23rdInternational Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0302
COMPARING THE LVRT CAPABILITY OF GRID CONNECTED WIND ENERGY
CONVERSION SYSTEM WITH DIFFERENT FACT DEVICES
Mona N. Eskander
Maged N. F. Nashed
Electronics Research Institute, Cairo, Egypt
E-mail: eskander@eri.sci.eg
ABSTRACT
This paper compares the effectiveness of installing two
types of FACTS devices, namely the Static Synchronous
Compensator "STATCOM", and the Unified Power
Quality Controller "UPQC" to enhance the low voltage
ride through "LVRT" capability of a grid-connected wind
energy conversion systems (WECS) employing a doubly
fed induction generator (DFIG). These devices are
applied to protect the system from ground faults, allow
fast recovery, improve the system power factor, and
prevent the system from rotor over-current and dc-link
overvoltage. Comparing the responses of the stator and
rotor currents, electric torque, rotor speed, active power,
and reactive power of the two investigated systems during
grid faults is presented. Results revealed the better
performance of the WECS with UPQC. Faster recovery,
smoother speed and electric torque variations, smoother
current profiles, and electric generator protection are
proven when employing the UPQC. The advantages of
STATCOM are confined to lower cost and less complexity
of control systems due to the usage of one converter only.
These instructions give the author basic guidelines to
prepare the final version of the paper to be included in
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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), the 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]. During faults, when crowbar is engaged, the
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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 technique
makes the DFIG unable to supply substantial amounts of
reactive power, and thus increases the chance of system
instability. Therefore, static synchronous compensators
(STATCOMs) have been proposed to supply additional
reactive power and compensate the DFIG consumption
[5]. 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 [5-6]. To achieve full system
protection a unified power quality controller UPQC is
proposed 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.
Comparing the responses of the stator and rotor voltages,
stator and rotor currents, dc link voltage, electric torque,
rotor speed, active power, and reactive power of the
system employing the STATCOM and the system
employing the UPQC during grid faults is presented PI
controllers are tuned to allow maximum power tracking
and control the FACTS devices. Results revealed the
better performance of the WECS with UPQC.
II. SYSTEM DESCRIPTION
The DFIG system is presented in Fig. 1. The system
consists of a wound rotor induction generator with slip
rings and a back-to-back converter connected between the
rotor slip rings and 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
wind turbine, typically in the range of 10% as proved in
[7].
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23rdInternational Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0302
Figure 1, Schematic diagram of the DFIG-based wind
energy generation system
Where: ω is the synchronous angular speed of the
fundamental system voltage. Neglecting the voltage
harmonics produced by the inverter, and according to the
PWM technique, the voltage at the inverter output
terminals and the DC-side can be written as:
v d1 = Kmv dc sin α
(3)
v q1 = Kmv dc cos α
III. STATCOM MODELLING
The STATCOM connection with the WECS employing
the DFIG consists of a two level VSC, a DC energy
source, a passive filter, and a coupling transformer
connecting the VSC in shunt to the distribution network.
In general, a coupling transformer is installed between
the distribution system and the STATCOM for isolating
the low voltage STATCOM from the high-voltage
distribution system. The passive filter is installed at the
point of common connection to reduce harmonics and
allow better power quality. The VSC converts the DC
voltage across the storage device (a 250 V battery) into a
set of 3-phase AC output voltages. Suitable adjustment of
the phase and magnitude of the STATCOM output
voltages allows effective control of active and reactive
power exchanges between the STATCOM and the AC
system. The control scheme consists of phase locked loop
to track the magnitude and phase angle of the supply
voltage during normal operation to detect the occurrence
of voltage sag. A 250 V battery is applied to DC input of
VSC. The VSC switching strategy is based on a
sinusoidal PWM technique, which offers simplicity and
good response. The switching frequency is set at 2000
Hz. The controller input is an error signal obtained from
the reference voltage and the value rms of the terminal
voltage measured. Such error is processed by a PI
controller whose output is provided to the PWM signal
generator to drive the error to zero, i.e., the load rms
voltage is brought back to the reference voltage. Fig. 2
shows the STATCOM connected to the PCC in three
phase form, the differential equations for Fig. 2 in threephase form can be written as:
di a
= − Ri
dt
di b
L
= − Ri
dt
di c
L
= − Ri
dt
L
a
+ ( v a − v a1 )
b
+ ( v b − v b1 )
c
+ ( v c − v c1 )
(1)
where ia, ib, and ic are the AC line currents of the
STATCOM; va, vb, and vc are the PCC voltages; va1, vb1,
and vc1,and are the inverter terminal voltages; R and L
represent the equivalent conduction losses and the
inductance for the transformer and the filter. The threephase voltages and currents in a synchronously rotating
d-q frame:
di d
= − Ri d + ω Li q + ( v d − v d 1 )
dt
di q
L
= − Ri q − ω Li d + ( v q − v q 1 )
dt
L
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(2)
Figure 2, STATCOM equivalent circuit
The instantaneous active and reactive power at the PCC
is given as:
3
(v d i d + v q i q )
2
3
Q = (v d i q − v q i d )
2
P=
(4)
While the instantaneous power at the dc side is:
dv
(5)
P = v dc c dc
dt
From eqs. 4&5, the DC capacitor equation as function of
STATCOM current is:
dv dc
3
(6)
=
(v d i d + v q i q )
dt
2c
Assuming d-axis coincides with always coincident with
the instantaneous voltage vector Vq, and neglecting R, a
simplified model for the STATCOM is:
di
L d = ωLi q + ( v d − Kmv dc sin α)
dt
(7)
di q
L
= −ωLi d + Kmv dc cos α
dt
dv dc 3
(8)
= vdid
dt
2
The reactive power is given by
3
(9)
Q = vdiq
2
Therefore, the DC-voltage vdc can be regulated by
controlling iq, and id which is sufficient to control the
reactive power and hence the PCC voltage can be
regulated.
IV. SYSTEM WITH UPQC
The conventional UPFC, shown in Fig.(3), 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
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23rdInternational Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0302
converters are connected by a common DC link. 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.
Similarly, the real and reactive powers received at the
receiving bus are:
Po = −PR = −VR2 G + VS VR G cos(δS − δ R ) − VS VR Bsin(δS − δ R )
Qo = −QR = VR2 B − VS VR G sin(δS − δR ) − VS VR Bcos(δS − δ R )
For typical transmission line X>>R. Therefore, the
conductance G is usually neglected and suseptance B is
replaced by B=-1/X. Using these approximations, the
expression for real power transmitted over the line form
the sending to the receiving bus becomes:
PS = − PR = − VS VR B sin(δS − δ R ) = −
=
VS VR
B sin(δS − δ R )
X
VS VR
B sin δ = Po (δ)
X
The angle δ =δS −δR is called the power angle. The
reactive power sent to the line from the sending bus and
received from the line at the receiving bus are:
Figure 3, UPQC Schematic.
The objective of UPQC controlled by PI controllers is to
enhance system stability by fast recovery from
symmetrical ground faults and fast damping of low
frequency oscillations. The UPQC is placed between two
busses, as shown in Fig. (4), referred to as the sending
bus and the receiving bus. The RMS voltages of the
sending and receiving buses are:
Figure 4, Equivalent Power System.
Vs = VS ∠δ s
(10)
VR = VR ∠δ R
(11)
I*line is phasor current on the line, R and X are resistance
and reactance of the line respectively δs and δr are the
phase angles of sending and receiving ends respectively.
The complex power injected into the sending bus:
(12)
SS = PS + jQ S = VS I *line
Where PS and QS are the real and reactive powers injected
into the sending bus, * denotes conjugate complex value.
The line current can be written as:
VS2 − VSVR cos(δS − δR )
X
2
−
V
+
V
R
SVR cos(δS − δR )
− QR = −VR2 B − VSVR Bcos(δS − δR ) =
= Qo (δ)
X
QS = −VS2B + VSVR Bsin(δS − δR ) =
V. SIMULATION RESULTS AND DISCUSSION FOR 3-PHASE GROUND FAULTS
The performance of the considered power system is
modeled and its performance evaluated for both FACTS
devices, i.e. with STATCOM, and with UPQC, using the
Matlab software. A 3- phase voltage to ground fault is
assumed 0.15 sec duration. The simulation results are
given in Figs. 5 to Fig.9. Always Figure (a) show the
system performance with STATCOM, while Figure (b)
show the system performance with UPQC.
From these figures, the active and reactive power profiles
during and after fault recovery as shown in Figures 5a
and 5b, shows that the stator power does not fall to zero
during fault in the system employing the UPQC. This
performance is due to the series-connected transformer
coupling the series inverter of the UPQC, leading to
protection of the generator. This is not the case for the
system with the STATCOM.
4
3
x 10
2.5
2
I line =
VS − VR
= ( VS − VR )(G + jB)
R − jX
Where, G =
R
2
R +X
and
2
(13)
B=
X
R + X2
2
Stator P&Q
1.5
1
0.5
0
-0.5
-1
Combining equations (10) to (13), the following
expressions for the real and reactive powers injected into
the sending bus are obtained:
PS = VS2 G − VS VR G cos(δ S − δ R ) − VS VR B sin(δ S − δ R )
QS =
−VS2 B −
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VS VR G sin(δ S − δ R ) + VS VR B cos(δ S − δ R )
-1.5
-2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time
Figure 5a, Stator active and reactive power with
STATCOM
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23rdInternational Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0302
4
x 10
50
2
40
1.5
30
1
20
rotor current
stator P&Q
2.5
0.5
0
10
0
-0.5
-10
-1
-20
-1.5
-30
-2
0
0.1
0.2
0.3
0.4
0.5
0.6
-40
0.7
0
0.1
0.2
0.3
time
0.4
0.5
0.6
0.7
time
Figure 7b, Rotor current with UPQC.
Figure 5b, Stator active and reactive power with UPQC
80
60
60
40
40
Torque
stator current
20
20
0
-20
-20
-40
-60
0
-40
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-60
time
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
time
Figure 6a, Stator current with STATCOM.
Figure 8a, Electric torque with STATCOM
50
60
40
30
50
20
0
Torque
stator current
40
10
-10
30
20
-20
10
-30
-40
-50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-10
time
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
time
Figure 6b, Stator current with UPQC.
Figure 8b, Electric torque with UPQC.
60
200
180
40
160
140
rotor speed
rotor current
20
0
-20
120
100
80
60
-40
40
20
-60
0
0.1
0.2
0.3
0.4
0.5
0.6
time
Figure 7a, Rotor current with STATCOM.
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0.7
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
time
Figure 9a, Rotor speed with STATCOM.
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23rdInternational Conference on Electricity Distribution
Lyon, 15-18 June 2015
Paper 0302
200
REFERENCES
180
160
rotor speed
140
120
100
80
60
40
20
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
time
Figure 9b, Rotor speed with STATCOM.
This effect is also obvious in Figures 6a and b where the
fast recovery of the stator current to its rated value and its
non-zero magnitude during fault is demonstrated.
Figures 7 a and b reveals the smoother profile of the rotor
current as well as the fast recovery for the system
incorporating the UPQC.
The electric torque, demonstrated in Figures 8a and b, for
both systems shows a smoother profile for the system
with the system incorporating the UPQC.
The variation in the rotor speed during fault, shown in
Figures 9a and b is comparable for both systems.
However the recovery to its value before fault occurrence
is faster with the UPQC.
VI. CONCLUSION:
In this paper investigation of the effectiveness of
installing two types of FACTS devices, namely the Static
Synchronous Compensator "STATCOM", and the
Unified Power Quality Controller "UPQC" to enhance the
low voltage ride through "LVRT" capability of a grid
connected wind energy conversion systems (WECS)
employing a doubly fed induction generator (DFIG) is
presented. These devices are applied to protect the system
from ground faults, allow fast recovery, improve the
system power factor, and prevent the system from rotor
over-current and dc-link overvoltage. Comparing the
responses of the stator and rotor currents, dc link voltage,
electric torque, rotor speed, active power, and reactive
power of the two investigated systems during grid faults
is presented. Results revealed the safer operation of the
system using the UPQC, revealed from the non-zero
stator current during fault and the slight increase in rotor
speed. Faster recovery, and smoother profiles are proven
when employing the UPQC. The advantages of
STATCOM are confined to lower cost and less
complexity of control systems due to the usage of one
converter only.
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