The document discusses improving the fault ride-through capability of DFIG wind turbines by employing a voltage-compensation-type active superconducting fault current limiter (SFCL). It proposes using an active SFCL, which has higher controllability than a resistive or inductive SFCL. The active SFCL works by controlling the current injected in the secondary winding of superconducting transformers to limit fault currents and compensate voltage drops. Simulation results show the active SFCL can smooth power fluctuations in the DFIG during grid faults, strengthening the stability of the wind power system compared to an inductive SFCL.

1. 132 CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 2, SPRING 2015
Fault Ride-Through Capability Improvement of
DFIG-Based Wind Turbine by Employing a
Voltage-Compensation-Type Active SFCL
Amélioration de la robustesse aux défaillances des
tours éoliennes en utilisant le “DFIG” à
travers une compensation de tension active
Lei Chen, Member, IEEE, Feng Zheng, Changhong Deng, Zhe Li, and Fang Guo
Abstract—Based on the considerations in creating a smart grid roadmap, an integrated application of
renewable energy sources and superconducting power devices may bring more positive effects. This paper
suggests a voltage-compensation-type active superconducting fault current limiter (SFCL) to enhance the fault
ride-through capability of doubly fed induction generator (DFIG) for wind power generation. Since the active
SFCL has higher controllability and flexibility than a common resistive- or inductive-type SFCL, its application
may give better results. Related theory derivation, cost evaluation, and simulation analysis are conducted, and
a comparison between the active SFCL and an inductive SFCL is performed. From the results, the active
SFCL can limit the faulty currents flowing through the DFIG’s stator and rotor windings and compensate the
generator terminal voltage. The inductive SFCL may not evacuate the surplus active power during the grid
fault; however, the active SFCL can smooth the DFIG’s power fluctuation, and the stability of the wind power
system can be well strengthened.
Résumé—Dans l’objectif de mettre en place une feuille de route pour un réseau électrique intelligent, une
application intégrée pour les sources d’énergies renouvelables et appareils électriques supraconducteurs peut
avoir un impact positif. Ces travaux de recherche proposent une compensation de tension active pour limiter les
défauts provenant des limiteurs de courants actifs et super conducteur (SFCL) afin d’améliorer la robustesse
aux défaillances du générateur à induction à double alimentation (DFIG) pour la production d’énergie éolienne.
Puisque le SFCL a une contrôlabilité et une flexibilité supérieures à un SFCL résistif ou inductif, son utilisation
pourrait offrir de meilleurs résultats. D’après la théorie, les évaluations des coûts, et les simulations, des
analysesont été proposées et une comparaison entre le SFCL actif et le SFCL inductif est présentée. D’après les
résultats obtenus, le SFCL actif pourrait limiter les courants défectueux à travers les enroulements du stator
et du rotor du DFIG et compenser la tension aux bornes du générateur. Dans certains cas, le SFCL inductif
ne peut pas évacuer le surplus de puissance active lors d’un défaut du réseau. Toutefois, le SFCL actif peut
lisser la fluctuation de puissance du DFIG pour améliorer la stabilité du réseau d’énergie éolienne.
Index Terms—Doubly fed induction generator (DFIG), fault ride-through (FRT) capability, transient
simulation, voltage-compensation-type active superconducting fault current limiter (SFCL).
NOMENCLATURE
RE Renewable energy.
FRT Fault ride through.
Manuscript received August 29, 2014; revised December 29, 2014;
accepted February 20, 2015. Date of current version June 2, 2015.
This work was supported in part by the Fundamental Research
Funds for the Central Universities under Grant 2042014kf0011, in part
by the Wuhan Planning Projects of Science and Technology under
Grant 2013072304010827 and Grant 2013072304020824, and in part
by the Natural Science Foundation of Hubei Province, China, under
Grant 2014CFB706.
L. Chen, F. Zheng, C. Deng, and Z. Li are with the School of
Electrical Engineering, Wuhan University, Wuhan 430072, China (e-mail:
stclchen1982@163.com; 190991761@qq.com; deng_ch@whu.edu.cn;
1103080319@qq.com).
F. Guo is with the China Energy Engineering Group, Department of
Substation, Guang Dong Electric Power Design Institute, Guangzhou 510635,
China (e-mail: 34268545@qq.com).
Associate Editor managing this paper’s review: Houshang Karimi.
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/CJECE.2015.2406665
RSC Rotor side converter.
DFIG Doubly fed induction generator.
SFCL Superconducting fault current limiter.
SMES Superconducting magnetic energy storage.
SPWM Sinusoidal pulsewidth modulation.
STATCOM Static synchronous compensator.
Symbols
i Current [A].
I Effective phase current [A].
k Coupling factor [-].
L Inductance [H].
M Mutual inductance [H].
R Resistance [ ].
t Time [s].
T Time constant [s].
v Voltage [V].
0840-8688 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

2. CHEN et al.: FRT CAPABILITY IMPROVEMENT OF DFIG-BASED WIND TURBINE 133
V Effective phase voltage [V].
X Reactance [ ].
Z Impedance [ ].
ω Angular velocity [rad/s].
ψ, Flux [T].
Subscripts
A Phase A.
B Phase B.
C Phase C.
s1 Superconducting transformer’s primary coil.
s2 Superconducting transformer’s secondary coil.
st Superconducting transformer.
f Filter.
g Grid.
co Converter’s output.
r Rotor.
s Stator.
σ Leakage.
I. INTRODUCTION
THE smart grid is an electrical system that uses informa-
tion, two-way, cyber-secure communication technologies,
and computational intelligence in an integrated fashion across
the entire spectrum of the energy system from the generation
to the end points of consumption of the electricity [1]–[3].
Although there may be varying definitions to suit different
countries’ specific features, the overall goal of building the
smart grid is to improve the efficiency, reliability, economics,
and sustainability of the production and distribution of
electricity. Concerning the smart grid’s technical framework,
RE sources and superconducting power devices are two types
of crucial components. For the former, RE sources that
have vast potential to reduce dependence on fossil fuels and
their high penetrations into power distribution systems have
attracted increasing attention around the world [4]. As for the
latter, superconducting power devices can help to enhance an
electric system’s operating performance from several aspects,
such as current limitation and improvement of transient stabil-
ity as well as power quality [5]–[7]. In a sense, an integrated
application of RE generation and superconducting power may
bring more promising effects [8]–[11].
Currently, with regard to the rapid expansion of wind
power generation, DFIG attracts a wide public interest and
offers many advantages, such as reduced inverter and output
filter costs, due to low rotor- and grid-side power conversion
ratings (25%–30%) [12], [13]. However, DFIG suffers
from high sensitivity to grid disturbances, especially grid
faults [14]–[16]. To guarantee the FRT capability of DFIG,
different kinds of measures have been suggested [17]–[26].
Among the solutions, not only superconducting power
devices but also conventional apparatuses and advanced
control strategies are considered. A brief comparison of them
is given as follows.
The advanced control strategies can be regarded as the
software solutions, but they will not provide the FRT capability
under severe grid fault conditions, since the RSC cannot
supply the voltage as high as rotor back electromotive force
voltage because of dc-link voltage limitation [17], [18].
Fig. 1. Structure of the voltage-compensation-type active SFCL.
The crowbar circuit is the most popular hardware solution
to improve the FRT capability. This solution can protect the
RSC from overcurrent. However, it does not support the utility
to resume normal operation and is not allowed by the grid
connection requirement [19], [20]. STATCOM can be used
for dynamic power compensation, but it can only control the
reactive power and minimize the voltage fluctuation after fault
occurrence [21], [22]. Applying an SMES can stabilize the
dc-link voltage and provide a voltage protection to the RSC,
but it cannot suppress the overcurrent and electromagnetic
torque oscillations [23], [24]. In contrast, using an SFCL can
restrict the fault current, prevent the disruption of protective
equipment, and improve the dynamic performance of DFIG
more directly [25], [26].
Actually, our research group has proposed a voltage-
compensation-type active SFCL adopting high-temperature
superconductor (HTS) materials and power electronic
elements [27]. After the introduction of modern control theory,
the active SFCL offers a variety of operation modes and has
the abilities of controlling fault current and compensating
voltage sag within the expected ranges [28]–[31]. Since the
active SFCL has higher controllability and flexibility than a
common resistive- or inductive-type SFCL, its application in
enhancing the FRT capability of DFIG may play better results.
This paper is organized as follows. Section II introduces the
active SFCL’s structural principle as well as control strategy,
and discusses the influence mechanism of the active SFCL
to the FRT capacity of DFIG. Section III is devoted to
the modeling and simulation of a DFIG-based wind turbine
integrated with the active SFCL, where a comparison between
the active SFCL and an inductive SFCL is also performed.
Finally, the conclusion is drawn in Section IV.
II. THEORETICAL ANALYSIS
A. Structural Principle of the Active SFCL
Fig. 1 shows the topological structure of the voltage-
compensation-type active SFCL, which is composed of three

3. 134 CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 2, SPRING 2015
air-core superconducting transformers and a three-phase
four-wire converter. Ls1 and Ls2 are the winding self-
inductances, and Mst is the mutual inductance. Z1 is
the circuit impedance, and Z2 is the load impedance.
Cdc1 and Cdc2 are the split dc-link capacitors. L f and C f are
used to filter the high-order harmonics caused by the converter.
In normal (no fault) state, the injected current in the
secondary winding of each phase superconducting transformer
will be controlled to keep a certain value, where the magnetic
field in the air-core can be compensated to zero, so that the
active SFCL will not affect the main circuit. When a short-
circuit fault is detected, the injected current corresponding
to the faulted phase will be timely adjusted in amplitude or
phase angle, so as to control the series compensation voltage
and suppress the fault current.
It should be noted that the short-circuit fault can be
mainly classified as four types: 1) the single-phase grounded
fault; 2) double-phase ungrounded fault (phase-to-phase);
3) double-phase grounded fault; and 4) three-phase fault.
In accordance with the presented current-limiting principle,
the injected currents (is2A, is2B, and is2C) will be unbalanced
in the case that an unsymmetrical fault occurs. Therefore,
a three-phase four-wire voltage source inverter (VSI)
converter, which has split dc-link capacitors, a few electronic
devices, and great function with unbalanced load, serves as a
critical component included in the active SFCL.
Herein, taking the A-phase circuit for example to expound
the active SFCL’s operation mode, the mathematical relation
between the grid-side source voltage VgA and the main current
Is1A can be expressed as
˙VgA = ˙Is1A(Z1 + Z2) + jωLs1 ˙Is1A − jωMst ˙Is2A. (1)
The current is2A is controlled to make jωLs1 ˙Is1A −
jωMst ˙Is2A = 0, and the A-phase superconducting trans-
former’s primary voltage vs1A is regulated to zero. Thus, the
active SFCL’s equivalent impedance in series with the A-phase
circuit is zero, and is2A can be set as
˙Is2A = ˙Is1A Ls1/Mst =
˙Is1A
√
Ls1/Ls2
k
=
˙VgA
√
Ls1/Ls2
(Z1 + Z2)k
(2)
where k is the coupling coefficient and k = Mst/(Ls1Ls2)1/2.
When the node U is grounded and a single-phase fault
happens, the line current will rise from is1A to is1A−F, and
the primary and secondary voltages of the A-phase super-
conducting transformer will increase to vs1A−F and vs2A−F ,
respectively. Then, the following equations can be obtained:
˙Is1A−F =
˙VgA + jωMst ˙Is2A
Z1 + jωLs1
(3)
˙Vs1A−F = jωLs1 ˙Is1A−F − jωMst ˙Is2A
=
˙VgA( jωLs1) − ˙Is2A Z1( jωMst)
Z1 + jωLs1
(4)
˙Vs2A−F = jωLs2 ˙Is2A − jωMst ˙Is1A−F
= jωLs2 ˙Is2A − jωMst
˙VgA + jωMst ˙Is2A
Z1 + jωLs1
. (5)
From (4), changing is2A can play a role in regulating the
primary voltage, and further the current-limiting impedance
ZSFCL can be controlled in
ZSFCL =
˙Vs1A−F ˙Is1A−F = jωLs1−
jωMst ˙Is2A(Z1+ jωLs1)
˙VgA + jωMst ˙Is2A
.
(6)
According to the difference in the regulating objectives
of is2A, there are three operation modes.
1) Making is2A remain the original state, and
ZSFCL−1 = Z1( jωLs1)/(Z1 + Z2 + jωLs1).
2) Controlling is2A to zero, and ZSFCL−2 = jωLs1.
3) Regulating the phase angle of is2A to make the angle
difference between ˙VgA and jωMst ˙Is2A be 180°. Setting
jωMst ˙Is2A = −c ˙VgA, and ZSFCL−3 = cZ1/(1 − c) +
jωLs1/(1 − c).
As the other crucial component adopted in the active SFCL,
the air-core superconducting transformer has many advantages,
such as the absence of iron losses and magnetic saturation,
and it has more possibility of reduction in size, weight, and
harmonic than the conventional iron-core superconducting
transformer [32], [33]. On the other hand, the air-core-type
coupling coils might serve as a shunt reactor with large
magnetizing current, and it can be applied in an inductive
pulsed-power supply to decrease energy loss and improve
transfer efficiency [34], [35]. One point should be noted
particularly, and it is essential that lack of flux saturation in the
air-core can help to ensure the current-limiting impedance’s
linearity and reduce the possible high-order harmonics.
B. Control Strategy for the Converter Included in the SFCL
To make the active SFCL have higher controllability and
flexibility, this section suggests a suitable control strategy for
the converter. Assuming that the switching devices are ideal
and Cdc1 = Cdc2, the following mathematical equations can
be derived (R denotes the equivalent resistance of L f ):
⎧
⎪⎪⎪⎪⎪⎨
⎪⎪⎪⎪⎪⎩
L f
dicoA
dt
+ RicoA = −vs2A + vcoA
L f
dicoB
dt
+ RicoB = −vs2B + vcoB
L f
dicoC
dt
+ RicoC = −vs2C + vcoC
(7)
⎧
⎪⎪⎪⎪⎨
⎪⎪⎪⎪⎩
C f
dvs2A
dt
= icoA − is2A
C f
dvs2B
dt
= icoB − is2B
C f
dvs2C
dt
= icoC − is2C.
(8)
Furthermore, the mathematical equations in dq0 reference
frame can be obtained
⎧
⎪⎪⎪⎪⎪⎨
⎪⎪⎪⎪⎪⎩
L f
dicod
dt
= −Ricod + ωL f icoq − us2d + ucod
L f
dicoq
dt
= −Ricoq − ωL f icod − us2q + ucoq
L f
dico0
dt
= −Rico0 − us20 + uco0
(9)
⎧
⎪⎪⎪⎪⎨
⎪⎪⎪⎪⎩
C f
dvs2d
dt
= ωC f vs2q + icod − is2d
C f
dvs2q
dt
= −ωC f vs2d + icoq − is2q
C f
dvs20
dt
= ico0 − is20.
(10)

4. CHEN et al.: FRT CAPABILITY IMPROVEMENT OF DFIG-BASED WIND TURBINE 135
Fig. 2. Block diagram of the control strategy designed for the three-phase
four-wire VSI converter included in the active SFCL.
After dq0 transformation, the control objects (vs2d and vs2q)
can be determined, and the control system designed for the
three-phase four-wire converter should be consisting of two
subsystems: 1) coupling dq-axis system that needs to be
decoupled and 2) 0-axis system.
Since the injected currents (is2A, is2B, and is2C) will be
unbalanced under unsymmetrical fault conditions, not only
the sum of Vdc1 and Vdc2 should be essentially controlled
to maintain a constant value, but also the voltage balance
control for the split dc-link capacitors (Cdc1 and Cdc2) should
be considered [36]. The balance equation is arranged in
⎧
⎨
⎩
vdc1 + vdc2 = vdc
Cdc2
dvdc2
dt
− Cdc1
dvdc1
dt
= 3 is20 + C f
dus20
dt
.
(11)
In the light of (9)–(11), Fig. 2 shows the control block
diagram of the converter. From this figure, our research group
presents a double-loop control strategy, including an outer
loop for voltage control and an inner loop for current control.
After a series of computations from the referenced signals, the
calculated voltages (vcod, vcoq, and vco0) will be transferred to
the module of SPWM, and the pulse signals will be produced
to drive the converter.
C. Influence Mechanism of the Suggested SFCL
to the FRT Improvement of DFIG
Fig. 3 shows the schematic of a DFIG-based wind
turbine integrated with the active SFCL. Herein, the DFIG
is connected to the electric power grid through a step-up
transformer and the active SFCL is introduced before the
transformer. Supposing that a three-phase short-circuit
happens, the SFCL’s influence on the DFIG is conducted.
Based on the equivalent circuit of a DFIG for transient
analysis [37], the following equations can be obtained:
−→
Vs = Rs
−→
is + d
−→
ψs/dt + jωs
−→
ψs (12)
−→
Vr = Rr
−→
ir + d
−→
ψr /dt + j(ωs − ωr )
−→
ψr (13)
−→
ψs = Ls
−→
is + Lm
−→
ir (14)
−→
ψr = Lm
−→
is + Lr
−→
ir (15)
Fig. 3. Schematic of a DFIG-based wind turbine with the active SFCL.
where V, i, ψ, R, and L are, respectively, the voltage, current,
flux, resistance, and inductance of the DFIG, subscripts s and r
represent, respectively, the DFIG’s stator and rotor windings.
Besides, knowing that Ls = Lsσ + Lm and Lr = Lrσ + Lm,
Lsσ /Lrσ is the leakage inductance.
According to (14) and (15), the currents flowing through
the DFIG’s stator and rotor windings can be expressed as
−→
is =
−→
ψs/Ls − kr
−→
ψr /Ls (16)
−→
ir = −ks
−→
ψs/Lr +
−→
ψr /Lr (17)
where Ls = Ls − L2
m/Lr , Lr = Lr − L2
m/Ls, and ks, kr are,
respectively, signified as ks = Lm/Ls, kr = Lm/Lr .
In the fault state, a series of transient electromagnetic
changes will be caused, and the changes of stator and rotor
fluxes are critical. Due to the power transformer, the DFIG’s
terminal voltage will not directly drop to zero, and after a
stage of concussion, the terminal voltage will tend to a steady
value.
Consequently, a voltage-drop coefficient A1 (0 ≤ A1 ≤ 1)
is introduced, and the terminal voltage is described as
−→
Vs = (1 − A1)Vsejωst(t ≥ t0). According to the constant-
linkage theorem [38], [39], the stator flux can be indicated as
−→
ψs =
⎧
⎪⎪⎪⎪⎨
⎪⎪⎪⎪⎩
LsVs
Rs + jωs Ls
ejωst
+
Rs Ls Ir
Rs + j ωs Ls
ejωst
t < t0
Ls(1 − A1)Vs
Rs + jωs Ls
ejωst
+
Rs Ls Ir
Rs + j ωs Ls
ejωst
+
−→
ψs0e−Rs t/Ls t ≥ t0
(18)
where
−→
ψs0 = Ls A1Vsejωst0 /(Rs + jωs Ls). In a similar
way, the rotor flux is
−→
ψr = (ks LsVs)/(Rs + jωs Ls)ejωst +
Ir Lr ejωst0 e−Rr t/Lr t ≥ t0.
By neglecting the stator resistance and substituting the stator
and rotor fluxes in (16) and (17), the fault currents flowing
through the stator and rotor windings can be expressed as
−→
is =
(1 − A1)Vs
jωs
ejωst
+
−→
ψs0e−Rs t/Ls
/Ls
− kr
ksVs
jωs
ejωst
+ Ir Lr ejωst0
e−Rr t/Lr
/Ls (19)
−→
ir = −ks
(1 − A1)Vs
jωs
ejωst
+
−→
ψs0e−Rst/Ls
/Lr
+
ksVs
jωs
ejωst
+ Ir Lr ejωst0 e−Rr t/Lr
/Lr . (20)

5. 136 CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 2, SPRING 2015
In the case that the active SFCL is applied, the contribution
of introducing the current-limiting impedance ZSFCL is to
improve Ls and meanwhile to reduce ks. Taking mode 2 for
example, the current-limiting inductance Ls1 will be added
into the main circuit, and the expressions of Ls and ks will be
changed into Ls1 + Ls − L2
m/Lr and Lm/(Ls + Ls1), respec-
tively. From (19) and (20), the fault currents flowing through
the DFIG’s stator and rotor sides can be both suppressed.
Regarding the voltage compensation from the active SFCL,
the desired voltage boosting level should be considered.
In terms of the presented three operation modes, the
performance behavior under mode 3 is the best, but the
control philosophy under mode 1 is easy to implement
(without additional controls for the injected current’s initial
amplitude or phase angle). To make the active SFCL carry
out a mode switching smoothly, a specific criterion related
to voltage dip may be used. For instance, mode 1 can play
the role in response to grid faults that produce a voltage
dip of 30% and low, and in case that the dip is relatively
larger, mode 1 will be replaced by mode 2 or mode 3.
It should be noted that if the voltage dip is 100%, the active
SFCL can undertake part compensation, which is determined
by the air-core transformer’s design parameters and the
VSI converter’s rated capacity, and the remaining voltage
compensation can be provided through the DFIG converters as
injected reactive current to fulfill the grid code requirements.
D. Technical Discussions on the Active SFCL’s Cost
In view of that cost is an important factor affecting a
superconducting power device’s practical application, some
technical issues related to the active SFCL’s cost are discussed
in this section.
According to the previous studies of technical and economic
impacts on a power system by introducing a SFCL [40]–[42],
the annual cost of a SFCL is commonly composed of invest-
ment cost, maintenance cost, refrigeration cost, and so on.
Applying a SFCL may also achieve prospective benefits and
operation cost savings, in the case of considering the factors
such as increase of system reliability and reduction of expected
fault current. With regard to a SFCL’s installation site, the
most promising locations are found in distribution network,
low voltage (400 V), and industrial systems [40]. From the
reports in [43], the comparison between a dynamic voltage
restorer and a hybrid-type SFCL for the FRT improvement
is conducted, and it is stated that, when a SFCL is applied
for individual wind generator, the cost is expensive and
unacceptable. But in our opinion, regarding the FRT capacity
enhancement of a wind farm, including multiple generators,
the technoeconomic feasibility of using a SFCL may be
preferably demonstrated.
On the basis of the annual cost method, our research group
roughly evaluates the cost of the active SFCL at a 35-kV
power distribution network [42], where a large wind farm
(50-MW capacity) is accessed. During the calculation, the
second-generation HTS is used, and the calculation data come
from the voltage-compensation-type active SFCL developed
by Huazhong University of Science and Technology, China.
TABLE I
PARAMETERS FOR ECONOMICAL EVALUATION
The active SFCL’s annual cost cSFCL can be computed in [44]
cSFCL = ci + cm + co − ce (21)
where ci, cm, co, and ce are, respectively, the annual invest-
ment cost, maintenance cost, operating cost (including loss
cost as well as refrigeration cost), and economical benefits.
Considering the reduction of expected fault current (lower
network losses), enhancement of power quality, delay of
system improvement, and lower dimensioning of installa-
tions and devices resulted from the active SFCL, ce can be
expressed as [45]
ce =
k2Tn Pn
r2
+
cn
(P/A, b, n)
−
cn
(P/A, b, n + n1)
+ k3Cb
(22)
where (P/A, i, n) is the annual present value factor; Tn and Pn
are, respectively, the annual operating time and the rated
power of the network; r is the factor obtained by the ratio
of maximum power to the rated power; n1 is the delay year
number; and k3 and Cb are, respectively, the ratios of the
reduced dimensioning of installations and devices to the initial
one, and the installation cost as well as engineering cost.
The main parameters for an economical evaluation are listed
in Table I. In combination with the formula calculating and the
conceptual design of the active SFCL in a power distribution
system [46], the total cost of a 35-kV class three-phase active
SFCL is ∼4049 k¥ ($650 k) that includes HTS materials
cost 790 k¥, cryogenic vessel cost 154 k¥, converter cost
3000 k¥, and so on. As a matter of fact, in terms of the
active SFCL’s application in enhancing the wind farm’s FRT
capacity, improving this mean’s economical efficiency will
be considerably beneficial to demonstrating the feasibility.
A practicable method of reducing the total cost is to optimize
the superconducting transformer’s winding structure and
coupling factor, and accordingly the compensation capacity
contributed by the three-phase four-wire converter can
be decreased. The detailed technoeconomic analysis and
optimization calculation will be carried out in the near future,
and the results will be reported in later articles. Section III
is devoted to investigating the active SFCL’s performance
behaviors by electromagnetic transient simulation.

6. CHEN et al.: FRT CAPABILITY IMPROVEMENT OF DFIG-BASED WIND TURBINE 137
Fig. 4. Fault detection method applied to the simulation model for the active
SFCL’s operation mode switching.
TABLE II
MAIN SIMULATION PARAMETERS OF THE SYSTEM MODEL
III. SIMULATION STUDY
For quantitatively evaluating the active SFCL’s effects on the
DFIG’s FRT capability, the simulation model corresponding to
Fig. 3 is created through MATLAB software, where the fault
detection method is shown in Fig. 4. During the simulation,
current-limiting mode 1 and mode 3 are considered (the former
will be triggered automatically, and the latter will depend on
the fault detection). In addition, parts of simulation parameters
are expressed in Table II.
A. Symmetric Fault Simulation
In normal condition, the line current’s peak value
is ∼1.75 kA. Supposing that a three-phase grounded fault
happens at t = 1 s, and the fault duration/resistance is
0.4 s/0.1 , Figs. 5 and 6 show the operational characteristics
of the DFIG-based wind turbine without and with the active
SFCL (only mode 1 plays its role). From the figures, applying
the active SFCL can limit the first peak of the fault currents
(iAf, iBf, and iCf) to 2.62, 3.41, and 3.12 kA, respectively,
in contrast with 6.91, 9.26, and 6.59 kA without SFCL.
Fig. 5. Stator current waveform of the DFIG under the symmetric fault.
(a) Without SFCL. (b) With the active SFCL (mode 1).
Fig. 6. Stator voltage waveform of the DFIG under the symmetric fault.
(a) Without SFCL. (b) With the active SFCL (mode 1).
The reduction rate of the expected fault currents will be about
62.1%, 63.2%, and 52.7%, respectively. It is observed that,
employing the active SFCL can also compensate the DFIG’s
terminal voltage to 60% of normal value. By contrast, the
terminal voltage will decrease to 7% of normal value without
any additional current-limiting or voltage-compensation
devices.
Furthermore, owing to the mode switching driven by the
fault detection, mode 3 will smoothly replace mode 1 to
improve the active SFCL’s performance, and the A-phase
circuit is selected to state the wind power integrated system’s
transient characteristics. Fig. 7(a) reflects the active SFCL’s
effects on the line fault current (the DFIG’s stator current)
under mode 3, and Fig. 7(b) shows the injected current in the
A-phase superconducting transformer’s secondary winding.
It is clear that the mode switching is effectively implemented.
As mode 3 appears and plays its role, the first peak of the
A-phase fault current can be limited to 1.95 kA, and the
reduction rate of the expected fault current is ∼71.8%.

7. 138 CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 2, SPRING 2015
Fig. 7. Currents flowing through the A-phase superconducting transformer
under the symmetric fault. (a) Primary winding. (b) Secondary winding.
Fig. 8. Effects of the fault resistance on the DFIG’s terminal-voltage under
the symmetric fault. (a) Without SFCL. (b) With the active SFCL (mode 3).
Since the fault resistance may directly affect the DFIG’s
transient behaviors, relevant simulation analysis is performed
and denoted in Fig. 8. When the fault resistance is,
respectively, set as 1, 5, 10, and 15 , the DFIG’s terminal-
voltage without SFCL will be down to 12.5%, 48.3%, 68.8%,
and 79.2% of normal value, respectively. In the case that the
active SFCL operates under its current-limiting mode 3, the
DFIG’s terminal-voltage can be, respectively, compensated to
62.1%, 86.7%, 95.2%, and 97.4% of normal value.
Note that, to show a comparison between the active SFCL
and other methods for improving the DFIG’s FRT capacity,
a flux-coupling-type SFCL (an inductive SFCL) is introduced
and built in MATLAB [47], [48], and its inductance
parameters are similar to the imitated air-core superconducting
transformer. Accordingly, the two different SFCLs’ effects on
the DFIG’s operating characteristics are both assessed and
stated as follows.
The evaluation of rms voltage in pu at the terminals of the
wind turbine generator is shown in Fig. 9, where the fault
Fig. 9. Influence of the two different SFCLs on the DFIG’s terminal-voltage
under the symmetric fault. (a) Active SFCL. (b) Inductive SFCL.
Fig. 10. Influence of the two different SFCLs on the DFIG’s power fluctuation
under the symmetric fault. (a) Active SFCL. (b) Inductive SFCL.
duration/resistance is 0.4 s/0.1 . It is recognized that the
DFIG’s terminal voltage during the fault can be maintained at
78% of normal value. As the highest minimum voltage level
required worldwide is ∼0.25 pu [49], the introduction of the
active SFCL (mode 3) can well meet the demand for ensuring
the DFIG’s low-voltage ride-through capacity. Concerning the
inductive SFCL’s impacts on improving the DFIG’s terminal-
voltage, its contribution is between the active SFCL’s mode 1
and mode 3.
Increased current in the rotor circuit is very harmful to the
RSC, and the DFIG’s power fluctuation is closely related to
the wind power system’s stability. Hence, the DFIG’s power
fluctuation and rotor current are selected as the key indicators
to compare the two SFCLs’ performance behaviors, and the
detailed simulation results are shown in Figs. 10 and 11, where
the change of the rotor current is described. From the given
two figures, employing the active SFCL and the inductive
SFCL can both make the DFIG’s rotor current be limited
within a safety margin, and consequently, the probability of

8. CHEN et al.: FRT CAPABILITY IMPROVEMENT OF DFIG-BASED WIND TURBINE 139
Fig. 11. Effects of the two different SFCLs on the DFIG’s rotor current
under the symmetric fault. (a) Active SFCL. (b) Inductive SFCL.
Fig. 12. Stator current waveform of the DFIG under the unsymmetrical fault.
(a) Without SFCL. (b) With the active SFCL (mode 3).
the DFIG being disconnected from the electric power grid
can be well reduced. It is also observed that, applying the
inductive SFCL can only restrain the DFIG’s power oscillation
amplitude to a certain extent, but it may not provide a more
positive effect on evacuating the surplus active power during a
fault incident. By comparison, the active SFCL’s contributions
include consuming the DFIG’s active power and decreasing the
output power oscillation, and thus the operational stability of
the wind power integrated system can be effectively enhanced.
B. Unsymmetrical Fault Simulation
In this section, the DFIG’s dynamic response characteristics
under an unsymmetrical fault are imitated. Supposing that a
phase-to-phase (B-phase and C-phase) fault happens at t = 1 s,
and the fault duration is 0.4 s, Figs. 12 and 13 show the DFIG’s
stator current and stator voltage behaviors under the fault,
where the active SFCL’s current-limiting mode 3 is applied.
Fig. 13. Stator voltage waveform of the DFIG under the unsymmetrical fault.
(a) Without SFCL. (b) With the active SFCL (mode 3).
Fig. 14. Effects of the active SFCL on the DFIG’s terminal-voltage under
the unsymmetrical fault. (a) B-phase voltage. (b) C-phase voltage.
As the aforementioned comparison between the active SFCL
and the inductive SFCL can describe the essential difference
between the two means, this section does not simulate the
inductive SFCL’s current-limiting characteristics.
In the case of installing the active SFCL, reduction of
the fault-phase currents and improvement of the terminal-
voltage can both be achieved. Aiming at the DFIG’s
B-phase and C-phase voltages, the evaluation of the rms
characteristics in pu is shown in Fig. 14. By observing the
characteristic curves at t = 1.3 s, the two phase voltages
will, respectively, drop to 70% and 24.3% of normal value
without SFCL. When the active SFCL’s mode 3 comes into
play and the transient impedance is added into each of the
fault-phase circuit, the DFIG’s B-phase and C-phase voltages
can be compensated to 94.1% and 87.2% of normal value,
respectively. Herein, it should be pointed out that, for dealing

9. 140 CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 2, SPRING 2015
Fig. 15. Effects of the active SFCL on the DFIG’s operational characteristics
under the unsymmetrical fault. (a) Power fluctuation. (b) Change of rotor
current.
with the unsymmetrical fault more efficiently (since the
unbalance between the two fault-phases is relatively obvious),
not only the angle of the current flowing through the
C-phase superconducting transformer’s secondary winding
will be adjusted, but also the amplitude will be properly
enlarged by the converter. As a result, the active SFCL can
act on the two fault phases more balancedly.
Concerning the DFIG’s power fluctuation and rotor fault
current under the unsymmetrical fault, the active SFCL’s
effects on them are indicated in Fig. 15. From this figure,
applying the active SFCL can availably reduce the DFIG’s
power fluctuation, and the inhibition rate is ∼32.1%.
Meanwhile, the increase of the circuit impedance will slightly
lengthen the duration of the fluctuation process. On the other
hand, the unsymmetrical fault will make some additional
ac and dc components be induced in the DFIG’s rotor
current. Introducing the active SFCL can also suppress
their amplitudes, and the maximum reduction ratio can be
up to 68.9%.
IV. CONCLUSION
In this paper, according to theory derivation, cost
evaluation, and simulation analysis, our research group
studies the application of a voltage-compensation-type active
SFCL for improving the FRT capability of DFIG-based
wind turbine. In addition, a brief comparison between the
active SFCL and a common inductive SFCL is conducted
to estimate the difference. From the results, the following
conclusions can be obtained.
1) Installing the active SFCL can effectively limit the fault
currents flowing through the DFIG’s stator and rotor
windings.
2) Applying the active SFCL can evidently compensate the
DFIG’s terminal-voltage and decrease its output power
fluctuation, which helps to strengthen the operational
stability of the wind power integrated system.
3) Employing the inductive SFCL can only restrain the
DFIG’s power oscillation amplitude, but it may not
provide a more promising effect on evacuating the
surplus active power during the grid fault.
Admittedly, concerning the proposed technical idea that is
used to enhance the DFIG’s transient performance, a number
of aspects should be enriched, such as optimization design and
technoeconomic analysis of the active SFCL, and protection
cooperation considered self-adaptive adjustment. Moreover,
for a wind farm, which may be consisting of hundreds of
wind turbines, the active SFCL should be installed for the
whole wind farm instead of individual wind turbines, so as to
reduce the cost as far as possible These research tasks will be
carried out in the near future.
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Lei Chen (M’12) was born in Jingzhou, China,
in 1982. He received the B.S. and Ph.D. degrees in
electrical engineering from the School of Electrical
and Electronic Engineering, Huazhong University
of Science and Technology (HUST), Hubei, China,
in 2004 and 2010, respectively.
He was with the Post-Doctoral Scientific Research
Workstation, Hubei Electric Power Company, Hubei,
from 2011 to 2013. Since 2013, he has been a
Teacher with the School of Electrical Engineering,
Wuhan University, Wuhan, China. He has authored
over 30 articles. His current research interests include power system real-time
simulation, smart grid, and superconducting power application.
Feng Zheng was born in Wenzhou, China, in 1983.
He received the B.S. degree in automation and the
M.S. degree in electric power system and automation
from the School of Electrical and Information, China
Three Gorges University, Yichang, China, in 2006
and 2009, respectively. He is currently pursuing the
Ph.D. degree in electric power system and automa-
tion with Wuhan University, Wuhan, China.
His current research interests include microgrid
and superconducting fault current limiter.

11. 142 CANADIAN JOURNAL OF ELECTRICAL AND COMPUTER ENGINEERING, VOL. 38, NO. 2, SPRING 2015
Changhong Deng was born in Hubei, China,
in 1963. She received the B.S. degree in electrical
engineering from China Three Gorges University,
Yichang, China, in 1982, and the M.S. and
Ph.D. degrees in electrical engineering from Wuhan
University, Wuhan, China, in 1989 and 2005,
respectively.
She has been a Teacher with the School of Elec-
trical Engineering, Wuhan University, since 2002.
She is currently a Professor and Ph.D. Supervisor.
She has authored over 50 articles. Her current
research interests include power system simulation, distributed generation,
and smart grid.
Zhe Li was born in Zhoukou, China, in 1989. He received the
B.S. and M.S. degrees in electrical engineering from the School of Electrical
Engineering, Wuhan University, Wuhan, China, in 2012 and 2014,
respectively.
His current research interests include microgrid and fault reconstruction.
Fang Guo was born in Jingzhou, China, in 1983. She received the
B.S. and Ph.D. degrees in electrical engineering from the School of Electrical
and Electronic Engineering, Huazhong University of Science and Technology,
Hubei, China, in 2004 and 2010, respectively.
She is currently with the Guang Dong Electric Power Design Institute,
Guangzhou, China. Her current research interests include superconducting
power application and energy storage technology.