00_HCMUT-WASEDA_0182

Modified Controls for Grid-Connected Wind-Turbine Doubly Fed Induction Generator
under Unbalanced Voltage Dip for Torque Stability and Reduction of Current Harmonic
Truc Pham-Dinh1
, Hai Nguyen-Thanh2
, Kenko Uchida3
, Nguyen Gia Minh Thao4
1
Faculty of Electrical-Electronic Engineering, Ho Chi Minh City University of Technology, Ho Chi Minh City, Vietnam
(E-mail: pdtruc@hcmut.edu.vn)
2
Department of Technology, Le Hong Phong High School, Ho Chi Minh City, Vietnam
(E-mail: hoc_vien@yahoo.com.vn)
3,4
Department of Electrical Engineering and Bioscience, Waseda University, Tokyo, Japan
(E-mail: 3
kuchida@uchi.elec.waseda.ac.jp ; 4
thao@uchi.elec.waseda.ac.jp)
Abstract: This paper presents modified Stator Voltage Oriented Control (SVOC) for Doubly Fed Induction Generator
(DFIG) in wind turbines to reduce torque pulsation during unbalanced voltage dips, and current waveforms are also
improved with the decreasing of harmonics. The proposed schemes utilize multiple PI controllers with anti-windup to
obtain commanded rotor currents and also introduce extra commanded values for rotor current; as well, Notch filters are
used to eliminate the second-order harmonic components. The designed system consists of a wound-rotor induction
generator and back-to-back power-electronic converters connected to both rotor and grid sides. Wherein, the
modifications are applied to the control of rotor side converter (RSC). Simulations in Matlab/Simulink illustrate the
enhanced stability of torque response and the improvement of current waveform. Comparisons of the simulation results
with a traditional Stator Flux Oriented Control (SFOC) and a previously suggested modification of SFOC for operation
under unbalanced voltage dip are provided to evaluate the newly proposed methods in the paper.
Keywords: DFIG, Unbalanced Voltage Dip, PI controller, Anti-windup, Wind Turbine, SFOC, SVOC.
1. INTRODUCTION
Doubly fed induction generators (DFIG) are common
choices for grid-connected wind power generation. The
reasons for this popularity are the low cost of power
electronic circuits needed to allow independent
controlling of powers (active and reactive) delivered to
the grid and variable speed constant frequency operation
[1,2]. DFIG is the cheapest solution for on-shore wind
farms when the whole systems are considered [3]. There
are more and more wind farm’s connections to the grid
and the penetration has been up to more than 50% in
several countries [4]. However, the grids often
experience problems such as unbalanced voltage dips,
which result in the increase of winding temperature,
pulsation of torque and power, oscillations of stator/rotor
currents, and mechanical stress on the gear-box [5,6].
Technical limitations for connected wind farms to
maximize generator’s output include reactive power and
voltage control, frequency maintaining, and fault
ride-through capabilities [4].
The stator voltage’s magnitude is determined by the
flowing of reactive power between generator and the
grid while the phase difference is controlled by active
power [4]. Therefore, power balance must be maintained
on the grid. A voltage drop is proportional to current and
radial distance to the substations happens when a fault
occurs. Due to the remote location of the wind farms, the
voltage difference may be well out of the limits, and this
could result in multiple disconnections of the wind farms
[4].
The active power output of the generator relies on the
input mechanical power provided by the wind turbine.
Therefore, a mismatch in power supply and demand on
the distribution network could lead to variation in
rotational energy stored in the generator. This will result
in the discrepancy of the frequency [4]. A shortage in
generation results in frequency decrease while the
abundance of the generator causes the increase of
frequency [4].
Fault ride-through capabilities are necessary for the
wind farms to stay connected to protect the network
securities. During a voltage dip, DFIG will increase the
demand of reactive power to a level that could cause
further suppression of the grid voltage [4]. Wind farm
disconnection as a result of this will cause a mismatch of
power supply and demand, and then result in frequency
drop. Spinning power reserves have to be established for
the grid if the generators are unable to ride through faults.
Hence, modification of the control system is necessary.
In addition, to maintain the connection to distribution
network during voltage unbalance, generators need to
keep providing sufficient powers with acceptable
qualities, a modified Stator Flux Oriented Control
(SFOC) based control method is proposed in [6] which
uses four command values of rotor current components
such as * * * *
, , ,dr qr dr qri i i i+ + − −
+ + − −
to achieve independent
control of P and Q as well as constant torque, or constant
active power, or balance stator current, or no oscillation
of rotor current.
A further modification for unbalance voltage
ride-through based on the above method is proposed in
[7] which uses Stator Voltage Oriented Control (SVOC),
PI controller with resonance, and * *
,dr qri i+ +
+ +
based
expressions for calculating of * *
,dr qri i− −
− −
. Different order
of coordinate transformation for rotor current
components is suggested in [7].
SICE Annual Conference 2014
September 9-12, 2014, Hokkaido University, Sapporo, Japan
978-4-907764-45-6 PR0001/14 ¥400 © 2014 SICE 1493

This paper presents two new SVOC based control
schemes, which use PI controllers with anti-windup to
deduce * *
,dr qri i+ +
+ + from active and reactive power errors.
These PI controllers provided simplicity to the control
system and also increase the independence of the system
with parameter’s variations. The commanded values of
* *
,dr qri i− −
− − are calculated from feedback quantities. Both
PWM current controller and hysteresis current controller
are utilized in these new methods.
2. CONTROLSTRUCTURE AND
MODELLING
The structures of the wind turbine and the mechanical
model have been presented in [8] and [9], they are not
included in this paper. This section discusses the control
structure for field oriented control of grid-connected
doubly fed induction generator. Stator Flux Oriented
Control is used for controlling schemes in [6,8,9], while
the structures in [7] and this paper utilize Stator Voltage
Oriented Control.
Dynamic model of DFIG with balanced grid voltage
in a generally d-q rotating reference frame [2] are
considered in this paper. Furthermore, positively and
negatively rotating reference frames, which are denoted
as dq+ and dq− respectively, are also used to develop a
control model for DFIG during unbalanced voltage dip.
These reference frames are presented in Fig. 1 below.
Fig. 1: Relationships between (α,β)s , (α,β)r , dq+ and
dq− reference frames [6,7].
In a rotating d-q reference frame, calculation of the
active and reactive powers is as following:
3 3
( ); ( )
2 2
s ds ds qs qs s qs ds ds qsP v i v i Q v i v i= + = −　 (1)
In a SFOC reference frame, where the d axis is
attached to the stator flux space vector, the following
characteristics are obtained:
msmsds iL== ψψ , 0=qsψ (2)
The stator voltage equations of DFIG in a generally
rotating d-q reference frame as shown in (3.1) and (3.2)
can be approximately reduced to the forms shown in
(4.1) and (4.2) in a stator flux oriented reference frame.
ds s ds s qs dsv R i d dtω ψ ψ= − + (3.1)
qs s qs s ds qsv R i d dtω ψ ψ= + + (3.2)
0≈dsv (4.1)
smsmsdssqs ViLv ==Ψ≈ ωω (4.2)
Therefore, the equations for active and reactive
powers in the stator flux reference frame are shown in
(5.1) and (5.2).
( ) qr
s
m
sqsqsqsqsdsdss i
L
L
VivivivP
2
3
2
3
2
3
−==+=
(5.1)
( ) 







−==−= dr
ms
s
s
m
sdsqsqsdsdsqss i
L
V
L
L
VivivivQ
ω2
3
2
3
2
3
(5.2)
The equations above have shown that independent
control of P and Q can be obtained by controlling idr and
iqr in SFOC. Similarly, SVOC can also be used to
independently control P and Q by using idr and iqr [10].
If the magnitudes of stator voltage and flux space
vectors are constant, the equations of rotor voltage in
the synchronously rotating reference frame are reduced
as following [11]:
( )
( )
dr
dr r dr r s r dr
qr
qr r qr r s r qr
di
v R i L
dt
di
v R i L
dt
σ ω ω ψ
σ ω ω ψ
= + − −
= + + −
(6)
Where ( )rsm LLL /1 2
−=σ
The expressions of active and reactive powers in SVOC
can be approximated as following [11]:
( )
1.5 /
1.5
m s dr s
s
s m qr s
s r
P L V i L
V
Q V L i L
ω ω
≈
 
≈ − +  − 
(7)
Fig. 2 shows the control structure of SFOC while Fig.
3 and Fig. 4 demonstrate the structure for proposed
SVOC’s. The quantities * *
,dr qrv v+ +
in Fig. 3 can be
calculated with the following equations in [7], where
* *
,dr qru u+ +
are the outputs of the PI controllers plus
anti-windup.
dqr r dqr dqrv L u eσ+ + +
= + (8)
( )
( )
m
dqr dqs s dqs s dqs
s
s r dqr r dqr
L
e v R i j
L
j R i
ω ψ
ω ω ψ
+ + + +
+ +
= − −
+ − +　　　
(9)
3. THE PROPOSED CONTROL METHODS FOR
IMPROVED TORQUE STABILITY AND
REDUCED CURRENT HARMONICS
The proposed SVOC based systems in Figs. 3 and 4
are different from the traditional SFOC in terms of using
September 9-12, 2014, Sapporo, Japan
1494

Notch filter to eliminate the 2nd
order harmonic which
causes power and torque pulsation and addition of extra
commands * *
,dr qri i− −
− −
to improve torque performance.
However, they are similar to the traditional one in using
PI plus anti-windup, as presented in Fig. 5, to obtain
* *
,dr qri i+ +
+ + from the errors of reference and estimated
powers. The control structure of the traditional SFOC is
similar to the scheme in Fig. 2 except the uses of Notch
filters and Sequence Component Controller.
Fig. 2: Control structure of SFOC with Sequence
Component Controller and PI + anti-windup.
Fig. 3: Proposed control structure with SVOC, PWM
current controller, and PI + anti-windup.
The two proposed control methods are different to the
methods in [6] and [7] in term that the reference values
* *
,dr qri i+ +
+ + are the outputs of two PI controllers with
anti-windup, instead of being calculated from * *
,P Q
(as shown in (10) from [7]). The PI controllers will
provide the independences with parameter variations for
the commanded values * *
,dr qri i+ +
+ + . Robust responses of
* *
,dr qri i+ +
+ + to the variation of * *
,P Q can also be obtained.
The expressions to calculate * *
,dr qri i− −
− − in the suggested
ones are not the same as expressions in [6, 7] although
the same control target of zero torque pulsations is
applied. Components of feedback voltage and rotor
current are used for the calculations in this paper to
provide reliability and quick adjustment for controller, as
shown in (11).
The two methods are also different with the one in [6]
by using SVOC and the difference in the order of
coordinate transformation to obtain *
rvαβ (for the case
with Pulse Width Modulation (PWM) current controller)
as shown in Fig. 3.
Fig. 4: Proposed control structure with SVOC,
Hysteresis current controller, and PI + anti-windup.
Fig. 5: The layout of PI controller with anti-windup in
Matlab/Simulink.
0
0
sin 2
sin 2
cos 2
cos2
0 0 0 0
3
2
0 0 0 0
0 0 0 0
s
qss qs ds qs ds
s dsqs ds qs ds
s s s qsds qs ds qs
s
ds
s
P
vQ v v v v
P vv v v v
Q L vv v v v
P
v
Q
ω
++ + − −
++ + − −
+− − + +
+− − + +
−− − + +
−− − + +
−
−
  
    − −        − − − −   = ×     − −        
        
3
2
ds qs ds qs
qs ds qs ds dr
ds ds qs ds qrm
s ds qs ds qs dr
qrds qs ds qs
qs ds qs ds
v v v v
v v v v i
v v v v iL
L v v v v i
iv v v v
v v v v
+ + − −
+ + − −
+ + − − +
+ + − − +
− − + + +
− − + + +
− − + + −
− − + + −
−− − + +
−− − + +
− − + +
− − + +
 
 
 − − 
  
 − − 
 + ×
  
  
− −  
 
 − − 





 

(10)
A totally different current control method, which is
1495

hysteresis control, is also presented in this paper as
shown in Fig. 4.
The application of Notch filters for removing the 2nd
harmonic order is the similarity between the proposed
ones and schemes in [6,7].
The values of * *
,dr qri i+ +
− − and then * *
,dr qri i+ +
as in Figs.
3-4 can be done by using (12.1) and (12.2).
* *
;
qs qsds ds
dr dr qr qr dr qr
ds qs ds qs
v vv v
i i i i i i
v v v v
− −− −
− −− + + − + +− −
− + + − + ++ + + +
+ + + +
= + = − (11)
2* * * * * sj
dr dr dr dr dri i i i i e θ−+ + + + −
+ − + −= + = + (12.1)
2* * * * * sj
qr qr qr qr qri i i i i e θ−+ + + + −
+ − + −= + = + (12.2)
The next session will verify the performance of the
proposed methods.
4. SIMULATION RESULTS
Simulations of the proposed control methods for the
2.3MW grid-connected DFIG are carried out with the
generator's parameters as given by Table 1. The
commanded values of P and Q are changed after 50s,
reference value of P is changed from 1.5 MW to 2.0 MW
while the reference value of Q is changed from 1.2
MVAR to 800 KVAR. The grid voltages are balanced
until the 60th
second, one of the phase voltages is
reduced by 10%, then they are balanced again from the
80th
second. The proposed control methods are for
variable speed and constant frequency of DFIG, without
loss of generality, the rotor speed in the simulation is
super-synchronous and at a particular value of 1600 rpm.
The wind speed’s variation is shown in Fig 6.
Table 1. Parameters of the 2.3MW DFIG
Parameter Symbol Value
Stator inductance LS 159.2 (µH)
Rotor inductance Lr 159.2 (µH)
Magnetic inductance Lm 5.096 (mH)
Stator resistance RS 4 (mΩ)
Rotor resistance Rr 4 (mΩ)
Number of pole pairs P 2
Frequency (angular) ωS 100π (rad/s)
Inertia J 93.22 (kg.m
2
)
Inertia of Rotor Jrot
4.17×10
6
(kg.m
2
)
Fig. 6: Random variation of the wind speed.
The simulations are assumed that the DFIG has been
operating in the steady state for a long time, after starting
and grid synchronization.
The proposed control method with hysteresis current
controller has a hysteresis band of 1% rotor current
obtained when the generator is delivering rated active
power and not delivering reactive power using PWM
current controller.
Figures 7-14 present the responses of active power,
reactive power, stator current, rotor current and torque.
In each figure, there are four sub-figures for the
responses obtained with traditional SFOC and PWM
current control (a), previously proposed method with
SFOC, Sequence Component Control and PWM current
control (b), currently proposed method with SVOC and
PWM current control (c), currently proposed method
with SVOC and hysteresis current control (d).
49 50 51
-2.1
-2
-1.9
-1.8
-1.7
-1.6
-1.5
-1.4
x 10
6
Time [s]
(a)
Ps[W]
49 50 51
-2.1
-2
-1.9
-1.8
-1.7
-1.6
-1.5
-1.4
x 10
6
Time [s]
(b)
49 50 51
-2.1
-2
-1.9
-1.8
-1.7
-1.6
-1.5
-1.4
x 10
6
Time [s]
(c)
49 50 51
-2.1
-2
-1.9
-1.8
-1.7
-1.6
-1.5
-1.4
x 10
6
Time [s]
(d)
Fig. 7: Active power during transient state.
60 70 80 90 100
-2.1
-2
-1.9
-1.8
-1.7
x 10
6
Time [s]
(a)
Ps[W]
60 70 80 90 100
-2.1
-2
-1.9
-1.8
x 10
6
Time [s]
(b)
60 70 80 90 100
-2.05
-2
-1.95
-1.9
-1.85
x 10
6
Time [s]
(c)
Ps[W]
60 70 80 90 100
-2.1
-2
-1.9
-1.8
x 10
6
Time [s]
(d)
Fig. 8: Active power during unbalanced voltage.
49 49.5 50 50.5 51
-1.3
-1.2
-1.1
-1
-0.9
-0.8
-0.7
x 10
6
Time [s]
(a)
Qs[VAR]
49 49.5 50 50.5 51
-1.3
-1.2
-1.1
-1
-0.9
-0.8
-0.7
x 10
6
Time [s]
(b)
49 49.5 50 50.5 51
-1.3
-1.2
-1.1
-1
-0.9
-0.8
-0.7
x 10
6
Time [s]
(c)
Qs[VAR]
49 49.5 50 50.5 51
-1.3
-1.2
-1.1
-1
-0.9
-0.8
-0.7
x 10
6
Time [s]
(d)
Fig. 9: Reactive power during transient state.
The red lines in the figures above are the commanded
values of P and Q. The average values over one period
are observed for active and reactive power. However,
1496

instantaneous values are collected for stator current,
rotor current and torque. Harmonics analyses of the rotor
currents are shown in Figs. 15-20 for the traditional
SFOC and the two proposed control methods. Figs.
21-26 present the harmonic contents as well as Total
Harmonic Distortion (THD) of stator current in the three
schemes, for both balanced and unbalanced voltages.
60 70 80 90 100
-9
-8.5
-8
-7.5
-7
-6.5
x 10
5
Time [s]
(a)
Qs[VAR]
60 70 80 90 100
-9
-8.5
-8
-7.5
-7
-6.5
x 10
5
Time [s]
(b)
60 70 80 90 100
-9
-8.5
-8
-7.5
-7
-6.5
x 10
5
Time [s]
(c)
Qs[VAR]
60 70 80 90 100
-9
-8.5
-8
-7.5
-7
-6.5
x 10
5
Time [s]
(d)
Fig. 10: Reactive power during unbalanced voltage.
Fig. 11: Stator current before and during unbalanced
voltage.
59.5 60 60.5
-3
-3
-3
0
1
2
3
Time [s]
(a)
Iabcr[kA]
59.5 60 60.5
-3
-2
-1
0
1
2
3
Time [s]
(b)
59.5 60 60.5
-3
-3
-1
0
1
2
3
Time [s]
(c)
Iabcr[kA]
59.5 60 60.5
-3
-2
-1
0
1
2
3
Time [s]
(d)
Fig. 12: Rotor current before and during unbalanced
voltage.
30 40 50 60 70 80 90 100
-1.3
-1.2
-1.1
-1
-0.9
x 10
4
Time [s]
(a)
Te[kN.m]
30 40 50 60 70 80 90 100
-1.3
-1.2
-1.1
-1
-0.9
x 10
4
Time [s]
(b)
30 40 50 60 70 80 90 100
-1.3
-1.2
-1.1
-1
-0.9
x 10
4
Time [s]
(c)
Te[k.N.m]
30 40 50 60 70 80 90 100
-1.3
-1.2
-1.1
-1
-0.9
x 10
4
Time [s]
(d)
Fig. 13: Generator torque during transient state and
unbalanced voltage.
60 70 80 90 100
-1.3
-1.2
-1.1
-1
x 10
4
Time [s]
(a)
Te[kN.m]
60 70 80 90 100
-1.35
-1.3
-1.25
-1.2
-1.15
-1.1
x 10
4
Time [s]
(b)
60 70 80 90 100
-1.35
-1.3
-1.25
-1.2
-1.15
-1.1
x 10
4
Time [s]
(c)
Te[k.N.m]
60 70 80 90 100
-1.35
-1.3
-1.25
-1.2
-1.15
x 10
4
Time [s]
(d)
Fig. 14: Generator torque during unbalanced voltage.
0 50 100 150 200
0
500
1000
1500
2000
2500
Frequency (Hz)
Fundamental (50Hz) = 18.04 , THD= 155.40%
Mag(%ofFundamental)
Fig. 15: Rotor current of the traditional SFOC with
PWM current control during balanced voltage.
0 50 100 150 200
0
500
1000
1500
Frequency (Hz)
Mag(%ofFundamental)
Fig. 16: Rotor current of SVOC with hysteresis control
during balanced voltage.
Fig. 17: Rotor current of SVOC with PWM control
59.95 60 60.05
-3
-3
-1
0
1
2
3
Iabcs[kA]
Time [s]
(a)
59.95 60 60.05
-3
-2
-1
0
1
2
3
Time [s]
(b)
59.95 60 60.05
-3
-2
-1
0
1
2
3
Time [s]
(c)
59.95 60 60.05
-3
-2
-1
0
1
2
3
Time [s]
(d)
1497

Fig. 18: Rotor current of the traditional SFOC with
PWM current control during unbalanced voltage.
Fig. 19: Rotor current of SVOC with hysteresis control
during unbalanced voltage.
0 50 100 150 200
0
500
1000
1500
Frequency (Hz)
Mag(%ofFundamental)
Fig. 20: Rotor current of SVOC with PWM control
0 50 100 150 200
0
0.1
0.2
0.3
0.4
0.5
Frequency (Hz)
Fundamental (50Hz) = 1601 , THD= 1.97%
Mag(%ofFundamental)
Fig. 21: Stator current of the traditional SFOC with
PWM current control during balanced voltage.
0 50 100 150 200
0
0.05
0.1
0.15
0.2
0.25
0.3
Frequency (Hz)
Mag(%ofFundamental)
Fig. 22: Stator current of SVOC with hysteresis control
0 50 100 150 200
0
0.1
0.2
0.3
0.4
0.5
0.6
Frequency (Hz)
Mag(%ofFundamental)
Fig. 23: Stator current of SVOC with PWM control
Fig. 24: Stator current of the traditional SFOC with
PWM current control during unbalanced voltage.
Fig. 25: Stator current of SVOC with hysteresis control
1498

0 50 100 150 200
0
0.5
1
1.5
2
Frequency (Hz)
Mag(%ofFundamental)
Fig. 26: Stator current of SVOC with PWM control
5. DISCUSSION
The Fig. 7 shows that both the proposed methods
have insignificant steady-state errors in active power
response during balanced voltage, especially compared
with the steady-state error of the traditional SFOC
method. During unbalanced voltage, they both give
power responses with less pulsation than the other two
methods and also small steady-state errors; the
performance is much better with SVOC using hysteresis
current controller, as shown in Fig. 8.
The responses of reactive powers are quite similar to
each other for the four methods during the transient
state and steady state under balanced voltage. The
results in Fig. 9 and 10 also shows better reactive power
for the two proposed methods which are less oscillated.
Reactive power delivered with SVOC using the
hysteresis current control has much better performance.
The waveforms of stator current in the three modified
control methods are less distorted when voltage
unbalance happens, compared with the traditional SFOC.
The waveform of SVOC with hysteresis current control
is the least affected one as shown in Fig. 11. Similarly,
Fig. 12 demonstrates the small distortion in the rotor
current waveform of SVOC with hysteresis control
although higher rotor current is observed for this
method.
The performance of generator torque is much better
for the two proposed control scheme in this paper
during unbalanced voltage as shown in Fig. 13 and 14.
These methods give less torque pulsation compared to
the other two methods with SFOC, even modification
for coping with voltage unbalance is included. The
reduction of torque’s variation helps to decrease the
mechanical stresses on wind turbine systems.
Harmonic analysis of rotor current has shown little
difference in frequency spectrum of the three control
methods (the traditional SFOC with PWM current
control and the two proposed SVOC in this paper)
during the balanced voltage. Rotor frequency is about
3.33 Hz when the rotor speed is 1600 rpm. The energy
contents in higher-order harmonics are quite small
during the balance as shown in Fig. 15-17. However, the
magnitudes of higher harmonics in traditional SFOC
and SVOC with the hysteresis current control increase
significantly under voltage unbalance as demonstrated
in Fig. 18 and 19. It’s observed in Fig. 20 that frequency
spectrum for SVOC with PWM current control is not
much changed during the unbalance.
Harmonic contents of stator current during balanced
voltage are quite good for the three control schemes
above as shown in Fig. 21-23. The Total Harmonic
Distortion’s (THD) are almost the same in these figures.
However, during voltage unbalance, SVOC with PWM
current control gives the best performance in terms of
THD as shown in Fig. 23-26. Table 2 illustrates the
comparison of THD in the three methods for both
operating conditions, balanced unbalanced voltages.
Table 2. THD comparison for stator current.
THD
Traditional
SFOC
SVOC +
(PI+A) +
PWM
SVOC +
(PI+A) +
Hysteresis
Balanced
grid voltage
1.97 1.99 1.84
0% 1% -7%
Unbalanced
Grid
voltage
5.77 3.06 4.03
0% -47% -30%
(%)Tradtional SFOC
Tradtional SFOC
THD THD
Deviation
THD
−
=
Total harmonic distortion of the two new control
schemes has been significantly reduced during the
unbalanced voltage (47% for SVOV with PWM control
and 30% for SVOC with hysteresis control), when
compared with the THD in the traditional SFOC. All the
THD values of stator current are increased during the
unbalance.
Although the controlling target of the proposed
methods in this paper is constant generator’s torque to
reduce mechanical stresses, the obtained results are
satisfactory not only for torque but also for stator and
rotor current harmonics as well as active, reactive
powers.
The proposed methods give better performances of
torque and current waveform due to the presence of
reference values for rotor current’s negative sequence
components in the negatively rotating reference frame as
calculated in (11). The effects of negative sequence
components on rotor current during the unbalance, which
are the major causes for torque’s under-performance and
harmonics, are limited when the actual current
components in the positively rotating reference frame are
driven to the equivalent commanded values deduced
from these reference values as shown in (12). Another
reason for improved responses is the utilization of PI
plus anti-windup controllers used for driving the current
errors to zero. They give the advantages of quick
damping for current’s overshoots and oscillation, which
frequently happen during the voltage unbalance. The
number of PI controllers is not increased when compared
with other methods. This helps to give fast responses.
1499

6. CONCLUSION
Two new SVOC-based control methods, which use PI
controllers with anti-windup to deduce rotor current’s
commanded positive components in positively rotating
reference frame and two extra commanded values for
rotor current’s negative components in the negatively
rotating reference frame * *
,dr qri i− −
− −
, are proposed in the
paper. Verifications of the control schemes by
Matlab/Simulink during balanced and unbalanced
voltage of 10% in one phase, steady and transient states,
have also been presented. The results have shown
significantly reduced torque pulsation, especially for
SVOC with hysteresis current control, and decreased
stator and rotor current harmonics, especially for SVOC
with PWM current controller. Improved responses of
active and reactive powers are also observed for the
proposed ones.
The results are also compared with the others
obtained from simulation of the traditional SFOC and
modified SFOC using Sequence Component Controller
and PI controller with anti-windup. Both these control
methods utilized PWM current controller.
In the future, simulations of the proposed control
structures with other expressions of the rotor current
commands to achieve three other targets suggested in
[6] and [7] (constant active power, no oscillation of
rotor current, and balanced stator current) should also be
done. Experimental verifications should also be
implemented.
NOMENCLATURE
,s rv v Stator, rotor voltage vectors.
,s ri i Stator, rotor current vectors.
,s rψ ψ Stator, rotor flux vectors.
sω Stator angular frequency.
rω Rotor speed.
,s sP Q Stator output active and reactive power.
mL Mutual inductance.
,s rL Lσ σ Stator, rotor leakage inductances.
,s rL L Stator, rotor self inductances.
,s rR R Stator, rotor resistance
rθ Rotor angle
sθ Stator flux angle in SFOC, stator voltage
angle in SVOC.
slθ Slip angle, sl s rθ θ θ= −
Superscripts
,+ − Positively, negatively (dq) rotating reference
frames.
∗ Reference value for controllers.
Subscripts
,α β Stationary α-β axis.
,r rα β Rotor αr-βr axis.
,d q Rotating d-q axis.
,s r Stator, rotor.
,+ − Positive, negative components.
REFERENCES
[1] Ackermann, T. ; Wind power in power systems; John
Wiley and Sons, USA, 2003.
[2] Leonhard, W.; Control of electric drives;
Springer-Verlag, 3rd
edition, USA, 2001.
[3] Wenske, J.; “Special report direct drives and
drive-train development trends”; Wind Energy
Report Germany 2011, Siemens Press Picture, 2011.
[4] Alegría, M. I., Andreu, J., Martín, L. J., Ibanez, P.,
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requirements and regulation”; Renewable and
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[5] Muljadi, E., Yildirim, D., Batan, T., and Butterfield,
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[6] Xu, L., Wang, Y.; “Dynamic modeling and control
of DFIG based wind turbines under unbalanced
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2, pp. 439-451, 2009.
[8] Pham-Dinh, T., Nguyen, A. N., Nguyen-Thanh, H.;
“Improving stability for independent power control
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SFOC and DPC during grid unbalance”;
Proceeding of IPEC 2012, pp. 155-160, Ho Chi
Minh City, Vietnam.
[9] Pham-Dinh, T., Nguyen-Thanh, H., Uchida, K.,
Nguyen, G. M. T.; “Comparison between
modifications of SFOC and PDC in control of
grid-connected doubly fed induction generator
under unbalanced voltage dip”; Proceeding of SICE
2013, pp. 2581-2588, Nagoya Japan.
[10]Chondrogiannis, S.; Technical aspects of offshore
wind farms employing doubly fed induction
generators, PhD Thesis, Faculty of Engineering and
Physical Sciences, The University of Manchester,
United Kingdom, 2007.
[11] Yikang, H., Jiabing, H., Rende, Z.; “Modelling and
control of wind-turbine used DFIG under network
fault conditions”; Proceeding of ICEMS 2005, Vol.
2, pp. 986-991, Nanjing, China.
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