International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1925-1930
ISSN 2078-2365
http://www.ieejournal.com/
Superiority of LVRT of Grid Connected
Wind Energy Conversion System Using
Unified Power Quality Controller
M. N. Eskander, M.A.Saleh, Maged N.F.Nashed, S. Amer,
Department of Power Electronics & Energy Conversion
Electronics Research Institute, Cairo, Egypt
eskandermona@yahoo.com, salehmahmoud36@yahoo.com, amersanaa@yahoo.com, magederi@yahoo.com

Abstract— The Unified Power Quality Controller (UPQC)
device is an integration of a series active filter and a shunt
active filter. The main purpose of a UPQC is to compensate for
voltage flicker, reactive power, and harmonics. It has the
capability of improving power quality at the point of installation
on power distribution systems or industrial power systems.
This paper utilizes the UPQC to enhance the low-voltage
ride-through (LVRT) capability of the doubly fed induction
generator (DFIG)-based wind energy conversion system
(WECS) according to the grid connection requirement. UPQC is
applied to protect the system from ground faults, allows fast
restoring of generation system steady state characteristics,
improves the system power factor, and prevent the system from
rotor over-current and dc-link overvoltage. A comparison
between the LVRT results using the UPQC and the results when
using STATCOM or DVR is also presented.
Index Terms— Unified Power Quality Controller, doubly fed
induction generator, low-voltage ride-through, static
synchronous compensators, dynamic voltage restorers,
MATLAB/Simulink
I. INTRODUCTION
Wind power generation has been growing rapidly around
the world over the past decade and has become one of the
most mature renewable generation technologies. This rapid
growth has motivated the establishment of grid codes for the
interconnection of wind power plants. The performance of the
doubly fed induction generator (DFIG) during grid faults is
attracting much interest due to the proliferation of wind
turbines that employ this technology. International grid codes
specify that the generator must exhibit a fault-ride-through
(FRT) capability by remaining connected and contributing to
network stability during a fault [1]. In wind energy systems
employing double fed induction generators (DFIGs), where
the stator winding connects to the grid and the rotor windings
are connected to the grid via back-to-back converters that
control the rotor current and inject current into the grid, the
Eskander et. al.,
system is sensitive to grid disturbances. Therefore, proper
protection schemes and FRT techniques are necessary in
order to withstand grid faults. Active crowbars are commonly
utilized to protect the rotor-side converters (RSCs) against
voltage and current transients, which are caused by voltage
dips in the stator side [2-3]. When a fault happens and this
crowbar is engaged, the DFIG behaves like an induction
machine since the rotor winding is short-circuited by shunt
resistors and the RSC is disabled. The main drawback of
crowbar protection is that the DFIG consumes reactive power
when it is activated and this aggravates the grid voltage dip
during a fault. In [4], series dynamic resistors are utilized to
restrain significant rotor currents so that the RSC and rotor
circuits can be effectively protected by way of coordination
control of the chopper resistor and the crowbar. However, if
the wind farm is connected to a weak power network, the
employment of such techniques prevents the DFIG from
supplying substantial amounts of reactive power, and thus
increases the chance of system instability. Other researches
investigated the application of dynamic voltage restorers
(DVR) to compensate for voltage sags and swells as given in
[5-8].
Therefore,
static
synchronous
compensators
(STATCOMs) have been proposed to supply additional
reactive power and compensate the DFIG consumption
[9-13]. Nevertheless, a STATCOM is inadequate to prevent
the system from either rotor over-current or dc-link
overvoltage. To achieve full system protection, this scheme is
usually combined with other protective elements, such as
stator braking resistors and Crowbar, which increase system
complexity and additional cost.
In this paper protection of a grid-connected wind energy
conversion system based on doubly fed induction generator
(DFIG) against grid faults is proposed. To achieve full system
protection a unified power quality controller UPQC is applied
to protect the system from ground faults, allows fast recovery,
improve power factor, and prevent the system from rotor
over-current and dc-link overvoltage. PI controllers are
designed to allow maximum power tracking and control the
1925
Superiority of LVRT of Grid Connected Wind Energy Conversion System Using Unified Power Quality Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1925-1930
ISSN 2078-2365
http://www.ieejournal.com/
application of the UPQC. Simulation results of voltages,
currents, active and reactive power prove the fast recovery,
lower transients, and improvement of system power quality
when applying UPQC. A comparison between the system
performance for LVRT when using STATCOM and DVR is
given to prove the superiority of UPQC over other FACTS
devices.
II. SYSTEM DESCRIPTION
The DFIG system is presented in Fig. 1. The system
consists of a wound rotor induction generator whose Rotor
circuit is connected via the slip rings and a back-to-back
converter to the grid. The stator is directly connected to the
grid through a three-winding transformer. The rotor side
converter (RSC) is utilized to control the electromagnetic
torque and the excitation of the generator, while the generator
side converter (GSC) controls the dc-link voltage. Since only
the slip power is handled by the converter, the power rating of
the converter can be only a fraction of The rated power of the
rotor converters is typically in the range of 10% as proved by
the authors of this paper in a previously published research
paper [14].
d ds
 e  qs
dt
d qs
v qs  R s i qs 
 e  ds
dt
d
v dr  R r i dr  dr  (e   r )  qr
dt
d qr
v qr  R r i qr 
 (e   r )  dr
dt
v ds  R s i ds 
(1)
Where vds and vqs are the d &q stator voltages,
vdr and vqr are the d &q rotor voltages,
ids and iqs are the d &q stator currents,
idr and iqr are the d &q rotor currents,
ωe is the supply angular frequency,
ωr is the rotor angular frequency,
λds and λqs are the d &q stator flux linkage, and
λdr and λqr are the d &q rotor flux linkages:
 ds  L s i ds  L m i dr
 qs  L s i qs  L m i qr
 dr  L m i ds  L r i dr
(2)
 qr  L m i qs  L r i qr
Where Rs and Rr are the stator and rotor resistance,
Lls and Llr are the stator and rotor leakage
inductance,
Lm is the magnetizing inductance, and Ls = Lls+Lm,
Lr = Llr+Lm.
Under stator flux-oriented control, adjustment of the rotor
q-axis current controls the generator torque or the stator
active power of the DFIG as
Ps 
3
3 Lm
( v qs i qs  v ds i ds )  
v qs i qr
2
2 Ls
(3)
On the other hand, regulating the rotor d-axis current
component controls the stator reactive power as:
Fig. 1 Schematic diagram of the DFIG-based wind energy generation system.
The equations for the fluxes, currents, and voltages can be
expressed as:
Qs 
3
3 Lm
( v qs i ds  v ds i qs ) 
v qs (i mc  i dr ) (4)
2
2 Ls
Where ims is magnetizing current.
1926
Eskander et. al.,
Superiority of LVRT of Grid Connected Wind Energy Conversion System Using Unified Power Quality Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1925-1930
ISSN 2078-2365
http://www.ieejournal.com/
III. OUPQC MODEL
The conventional UPFC, shown in Fig. 2 is the
combination of two voltage source converters (VSC); one
converter connected to the power system through a shunt
transformer, while the second converter is inserted into the
transmission line via a series transformer. The converters are
connected by a common DC link. For the OUPFC proposed in
this paper the common DC link is replaced by separate DC
source in each converter, which allows shutting down any of
two converters if not needed. This scheme also helps in
reducing maintenance cost. It also allows individual dealing
with each customer.
Figure 3 shows the stator active and reactive power
variation due to the ground fault. The small deviation from the
P and Q original values, as well as the fast response is clear. It
is noticed that the active power decreases slowly at the
beginning of the fault, and its value reaches zero for shorter
time than the ground fault duration. This is due to the series
inductance of the transformer connecting the UPQC series
converter to the WECS. The low value of the reactive power
proves the high power factor achieved by WECS due to the
presence of UPQC.
1800
1600
1400
Ps
Stator P & Q
1200
1000
800
600
400
200
Qs
0
-200
0
Fig. 2 Conventional UPQC scheme.
The main objective of the series converter is to produce an
ac voltage of controllable magnitude and phase angle, and
inject this voltage at fundamental frequency into the
transmission line, exchanging real and reactive power at its ac
terminals through the series connected transformer. The shunt
converter provides the required real power at the dc terminals.
The two converters and associated transformers are simulated
with Matlab/Simulink. PI controllers are designed for each
converter, and the proposed system is tested under 3-phase
ground fault.
IV. SIMULATION RESULTS
The WECS employing the DFIG with the UPQC is
modeled with Matlab/Simulink. The model incorporates two
PI controllers for adjusting the amplitude and phase of the
rotor injected voltage via the two converters; RSC and GSC.
The injected voltage is adjusted to force the generator speed
to follow the turbine rotational speed to allow maximum
extraction of wind power. Another two PI controllers are
tuned to control the series and parallel VSC of the UPQC.
Three phase line to ground fault is applied to the proposed
system from t=0.3sec for 100msec. The LVRT of the
WECS-UPQC is tested at sub-synchronous speed
(slip=0.045, and applied rotor voltage Vr=0.2Vs).
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
Fig. 3 Stator P and Q of UPQC-WECS during fault.
To verify the validity of UPQC for LVRT, the response of
stator active and reactive power due to 3-phase ground fault
for a WECS employing STATCOM is shown in Fig. 4. The
slower LVRT and higher harmonic contents of the
STATCOM-WECS is apparent. It is also noted that the
reactive power magnitude is comparable to the active power
magnitude, which means that the STATCOM-WECS suffers
from low power factor.
To verify the superiority of UPQC over DVR for LVRT,
the response of stator active and reactive power due to
3-phase ground fault for a WECS employing DVR is shown in
Fig. 5. The slower LVRT and higher harmonic contents of the
DVR-WECS is clear.
However, the reactive power is compensated due to the
series coil of the DVR device.
Figure 6 shows the rotor active and reactive power variation
due to the ground fault. The fast recovery to the rotor applied
power is clear.
1927
Eskander et. al.,
Superiority of LVRT of Grid Connected Wind Energy Conversion System Using Unified Power Quality Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1925-1930
ISSN 2078-2365
http://www.ieejournal.com/
Figure 7 shows the electric torque response of
UPQC-WECS during fault. The low peak, and fast recovery
from the ground fault is clear. Similar to active power
response, the electric torque decreases slowly at the beginning
of the fault, and its value reaches zero for shorter time than the
ground fault duration. This is due to the series inductance of
the transformer connecting the UPQC series converter to the
WECS.
6000
2000
0
-2000
6
-4000
5
-6000
4
-8000
0
0.1
0.2
0.3
0.4
0.5
0.6
Electric Torque
Stator active & reactive power
4000
0.7
Time
Fig. 4 P and Q of STATCOM-WECS during fault.
3
2
4
4
x 10
1
3
0
2
-1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Stator P &Q
Time
1
Fig. 7 Torque of UPQC-WECS during fault.
0
-1
-2
-3
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
The stator current, shown in Fig.(8), reveals the fast recovery
after the fault. It is noticed that the current recovery is not
associated with transient peaks, which proves the benefits of
using the UPQC. It is also noted that the stator current did not
reach zero during ground fault, as expected. This is due to the
inductance of the series transformer connected to the series
converter of the UPQC. This performance allows protection
to the generator windings.
Fig. 5 P and Q of DVR-WECS during fault.
10
180
8
160
6
140
Pr
4
Sttaor Current
Rotor P & Q
120
100
80
Qr
2
0
-2
60
-4
40
-6
20
-8
0
-20
-10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
Fig. 6 Rotor P and Q of UPQC-WECS during fault.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
Fig. 8 Stator current of UPQC-WECS during fault.
1928
Eskander et. al.,
Superiority of LVRT of Grid Connected Wind Energy Conversion System Using Unified Power Quality Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1925-1930
ISSN 2078-2365
http://www.ieejournal.com/
Figure (9) shows the rotor currents for the studied system.
Since stator current did not decrease during fault,
consequently the rotor current is not highly affected by fault.
Also the transient value is limited.
Figure 11 shows the dc link current due to the 3-phase ground
fault. A negative peak is noticed at the beginning of the fault.
However, the magnitude of the dc current peak is 20% of the steady
state dc current value. This percentage is within safety limits and
hence the rotor converters could not be damaged.
10
8
1.2
6
1
4
DC-Link Current
Rotor Current
0.8
2
0
-2
-4
0.6
0.4
0.2
-6
0
-8
-10
0
0.1
0.2
0.3
0.4
0.5
0.6
-0.2
0.7
Time
-0.4
Fig. 9 Rotor current of UPQC-WECS during fault
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
The effect of fault on the rotor speed is shown in Fig. (10).
The limited increase in the rotor speed due to the 3-phase
ground fault is minimal, showing that the system is protected
from overspeed. Fast recovery to the rotor speed value before
the fault is clear. The speed is set by the applied rotor voltage
magnitude and phase calculated to allow maximum power
extraction from the wind.
Fig. 11 DC link current of UPQC-WECS.
Figure (12) shows the Ac line voltage due to the 3-phase
ground fault. A negative peak is noticed at the edge of the
fault. However, the fast recovery of the voltage peak is
obvious, hence the stator coils could not be damaged.
400
200
300
180
200
Phase A Line Voltage
160
Rotor Speed
140
120
100
80
0
-100
-200
-300
60
-400
40
-500
20
0
100
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
Fig. (12) AC line voltage of UPQC-WECS
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
Fig. 10 Effect of fault on rotor speed.
1929
Eskander et. al.,
Superiority of LVRT of Grid Connected Wind Energy Conversion System Using Unified Power Quality Controller
International Electrical Engineering Journal (IEEJ)
Vol. 6 (2015) No.6, pp. 1925-1930
ISSN 2078-2365
http://www.ieejournal.com/
V. CONCLUSIONS
In this paper a unified power quality controller (UPQC) is
applied to a wind energy conversion system employing a
doubly fed induction generator to enhance the low voltage
ride through of the system during 3-phase ground fault. PI
controllers are used to allow variable speed operation of the
DFIG for maximum wind power extraction. Other PI
controllers are tuned to control the VSC of the UPQC.
The proposed system is modeled using Matlab/Simulink
and simulation results are presented. The results revealed the
fast recovery of stator, rotor, and dc link currents to their
steady state values after fault removal. The electric torque, the
active and reactive stator power, and the active and reactive
rotor power also reached their values prior to fault without
high transients. The fast recovery of line voltage peak due to
the ground fault proved the validity of UPQC.
The system power factor is high, and the increase in the
rotor speed due to the ground fault is limited, hence the
generation system is not endangered. The stator active and
reactive power during 3-phase fault for a WECS using a
STATCOM is compared with the results obtained in WECS
using UPQC. Results ensured the superior performance of
WECS employing the UPQC.
Also, the stator active and reactive power during 3-phase
fault for a WECS using a DVR is compared with the results
obtained in WECS using UPQC. Results ensured the superior
performance of WECS employing the UPQC with respect to
higher harmonics and longer recovery time. However, the
consumed reactive power is lower due to the series-connected
coil of the DVR device.
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Eskander et. al.,
Superiority of LVRT of Grid Connected Wind Energy Conversion System Using Unified Power Quality Controller