig.6 its clear that with due course of time the oscillations in dc-link voltage, active power and reactive
power reduces, and we are getting a constant dc-link voltage while reactive power reduces to zero, thus we will get
maximum total active power. It proves that the proposed control system controls; dc link voltage, active and reactive
power more accurately.
5. CONCLUSION
Enhanced control and operation of a DFIG-based wind Power generation system under unbalanced supply voltage
conditions have been investigated in this paper. A new coordinated control strategy for the RSC and GSC has been
proposed. Simulation results show that PI-R controller overcomes oscillations in dc link voltage, active and reactive
power. The RSC is controlled to eliminate the electromagnetic torque oscillation while the GSC compensates for the
oscillation of the DFIG stator output active power to eliminate the oscillation in the total active power generated from the
overall system. PI–R current controllers in the positive synchronous rotating reference frame have been proposed for
regulating the GSC’s and RSC’s positive and negative sequence currents. Such controllers can provide precise control of
both positive and negative sequence currents without involving the decomposition of positive and negative sequence
component. The control and operation of DFIG-based wind power system under unbalanced conditions can be
significantly improved by simultaneously eliminating torque and total generated active power oscillations.
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