1. Comparison between Modifications of SFOC and DPC in Control
of Grid-Connected Doubly Fed Induction Generator under Unbalanced Voltage Dip
Truc Pham-Dinh1
, Hai Nguyen-Thanh2
, Kenko Uchida3
, Nguyen Gia Minh Thao4
1
Faculty of Electrical-Electronics Engineering, Ho Chi Minh City University of Technology, Vietnam
2
Department of Technology, Le Hong Phong High School, Ho Chi Minh City, Vietnam
( Email: 1
pdtruc@hcmut.edu.vn ; 2
hoc_vien@yahoo.com.vn )
3,4
Department of Electrical Engineering and Bioscience, Waseda University, Tokyo, Japan
( Email: 3
kuchida@uchi.elec.waseda.ac.jp ; 4
thao@uchi.elec.waseda.ac.jp )
Abstract: This paper presents both the previously and newly modified Stator Flux Oriented Control (SFOC) and Direct
Power Control (DPC) structures for Doubly Fed Induction Generator (DFIG) in wind turbines to improve responses of
active power, reactive power and generator's torque during the grid voltage unbalance. In the newly proposed
SFOC-based scheme, which emphasizes on improvement of generator's torque performance, PI controllers with
anti-windup, Notch filters and the Sequence Component Controller (SCC) are utilized. The other control techniques use
single or multiple applications of PI controller with anti-windup, hybrid PI-Fuzzy controller with anti-windup and
Notch filter to eliminate the second-order harmonic components. The designed system consists of a wound rotor
induction generator and power-electronics converters at both rotor and grid sides. The modifications are applied to the
rotor side converter (RSC). Simulations in Matlab/Simulink illustrate the enhanced stability of torque, active and
reactive powers delivered by DFIG in both the SFOC-based and DPC-based schemes. Moreover, comparisons in
simulation results, obtained separately from all the presented control structures, are provided to evaluate the
effectiveness of the newly proposed scheme.
Keywords: DFIG, Unbalanced Voltage Dip, PI-Fuzzy, Wind Turbine, SFOC, DPC.
1. INTRODUCTION
Nowadays, Doubly Fed Induction Generators, which
are wound rotor induction machines with slip rings on
the rotor shaft connected with the electrical grid through
power-electronics circuits in addition to the stator's grid
connection, have been commonly used in wind power
generation due to the advantages of power-electronics
circuit's low rating and independent control of delivered
active and reactive powers [1]. They also have the
capability to supply power on both stator and rotor in the
super-synchronous speed region while it is consumed by
rotor and generated by stator in sub-synchronous speed
region [1]. These powers of DFIG can be controlled
independently by the two most popular methods: SFOC
and DPC, which are designed for operations with the
balanced grid voltage [2]. However, it happens very
often that the grids experience the problems of voltage
unbalance, which raise the winding temperature and
cause pulsations of torque and power, oscillations of
stator/rotor currents, the electrical stress on the RSC and
the mechanical stress on the gear box [3,4,5,6].
In fact, there have been several modifications
proposed to ameliorate the performances of the DFIG
based wind turbines under the unbalance voltage dips.
Current controllers are designed to control positive and
negative sequences of the rotor current on the basic of
machine models in positive and negative d-q reference
frames in order to achieve four targets such as: the
balance of stator current, constant output active power,
invariant electromagnetic torque for reducing the
mechanical stress, and oscillation of the rotor current [5].
In detail, both positive and negative sequence
components are used in this scheme. Referring in [6],
dynamic programming power control plus which applies
the Bellman theory for optimal control of discrete time
systems is suggested to obtain correctly the converter
switching sequence for controlling the decoupled active
and reactive powers under the network unbalance
condition. The optimum policy is determined from the
pre-defined quadratic time-domain performance criterion
and cost function for any operating point. Nonetheless,
more verifications for MW-rank DFIG need to be done
for this scheme, and computational tasks are increased.
According to [7], an additional hardware is introduced
such as the series voltage compensation on stator of
DFIG for ramp-function voltage injection. However, if
the period of unbalance is prolonged, the problem of
energy storage will arise. Besides, another hardware
modification of the active crowbar in conjunction with
Direct Torque Control is also suggested in [8].
Nevertheless, this modification does not guarantee the
continuous connection of DFIG to the grid and requires a
special rating design [6,7].
A modified control scheme to reduce the over current
and over DC link voltage of the converters during grid
voltage disturbance has been proposed for SFOC and
stator voltage oriented control by considering the
dynamic response of magnetizing current [9]. The effect
of the core saturation on the performance of DFIG
during voltage unbalance is also investigated [10]. A new
model of DFIG whose magnetizing inductance is a
function of magnetizing current is developed to propose
a new control scheme, considering the dynamic response
of magnetizing current to compensate the effect of core
saturation [10]. However, the complexity of this control
system will increase with the inclusion of magnetizing
current's dynamic control.
SICE Annual Conference 2013
September 14-17, 2013, Nagoya University, Nagoya, Japan
2581 PR0001/13/0000-2581 ¥400 © 2013

2. This paper will investigate the qualities of active
powers, reactive powers, and generator's torques under
the unbalanced grid voltage dip during transient and
steady states for the traditional and modified SFOC and
DPC methods of DFIG. In detail, one newly modified
control scheme is proposed in this study, and two other
control structures were previously suggested in [16] by
the authors. The modifications are single or combined
applications of PI controller, hybrid PI-Fuzzy controller,
Notch filter and SCC to eliminate the negative sequence
components. In which, the PI controllers with
anti-windup are always used to replace the classical PI
controllers even in the traditional SFOC or DPC. The
grid's voltage dip is modeled with a reduction of 25% of
the rated voltage in one phase. Meanwhile, the wind
speed is allowed to vary randomly during the process.
2. MATHEMATICAL MODELOFWIND
TURBINE
The model of wind turbine and its formulas of power
transferred to generator are presented in this session.
According to [11], the mechanical system of wind
turbine is shown in Fig. 1. Specifications of the wind
turbine are discussed in Section 4 of this paper.
Fig.1 The mechanical model of wind turbine [11].
The power extracted from the wind is
),(
2
1 3
 pwturb
CAvP  (1)
Where:
ρ (kg/m3
), is the air density. A = R2
(m2
), is the
cross-sectional area through which the wind passes.
R (m) is the length of turbine's blades. vw (m/s) is the
normal wind speed to the cross-session area A.
Cp( is the aerodynamic efficiency which depends
on the tip speed ratio λ, and the blade pitch angle β.
According to Betz's efficiency, the maximum theoretical
efficiency is 59.3% [12].
The tip speed ratio λ expressed in (2) is defined as the
speed at which the outer tip of the blade is moving
divided by the wind speed.
w
turb
v
R
  (2)
Where: ωturb (rad/s)is the angular velocity of turbine.
The turbine efficiency Cp given by (3) is the function of
the tip-speed ratio , and the pitch angle β.
i
eC
i
p




5.12
54.0
116
22.0),(









 (3)
3. DIRECT POWER CONTROLAND STATOR
FLUX ORIENTED CONTROLOF DFIG
3.1 Previously and newly proposed control schemes
The structure of our formerly modified control method
with DPC for DFIG is represented in Fig. 2. Besides, the
modified control scheme previously and newly proposed
one with SFOC are illustrated by Fig. 3 and Fig. 4,
respectively. In which, appropriate voltage vectors for
the RSC are selected to control the generated active and
reactive powers in DPC. Converters on the rotor side of
DFIG are controlled by SFOC to achieve the
independent control of active and reactive powers. In
Stator Flux Oriented Control, the equations for
controlling the active and reactive power are derived
from the machine model in a rotating reference frame
which is attached to the induction machine's stator flux
space vector. Therefore, the implementation of SFOC
requires continuously reference frame transformation.
According to [13], the control system, using hybrid
PI-Fuzzy controller, has provided better performances of
the generated powers. However, this is only verified with
the balanced grid voltage. To enhance the stability of the
powers during voltage unbalance situation, the inclusion
of Notch filter has been suggested by [14,15] and shown
in Fig. 2. In detail, Notch filters are used to eliminate
second-order harmonic components in positive and
negative sequences of the stator voltage. For the scheme
in Fig. 3, Notch filters are used with the positive
sequence of stator voltage and the negative sequence of
the rotor current [16].
Fig. 2 The typical configuration of the grid-connected
DFIG, using DPC with Notch Filter in [16].
On the other hand, as seen in Fig. 4, the control
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2582

3. scheme proposed in this study, applies SCC to eliminate
the negative sequences of the stator voltage which cause
distortions in power responses. Additionally, Notch filter
is also used to eliminate the second-order harmonic
component in the stator voltage. This suggested control
scheme reduces the number of current sensors and Notch
filter. The decreased amount of computational tasks is
achieved with PI controllers with anti-windup.
Fig. 5 shows the spatial relationships between the
stationary (α,β)s reference frame, the rotor (α,β)r
reference frame rotating at the speed of ωr, and the dq+
and dq−
reference frames rotating at the angular speed
of ωs and −ωs, respectively. In addition, as seen in this
figure, the d+
axis of the dq+
reference frame is fixed to
the positive sequence stator voltage ds
V


.
Fig. 3 The previous control scheme for RSC of DFIG
using PI-Fuzzy controllers and Notch filters in [16].
Fig. 4 The proposed control scheme for RSC of DFIG
using PI controllers, Notch filters and SCC in this study.
Referring in Fig. 5, the transformations between
(α,β)s, (α,β)r, dq+ and dq− reference frames are
expressed in (4).
( , ) ( , )
;S Sj t j t
dq s dq s
F F e F F e
 
   
 
  (4)
According to (4) and [4,14,17], variables shown in
Figs. 3-4, , , , ,dq dq sdq dr qr
I I V I I
    
, are given by (5) to (9).
Fig. 5 Relationships between (α,β)s , (α,β)r , dq+ and
dq− reference frames [14].
2 2
;S Sj t j t
dq dq dq dq
I I e I I e
    
  (5)
2 2
( , ) ( , )
;Sl Slj t j t
dq r dq r
I I e I I e
 
   
 
  (6)
sdq
sdq s sdq s sdq
d
V R I j
dt

 

  
   (7)
2 Sj t
dr dr dr dr dr
I I I I I e
    
   
    (8)
2 Sj t
qr qr qr qr qr
I I I I I e
    
   
    (9)
The active and reactive powers of stator, s
P


and s
Q


,
are expressed in (10) and (11), respectively.
 1.5s ds ds qs qs
P V i V i
    
    
  (10)
 1.5s ds qs qs ds
Q V i V i
    
    
  (11)
3.2 PI-Fuzzy controller for the scheme in Fig. 3
As presented in Fig. 6, PI-Fuzzy controllers are used
to control the errors between the set and actual values of
both the active and reactive powers delivered to the grid
by the generator. In which, the parameters of the PI
controller (Ti and KP) are tuned suitably by the fuzzy
logic controller (FLC) to obtain the finest output for
driving the errors to zero. The variable parameters of the
controllers, which are fixed in traditional PI controllers,
will help achieve the best performance of the system.
The outputs of these controllers are the commanded
values of d-q components of the rotor current in the dq+
reference frame. As illustrated by Fig. 3, these
commanded values of currents are used to regulate the
RSC for supplying the rotor phase voltage to DFIG.
3.3 Modifications in the newly proposed scheme
The proposed control scheme uses the PI controllers
with anti windup instead of a hybrid PI and Fuzzy
controllers as shown in the control scheme for SFOC in
[16]. The combination of Fuzzy logic in the PI controller
requires more computational time, especially in real-time
control for the scheme's practical application.
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2583

4. Fig. 6 PI-Fuzzy controller.
The newly proposed scheme also include a SCC
which help to eliminate the negative sequence
components of the fundamental frequency and all the
harmonics components of stator voltage. The Notch
filters are assigned to remove the negative sequence
components which cause oscillation in active power,
reactive power, and electromagnetic torque according to
equations (8) and (9) [4]. The oscillation in these
equations happens at twice the frequency of the positive
sequence component. However, the performance of
digitally designed Notch filters is not perfect. Therefore,
the inclusion of SCC helps to clear all the negative
sequence components.
SCC also functions as a current controller using PI
controllers to regulate the positive sequence components
of rotor current. Negative sequence components of rotor
current will increase the power rating of Rotor Side
Converter if being used to control generator's active and
reactive power [5].
4. SIMULATION RESULTS
Simulations of the modified control methods for the
2.3MW DFIG are carried out with the generator and
wind turbine's parameters as given by Table 1.
Operations in both sub-synchronous speed region
(70%-100% synchronous speed) and super-synchronous
speed region (up to 130% synchronous speed) are
simulated with the randomly variable wind speed shown
in Fig. 7. As illustrated by Fig. 8, the grid voltage
unbalance happens after the time t = 30 s. Meanwhile,
the commanded values of the active and reactive powers
change at the time t = 50 s. Any operation outside these
two speed regions will increase significantly the power
rating of converters connected between rotor and the
grid.
Comparisons of average values of active and reactive
powers (PS and QS) in the steady state with five different
controllers are presented in Table 2 and Table 3,
respectively. In detail, both actual values and the
percentage of references are also shown for evaluation.
In addition, the average electromagnetic torque of the
generator is described in Table 4.
Table 1. Parameters of the 2.3MW DFIG and wind
turbine in use.
Generator
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 of the
electric system
ωs 100π (rad/s)
Inertia of generator Igen 93.22 (kg.m2
)
Wind turbine
Power PS 2.3 (MW)
Radius R 40 (m)
Friction coefficient Kms 8.949×107
(Nm/rad)
Gear box 1: f 80
Inertia of turbine rotor IWTR
4.176×106
(kg.m2
)
Density of air ρ 1.225
(kg/m3
)
Damping coefficient Ignored
Fig. 7 Random variation of the wind speed.
29.95 29.96 29.97 29.98 29.99 30 30.01 30.02 30.03 30.04 30.05
-800
-600
-400
-200
0
200
400
600
800
Time [s]
Vabcs[V]
Fig. 8 Voltage unbalance after the time t = 30 s.
The mean, maximum, and minimum values of the
active power, reactive power and machine's torque
during the unbalanced voltage from the 39th
second to
the 49th
second, are represented in Tables 2 to 3. In
detail, the statistics of operations at the sub-synchronous
speed nr = 1400 rpm and the super-synchronous speed
nr = 1600 rpm, are also illustrated by these tables.
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2584

5. Table 2. Average values of active power (PS) in the
steady state for five controllers.
 %
S Sref
Sref
P P
Deviation
P


During the unbalanced voltage, best performances of
active power are observed for DPC with Notch Filter,
then the traditional DPC without Filter.
In detail, the lowest value of PMax for DPC with
Notch filters is 5.0% of the commanded value when
nr =1400 rpm, and is 4.3% when nr = 1600 rpm.
The highest value of PMin for DPC with Notch Filter
is -4.2% of the commanded value when nr =1400 rpm,
and is -5.5% when nr = 1600.
Table 3. Average values of reactive power (QS) in the
steady state for five controllers.
 %
S Sref
Sref
Q Q
Deviation
Q


During the unbalanced voltage, best performances of
reactive power are observed for the DPC with Notch
Filter, then the traditional DPC without Filter.
In detail, the lowest value of QMax for DPC with
Filter is 9.87% of the commanded value when nr =1400
rpm, and is 7.2% when nr = 1600 rpm.
The highest value of QMin for the DPC with Filter is
-11.8% of the set value when nr =1400 rpm, and is
-8.3% when nr = 1600 rpm.
Table 4. Average values of generator's torque in the
steady state for the five controllers.
According to Table 4, during the unbalanced voltage,
the best performances of machine's torque are observed
for the SFOC with PI and SCC, then the SFOC with PI.
In detail, the lowest value of TMax for SFOC with PI
and SCC is 15514 N.m when nr =1400 rpm, and is
14295 N.m when nr = 1600 rpm.
The highest value of TMin for SFOC with PI and SCC
is 9783 N.m when nr =1400 rpm, and is 7645 N.m when
nr = 1600 rpm.
The results of five control schemes are shown in Figs.
9 to 14 for the active and reactive powers.
20 30 40
1
1.5
2
2.5
3
3.5
TIME [S]
Ps[MW](nr=1400)
SFOC WITH PI
20 30 40
1
1.5
2
2.5
3
3.5
SFOC WITH PI & SCC
TIME [S]
20 30 40
1
1.5
2
2.5
3
3.5
SFOC WITH PI-F & FILTER
TIME [S]
20 30 40
0.5
1
1.5
2
2.5
3
SFOC WITH PI
TIME [S]
Ps[MW](nr=1600)
20 30 40
0.5
1
1.5
2
2.5
3
SFOC WITH PI & SCC
TIME [S]
20 30 40
1
1.5
2
2.5
3
3.5
SFOC WITH PI-F & FILTER
TIME [S]
20 30 40
1.9
1.95
2
2.05
2.1
2.15
Time [s]
Ps[MW](nr=1400)
DPC normally
20 30 40
1.9
1.95
2
2.05
2.1
2.15
DPC WITH FILTER
Time [s]
20 30 40
1.9
1.95
2
2.05
2.1
2.15
DPC normally
Time [s]
Ps[MW](nr=1600)
20 30 40
1.9
1.95
2
2.05
2.1
2.15
DPC WITH FILTER
Time [s]
Fig. 9 Active power of DFIG when the voltage
unbalance happens from the time t = 30 s.
20 40 60 80
0
0.5
1
1.5
2
2.5
3
3.5
TIME [S]
Ps[MW](nr=1400)
SFOC WITH PI
20 40 60 80
0
0.5
1
1.5
2
2.5
3
3.5
SFOC WITH PI & SCC
TIME [S]
20 40 60 80
0
0.5
1
1.5
2
2.5
3
3.5
SFOC WITH PI-F & FILTER
TIME [S]
20 40 60 80
-0.5
0
0.5
1
1.5
2
2.5
3
SFOC WITH PI
TIME [S]
Ps[MW](nr=1600)
20 40 60 80
-0.5
0
0.5
1
1.5
2
2.5
3
SFOC WITH PI & SCC
TIME [S]
20 40 60 80
-0.5
0
0.5
1
1.5
2
2.5
3
SFOC WITH PI-F & FILTER
TIME [S]
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Time [s]
Ps[MW](nr=1400)
DPC normally
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC WITH FILTER
Time [s]
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC normally
Time [s]
Ps[MW](nr=1600)
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC WITH FILTER
Time [s]
Fig. 10 Active power of DFIG during transient state.
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2585

6. 49.5 50 50.5
0
0.5
1
1.5
2
2.5
3
TIME [S]
Ps[MW](nr=1400)
SFOC WITH PI
49.5 50 50.5
0
0.5
1
1.5
2
2.5
3
SFOC WITH PI & SCC
TIME [S]
49.5 50 50.5
0
0.5
1
1.5
2
2.5
3
SFOC WITH PI-F & FILTER
TIME [S]
49.5 50 50.5
0
0.5
1
1.5
2
2.5
SFOC WITH PI
TIME [S]
Ps[MW](nr=1600)
49.5 50 50.5
0
0.5
1
1.5
2
2.5
SFOC WITH PI & SCC
TIME [S]
49.5 50 50.5
0
0.5
1
1.5
2
2.5
SFOC WITH PI-F & NOTCH
TIME [S]
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Time [s]
Ps[MW](nr=1400)
DPC normally
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC WITH FILTER
Time [s]
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC normally
Time [s]
Ps[MW](nr=1600)
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC WITH FILTER
Time [s]
Fig. 11 Dynamic responses of DFIG's active power
during transient state under the voltage unbalance.
20 30 40
0.5
1
1.5
2
2.5
TIME [S]
Qs[MVAR](nr=1400)
SFOC WITH PI
20 30 40
0.5
1
1.5
2
2.5
SFOC WITH PI & SCC
TIME [S]
20 30 40
0.5
1
1.5
2
2.5
SFOC WITH PI-F & FILTER
TIME [S]
20 30 40
0
0.5
1
1.5
2
SFOC WITH PI
TIME [S]
Qs[MVAR](nr=1600)
20 30 40
0
0.5
1
1.5
2
SFOC WITH PI & SCC
TIME [S]
20 30 40
0
0.5
1
1.5
2
SFOC WITH PI-F & FILTER
TIME [S]
20 30 40
0.85
0.9
0.95
1
1.05
1.1
1.15
Time [s]
Qs[MVAR](nr=1400)
DPC normally
20 30 40
0.85
0.9
0.95
1
1.05
1.1
1.15
DPC with FILTER
Time [s]
20 30 40
0.85
0.9
0.95
1
1.05
1.1
1.15
DPC normally
Time [s]
Qs[MVAR](nr=1600)
20 30 40
0.85
0.9
0.95
1
1.05
1.1
1.15
DPC with FILTER
Time [s]
Fig. 12 Reactive power of DFIG when the voltage
unbalance happens from the time t = 30 s.
20 40 60 80
0.5
1
1.5
2
2.5
TIME [S]
Qs[MVAR](nr=1400)
SFOC WITH PI
20 40 60 80
0.5
1
1.5
2
2.5
SFOC WITH PI & SCC
TIME [S]
20 40 60 80
0.5
1
1.5
2
2.5
SFOC WITH PI-F & FILTER
TIME [S]
20 40 60 80
0
0.5
1
1.5
2
2.5
SFOC WITH PI
TIME [S]
Qs[MVAR](nr=1600)
20 40 60 80
0
0.5
1
1.5
2
2.5
SFOC WITH PI & SCC
TIME [S]
20 40 60 80
0
0.5
1
1.5
2
2.5
SFOC WITH PI-F & FILTER
TIME [S]
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Time [s]
Qs[MVAR](nr=1400)
DPC normally
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC with FILTER
Time [s]
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC normally
Time [s]
Qs[MVAR](nr=1600)
20 40 60 80
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC with FILTER
Time [s]
Fig. 13 Reactive power of DFIG during transient
states when the commanded values change.
49.5 50 50.5
0.5
1
1.5
2
2.5
TIME [S]
Qs[MVAR](nr=1400)
SFOC WITH PI
49.5 50 50.5
0.5
1
1.5
2
2.5
SFOC WITH PI & SCC
TIME [S]
49.5 50 50.5
0.5
1
1.5
2
2.5
SFOC WITH PI-F & FILTER
TIME [S]
49.5 50 50.5
0
0.5
1
1.5
2
2.5
SFOC WITH PI
TIME [S]
Qs[MVAR](nr=1600)
49.5 50 50.5
0
0.5
1
1.5
2
2.5
SFOC WITH PI & SCC
TIME [S]
49.5 50 50.5
0
0.5
1
1.5
2
2.5
SFOC WITH PI-F & FILTER
TIME [S]
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
Time [s]
Qs[MVAR](nr=1400)
DPC normally
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC with FILTER
Time [s]
49 49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC normally
Time [s]
Qs[MVAR](nr=1600)
49.5 50 50.5
0.8
1
1.2
1.4
1.6
1.8
2
2.2
DPC with FILTER
Time [s]
Fig. 14 Dynamic responses of DFIG's reactive power
during transient state under the voltage unbalance.
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2586

7. The above figures demonstrate the power responses
when the voltage unbalance happens (from the time t =
30 s) and when the commanded values of powers
change (at the time t = 50 s) under the voltage unbalance.
Besides, Fig. 15 illustrates the torque response of the
generator.
The active power response of DFIG with SFOC is
improved when the either modification is included.
However, the improvements of SFOC's performance are
not as good as DPC's performance. Oscillation of active
power is strongly reduced with DPC for both speed
regions. Similarly, reactive power responses of DFIG
with SFOC are not as good as the responses of DPC as
shown in Figs 12 to 14.
20 40 60 80
2
4
6
8
10
12
14
16
18
TIME [S]
Te[kN.m](nr=1400)
SFOC WITH PI
20 40 60 80
2
4
6
8
10
12
14
16
18
SFOC WITH PI & SCC
TIME [S]
20 40 60 80
2
4
6
8
10
12
14
16
18
SFOC WITH PI+F & FILTER
TIME [S]
20 40 60 80
-0.2
1
4
7
10
13
16
SFOC WITH PI
TIME [S]
Te[KN.m](nr=1600)
20 40 60 80
-0.2
1
4
7
10
13
16
SFOC WITH PI & SCC
TIME [S]
20 40 60 80
-0.2
1
4
7
10
13
16
SFOC WITH PI-F & FILTER
TIME [S]
20 40 60 80
0
2
4
6
8
10
12
14
16
18
Time [s]
Te[kN.m](nr=1400)
DPC normally
20 40 60 80
0
2
4
6
8
10
12
14
16
18
DPC with FILTER
Time [s]
20 40 60 80
0
2
4
6
8
10
12
14
16
18
DPC normally
Time [s]
Te[kN.m](nr=1400)
20 40 60 80
0
2
4
6
8
10
12
14
16
18
DPC with FILTER
Time [s]
Fig. 15 The torque response of DFIG.
On the contrary with active and reactive powers, the
generator's torque responses in SFOC are much better
than the ones in DPC as shown in Fig. 15, especially
when SCC and PI with anti windup are used. Significant
reduction of torque's oscillation is observed for the
newly proposed control scheme when voltage unbalance
happens. All the performances of torque with SFOC are
better in the sub-synchronous speed region. The control
method proposed in this paper gives slightly better
result during the transient state under voltage unbalance
in super-synchronous speed region.
Significant oscillations are observed in the responses
of torque in DPC's performance during transient state
under voltage unbalance. Variation of torque is also
higher when voltage unbalance happens. The
observations are consistent for both speed regions.
5. DICUSSION
As shown in Table 2, two DPC-based methods have
shown good steady-state active power responses during
the voltage unbalance. In detail, the deviation of the
mean value of active power from the reference value is
almost zero percent with the inclusion of Notch filter;
and the deviation of the maximum and minimum values
from the mean value are within 5% at both the speed
regions above or below the synchronous speed.
Similarly, SFOC with PI-Fuzzy controller and Notch
filter is also giving the good performance with small
deviation of mean values from reference values (about
2.7% at the sub-synchronous speed and 7.6% at the
super-synchronous speed), and a minor fluctuation from
mean values of the maximum and minimum values in
both the speed regions. The oscillation of the active
power is observed to be smallest for DPC with Notch
filter during the voltage unbalance.
The traditional SFOC's active power response when
the voltage unbalance happens has higher ripples in
both speed regions, while the responses obtained with
the two DPC schemes and SFOC with PI-Fuzzy
controller and Notch filter are not significantly distorted
as shown in Figs 9 and 10. The fluctuation of active
power with the newly proposed method is less than the
one with traditional SFOC but higher than the one with
SFOC including PI-Fuzzy and Notch filter from the
table and figures.
Figure 11 shows that the dynamic response of active
power for SFOC with PI-Fuzzy controller and Notch
filter during the transient state under voltage unbalance
is slower than the one for the proposed control scheme.
As seen in Table 3, steady-state responses of the
reactive power are also very good when Notch filters
are included in DPC. In detail, the deviations are 0.05%
and 0.1% respectively for operations at below and
above the synchronous speed. Besides, the deviations of
reactive power's mean values for SFOC with PI-Fuzzy
controller and Notch filter are also reasonably small
during the voltage unbalance (0.05% and 1.2% for the
sub-synchronous and super-synchronous speed regions,
respectively). As illustrated by Fig. 12, the fluctuation is
observed to be smallest for DPC with Notch filter.
Additionally, higher ripples are observed in reactive
power responses of the traditional SFOC when the
voltage unbalance occurs as described in Fig. 12. The
observation is also consistent with statistics in Table 3.
As shown in the table and figures, the oscillation of
reactive power in the proposed method is improved
when compared with the one in traditional method.
Fig. 13 and Fig. 14 shows the dynamic responses of
reactive powers during transient states. In detail, the
slower dynamic response is also observed with SFOC
with PI-Fuzzy and Notch filter when compared with the
response in SFOC incorporating SCC and PI with
anti-windup.
Besides, as illustrated by Table 4, the proposed SFOC
with PI and SCC (shown in Fig. 4) gives the smallest
torque variation during voltage unbalance for both the
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2587

8. super- and sub-synchronous speed regions. And this
characteristic is highlighted with the red color in Fig. 15.
Furthermore, as represented in Fig. 11 and Fig. 14, the
dynamic responses of active and reactive powers of the
proposed control method are as fast as the responses of
DPC methods. Torque responses observed in Fig. 15 are
also consistent with statistics described in Table 4.
6. CONCLUSION
The proposed SFOC-based scheme for DFIG with the
inclusion of SCC has elevated the stability of the torque
response during the grid voltage unbalance when being
compared with other modifications of SFOC and DPC
for better stabilities during the unbalanced voltage dip.
This improvement helps reduce the electrical stress on
converters and the mechanical stress on the gear box.
Furthermore, the responses of active and reactive power
are ameliorated when being compared with a traditional
SFOC, although the oscillations are still quite high.
In this study, the observations are made during the
occurrence of the voltage dip in one phase, transient
states as well as steady states of the powers and torque
under the unbalanced condition. When being compared
with responses from DPC, the proposed scheme also
gives fast responses of active and reactive powers
during transient states under the voltage unbalance.
In all the observations, the independent controls of the
powers are still maintained for the suggested scheme.
Responses of the active power, reactive power, and
torque from all the control schemes are observed at the
sub-synchronous speed operation when the active power
is consumed on the rotor and delivered on the stator of
DFIG, and at the super-synchronous speed operation
when the active power is delivered on both the stator
and rotor of DFIG.
In the future, the experimental verification of the
proposed control scheme should be carried out to
validate the results obtained in simulations. Control
methods to reduce the oscillations in stator current and
to regulate the switching states of Grid Side Converter
should also be researched.
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] Muljadi, E., Yildirim, D., Batan, T., and Butterfield,
C.P.; “Understand the unbalanced-voltage problem
in wind turbine generation”; Proceeding of IEEE
Industry Application Conference, Phoenix, USA,
pp.1359-1365, 1999.
[4] Baggu, M. M.; “Advanced control techniques for
doubly fed induction generator – based wind turbine
converters to improve low voltage ride- throught
during system imbalances”; PhD Thesis, Missouri
University of Science and Technology, 2009
[5] L. Xu, Y. Wang; “Dynamic modeling and control of
DFIG based wind turbines under unbalanced
network conditions”; IEEE Trans. Power Syst., Vol.
22, No. 1 , pp. 314–323, 2007.
[6] Santos-Martin, D., Rodriguez-Amenedo, J. L., and
Arnaltes, S.; “Providing a Ride-Through Capability
to a Doubly Fed Induction Generator Under
Unbalanced Voltage Dip”; IEEE Trans. of Power
Electronics, Vol. 24, No. 7, pp. 1747-1757, 2009.
[7] Zhang, S., Tseng, K. J., Choi, S. S., Nguyen, T. D.,
and Yao, D. L.; “Advanced Control of Series
Voltage Compensation to Enhance Wind Turbine
Ride Through”; IEEE Transactions of Power
Electronics, Vol. 27, No. 2, pp. 763-772, 2012.
[8] Seman, S., Niiranen, J., and Arkkio, A.;
“Ride-Through Analysis of Doubly Fed Induction
Wind-Power Generator Under Unsymmetrical
Network Disturbance”; IEEE Transactions of
Power Systems, Vol. 21, No.4, pp.1782-1789, 2006.
[9] Yikang, H., Jiabing, H., Rende, Z.; “Modeling and
Control of Wind-Turbine Used DFIG under
Network Fault Conditions”; Proceeding of ICEMS
2005, pp. 986-991, 2005.
[10] Zhao, J., Zhang, W., He, Y., and Hu, J.; “Modeling
and Control of a Wind-Turbine Driven DFIG
Incorporating Core Saturation During Grid Voltage
Dips”; Proceeding of ICEMS, pp. 2438-2442, 2008.
[11] Sorensen, P., Hansen, D.A.,Christensen, P., Mieritz,
M.; Bech, J., Bak-Jensen, B., Nielsen, H.;
“Simulation and Verification of Transient Events in
Large Wind Power Installation”; Project Report,
Risø National Laboratory, Roskilde, Norway, 2003.
[12]Masters, M. G.; Renewable and Efficient Electric
Power Systems; John Wiley and Sons, Inc.,
Publication, 2004.
[13]Pham-Dinh, T., Pham-Trung, H., Le-Thanh, H.,
“PI-Fuzzy Controller for Doubly Fed Induction
Generator Wind Turbine”; Proceedings of ASAC-
2011; pp. 79 – 81, 2011.
[14]Phan, V. T., Lee, H. H., Chun, T. W; “An Effective
rotor current controller for unbalanced stand – alone
DFIG systems in the rotor reference frame”; Journal
of Power Electronics, Vol.10, No.6, pp. 194-202,
2010.
[15]Jia-bing HU, Yi-kang HE, Lie X; “Dynamic
modeling and direct power control of wind turbine
driven DFIG under unbalanced network voltage
conditions”; Journal of Zhejiang University
SCIENCE, 2008.
[16]Pham-Dinh, T., Nguyen-Thanh H., Nguyen-Anh
N.; “Improving Stability For Independent Power
Control Of Wind-Turbine Doubly Fed Induction
Generator with SFOC and DPC During Grid
Unbalance”; Proceeding of 10th
IPEC, pp.155 – 160,
2012.
[17]Peterson, A., Harnefors, L., and Thiringer, T.;
“Comparison between stator-flux and grid flux
oriented rotor current control of doubly-fed
induction generators”; Proceeding of the 35th
Annual IEEE Power Electronics Specialist
Conference, Vol. 1, pp. 482–486, 2004.
SICE Annual Conference 2013
September 14-17, 2013, Nagoya, Japan
2588