Hardware in the Loop-Based Testing of Three Schemes for Mitigation
the Effect of Unsymmetrical Grid Faults on DFIG
ESSAMUDIN ALI EBRAHIM, MAGED N. F. NASHED, MONA N. ESKANDER
Power Electronics and Energy Conversion Department,
Electronics Research Institute,
Joseph Tito St., Huckstep, Qism El-Nozha, Cairo Governorate, Cairo,
EGYPT
Abstract: - This paper presents three-proposed schemes to mitigate the effect of unsymmetrical voltage sag
fault on a wind-driven grid-connected Double Fed Induction Generator (DFIG). The first tested scheme
comprises a static compensator (STATCOM) connected to the DFIG stator, while a three-phase parallel RL
external impedance is connected to the rotor circuit in the second scheme. The STATCOM and the added rotor
impedance are connected simultaneously in the third scheme. The effect of applying the three schemes on the
responses of the stator and rotor voltages and currents, the dc-link voltage and current, the electrical torque, and
the rotor speed during an unsymmetrical voltage sag are presented and compared at sub-and super-synchronous
speeds. All systems were emulated, implemented, and tested through an OPAL RT-4510 Digital Real-Time
Simulator (DRTS) in a Hardware-In-the-Loop (HIL) application. The internal Field-Programmable Gate Array
(FPGA) chip assisted in using this platform as a Rapid Control Prototyping (RCP) for virtual mitigation control
and testing. The Matlab/ Simulink RT-lab software packages combination helped in the RT development
environment. All real-time waveforms of parameters for the proposed scenarios were monitored through the
HIL-controller and data acquisition interface and then compared with the simulated results. The results reveal
that the simulation waveforms and the real time waveforms are congruent. Results prove the better performance
of the DFIG during unsymmetrical voltage sag for sub-synchronous speed when applying both protection
schemes, while best results are obtained when using only the rotor impedance at super-synchronous speed
operation of the DFIG.
Key-Words: - Double Fed Induction Generator (DFIG), Hardware In the loop (HIL), Rapid Control Prototyping
(RCP), Static Compensator (STATCOM), Unsymmetrical grid fault, Voltage sag, Wind Energy
Received: September 9, 2022. Revised: August 29, 2023. Accepted: September 27, 2023. Published: October 30, 2023.
1 Introduction
The double-fed induction generator (DFIG) is
employed in variable speed wind energy conversion
systems (WECS), where the DFIG stator is directly
connected to the grid, and its rotor is connected to
the grid via two back-to-back converters and a
transformer. The DFIG has a large share in the wind
energy conversion systems market due to its
benefits of variable speed operation, lower cost rotor
converters, and active and reactive power
independent control, [1]. However, its low voltage
ride-through (LVRT) capability is due to its stator’s
direct connection to the grid, [2]. Several researches
were done to solve the problems associated with
symmetrical grid faults, [3].
The unsymmetrical grid faults lead to high
electric torque pulsations, which adversely affect the
wind turbine and the associated mechanical
components such as the gearbox and bearings, [4].
These faults also cause high DC link voltage ripples,
which shorten the lifetime of the DC link capacitor,
and lead to high rotor current transients which
damage the rotor converter. Reference, [5],
proposed various techniques to mitigate the effect of
unsymmetrical grid faults on the performance of
grid-connected DFIG. The authors compared the
effects of both abrupt and discrete grid voltage sags
on the doubly-fed induction generator performance
without proposing a solution for mitigating these
effects.
In, [6], the authors proposed a control strategy
for both the rotor side and the grid side converters of
a DFIG-based wind turbine system to enhance the
low voltage ride through non-ideal proportional
resonant (PR) controllers for both converters to limit
rotor current surges and to keep the dc-link voltage
nearly constant during the grid faults. However,
sharp peaks were still obvious in DC voltage.
A gate-controlled series capacitor (GCSC) was
proposed in, [7], to be inserted in series with the
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E-ISSN: 2224-350X
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generator rotor as soon as a voltage dip occurs,
increasing the voltage seen by the rotor winding and
consequently, limiting the rotor over-current and
protecting the rotor side inverter. This method
ignored the effect of the faults on the stator
windings.
In, [8], an inductance emulating control design
was proposed for the FRT of a DFIG-based wind
turbine. A feed-forward current reference control
was proposed in, [9], to improve the transient
performance of a DFIG-based wind turbine during
grid faults. A DVR was proposed in, [10], to
compensate for voltage sags, which have better
performance during symmetrical faults. Sliding
mode control was proposed in, [11], by applying an
infinite-frequency switching control to keep output,
the sliding variable, at zero. Such high-frequency
control switching generates undesirable system
vibrations. A combination of series-dynamic-
resistors and stator-side-fault-current limiters were
proposed in, [12], and a STATCOM-based approach
was proposed in, [13]. However, those designs
cannot ensure the dynamic stability of the system
during grid faults. A multi-target control strategy for
DFIG using a virtual synchronous generator (VSG)
under unbalanced grid voltage is proposed in, [14]
by introducing the extended power method and
resonant controller into the conventional VSG.
Electric torque variations were still present in the
presented results. In, [15], the authors focused on
control strategies to reduce the harmonics generated
in the DFIG- wind turbine system due to
unsymmetrical grid fault. The current and voltage
peaks were not considered.
The authors in, [16], proposed a modified
dynamic model of the DFIG subjected to
symmetrical and unsymmetrical fault. The results
showed lower oscillations during the three-phase
fault (symmetrical fault), but large oscillations with
relatively small duration, occurred during the
unsymmetrical grid fault. The authors in, [17]
proposed a modified DVR topology to regulate the
stator voltage through the rotor power converters to
improve fault-ride-through of grid-connected DFIG-
WECS.
A saturated-core fault-current-limiter that is
based on the change in the permeability between the
saturated and unsaturated conditions has been
presented in, [18]. The occurrence of unsymmetrical
grid fault, results in a negative sequence voltage
component that generates a second-order voltage
harmonic interference in the d–q transformation of
the grid voltage. Thus a low-pass filter was
proposed and applied for the determination of the q-
axis component of the grid voltage, according to,
[19].
Series compensated grid-connected topology for
the DFIG is used for minor voltage sags and
unbalanced and distorted grid disturbances, [20],
[21], [22]. The performance during symmetrical and
unsymmetrical faults is observed in, [23]. Rotor
current limitation during the fault, using a rotor-side
converter control scheme, improved the
performance of the DFIG in, [24]. However, an
increase in the rotor speed and oscillations are
observed in the power and electromagnetic torque.
A common- STATCOM topology with a PQ-
based control strategy is proposed in, [25], where a
common capacitor between the rotor-side converter
and the grid-side converter is shared by the
STATCOM. In, [26], the voltage dips and voltage
harmonics were compensated via a modified DVR
topology in WECS-DFIG system.
The paper is focused on symmetrical faults,
while a short study was presented for unsymmetrical
voltage sag, hence the effect of DVR was not clear.
In, [27], the authors proposed an enhanced
dynamic voltage restorer (EDVR) to improve the
voltage stability of a microgrid. In, [28], the authors
proposed a fault ride through scheme for DFIG-
based wind energy systems combining the
properties of fractional order sliding mode control
with the active/reactive power control capability of
the dynamic voltage restorer, and the high-power
density of superconducting magnetic material. This
system requires complex control strategy and high
cost. A control strategy based on super-capacitors
was proposed for simultaneous consideration of
voltage ride through and frequency regulation under
power grid faults, [29].
In this paper, three control schemes are
proposed to mitigate the adverse effects of
unsymmetrical voltage-to-ground fault on the
electric torque, stator, and rotor current transients,
and the DC link voltage ripples of a wind-driven
DFIG. In the first strategy, a STATCOM is
connected to the stator terminals, while in the
second proposed strategy a three-phase impedance
is connected to the rotor circuit during fault
conditions only. The third scheme involved both the
STATCOM and added the rotor impedance. The
performance of the DFIG under single-phase
voltage sag is simulated for each scheme using
Matlab/ Simulink. Simulation results for the three
schemes are compared. The compared simulation
results include the stator voltage and current, the
rotor voltage and current, the dc-link voltage and
current, the rotor speed, and the electrical torque. In
addition, theoretical simulation results are compared
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for the sub and super-synchronous range of
operation of the DFIG. It is well known that the
current process for testing wind-driven generators
under grid fault conditions is time and cost-
intensive, as it requires the setup of the wind turbine
and generator in the field. In addition, the
complexity of the control algorithms and modelling
of the overall DFIG system, especially during grid
faults is another problem. Hence, a laboratory real-
time simulator is essential for the test process. So,
in this manuscript, a real-time simulator is employed
to simulate a virtual DFIG subjected to an
unsymmetrical grid voltage sag. The used real-time
simulator is known as OPAL RT-4510. It is
considered hardware-in-the-loop (HIL) and at the
same time is a rapid control prototyping (RCP)
platform. It is used for real-time emulation of the
three protection schemes proposed to mitigate the
electric torque oscillations, decrease rotor current
transients, and decrease the DC link voltage ripples,
resulting from unsymmetrical grid-voltage sag. The
three systems models are then applied to the real-
time simulator OPAL RT-4510 with down-sized
signals compared with Matlab results. Emulation
results are presented, showing good matching with
the simulation results, hence proving the validity of
the real-time emulator.
The contribution in this paper can be summarized
as:
1- Using both STATCOM and external rotor
impedance simultaneously for mitigating the
effect of the unsymmetrical voltage sag.
2- Concluding the suitable protection scheme for
the sub-synchronous speed range, which is
found to be different from the suitable
protection scheme for the super-synchronous
speed range.
3- Emulation of the theoretical results, with exact
matching, hence ensuring the feasibility of the
results
The paper is organized as follows:
Section 1 is an introduction and the system
description is explained in section 2. Section 3
implies the simulation tools with the HIL
implementation. Simulation and real-time results for
DFIG-operation at both sub- and super-synchronous
speed are included in sections 4 and 5 respectively.
Finally, the conclusion, and recommendations are
included in section 6.
2 Description of the Proposed System
The DFIG, shown in Figure 1, is a three-phase
wound-rotor induction machine whose stator
windings are coupled to the power grid, while its
rotor-windings are connected to the grid via two
back to back converters. The wind-energy
conversion systems (WECS) employing a DFIG are
widely used as a variable-speed wind energy
system. This is due to many advantages. One of
these advantages is the easy control of the frequency
and magnitude of the rotor current. Another
advantage is its operation within wide range of rotor
speeds, hence increasing the captured wind energy.
The advantage, that makes the DFIG- based wind
power system dominates, is that the rotor converters'
rated power is only 30% of the generator’s rated
power, so this method is advantageous from a cost
perspective [28].
2.1 Wound Rotor Induction Machine
Equations
The d-q axes equations describing the DFIG shown
in Figure 1 are given as;
Vds = RsIds + p
ds - ω
qs (1)
Vqs = RsIqs + p
qs + ω
ds (2)
Vdr = Rr Idr + p
dr - ωr
qr (3)
Vqr = RrIqr + p
qr - ωr
dr (4)
ds = LsIds + MIdr (5)
qs = Ls Iqs + M Iqr (6)
dr = Lr Idr + M Ids (7)
qr = Lr Iqr + M Iqs (8)
Electromagnetic torque equation:
Te = 1.5 P M (
qs Idr –
ds Iqr)/ Ls (9)
Where suffixes s and r stand for stator and rotor
parameters respectively, V is the voltage, I is
current, is the flux, M is the mutual inductance, L
is the self-inductance, R is the resistance per phase,
ω is the synchronous speed, P number of pole pairs,
and "p" is the d/dt operator.
2.2 Description of Fault Ride Through
Systems
When the grid voltage drops, the active power
generation decreases, leading to rapid rise in rotor
current to compensate for the reduced active power.
The high rotor current may exceed the converters'
ratings, which result in tripping the WECS from the
grid, leading to instability of the utility system.
To overcome this scenario Fault Ride Through
(FRT) systems are imposed. The role of these
systems is to sustain grid connectivity and inject
reactive power during outages. A widely used
technique for enabling FRT is FACTS devices. One
of these devices that can generate or consume
reactive power from the electric grid is a static-
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synchronous compensator (STATCOM). Another
protection circuit proposed in the past literature is a
crowbar circuit equipped with RL impedance in the
rotor circuit. The crowbar allows a low resistance
path during grid fault, thereby protects the rotor-side
converter from over-currents. Hence the DFIG will
start operating as a squirrel-cage induction machine,
consuming reactive power, adversely affecting the
grid voltage. To overcome the limitations caused by
the crowbar, a simpler protection circuit, an R-L
impedance is connected to the rotor circuit. This
technique prevents the DFIG from acting as a
squirrel cage induction machine. The FRT
capability of these two systems are investigated and
compared.
2.2.1 The DFIG with STATCOM
The STATCOM connection with the DFIG is shown
in Figure 1, while its equivalent circuit is shown on
Figure 2 connected to the point of common
connection PCC. It consists of a two-level voltage
source converter (VSC), a DC energy source (e.g. a
capacitor), a passive filter, and a coupling
transformer connecting the VSC in shunt with the
distribution network. If the system voltage is less
than the voltage at the STATCOM terminals, the
STATCOM acts as a capacitor and reactive power is
injected from the STATCOM to the system. On the
other hand, if the system voltage is higher than the
voltage at the STATCOM terminal, the STATCOM
behaves as an inductor and the reactive power
transfers from the system to the STATCOM. Under
normal operating conditions, both voltages are equal
and there is no power exchange between the
STATCOM and the AC system. The differential
equations for Figure 2 in three-phase form are, [27]:
(10)
where ia, ib, ic are the AC line currents of the
STATCOM; va, vb and vc are the PCC voltages; υa1,
υb1 and υc1 are the inverter terminal voltages; R and
L represent the equivalent conduction losses and the
transformer and filter. Inductance. The above
equations are converted into a d-q synchronously
rotating frame as:
(11)
Where ω is the synchronous angular speed of the
fundamental system voltage. Neglecting the inverter
voltage harmonics, the voltage at the inverter output
terminals are:
Fig. 1: Schematic diagram of a wind-driven grid-
connected DFIG with STATCOM [1].
Fig. 2: The equivalent circuit of the STATCOM [1].
Fig. 3: STATCOM PI control block diagram [1].
Where K is the inverter constant, determined by
the inverter structure, m is the modulation index of
the PWM switching technique, υdc is the DC voltage
across the STATCOM capacitor, and α is the
inverter switching angle given as:
(12)
The instantaneous active and reactive power at the
PCC is:
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(13)
The active and the reactive power exchange
between the STATCOM and the AC system is
controlled via the VSC firing angle "α" and the
modulation index "m" to maintain the voltage at the
point of connection and the DC-voltage within
permissible limits. The system model given by the
above equations, is used along with the PI controller
to regulate the PCC voltage as shown in Figure 3,
[4].
2.2.2 The DFIG with External Rotor Impedance
One of FRT methods in the literature is to install a
crow bar together with a R-L impedance in series
with the rotor windings of DFIG to limit both the
stator and rotor currents. Since the crowbar
activation causes the DFIG to consume reactive
power, the FRT capability of a R-L impedance in
series with the rotor windings, shown in Figure 4, is
investigated. The rotor impedance consists of
parallel R-L branch, which considerably reduces the
rotor and stator over-current at the instants of grid
faults occurrence. The proposed scheme decreases
the electromagnetic torque oscillations and DC-link
over-voltage.
Fig. 4: DFIG with added RL rotor impedance [1].
3 Matlab/ Simulink Simulation and
Real-Time Test
In the following sections, Matlab/Simulink results
are presented and compared with real-time emulated
results, showing reasonable matching, proving the
validity of the proposed schemes. The data of the
test machine is given in appendix a. Figure 5a show
OP4510 block diagram, and Figure 5b is a photo of
the experimental rig of the proposed system, Real-
time digital hardware emulation technology is
important in designing and testing most electrical
schemes such as power systems, smart grids, motor
drives, and power quality techniques. This
technology saves time, protects real systems and
operators from any dangerous effects, and
introduces many test functionalities. This platform
enables the user of the Matlab/ Simulink model to
convert it to the real-time workshop and then
download the real-time simulation into multicore
multiprocessor hardware design. So, in this paper,
one of the more efficient real-time simulators is
used known as OPAL RT-4510. It is considered a
hardware-in-the-loop (HIL) and at the same time is
a rapid control prototyping (RCP) platform. This
includes a Kintex-7 FPGA with Intel Xeon 4-core
CPU processor, as shown in Figure 5a, [30]. In
addition, it implies 32-channels for digital
input/outputs and 16 analog channels. The output
parameters for the proposed system are monitored
through analog channels and measured as a real
signal through a digital oscilloscope as illustrated in
Figure 5b, [31]. The maximum output voltage signal
for the analog channel is 16V, so, each output signal
is scaled through the Simulink model according to
its value. For example, the stator voltage and current
waveforms are scaled down by 100 and 20
respectively. On the other hand, the rotor voltage
and current waveforms are both scaled down by 50.
But, torque and speed signals are scaled down by 50
and 20. Finally, the dc-link voltage and current
waveforms are also scaled down by 20 and 5
respectively.
4 DFIG at Sub-Synchronous Speed
4.1 DFIG with STATCOM during Fault
A STATCOM is connected to the AC stator side. It
consists of a voltage-controlled inverter fed via
batteries and controlled by a proportional-integral
(PI) controller to achieve automatic voltage
regulation. A single-phase voltage to ground fault is
assumed from time t= 0.5sec to 0.7 sec.
Figure 6a shows the Matlab/Simulink results of
the stator voltage and current with STATCOM
connected. The stator voltage and current restored
their magnitude as the ground fault ended. Figure
6b presents the Real-time emulator results of the
stator voltage and current with STATCOM
connected, showing exact matching. Matlab/
Simulink results in Figure 7a shows a rise in the
frequency of the rotor voltage and current during the
fault, while their magnitudes are the same as in the
normal operating condition. It is noted that sharp
peaks occurred in the rotor current at the end of the
fault. Figure 7b presents the Real-time emulator
results of the rotor voltage and current, showing
good matching.
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(a) OP4510 block diagram [30].
(b) Overall setup [31].
Fig. 5: Experimental rig for the proposed system.
Fig. 6a: Simulation of stator voltage and current
with STATCOM at sub-sync speed.
Stator voltage
Stator current
Fig. 6b: Emulation of stator voltage and current with
STATCOM during faults at sub-sync speed.
Fig. 7a: Simulation of rotor voltage and current with
STATCOM during fault at sub-sync speed.
4.2 DFIG with Three-Phase Rotor
Impedance
A three-phase RL impedance is added to the rotor
circuit during the grid fault, with their values
adjusted according to the machine rotor impedance
to give optimum results. Figure 8a shows the
simulation results of stator voltage and current,
while Figure 8b presents the corresponding emulator
results, with exact matching. The stable steady
values of both components prove the validity of
such a protection scheme. Figure 9a shows
simulation results of the rotor voltage and current
showing the complete cancellation of the effect of
grid to ground fault, while Figure 9b shows the
corresponding emulation results. However, Figure
10a showing the simulation results of the DC link
voltage and current, reveal high peak in the current
magnitude, and ripples in the DC voltage, while
Figure 10b presents the matching corresponding
emulation results. Hence additional protection
method has to be added. Figure 11a shows the
simulation results of rotor speed and electric torque
respectively, with their emulation results shown in
Figure 11b. Low ripples are presented in the torque
during the single-phase fault, while a smooth speed
profile is obtained.
Rotor voltage
Rotor current
Fig. 7b: Emulation of rotor voltage and current with
STATCOM during fault at sub-sync speed.
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Fig. 8a: Simulation of stator voltage and current
with three-phase RL impedance during faults.
Stator voltage
Stator current
Fig. 8b: Emulation of stator voltage and current with
three-phase RL impedance during faults.
Fig. 9a: Simulation of rotor voltage and current with
three-phase RL impedance during fault.
Rotor voltage
Rotor current
Fig. 9b: Emulation of rotor voltage and current with
three-phase RL impedance during fault.
Fig. 10a: Simulation of DC link voltage and current
with 3-phase RL impedance during fault.
Ch. A (Voltage) Ch. B (Current)
Fig. 10b: Emulation of DC link voltage and current
with 3-phase RL impedance during fault.
Fig. 11a: Simulation of rotor speed & electric torque
with3-phase RL impedance during fault.
Ch. A (Speed) Ch. B (Torque)
Fig. 11b: Emulation of rotor speed & electric torque
with3-phase RL impedance during fault.
4.3 DFIG with Three-Phase Rotor
Impedance and STATCOM
The three-phase RL impedance is added to the rotor
circuit during the grid fault in addition to the
STATCOM device described previously. Figure 12a
shows the Matlab/ Simulink results of rotor voltage
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and current showing the complete cancellation of
the effect of grid to ground fault. Figure 12b shows
the corresponding Real-time simulator results,
where matching is clear. Figure 13a shows the
Matlab/ Simulink DC link voltage and current, still
containing a high peak in the DC current during the
fault, and low ripples in the DC voltage. It is noted
that the peak in the DC current is lower than that
when applying the rotor impedance only. Figure 13b
shows the emulation results of DC link voltage and
current with clear matching. Figures 14a and Figure
14b show the Matlab rotor speed and electric torque
and real-time simulation respectively, showing good
matching. High torque peaks occurred at the
beginning and the end of the ground fault.
Fig. 12a: Matlab Rotor voltage and current with
STATCOM and three-phase RL impedance during
faults.
Rotor Voltage
Rotor Current
Fig. 12b: Real-time simulation of Rotor voltage and
current with STATCOM and three-phase RL
impedance.
Fig. 13a: Matlab DC link voltage and current with
STATCOM and three-phase RL impedance.
Ch. A (Voltage) Ch. B (Current)
Fig. 13b: Real-time simulation DC link voltage with
STATCOM and three-phase RL impedance.
(a) Simulation Results
(b) RT-results: Ch. A (Speed) Ch. B (Torque)
Fig. 14a,b: Matlab and RT-Lab results for rotor
speed and electric torque with STATCOM and
three-phase RL impedance.
5 DFIG at Super-Synchronous Speed
5.1 DFIG with STATCOM during Fault
Figure 15a and Figure 15b show simulation and
emulation results of stator voltage and current
respectively. The magnitude of the stator current
slightly increased than its steady-state value.
Figure 16a and Figure 16b present the
simulation and emulation results of the DC link
voltage and current. The figures show high ripples
in the DC link voltage and low ripples in DC
current. These current ripples lead to unstable
electrical torque as shown in Figure 17a and Figure
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17b. Hence using STATCOM as a protection
method is not valid for mitigating the effect of
unsymmetrical fault at the super-synchronous
operation of the DFIG.
Fig. 15a: Simulation of stator voltage ¤t at
super-sync speed with STATCOM.
Stator voltage
Stator current
Fig. 15b: RT results of stator voltage ¤t at
super-sync speed with STATCOM.
Fig. 16a: Simulation results of DC link voltage and
current with STATCOM only during faults.
Ch. A (Voltage) Ch. B (Current)
Fig. 16b: RT results of DC link voltage and current
at super-sync speed with STATCOM only during
faults.
5.2 DFIG with External Rotor Impedance
during Fault
Faster restoration of rotor voltage and current
magnitudes are noticed than when using STATCOM
as shown in Figure 18a and Figure 18b. However,
considerable ripples are present in the rotor voltage.
Consequently, the ripples in the DC current and DC
voltage, as shown in Figure 19a and Figure 19b
(simulation and emulation results) decreased than
when using STATCOM only. Also, the ripples are
low in the simulation and emulation results of
electric torque and rotor speed shown in Figure 20a
and Figure20b. Hence this protection method is
effective for mitigating the unsymmetrical grid-to-
ground fault at the super-synchronous range of
operation of the DFIG.
(a) Simulation
(b) Real-time
Fig. 17: Waveforms of speed and torque with
STATCOM at super-synchronous speed..
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Fig. 18a: Simulation results of rotor voltage &
current with 3-phase RL impedance at super-sync
Speed.
(a) Simulation
(b) Real-time: Ch. A (Voltage) and Ch. B (Current)
Fig. 19: DC-link voltage and current waveforms
with 3-phase RL impedance at super-sync speed.
5.3 DFIG with Both STATCOM and
External Rotor Impedance during Fault
The stator voltage and current restore their original
values, as shown in Figure 21a and Figure 21b.
However, the magnitude of the stator current
fluctuates throughout the examined time, i.e. before
and after the fault, as shown. The ripples in the DC
current and DC voltage are shown in Figure 22a and
Figure 22b are not reduced as in the scheme with
STATCOM only which consequently affected the
electrical torque shown in Figure 23a and Figure
23b. Hence, this protection scheme is not valid for
mitigating the effect of unsymmetrical voltage to
ground fault at super-synchronous speed range of
operation.
(a) Simulation
(b) Real-time: Ch. A (speed), Ch. B (torque), Ch. D
(current)
Fig. 20: Rotor speed-torque waveforms with three-
phase RL impedance at super-sync speed.
Fig. 21a: Simulation of stator voltage & current with
STATCOM &rotor impedance at super-sync speed.
Stator Voltage
Stator Current
Fig. 21b: Emulation of stator voltage & current with
STATCOM &rotor impedance at super-sync speed.
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.22
Essamudin Ali Ebrahim,
Maged N. F. Nashed, Mona N. Eskander
E-ISSN: 2224-350X
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Volume 18, 2023
(a) Simulation
(b) Real-time: Ch. A (Voltage), Ch. B (Current)
Fig. 22: DC-link voltage and current with
STATCOM and 3-phase rotor impedance at super-
synchronous speed.
(a) Simulation
(b) Real-time: Ch. A (Speed), Ch. B (Torque), Ch.
D (current)
Fig. 23: Results of rotor speed and torque with
STATCOM and 3-phase RL impedance at super-
synchronous speed.
6 Conclusion
In this paper, three fault-ride-through schemes are
proposed to mitigate the effect of unsymmetrical
voltage to ground fault on a grid-connected DFIG
coupled to a wind turbine. The first scheme
comprises a STATCOM connected to the DFIG
stator, while a three-phase external impedance is
connected to the rotor circuit in the second scheme.
The third scheme comprised both the STATCOM
and the rotor impedance. Matlab/ Simulink
simulation results of the stator and rotor voltages
and currents, the dc-link voltage and current, the
electrical torque, and the rotor speed are presented
at sub-and super-synchronous speeds for the three
proposed schemes. All systems were emulated,
implemented, and tested through an OPAL RT-4510
Digital Real-Time Simulator (DRTS) in a
Hardware-In-the-Loop (HIL) application. All real-
time waveforms of parameters for the proposed
scenarios were monitored through the HIL-
controller and data acquisition interface and then
compared with the simulated results. The results
reveal that both are congruent. The unsymmetrical
ground fault effect on the DFIG voltages, currents,
electrical torque, and speed is mitigated when using
both devices at the sub-synchronous range of
operation. While, the unsymmetrical ground fault
effect on the DFIG voltages, currents, electrical
torque, and speed is mitigated when using the
external rotor impedance only at super-synchronous
speed.
APPENDIX A
The main data of the test DFIG is:
S=3.7 KVA, =460V, f =60 Hz, ,
=1.083Ω, H, =0.203 H
J=0.02 Kg.m², B=0.005752 N.m.s.
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DOI: 10.37394/232016.2023.18.22
Essamudin Ali Ebrahim,
Maged N. F. Nashed, Mona N. Eskander
E-ISSN: 2224-350X
213
Volume 18, 2023
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Contribution of Individual Authors to the
Creation of a Scientific Article (Ghostwriting
Policy)
- Essamudin Ali Ebrahim has written and
implemented of the part Real Time. He organized
and executed the experimental work with paper
revision.
- Maged N. F. Nashed has carried out the
simulation.
- Mona Eskander was the one who presented the
idea and wrote the article.
Sources of Funding for Research Presented in a
Scientific Article or Scientific Article Itself
No funding was received for conducting this study.
Conflict of Interest
The authors have no conflicts of interest to declare.
Creative Commons Attribution License 4.0
(Attribution 4.0 International, CC BY 4.0)
This article is published under the terms of the
Creative Commons Attribution License 4.0
https://creativecommons.org/licenses/by/4.0/deed.en
_US
WSEAS TRANSACTIONS on POWER SYSTEMS
DOI: 10.37394/232016.2023.18.22
Essamudin Ali Ebrahim,
Maged N. F. Nashed, Mona N. Eskander
E-ISSN: 2224-350X
215
Volume 18, 2023
