3302 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 28, NO.

3, AUGUST 2013

A Comprehensive LVRT Control Strategy

for DFIG Wind Turbines With Enhanced
Reactive Power Support
Dongliang Xie, Zhao Xu, Senior Member, IEEE, Lihui Yang, Jacob Østergaard, Senior Member, IEEE,
Yusheng Xue, Member, IEEE, and Kit Po Wong, Fellow, IEEE

Abstract—The paper presents a new control strategy to enhance

the ability of reactive power support of a doubly fed induction
generator (DFIG) based wind turbine during serious voltage dips.
The proposed strategy is an advanced low voltage ride through
(LVRT) control scheme, with which a part of the captured wind
energy during grid faults is stored temporarily in the rotor’s in-
ertia energy and the remaining energy is available to the grid while
the DC-link voltage and rotor current are kept below the dan-
gerous levels. After grid fault clearance, the control strategy en- Fig. 1. Typical LVRT and reactive power requirements for wind turbines
sures smooth release of the rotor’s excessive inertia energy into the [1]–[4].
grid. Based on these designs, the DFIG’s reactive power capacity
on the stator and the grid side converter is handled carefully to sat-
isfy the new grid code requirements strictly. Simulation studies are
E.ON requires the LVRT to be much longer than 1 s. Other
presented and discussed.
examples exist in USA (FERC) and Canada (AESO), where the
Index Terms—Doubly fed induction generator (DFIG), low required LVRT duration can be even longer, and the duration
voltage ride through (LVRT), power system fault, reactive power.
with the minimum voltage at 0.15 p.u. can be more than 500
ms [2].
I. INTRODUCTION Traditionally, in order to defend the grid against large distur-

T O minimize the negative effects of large-scale wind bances on voltage and frequency, traditional thermal generators
power integration on the reliability of power grids, dif- must fulfill the LVRT requirement as well as providing emer-
ferent countries have defined different low voltage ride through gency power supports. With the increased wind penetration, grid
(LVRT) requirements for wind turbines (WT) in their grid connection requirements for WTs are getting reinforced to re-
codes. Fig. 1(a) combines the LVRT requirements according quire reactive power support by WTs during grid faults as well
to German, Scottish and Irish grid codes where WTs must stay [1]–[6]. For example, the Spanish grid code demands WTs to
connected when the terminal voltage remains inside or above start voltage control within 20 ms after fault recognition by pro-
the shadow area [1]. As illustrated in the figure, in Germany, viding additional reactive current amounting to at least 2% for
each percent of the voltage dip [3]. In Ireland [5] and the U.K.
[6], WTs must provide the maximum reactive current during a
Manuscript received August 05, 2012; revised August 23, 2012, November fault. Fig. 1(b) plots the worst case of reactive current require-
25, 2012, and January 05, 2013; accepted January 06, 2013. Date of publication
February 04, 2013; date of current version July 18, 2013. This work was sup- ments according to Spanish [3] and German [4] codes where a
ported in part by Hong Kong RGC GRF grants no. 515110 and 528412, in part WT’s reactive power support capability must remain inside or
by Hong Kong Polytechnic University via grants 4-ZZ8Z and G-YK80, and in right to the shadow area.
part by National Natural Science Foundation of China (No. 51207122). Paper
no. TPWRS-00912-2012. Doubly fed induction generator (DFIG) based WTs are
D. Xie is with the Department of Electrical Engineering, Hong Kong widely used because of high efficiency, variable speed oper-
Polytechnic University, Hong Kong, and also with State Grid Electric ation, as well as the use of converters with partial capacity
Power Research Institute, Nanjing, China (e-mail: eexdl@polyu.edu.hk;
xiedongliang@sgepri.sgcc.com.cn).
that enables independent control on active and reactive power.
Z. Xu is with the Department of Electrical Engineering, Hong Kong Poly- To realize LVRT for DFIGs, several technical concerns arisen
technic University, Hong Kong (e-mail: eezhaoxu@polyu.edu.hk). due to the grid faults must be properly addressed including
L. Yang is with State Key Laboratory of Electrical Insulation and Power the overcurrent in the stator and rotor circuits, the overvoltage
Equipment, Xi’an Jiaotong University, Xi’an 710049, China (e-mail:
lihui.yang@mail.xjtu.edu.cn). of the DC-link connecting the rotor side converter (RSC) and
J. Østergaard is with the Centre for Electric Power and Energy, Department the grid side converter (GSC), and the overloading of these
of Electrical Engineering, Technical University of Denmark, DK-2800 Lyngby, converters. To meet the challenging requirements of reactive
Denmark (e-mail: joe@elektro.dtu.dk).
Y. Xue is with State Grid Electric Power Research Institute, Nanjing, China power support during voltage dips, the reactive power output
(e-mail: xueyusheng@sgepri.sgcc.com.cn). from DFIGs needs to be increased as much as possible.
K. P. Wong is with the University of Western Australia, Crawley 6009, The existing LVRT solutions for DFIGs can be generally cat-
Western Australia (e-mail: kitpo@ieee.org).
Color versions of one or more of the figures in this paper are available online
egorized into two types including
at http://ieeexplore.ieee.org. — The crowbar protection [7]–[11]: Additional crowbar
Digital Object Identifier 10.1109/TPWRS.2013.2240707 resistors will be inserted into DFIG rotor circuits to limit

0885-8950/$31.00 © 2013 British Crown Copyright

XIE et al.: A COMPREHENSIVE LVRT CONTROL STRATEGY FOR DFIG WIND TURBINES WITH ENHANCED REACTIVE POWER SUPPORT 3303

Fig. 3. Different stages of voltage development during a typical fault.

the stage of voltage stabilizing, the basic component of rotor

Fig. 2. Schematic diagram of a DFIG WT system.
current should be however controlled according to the depth of
voltage dip. A staged reactive power controller is proposed to
distribute reactive power support between the stator and grid
the overcurrent and the RSC will be disabled temporarily (converter) sides automatically. The proposed control strategy
during fault periods. The detailed crowbar control strategy has been implemented using MATLAB/Simulink for a practical
and its industrial applications have been described in [12]. 1.5-MW DFIG WT. It has been demonstrated that the new
Although the crowbar solution can guarantee successful control strategy can fulfill the LVRT requirements and provide
fault ride through, it will however involve additional enhanced reactive supports to the grid in line with stringent grid
investments, energy dissipations by crowbar resistors, code requirements. In addition, simulation results of various
and particularly loss of control to active and reactive test scenarios have not only validated the effectiveness but also
power outputs. When the crowbar is activated, the DFIG the superior performances of the proposed control strategy with
will absorb large amount of reactive power, which can respect to both active and reactive power control.
be harmful to the grid. Therefore, some researches on
the improvements of reactive power performance of the II. MODELING OF A DFIG WIND TURBINE
crowbar scheme have been conducted [8], [9]. The schematic diagram of a grid-connected DFIG WT
— The demagnetizing method [13]–[17]: In order to de- system is shown in Fig. 2. The system includes the wind tur-
crease rotor current during a fault, demagnetizing methods bine, the shaft system, the induction generator, the back-to-back
try to eliminate transients of the induced electromagnetic PWM converters and the control system. Inside the DC-link, a
force in the rotor circuit by controlling the RSC output to dc-chopper is installed to protect the capacitor and converters
trace and counteract the oscillations of stator flux. How- from overvoltage [12]. The stator of the induction generator
ever, most methods so far are too complicated to be prac- and the grid-side converter are directly connected to the grid
tically implemented by industries. This is mainly due to synchronously. The control system consists of two subsystems
the limitations in converter capacity, the concerns of the for WT and DFIG respectively. The WT controller generates
algorithm reliability, and the estimations involved for the the rotor’s reference speed based on the optimum power-speed
control parameters like, e.g., rotor current, rotor flux and characteristic curve in order to capture maximum wind power,
rotor voltages, etc. [13]–[15]. which is termed as maximum power point tracking (MPPT)
Recognizing the limitations of existing LVRT solutions, control. During high winds, the pitch control will ensure the
the authors have successfully developed an innovative control WT working at the rated speed until it is stopped at the cut-off
strategy featuring improved energy efficiency and algorithm wind speed. Based on vector control techniques, the DFIG
simplicity [18]. By allowing the DFIG rotor to temporarily controller operates rotor and grid side converters to regulate
accelerate, wind energy continuously captured by a WT during active and reactive power output of the DFIG independently
a fault can be stored in the rotor as inertia energy, which will be according to the rotor’s reference speed and power factor
released back to the grid after fault clearance. The simulation requirement set by the grid code [19]. In the following analysis
results indicate that the method in [18] can decrease the rotor and simulation, the detailed fifth-order model of a DFIG WT is
current effectively. Its performance of limiting rotor current used [12]. The used MPPT control scheme is described in [18].
is even better than the crowbar protection, not to mention the
fact that the latter dissipates the captured wind energy as heats III. PROPOSED CONTROL STRATEGY FOR LVRT
during the faults. Therefore, the method in [18] is considered AND ENHANCED REACTIVE POWER SUPPORT
with high potentials of industrial applications in the future. A power system fault often consists of stages where voltage
Focusing on reinforcing the LVRT capability with reactive is falling, stabilizing and recovering, as illustrated in Fig. 3. At
power supports, this paper will further develop the control stage of voltage falling, the curve of rotor current often includes
strategy in [18] in order to fully utilize a DFIG’s potentials to large spikes. At stage of voltage recovering, the transient rotor
generate power effectively and securely considering a wider current is often of high amplitude. Based on this knowledge,
range of fault scenarios. Different control schemes can be special control schemes should be applied and catered for cer-
designed for different voltage stages. At stages of voltage tain stages.
falling and recovering, large rotor or stator current spikes The difficulties of LVRT control techniques for DFIG WTs
due to electromagnetic transients should be suppressed. At exist in the rapid change of the stator flux caused by the step
3304 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 28, NO. 3, AUGUST 2013

q-axis current in the rotor immediately. As a result, the increase

of the rotor speed finally absorbs wind energy as the rotor
inertia energy during the faults. Ignoring the friction losses,
the stored mechanical energy in the generator rotor
at time is relevant to the increase of rotor speed and can be
expressed as follows:

(1)
Fig. 4. Control scheme of the rotor side converter in normal operations.
where is the fault occurrence time, and is the rotor in-
ertia. With high wind speeds, the pitch control will be immedi-
change of grid voltage. This is the main reason for the observed ately triggered once the turbine speed exceeds the rated value,
rotor current oscillations. In view of this, the authors have tried so that the captured wind energy will be reduced effectively.
to decrease the basic component of the rotor current through Therefore the negative effect of increased mechanical stress to
cutting off the energy injection into the rotor circuit in [18], a the WT’s construction can be little as the pitch control will pre-
different way with practical merits to achieve the same goal of vent the rotor speed from over-speeding.
the crowbar protection. Under variable speed mode, the MPPT control is realized by
Once activated, however, the crowbar protection will convert setting the generator rotor’s reference speed as [20]
the DFIG into a typical induction motor [12]. Many demagne-
tizing methods make full use of the control loops for both d-
(2)
and q-axis currents in the rotor circuit. Compared with these
methods, the LVRT control strategy proposed by the authors in
[18] reserves considerable potential reactive power capacity so where is the output power, is the electromagnetic torque,
that a well-designed reactive controller can largely enhance the and is a construction dependent constant.
reactive power delivery to the grid, which is the focus of this To prevent overcurrent in the stator and rotor, the basic
paper. component of active current should be restricted. Accordingly,
The following sections will introduce the newly proposed the maximum allowed active power injected into the generator
control strategy in detail, which integrates the LVRT and re- during a fault is therefore restricted as below, based on (2)
active power control. Simulation validations as well as com-
parisons with other existing LVRT and reactive power control (3)
methods for DFIGs are given in Section IV.
where is the RMS value of the current PCC voltage, and
A. Rotor Side Control is the rated value of the voltage.
The reference value of q-axis current is expressed as a func-
In normal operations, the decoupling of active and reactive tion of the reference value of electromagnetic torque as in Fig. 4
power control is realized through the vector control technique [20]
with a reference frame oriented along the stator voltage vector
[20], resulting in the relatively independent d- and q-axis control (4)
loops as shown in Fig. 4. The q-axis loop controls the electro-
magnetic torque whose input signal is given by a PI controller The stator flux is hard to be measured accurately, since it
comparing the actual rotor speed with the reference one. The changes rapidly and cannot be controlled directly. Equation (4)
d-axis loop, corresponding to the control of magnetizing cur- indicates that this will lead to errors in the reference value for
rent, uses the desired reactive power as its input signal. The the rotor current. The first-order partial derivative of on
stator flux in Fig. 4 is estimated by the stator voltage [20]. is as follows:

At the stage of voltage recovering, starting when the voltage (5)

at the point of common coupling (PCC) increases above certain
threshold, the excess inertia energy is sent out to the grid through Because of (5), the impacts due to inaccurate stator flux could
a step-change signal given by the speed comparator in the q-axis be reduced by decreasing the reference torque , which ac-
loop. The stored inertia energy will increase due to the increase cordingly slows down the transformation from the mechanical
of the wind speed, the magnitude of voltage drop, or the fault and also magnetic energy to the electrical one in DFIGs. Based
duration time. The energy released in very short time duration on this consideration and (3), the reference value of electromag-
will cause overcurrent if without any special control. Therefore, netic torque can therefore be derived as
the terms and should be decreased in amplitudes
at this stage to prevent overcurrent as soon as a voltage recovery (6)
is detected.
During voltage dips, the power output of a DFIG is certainly where the factor is determined to effectively suppress
decreased. Accordingly the power input should be reduced the current spikes in the rotor and stator at the stage of voltage
as well to restore the power balance, which can be done by falling, and ensure safe LVRT. Considering a large may in-
reducing the electromagnetic torque. Known from Fig. 4, the crease the rotor current, the typical value, 0–0.5, is chosen for
decrease of electromagnetic torque will cause the decrease of this stage. At the stage of voltage stabilizing, a too small may
XIE et al.: A COMPREHENSIVE LVRT CONTROL STRATEGY FOR DFIG WIND TURBINES WITH ENHANCED REACTIVE POWER SUPPORT 3305

Fig. 6. Control scheme of the grid side converter in normal operations.

Fig. 5. Control scheme of the rotor side converter during voltage dips.

not ensure the rotor speed within the normal range, especially
under the high wind scenarios with the required LVRT dura-
tion of more than 1 s as shown in Fig. 1(a). With the proposed
control, DFIG will continue the active power production during
faults which is encouraged by the British grid code [6]. The
rotor-side control scheme during voltage dips is illustrated in
Fig. 5.
The new scheme in Fig. 5 is still a first-order system. Only
Fig. 7. Control scheme of the grid side converter during voltage dips.
the measurements of electric variables e.g. the grid voltage are
involved. Though the inertia energy is changing during this con-
trol process, there is no inertia response [21], [22] considered,
measured or directly controlled. Therefore only a small time (9)
delay is possibly involved in the closed control loop. PI con-
trollers with large bandwidths would be better for dealing with (10)
the fast oscillations of rotor current during voltage dips. How-
ever a larger bandwidth controller often has lower stability. The
bandwidth of a first-order system can be estimated from the where , and are the currents of the three branches con-
10%–90% rise time as follows [23]: necting the DC-link circuit and the RSC, the GSC and the capac-
itor , respectively. is the d-axis component of the grid-side
(7) circuit current. is the voltage cross the capacitor . is the
instantaneous active power injecting into the DC-link from the
In the following simulations, PI controllers with a bandwidth rotor side. , nominally 0.75, is the factor of PWM modulation
of about 35 Hz, i.e., the rise time of 10 ms, are enough to achieve depth [20].
good performances. During voltage dips, intense electromagnetic transients will
Similar to the demagnetizing methods, the control perfor- cause large . The PI controller in Fig. 6 cannot follow the
mance could also be affected if the voltage magnitude of RSC rapid changes of the current in time. A compensation term is
reaches its limit. Since the proposed method will not fully stop required to be added to represent the instantaneous variations
the DFIG from converting the mechanical energy to the elec- in . The term is used as the compensation signal
trical one during faults, the RSC voltage limit may not be ex- in [24], but the stator voltage may reduce to zero, consequently
ceeded at all while keeping the rotor current below 2 p.u. during resulting in overcompensation. Based on (8) and (9), the term
the short durations of LVRT. is therefore added to the d-axis control loop as shown
in Fig. 7, in order to smooth the fluctuations of DC-link voltage.
B. Grid Side Control
The reference frame of the GSC controller is also oriented
along the stator voltage vector based on the vector control tech- C. Reactive Power Control
nique. The d-axis loop controls the active power in order to Due to limited capacity of DFIG converters, the reactive
maintain a stable DC-link voltage. The q-axis loop controls re- power reference of the GSC is kept zero in normal operations,
active power generated from the grid side as illustrated in Fig. 6. in order to decrease the current and losses in both the con-
Under normal operations, the grid-side circuit current is much verters. Meanwhile, the response time of GSC is usually faster
smaller than the rotor circuit one. However, the grid-side circuit than RSC [12]. Therefore, the coordination for reactive power
current will rise significantly in low voltage events, where the generation between stator and GSC can be realized by using
GSC is an important delivery channel for the remaining active the following staged control scheme, where the output signal
power generated in the rotor circuit, and also a reactive power of the reactive power controller on the RSC is the input to the
source. one of the GSC. Therefore, the needed reactive power control
According to Kirchhoff’s circuit laws, the DC-link voltage due to PCC voltage variations can be distributed automatically
and related branch currents as denoted in Fig. 2, satisfy between both sides. It firstly uses the RSC to generate reactive
power until the fault becomes so serious that the GSC is also
(8) required to supply additional reactive power.
3306 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 28, NO. 3, AUGUST 2013

Fig. 8. Reactive power control scheme of a DFIG WT.

Fig. 10. Schematic diagram of the crowbar protection circuit.

Fig. 9. Single line chart of the test system.

The LVRT is a sort of power system transient security prob-

Fig. 11. Control scheme of the rotor side converter in Strategy B during voltage
lems under large disturbances, where voltage drops and needed dips.
reactive supports can be uncertain and rather fluctuating. There-
fore it is hard to obtain U-Q characteristic curves in advance for
power grids during those transients. The curves are often used TABLE I
to determine a schedule to distribute reactive power demands PARAMETER SETTINGS IN THE PROPOSED CONTROL STRATEGY
among different control resources in normal operations. Conse-
quently, a PCC-voltage feedback control is proposed for WTs
to enhance reactive power support in this paper, as illustrated in
Fig. 8.
In Fig. 8, denotes the reference RMS value of the PCC
voltage. and must be lower than the maximum “-” stands for not applicable
allowed reactive power of the stator and grid side accordingly.
For a fair comparison, the same reactive power control
strategy included in Strategy C is applied to Strategies A and
IV. SIMULATION RESULTS
B. For all the three strategies, the protection thresholds of rotor
As shown in Fig. 9, a simple test system has been estab- and stator currents are both set to 1.5 p.u., and the DC-chopper
lished consisting of a DFIG and a small transmission system. resistor is selected as 0.5 p.u.
The DFIG system is based on the models presented in Section II. Based on the worst profile, i.e., the lower boundary in
As shown in Fig. 9, a three-phase symmetric fault happens at Fig. 1(a), three scenarios have been simulated where low
the location denoted by the arrow, causing voltage dips at the voltage faults last for 1733 ms, 625 ms, or 150 ms if voltage
PCC bus. The following simulations were done in the Matlab/ drops 50%, 85%, or 100%, respectively. According to Fig. 1(b),
Simulink platform with all models built using standard blocks. the capacitive reactive current of WTs should be no less than
Major model parameters can be found in the Appendix. 90% of the rated value in the above three scenarios.
For comparisons, the DFIG WT has been simulated with 3
different control strategies considering possible scenarios with A. Scenarios With Longer Time Voltage Dips
different voltage dips. The focus is on their ability of LVRT and Fig. 12 plots transient time-response curves of terminal
reactive power support. These strategies are: voltage and active power etc. of the tested DFIG WT system
Strategy A: A crowbar protection equipped with a DC with Strategies A, B, and C. The first scenario has a 50%
chopper is illustrated in Fig. 10 [12]. The crowbar resis- voltage drop, fault duration time of 1733 ms and low wind
tors are all . speed (at 8 m/s). The fault occurs at 4 s and ends at 5.733 s.
Strategy B: A fast demagnetizing method proposed in The ratings for both the generation stator and rotor are listed in
[15]. The control schemes of RSC are given in Fig. 11 the Appendix.
for voltage dips and Fig. 4 for normal operations, respec- Fig. 12 shows that all the control strategies can fulfill the
tively. is connected to the signal in Fig. 4, while LVRT requirements, e.g., the stator, rotor and grid-side circuit
which is a modification from [15] in order to current should be lower than 2 times the rated value, the variance
enhance reactive support by this strategy. of DC-link voltage must be less than 15% of its rated value, the
Strategy C: The proposed control strategy described in rotor speed should be within a range from 70% to 130% of the
Section III. The settings of and in different stages synchronous speed (i.e., 1 p.u.), and the apparent power of both
of voltage development are listed in Table I. the converters need to be below 30% of the DFIG’s rated power
XIE et al.: A COMPREHENSIVE LVRT CONTROL STRATEGY FOR DFIG WIND TURBINES WITH ENHANCED REACTIVE POWER SUPPORT 3307

Fig. 13. Simulated transient responses of the studied DFIG WT at 50% voltage
drop and low wind speed (8 m/s) for control strategies A (crowbar), B (demag-
netizing method) and C (the proposed strategy). (a) Variation of total reactive
power. (b) Variation of reactive power from the stator side. (c) Variation of re-
active power from the GSC. (d) Variation of capacitive reactive current.

stator side during the fault. Strategies B and C provide 50%

rated reactive power from the stator side in Fig. 13(b) and give
almost 1 p.u. (100% of the rated value) reactive current shown
in Fig. 13(d). However, the current Strategy A can provide is
much less than the required minimum value, i.e., 90% of the
rated value, even if the reactive power of 0.3 p.u. is generated
from its GSC.

B. Scenarios With Shorter Time Voltage Dips

The testing scenario in Fig. 14 has an 85% voltage drop, fault
Fig. 12. Simulated transient responses of the studied DFIG WT at 50% voltage
duration of 625 ms and high wind speed (at 13 m/s).
drop and low wind speed (8 m/s) for control strategies A (crowbar), B (de- High wind speed and ultra-low fault voltage contribute to ob-
magnetizing method), and C (the proposed strategy). (a) Variation of terminal viously high oscillations in rotor current as shown in Fig. 14(c),
voltage. (b) Variation of active power. (c) Variation of rotor current. (d)Varia- leading to the increase of current in the rotor and grid sides as
tion of DC-link voltage. (e) Variation of grid-side circuit current. (f) Variation
of rotor speed. (g) Variation of the voltage on RSC. well as the DC-link voltage. Strategy C has lower and smoother
rotor current than Strategies A and B at the stage of voltage re-
covering. The increases of rotor speed in the three strategies are
[12]. The peak value of rotor current in Strategy C is much less all acceptable and far below 130% of the synchronous speed as
than the others, seen in Fig. 12(c), thanks to the control scheme shown in Fig. 14(f). Fig. 14(g) still shows that the voltage on
applied at the stage of voltage recovering. the RSC is below its rated value in both Strategies B and C.
These three control strategies lead to more obvious differ- In Fig. 15, where the results about reactive power and current
ences in power curves. With Strategy C, over 0.1 p.u. of ac- are shown, only Strategy C meets the requirement with reason-
tive power is delivered to the grid during the fault. Strategy A ably high capacitive current. The peak reactive current Strategy
absorbs active power when voltage drops, because the DFIG at A can provide is around 80% of the rated value. For Strategy
this moment has the same behavior as a typical induction motor. B, it is around 75%. Compared with the results in Fig. 13, the
This also leads to the differences in the rotor speed in Fig. 12(f), performance of Strategy B in providing reactive power is re-
where Strategy C largely suppresses the increase of the rotor duced, due to the conflict of control purposes between operating
speed, compared with the other two strategies. Fig. 12(g) shows the rotor current to trace the EMF’s oscillations and generating
the simulated voltage on the RSC, where the voltage magnitude smooth reactive current.
of both Strategies B and C is lower than the rated value, 0.3 Fig. 16 displays the rotor current and reactive current sup-
p.u., under the scenario. Strategy A introduces relatively high plied by the DFIG with the three control strategies when the
voltage on the rotor because the RSC is disconnected from the voltage at the PCC drops to zero. The LVRT requirement can
rotor circuit. be fulfilled by all the strategies and only Strategy C can satisfy
Fig. 13 shows the results of reactive power. Behaving like an the requirement of capacitive reactive current. Strategy B gives
induction motor, Strategy A absorbs reactive power from the a result around 80% of the rated value with high fluctuations,
3308 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 28, NO. 3, AUGUST 2013

Fig. 15. Simulated transient responses of the studied DFIG WT at 85% voltage
drop and high wind speed (13 m/s) for control strategies A (crowbar), B (demag-
netizing method) and C (the proposed strategy). (a) Variation of total reactive
power. (b) Variation of capacitive reactive current.

Fig. 14. Simulated transient responses of the studied DFIG WT at 85% voltage
drop and high wind speed (13 m/s) for control strategies A (crowbar), B (de-
magnetizing method) and C (the proposed strategy). (a) Variation of terminal
voltage. (b) Variation of active power. (c) Variation of rotor current. (d) Varia-
tion of DC-link voltage. (e) Variation of grid-side circuit current. (f) Variation
of rotor speed. (g) Variation of the voltage on RSC.

and Strategy A supplies almost zero reactive power during the

Fig. 16. Simulated transient responses of the studied DFIG WT at 100%
fault. voltage drop and high wind speed (13 m/s) for control strategies A (crowbar),
B (demagnetizing method) and C (the proposed strategy). (a) Variation of rotor
C. Robustness of the Proposed Strategy current. (b) Variation of capacitive reactive current.
This section shows the effect of different scenarios and con-
trol delays on the performance of the proposed control strategy.
Fig. 17 gives the simulated rotor currents when shifting the increase in both strategies inevitably, but the magnitudes are
starting time of the fault to the moment one quarter or half of a almost at the same level.
cycle after the original one. The resultant rotor current profiles
of high similarity indicate that the proposed method is not sen- V. CONCLUSIONS
sitive to the timing of a fault within a voltage cycle. The proposed control strategy has the following features that
The proposed strategy is a first-order closed-loop system. It distinguish it from other existing LVRT methods:
can use the same signals that trigger the crowbar protection to Multi-stage switching: The control topology or param-
realize the switching between the normal and the new LVRT eters will change at different stages of fault voltage de-
control schemes shown in Figs. 5 and 7. Fig. 18 compares the velopment, instead of the simple on-off switching in the
rotor current of the proposed strategy (Strategy C) with the crowbar protection. According to above simulations, the
crowbar protection (Strategy A) if the fault detection delays design provides better performance for all the stages, e.g.,
for 10 ms. The current spikes at the stage of voltage falling slower rotor acceleration at the stage of voltage stabilizing,
XIE et al.: A COMPREHENSIVE LVRT CONTROL STRATEGY FOR DFIG WIND TURBINES WITH ENHANCED REACTIVE POWER SUPPORT 3309

continuously operating resource of active and reactive power

and fulfill stringent grid code requirements during various
low voltage events. Being a simple control strategy without
additional hardware required, the proposed control is more
superior to the existing methods including the classical crowbar
protection both in performance and in the improvement of grid
reliability.

APPENDIX
The following gives the parameters used in the studied DFIG
WT model.
Fig. 17. Simulated rotor current of the studied DFIG WT at 85% voltage drop WT: Rated wind speed is 12 m/s; inertia constant ;
and high wind speed (13 m/s) for different starting time of a fault. damping coefficient ; shaft stiffness coefficient
; time constant of the pitch servo
;
DFIG: Base power, voltage and current are 1.5 MW, 575 V
and 1505 A accordingly; stator rated voltage and current are
575 V and 1505 A separately; rotor rated voltage and current
are 172.5 V and 1505 A accordingly; rotor rated speed is 1.1
p.u. (the synchronous speed is 1 p.u.); inertia constant
; friction coefficient ; stator resistance
; rotor resistance ; stator leakage
inductance ; rotor leakage inductance
; mutual inductance ; the rated voltage
on DC-link is 1200 V; the filtering inductance on the grid side
.

Fig. 18. Simulated rotor current of the studied DFIG WT at 85% voltage drop
and high wind speed (13 m/s) for control strategies A (crowbar) and C (the REFERENCES
proposed strategy) when the fault detection delays for 10 ms.
[1] W. Christiansen and D. R. Johnsen, Analysis of Requirements in Se-
lected Grid Codes, Section of Electric Power Engineering, Technical
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loop for the magnetizing current is excluded from the de produccion de regimen especial, PO 12.3. REE, Spain, Nov. 2005.
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Bayreuth, Germany, Apr. 2006.
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therefore obtained in the new design; plc, U.K., Jun. 2012.
[7] L. Peng, B. Francois, and Y. Li, “Improved crowbar control strategy of
Scenario-independent GSC control: Different from [24], DFIG based wind turbines for grid fault ride-through,” in Proc. IEEE
the compensation term added to the GSC controller, in 24th Annual Applied Power Electronics Conf. Expo., Washington, DC,
order to trace the rapid change of terminal voltage, can USA, Feb. 2009, pp. 1932–1938.
[8] A. H. Kasem, E. F. EI-Saadany, H. H. EI-Tamaly, and M. A. A. Wahab,
theoretically work under all LVRT scenarios including the “An improved fault ride-through strategy for doubly fed induction gen-
ones with serious voltage dips; erator-based wind turbines,” IET Renew. Power Gener., vol. 2, no. 4,
Feed-back and staged reactive power control: Simula- pp. 201–214, Dec. 2008.
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of wind turbines with doubly fed induction generator under symmet-
WT can be well and securely controlled by the proposed rical voltage dips,” IEEE Trans. Ind. Electron., vol. 56, no. 10, pp.
novel controller during grid faults, which have not been 4246–4254, Oct. 2009.
thoroughly studied in other LVRT methods anyhow; [10] M. Kayikçi and J. V. Milanović, “Assessing transient response of
DFIG-based wind plants-the influence of model simplifications and
No filtering component involved: There is no low-pass, parameters,” IEEE Trans. Power Syst., vol. 23, no. 2, pp. 545–554,
band-pass or high-pass filters used in the proposed May 2008.
method. Those filters are often seen in many demagne- [11] J. Yang, J. E. Fletcher, and J. O’Reilly, “A series-dynamic-resistor-
based converter protection scheme for doubly-fed induction generator
tizing methods, in order to identify specific components during various fault conditions,” IEEE Trans. Energy Convers., vol.
from electro-magnetic variables, which sometimes affect 25, no. 2, pp. 422–432, Jun. 2010.
the control performance significantly due to the introduced [12] V. Akhmatov, Induction Generators for Wind Power. Brentwood,
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[13] D. Xiang, L. Ran, P. J. Tavner, and S. Yang, “Control of a doubly fed
The simulation results showed that the proposed control induction generator in a wind turbine during grid fault ride-through,”
strategy can enable a safe LVRT of a DFIG based WT as a IEEE Trans. Energy Convers., vol. 21, no. 3, pp. 652–662, Sep. 2006.
3310 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 28, NO. 3, AUGUST 2013

[14] G. Abad, M. A. Rodríguez, J. Poza, and J. M. Canales, “Direct torque Lihui Yang received the Ph.D. degree in electrical
control for doubly fed induction machine-based wind turbines under engineering from Xi’an Jiaotong University, China,
voltage dips and without crowbar protection,” IEEE Trans. Energy in 2010.
Convers., vol. 25, no. 2, pp. 586–588, Jun. 2010. During 2008–2009, she was a visiting Ph.D.
[15] L. Yu, G. Chen, D. Cao, and G. Wu, “Low voltage ride-through con- student with the Centre for Electric Technology,
trol of doubly fed induction generator during symmetric voltage sag,” Technical University of Denmark. She is currently an
Elect. Mach. Control, vol. 14, no. 7, pp. 1–6, Jul. 2010. Assistant Professor with Xi’an Jiaotong University.
[16] S. Hu, X. Lin, Y. Kang, and X. Zou, “An improved low-voltage ride- Her research interests include stability and control
through control strategy of doubly fed induction generator during grid of wind power generation.
faults,” IEEE Trans. Power Electron., vol. 26, no. 12, pp. 3653–3665,
Dec. 2011.
[17] J. Liang, W. Qiao, and R. G. Harley, “Feed-forward transient current
control for low-voltage ride-through enhancement of DFIG wind tur-
bines,” IEEE Trans. Energy Convers., vol. 25, no. 3, pp. 836–843, Sep.
2010. Jacob Østergaard (M’95–SM’09) is Professor and
[18] L. Yang, Z. Xu, J. Østergaard, Z. Y. Dong, and K. P. Wong, “Advanced Head of the Centre for Electric Power and Energy,
control strategy of DFIG wind turbines for power system fault ride Department of Electrical Engineering, Technical
through,” IEEE Trans. Power Syst., vol. 27, no. 2, pp. 713–722, May University of Denmark. His research interests
2012. include integration of renewable energy, control
[19] M. Fujimitsu, T. Komatsu, K. Koyanagi, K. Hu, and R. Yokoyama, architecture for future power system, and demand
“Modeling of doubly-fed adjustable-speed machine for analytical side.
studies on long-term dynamics of power system,” in Proc. PowerCon Prof. Østergaard is serving in several professional
2000, Int. Conf. Power System Technology, Dec. 2005, vol. 4–7, pp. organizations including the EU SmartGrids advisory
25–30. council.
[20] R. Pena, J. C. Clare, and G. M. Asher, “Doubly fed induction gen-
erator using back-to-back PWM converters and its application to vari-
able speed wind-energy generation,” IEE Proc. Elect. Power Appl., vol.
143, no. 3, pp. 231–241, May 1996.
Yusheng Xue (M’87) received the Ph.D. degree in
[21] L. Holdsworth, J. Ekanayake, and N. Jenkins, “Power system fre-
electrical engineering from the University of Liege,
quency response from fixed speed and doubly fed induction generator
Belgium, in 1987. He is now the Honorary President
based wind turbines,” Wind Energy, vol. 7, pp. 21–35, 2004.
of State Grid Electric Power Research Institute
[22] A. Mullane and M. O’Malley, “The inertial response of induction-ma-
(SGEPRI), State Grid Corporation of China. His
chine-based wind turbines,” IEEE Trans. Power Syst., vol. 20, no. 3,
research interests include nonlinear stability, control
pp. 1496–1503, Aug. 2005.
and power system automation.
[23] L. Harnefors and H. Nee, “Model-based current control of AC ma-
Dr. Xue became a Member of the Chinese
chines using the internal model control method,” IEEE Trans. Ind.
Academy of Engineering in 1995.
Appl., vol. 34, no. 1, pp. 133–141, Jan./Feb. 1998.
[24] J. Yao, H. Li, Y. Liao, and Z. Chen, “An improved control strategy
of limiting the DC-link voltage fluctuation for a doubly fed induc-
tion wind generator,” IEEE Trans. Power Electron., vol. 23, no. 3, pp.
1205–1213, May 2008.
Kit Po Wong (M’87–SM’90–F’02) received the
Dongliang Xie received the M.Sc. degree from M.Sc, Ph.D., and higher doctorate D.Eng. degrees
Royal Institute of Technology (KTH), Sweden, from the University of Manchester, Institute of
and the Ph.D. degree in electrical engineering from Science and Technology, Manchester, U.K., in 1972,
Southeast University, China, in 2012. 1974, and 2001, respectively. He is a Winthrop
During 2011–2012, he was a research associate in Professor at with the Department of Electrical, Elec-
Hong Kong Polytechnic University. He is currently tronic and Computer Engineering, The University
working for State Grid Electric Power Research of Western Australia. His current research interests
Institute (SGEPRI), State Grid Corporation of China. include smart grid, power system analysis, planning
His research interests include analysis, simulation and operations.
and control for smart grid architectures and essentials Prof. Wong received three Sir John Madsen
consisting of renewable power generation, power Medals (1981, 1982, and 1988) from the Institution of Engineers Australia,
market and power system interactions, and demand elasticity. the 1999 Outstanding Engineer Award from IEEE Power Chapter Western
Australia, and the 2000 IEEE Third Millennium Award. He was General
Chairman of IEEE/CSEE PowerCon2000, IEE APSCOM 2003 and APSCOM
2009 international conferences. He was an Editor-in-Chief of IEE Proceedings
Zhao Xu (M’06–SM’12) received the Ph.D. degree in Generation, Transmission and Distribution and Editor (Electrical) of the
in electrical engineering from The University of Transactions of Hong Kong Institution of Engineers. He is serving as an
Queensland, Brisbane, Australia, in 2006. Editor-in-Chief of IEEE PES Letters.
From 2006–2009, he was an Assistant and later
Associate Professor with the Centre for Electric
Technology, Technical University of Denmark.
Since 2010, he joined Hong Kong Polytechnic Uni-
versity. His research interests include demand side,
grid integration of wind power, electricity market
planning and management.
Dr. Xu is now serving as the Secretary of PES/IAS/
PELS/IES Joint Chapter, IEEE Hong Kong Section.