1. Proceedings of the 2009 IEEE
International Conference on Mechatronics and Automation
August 9 - 12, Changchun, China
Doubly-Fed Induction Generator Control for
Variable-Speed Wind Power Generation System
YAO Xing-Jia ,LIU Shu,WANG
Chang-chun and XING Zuo-xia
Xiao-Dong, GUO
JIANG Hong-Liang
Wind Energy Institute of Technology Shenyang
University of Technology
Shenyang, Liaoning Province, China
Vision Automatic Identification Technology
Research Institute
Shenyang,Liaoning Province, China
jhljhw@163.com
lsjhl@163.com
Abstract- This paper presents control of doubly fed induction
II. VARIABLE SPEED OPERATION
generator for variable speed wind power generation. The control
scheme uses stator flux-oriented control for the rotor side converter
and grid voltage vector control for the grid side converter. A
complete simulation model is developed for the control of the active
and reactive powers of the doubly fed generator under variable
speed operation. Several studies are performed to test its operation
under different wind conditions. A laboratory test setup consisting of
a wound rotor induction generator by a variable speed de motor is
used to validate the software simulations,
Index Terms: Doubly-fed induction generator; Wind generation;
Control; Simulation
Fixed speed wind electric conversion systems (WECS)
generally use squirrel cage induction generators with direct
grid connection so as to maintain a fixed speed that matches
the electrical frequency of the grid. In order to operate the
fixed speed systems at low and high wind speeds efficiently,
pole changing is generally employed. Smaller number of pole
pairs is used at high wind speeds and higher number at lower
wind speeds. This allows the generator to operate at a
different mechanical speed without affecting its electrical
frequency. The advantage is that a cost-effective aerodynamic
control like stall control can be used. However, the drawbacks
in fixed speed systems are:
• it cannot optimally use the available wind power due to
constant speed operation;
• since there is no inherent reactive power control method
in this configuration, it must use capacitor banks instead of
drawing the reactive power from the grid;
• since the generator is made to run at a constant speed in
spite of fluctuations in wind speed, it will result in fluctuation
of generated voltage as well as output power.
In variable speed systems, the turbine rotor absorbs the
mechanical power fluctuations by changing its speed. So the
output power curve is smoother which greatly enhances the
quality of power. However, since variable speed operation
produces a variable frequency voltage, a power electronic
converter must be used to connect to the constant frequency
grid.
I. INTRODUCTION
Wind power generation is an important alternative to
mitigate this problem mainly due its smaller environmental
impact and its renewable characteristic that contribute for a
sustainable development. Doubly-fed induction generator
(DFIG) is able to supply power at constant voltage and
constant frequency while the rotor speed varies, that makes it
suitable for variable-speed wind power generation. Another
major advantage of the DFIG-based system is that the power
electronic equipments only need to handle a fraction of the
total system power, resulting the reduction of the power losses
and the cost of the power electronic equipments.
Most wind turbine manufacturers now equip their power
generating units with induction generators. These machines
are operated either at fixed speed or variable speed.
Generators driven by fixed speed turbines can be directly
connected to grid. However, variable speed generators need a
power electronic converter interface for interconnection with
the grid. Variable speed generation has better energy capture
than fixed speed generation. There are several other
advantages of using variable speed generation such as
mechanical stress reduction of turbine and acoustic noise
reduction. With recent developments in power electronic
converters, variable speed generation looks entirely feasible
and cost effective. The paper characterizes the performance of
a double-fed induction generator (DFIG) for variable speed
wind power generation.
978-1-4244-2693-5/09/$25.00 ©2009 IEEE
III .COMPOSITION OF THE WIND GENERATION SYSTEM
The induction generator converts the power captured by the
wind turbine into electrical power and transmits it to the grid.
The ac/dc/ac converter consists of the rotor-side converter
( Crotor)
and the grid-side converter (Cgrid). Both Cgrid and
C grid converters are voltage-sourced converters using forced
commutated power electronic devices to synthesize an ac
855

2. voltage from a de voltage source. A capacitor connected on
the de side acts as the de voltage source and a coupling
inductor L is used to connect the grid-side converter to the
grid. The three-phase rotor winding is connected to Crator by
slip rings and brushes and the three-phase stator winding is
directly connected to the grid.
The pitch angle command and the voltage command
signals Vr and
Vgc for C rotor
and Cgrid
converters,
Fig. 2. Crot control loop.
The control system for the double fed induction generator,
operating under maximum power extraction, is based on a
flux orientated control of the induction machine, in which the
dq current and voltage values are referred to the reference
frame aligned with air-gap flux.
respectively, are generated by the control system driving the
power of the wind turbine, the de bus voltage and the voltage
at the grid terminals .
The ac/dc/ac converter consists of the rotor-side converter
(Crotor) and the grid-side converter (Cgrid). Both Crotor
and Cgrid converters are voltage-sourced converters using
forced commutated power electronic devices to synthesize an
ac voltage from a de voltage source. A capacitor connected on
the de side acts as the de voltage source and a coupling
inductor L is used to connect the grid-side converter to the
grid. The three-phase rotor winding is connected to Crotor by
slip rings and brushes and the three-phase stator winding is
directly connected to the grid.
The pitch angle command and the voltage command
IV. CONTROL OF DFIG
A schematic diagram of the overall system is shown in
Fig. 3. Back-to-back PWM converters are connected between
the rotor of 2[MW] DFIG and the grid utility. The DFIG is
controlled in a rotating d-q reference frame, with the d-axis
aligned along the stator-flux vector as shown in Fig. 4. For
the stable control of the active and reactive power, it is
necessary to independently control them. The stator active
and reactive power of the DFIG is controlled by regulating
the current and voltage of the rotor windings. Therefore the
current and voltage of the rotor windings need to be
decomposed into components related to the stator active and
reactive power.
signals Vr and Vgc for Crotor and Cgrid converters,
respectively, are generated by the control system driving the
power of the wind turbine,the de bus voltage and the voltage
at the grid terminals.
The control system for the double fed induction
generator ,operating under maximum power extraction, is
based on a fluxorientated control of the induction machine , in
which the dq current and voltage values are referred to the
reference frame aligned with air-gap flux.
The control system is composed of two parts : in the first
part the wind turbine output power and the voltage at the grid
terminal are controlled by means of Crot; in the second part
the voltage at de bus capacitor is controlled by means of
Cgrid . The general scheme is illustrated in Fig. I while the
control loop of the Crot, modeled as a voltage source, is
shown in Fig. 2.
-
Turbine
Wind
..
Pow er
System
Fig.3. Basic configuration of DFIG wind turbine
L
=Sta tor
Induction
Generator
Th ree-ph ase
Grid
~~h.n~e --- - - ---- ---- _·
Fig.4.Vector diagram for stator flux-oriented control
A. Stator-Flux Oriented Control
Fig. 1. Wind turbin e and the doubly fed induction generator system
856

3. For the stator active and reactive power control, a d-q
reference frame synchron ized with the stator flux is chosen.
The stator flux: vector is adjusted to be aligned with the daxis. The flux linkages of the stator and rotor are expressed
as:
As = Ads = Lmi ms = L si ds + Lmidr
(3)
Rs
:
stator resistance;
Be : synchronous frame angle .
B. Grid side converter control
The grid-side converter serves to meet two purposes keeping
the DC-link voltage constant regardless of the magnitude and
direction of the rotor power;joining the reactive power
composition .The scheme of the grid-side converter is shown
in Fig. 5.
In the stationary three-phase coordinates,the state equation
of the converter is:
(4)
(5)
(12)
(6)
Where
Lm
:
Where Land R are the inductance and resistance of the line
reactors, respectively; p=dldt is derivative operator.
To facilitate independent control of the active and reactive
power flowing between the grid and the grid-side converter,
transformed into a dq coordinates with its q-axis fixed on the
grid voltage vector position, expressed as.
magnetizing inductance;
L, : stator self-inductance;
L, : rotor self-inductance;
}'ds ' }'qs :
stator d-q flux linkage;
}'dr'}'qr :
rotor d-q flux linkage;
ims ,ids,idr : magnetizing, stator and rotor d-axis urrents.
Rotor voltages in d-q reference frame can be expressed as a
function of rotor and magnetizing currents
0
Vdr = R rldr
or di
+ CJLr - dr -
or
Figure.5 Structure of the supply-side PWM converter
0
(7)
0
lIJ stCJ rlqr
L
dt
Where lIJ e is the synchronous rotating speed of the gridvoltage vector; subscripts d.q indicate the vector components
in the rotating(d-q) reference coordinates.
where
Vdn Vqr :
rotor d-q voltages;
Assuming
and
R; : rotor resistance;
lIJ sl :
eUXf
slip angular frequency .
=-
lIJLid
,(13) can be rewritten as :
The stator flux angle is calculated as follows:
(14)
(9)
Ads = f(VdS - R sids)dt
(10)
With feed forward control scheme for the grid voltage
eqd
and compensation control scheme for the cross-coupling
terms
(II)
where a superscript "s" represents quantities
reference frame and
In
ewq,wd'
i.e. only considering the impedance voltage
vq
and v~ ,the plant for the current control loops is given by
(15) :
stationary
857

4. converter-switching logic, which makes the converter look
like a current controlled, current source converter.
A . Rotor side converter control
The rotor side converter control begins with the stator and
rotor current transformation to the d-q reference frame
followed by both currents being transformed to the stator fluxoriented frame. Since the objective is to capture the maximum
energy available in the wind, the active power reference is
always made equal to the available wind turbine power. The
reactive power reference value was derived from the active
power reference and the desired value of the power factor.
The control uses the principle that in the stator flux-oriented
frame, the rotor current variations will reflect in stator current
variations and hence, by controlling the rotor current, the
stator active and reactive powers can be controlled.
A reference current i rx •rej was derived from the error
between the active power reference and the actual active
power by tuning a PI controller, as shown in Fig. 7. Similarly,
a reference current i rx •rif was obtained from the error between
the reactive power reference and the actual reactive power, as
shown in Fig.8. Then, both reference currents were
transformed to their natural reference frame that is the rotor
frame. These rotor current references, after a dq-to abc
transformation, were used for implementing the hysteresis
modulation on the rotor side three-phase converter.
(15)
Neglecting the loss and the voltage drop of inductors and
considering the power balance between the AC-side and DCside of the converter,the DC-link voltage can be
approximated by :
dVdc = ~(.J3 m i - id )
(16)
dt
C 2
q
c
Where m is the SVPWM modulation index .
From (16), it can be found that the DC-link voltage is
controlled by the current iq .Therefore the output of the
voltage control loop corrector is selected as the reference of
the q - axis component of the AC currents. The reference of
the d - axis component is dependent on the optimal reactive
power splitting.The space vector control scheme for the gridside converter is shown in Fig.6.
Figure.6. The control structure for grid-side converter
V. IMPLEMENTATION FF THESIMULATION
Iry.ref
There are many popular simulation tools such as ACSL,
MATLAB/Simulink and PSCAD that can be used to simulate
electric drives, power systems, power electronic circuits or a
combination of those. However, as is the case with any
software, there are certain advantages and disadvantages
associated with any one of the above mentioned software and
discretion has be used while choosing one of those depending
on the type of application it is used for.
PSCAD was selected for the simulations effort in this
study as it already has models for wind turbine , wind turbine
governor, and the wind resource in addition to having a
powerful simulation engine that is suitable for simulating
time domain instantaneous responses .
The main circuit developed in PSCAD contains a wound
rotor generator shaft connected to a wind turbine shaft by
means of a gearbox as modeled in PSCAD. The stator
terminals are directly connected to the grid and the rotor
terminals are connected to the grid by means of a back-to
back power converter bridge. The pitch angle adjustment for
the turbine is done by a wind turbine governor.
The rotor side power converter controls the stator active
and reactive powers and the grid side converter maintains the
de link voltage. To simulate these control schemes two
subsystems were developed-c-one for the rotor side converter
control and the other for the grid side converter control. A
hysteresis modulation strategy was used to implement the
Fig. 7. Obtaining quadrature axis rotor reference current in stator
flux-oriented frame
Fig. 8. Obtaining direct axis, quadrature axis rotor reference current in
stator flux-oriented frame.
B. Grid side converter control
The grid side converter control begins with transforming
the grid voltages to the stationary reference frame to obtain
the voltage vector angle as given by Eq. As seen before, the
de link voltage can be controlled by control of the direct axis
current ix in the voltage vector-oriented reference frame.
Thus, a reference current i rx,rej was derived from the de link
voltage error of the converter bridge by tuning a second PI
controller, as shown in Fig. 9. The current i rx,rej was forced
to zero so as to make the displacement equal to zero.
858

5. has proven that such a strategy will work well under wind
conditions.
The simulation model can also be used to test the
controller 's performance under network disturbances. A
single lineto-ground (SLG) fault will result in a dip in the de
link voltage, but the controller should be able to recover and
continue the tracking of stator and rotor currents after the
fault is removed. However, the line-line (LL), a double lineto-ground (LLG) or a three-phase fault is likely to have a
catastrophic effect on the controller as it may lose tracking of
the current thereby resulting in a possible collapse of the de
link voltage. Induction generators, in general, cannot sustain
an appreciable fault current for a fault at their terminals for a
long time due to the collapse of excitation source voltage
during the fault. However, they will inject a large amount of
current for a short transient period of time and this can
impact the power system.
The results obtained from the laboratory setup have
shown that the real power output of the induction generator
can be varied by controlling the power handled at the rotor.
This factor is helpful in optimally trapping the maximum
amount of wind energy available in an efficient manner.
Moreover, the power factor and hence the reactive power of
the generator can be controlled . Near unity power factors
have been achieved. The sub- and super-synchronous modes
of operation could be easily combined to provide a continuous
operation of the system at various speed ranges. The
laboratory results also validate the results obtained from the
software simulations. Similar rotor and stator power
variations may be seen in both results.
The reference currents in the grid voltage vector-oriented
frame were then transformed to their natural frame of
reference - the stationary frame. An inverse transformation
was used to obtain the reference currents as phase currents .
With the reference currents for both rotor side and grid side
converters, hysteresis modulation may then be implemented
for both converters.
I x.ref
Fig. 9. Obtaining tbe reference current
VI. SIMULATION RESULTS
Simulation studies for various wind conditions were
performed and the control response observed for each. Only
the following conditions are shown in this paper Fig. I0:
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Poor power factors in some fixed speed wind machines
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use of a power converter in conjunction with a DFIG, the
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859