Download Performance Improvement in a DFIG - based Wind Farm

International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 3, March 2016)
Performance Improvement in a DFIG - based Wind Farm Fed to
a Power System using STATCOM
Kalpana Patel1, Prof. Narayan Prasad Gupta2, Dr. Deepika Masand3
1,2
Department of Electrical and Electronics, OIST, Bhopal, Madhya Pradesh, India
Head of Department, Electrical and Electronics Eng., OIST, Bhopal, Madhya Pradesh, India
3
DFIG is a wound-rotor induction generator in which
stator is connected directly to the power system and the
rotor is connected to the power system by ac-dc-ac variable
frequency converters (VFC).The converters are rated for
25%-30% of DFIG nominal power. The VFC consist of
Grid Side Converter (GSC) and Rotor Side Converter
(RSC) which are connected back-to-back by a dc-link
capacitor [3].
Main disadvantage in wind turbines equipped with
DFIGs is their low voltage ride through capability or their
operation during faults in the power system. As a result of
grid fault, voltage drop occurs at the point of connection of
the wind turbine system. This leads to overcurrent in the
stator winding of DFIG, which then flows to the rotor
circuit due to magnetic coupling between stator and rotor.
As a result of this overcurrent, destruction of the converter
may occur [2]. During the fault conditions, the RSC is
generally blocked to protect it from overcurrent in the rotor
circuit. In weak power network during a grid fault, the GSC
is unable to provide sufficient reactive power and voltage
support because of its small power capability, thus there is
probability of voltage collapse in the system. The RSC
cannot restart and the wind turbine system has to be
disconnected from the grid to prevent such a contingency
and reconnect them after the fault is cleared [3]. FACTS
devices are widely used to overcome the voltage instability
problem and maintain the DFIG-based wind farm in service
during grid faults.
In this paper we have analyzed the role of Static
Synchronous Compensator (STATCOM) to supply reactive
power and voltage support to a DFIG-based wind farm
connected to a power system during grid faults. The
STATCOM is connected in shunt with the power system.
Abstract— Now-a-days technology for generating electricity
from renewable energy sources such as wind energy, has
greatly increased. Because of its non-polluting and
economically viable nature, wind energy is considered one of
the most important and promising source of renewable energy
all over the world. These days variable speed turbines
equipped with doubly-fed induction generator are mostly in
use. But during integration of DFIG-based wind farm to a
power grid, there arise many problems, one of which is the
voltage stability problem arising during grid disturbances.
This problem can be overcome by using FACT devices such as
STATCOM. This paper analyses the role of STATCOM in
improving voltage of a DFIG-based wind farm connected to a
power system. The STATCOM compensates the reactive
power at the point of common coupling and thus improves the
voltage and protects the system during and after the grid
faults. Simulink (MATLAB) tool is used for simulating the
developed system and results show that STATCOM improves
the voltage and maintains the uninterrupted operation of the
system during grid faults.
Keywords— Doubly Fed Induction Generator (DFIG),
Static Synchronous Compensator (STATCOM), Wind Farm.
I. INTRODUCTION
The concern about environmental pollution together with
lacking energy sources had led to the increased use of
renewable energy sources for electricity generation. Wind
power has grown most rapidly as compared to other
renewable energies. In recent years, the growth of wind
energy is about 30 percent annually. So wind energy has
become the most expanded technology today. The total
installed capacity in India from wind energy is 23439.26
MW which includes the states of Tamil Nadu, Gujarat,
Maharashtra, Karnataka, Madhya Pradesh, Andhra Pradesh,
Orissa, West Bengal and other states [1].
There are two types of Wind turbine systems (WTS):
fixed-speed and variable-speed wind turbines. Variable
speed wind turbine has an advantage that they are able to
independently control their active and reactive power.
Now-a-day variable speed turbines are mostly equipped
with DFIGs.
II. DFIG-BASED WIND TURBINE SYSTEM
Wind farm used in this paper consists of Doubly Fed
Induction Generator. DFIG consists of a wound rotor
induction generator having its stator directly connected
constant frequency 50 Hz grid and rotor is fed at variable
frequency through ac-dc-ac voltage source converters as
shown in fig 1.
127
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Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 3, March 2016)
The VSC consist of Rotor Side Converter (RSC) and
Grid side Converter (GSC) connected back-to-back by a
dc link capacitor [2].
The DFIG technology helps in extracting maximum
energy from the wind even for low wind speeds by
optimizing the turbine speed, and also minimizes
mechanical stresses on the turbine during gusts of wind.
One more advantage of the DFIG technology the power
electronic converters are able to generate or absorb reactive
power, thus eliminates the need for installing capacitor
banks as required in case of squirrel-cage induction
generator[5].
It consists of a voltage source converter, a dc capacitor
as energy storage and a coupling transformer which is
connected in shunt with the power system. The dc voltage
output of the storage device is converted into three phase ac
output voltage with the help of voltage source converter
[4]. The STATCOM is capable to generate/absorb reactive
power by changing the amplitude of the converter voltage
with respect to the system voltage such that controllable
current flows through the reactance which is connected in
between the STATCOM and the power system. When the
system voltage is low as compared to the STATCOM
voltage, the STATCOM generates reactive power
(capacitive nature) while when the system voltage is high
as compared to the STATCOM voltage, the STATCOM
absorbs reactive power (inductive nature) [2].
The equivalent circuit of STATCOM is shown in fig. 2.
The equations for real and reactive power injected by the
STATCOM are as under:
P = V1V2 sin δ/X and Q = V1 (V1-V2cosδ)/X
Where V1 is the system voltage to be controlled, V2 is
the voltage generated by the generator and δ is the angle
between V1 and V2. During steady state operation V1 is in
phase with V2 (δ =0), so only reactive power flows and
active power is zero.
Fig.1 Configuration of DFIG wind turbine
The role of RSC is to independently regulate the stator
active and reactive power at steady state while GSC
maintains the DC link capacitor voltage constant
irrespective of the magnitude and direction of the rotor
power. In DFIG as the stator is directly connected to the
grid, so during grid faults high currents will be induced in
the rotor windings and thud the RSC will be blocked by the
protection system of the turbine. The GSC is unable meet
the reactive power requirement and voltage support
because of its small power capability. As a result of which
voltage instability may occur and the DFIG has to be
immediately disconnected from the grid.
In order to minimize the effects of grid faults on the
DFIG-based wind farm and maintain its operation during
and after the fault, reactive power compensation is
required. In this paper STATCOM is used for reactive
power compensation.
In this paper 6 wind turbines and DFIGs are presented as
a single equivalent DFIG which is driven by a single
equivalent turbine. Each DFIG-wind turbine consists of 1.5
MW wind turbine generator system.
Fig.2 Equivalent Circuit of STATCOM [2]
Reactive power flows from V1 to V2 (STATCOM
absorbs reactive power) in case when V2 is lower than
V1.On the other hand reactive power flows from V2 to V1
(STATCOM generates reactive power) if V2 is higher than
V1.
III. STATCOM
Static synchronous compensator is the member of the
Flexible AC Transmission System (FACTS) family.
128
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Table 1
Wind Turbine Parameters Used In The System
On the other hand, if V2 is higher than V1, Q is flowing
from V2 to V1 (STATCOM is generating reactive power).
The amount of reactive power is expressed as:
Parameter
Rated Capacity
Rated Wind Speed
No. of Blades
Rotor Diameter
Swept Area
Rotor Speed
Q = V1 (V1- V2)/X
IV. DEVELOPED TEST MODEL
The single line diagram of the test model used in this
paper is shown in fig. 3. It conists of a 120 kV,50 Hz power
supply which feeds a 11kV distribution system through
120/11 kV 47 MVA step dowm transformer, which then
feeds a 440 V system through 11kV/440V, 12 MVA step
down transformer.The system consists of two loads: 2
MVA, 0.9 pf lagging at a distance of 30 km from the
transmission line and the other is the static load of 500 kW
at bus B440.The 30 km transmission line is of 11 kV and
nominal-π line. The DFIG-based wind farm has six wind
turbines each of 1.5MW, thus a total of 9 MW. It also has a
protection system connected at 440V bus which monitors
voltage, current, machine speed and the DC link voltage.
The wind speed from its initial value of 8 m/sec increases
gradually and reaches its final constant value of 12m/sec in
about 12 sec. All the tests conducted here are studied after
the system has reached its steady state. In DFIG, the GSC
helps to maintain the DC link voltage almost constant at a
value of 1200 V when the system is in its normal operating
condition. For providing dynamic compensation of reactive
power, STATCOM is connected in shunt at the sending end
of 11kV bus. Table 1, Table 2 and Table 3 show the
parameters of the wind turbine, generator and transmission
line respectively used in the system.
Value
(6 Turbines ×1.5MW) 9 MW
12 m/s
3
82.5
5346 m2
10.1-18.1 rpm
Table 2
Generator Parameters Used In The System
Parameter
Prated
rr
lls
llr
Lm
pf
Value
6Gen × 1.5 MW
0.005
0.171
0.156
2.9
0.9
Table 3
Transmission Line Parameters Used In The System
Parameter
Value
0.1153 Ω/km
0.00332 H/km
11.33 e-9 F/km
5.01 e-9 F/km
rl
lo
Cl
Co
In this paper we have analyzed the same cases on a
model implemented similar to Indian distribution and
transmission system using Simulink (MATLAB) tool. Also
the evaluation of STATCOM for providing dynamic
reactive power compensation during grid faults in a DFIGbased STATCOM is analyzed. Table 4 shows the
parameters of the STATCOM used.
Table 4
Statcom Parameters
Rating
3 MVA
Fig. 3 Single Line Diagram of the Test system
V. SIMULATION RESULTS AND DISCUSSION
Simulink (MATLAB) tool is used for the simulation
analysis. We have regenerated the results of reference
paper [2] which includes different cases such as effect of
short circuit fault and voltage sag.
129
Mode of operation
Voltage regulation mode
Voltage at the point of
common coupling
11 kV
DC-link voltage
4000 V
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 6, Issue 3, March 2016)
A. Steady State Response of the System:
Wind speed with initial value of 8 m/s is applied which
increases to a final value of 12 m/sec at t = 5 sec. as shown
in Fig. 4.
Time (s)
Fig.7 Voltage at the point of common coupling (Bus B440)
Ti
me (s)
Fig. 4 Wind speed profile
The control mode of DFIG-based wind farm block is set
to voltage regulation mode regulation mode with reference
voltage of 1 pu on the base of generator rating which is
6*1.5 MVA and V = 440 V at bus B440. At t= 5 sec, the
generated active power starts increasing with the increase
in wind speed and increases up to 7 MW as shown in
Fig. 5.
Ti
me (s)
Fig.8 Current at the point of common coupling (Bus B440)
Time (s)
Fig.9 DC link capacitor voltage
Time (s)
Fig. 10 shows the variation in turbine speed which
increases from 0.8 pu to 1.21 pu.
Fig.5 DFIG generated active power
Fig. 6 shows the generated reactive power of DFIG at
steady state, which is controlled to maintain the voltage at
PCC (Bus B440) at 1 pu shown in Fig. 7. and also
maintains the DC link capacitor voltage of DFIG constant
at 1200 V as shown in Fig. 9.
Time (s)
Fig.10 Turbine speed
B. System Performance during Single Line to Ground
(SLG) Fault with STATCOM and RSC Blocking:
The effect of grid fault on the wind farm is much severe
even if the fault is located far away from PCC of the wind
farm. The drop in voltage leads to the overcurrent in the
rotor circuit which may damage the system. Thus RSC of
DFIG must be blocked to protect it from damage due to
rotor circuit‟s overcurrent.
Time (s)
Fig.6 DFIG generated reactive power
Fig. 7 and Fig. 8 show the voltage and current
respectively at the point of common coupling (Bus B440)
130
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A temporary single line to ground fault is applied at t = 5
sec to the bus B3 (11kv) in Fig.3 for duration 5 sec. The
DFIG will not be able to exchange reactive power with the
power system as the RSC of DFIG-based wind farm is set
to var regulation mode with reference reactive power
command equal to zero. In this paper the protection system
of the wind turbine will trip the wind turbine when the PCC
voltage will fall to 50%.
Fig. 11 shows that the voltage at bus B440 drops to 0.54
pu when single line to ground fault is applied to bus B3 at t
= 5 sec.
The STATCOM provides the required reactive power to
maintain the wind warm in service during and after the
SLGF.
Time (s)
Fig.14 PCC (Bus B440) voltage during SLGF with STATCOM
Fig. 15 and Fig. 16 show the generated active and
reactive power respectively during SLGF with STATCOM.
Time (s)
Fig.11 PCC (Bus B440) voltage during SLGF
The wind turbine system will not trip as it is designed to
trip when the PCC voltage drops to 50% and continue to
generate active power as shown in Fig. 12.The DFIG will
not exchange any reactive power with the power system as
shown in Fig. 13 because the RSC of DFIG is blocked to
prevent overcurrent in the rotor circuit.
Time (s)
Fig.15 Generated active power during SLGF with STATCOM
Time (s)
Fig.16 Generated reactive power during SLGF with STATCOM
C. System Performance during Line to Line (L-L) Fault
with STATCOM and RSC Blocking:
A temporary line to line fault is applied at t = 5 sec to the
bus B3 (11kv) in Fig.3 for duration of 5 sec. The DFIG will
not be able to exchange reactive power with the power
system as the RSC of DFIG-based wind farm is set to Var
regulation mode with reference reactive power command
equal to zero.
Fig. 17 shows that the PCC voltage at bus B440 drops to
0.53 pu during the fault. In this case also The wind turbine
system will not trip as it is designed to trip when the PCC
voltage drops to 50% and continue to generate active
power.
Time (s)
Fig.12 Generated active power during SLGF
Time (s)
Fig.13 Generated reactive power during SLGF
After installing 3 MVA STATCOM at bus B3,the
voltage at PCC has improved to 0.68 pu as shown in
Fig. 14.
131
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Time (s)
Fig.20 The overshoot of the DC link voltage during L-L fault with
STATCOM
Time (s)
Fig.17 PCC (Bus B440) voltage during line to line fault
D. Voltage Sag of 30% at Bus 120 kV with STATCOM and
RSC Blocking:
A temporary voltage sag of 30% is applied to the grid at
bus 120kV in Fig.3.The voltage sag is applied at t = 5 sec
and duration of 0.5 sec.fig.21 shows that the voltage at bus
B440 dropped to 0.45 pu, due to which the wind turbine
protection system will trip the wind farm.
After installing 3 MVA STATCOM at bus B3,the
voltage at PCC has improved to 0.67 pu as shown in Fig.
19. The STATCOM provides the required reactive power
to maintain the wind warm in service during and after line
to line fault.
Time (s)
Fig.18 PCC (Bus B440) voltage during line to line fault with
STATCOM
Time (s)
Fig.21 PCC (Bus B440) voltage during a voltage sag of 30% at Bus
120kV
One more important requirement necessary in the
uninterrupted operation of the wind turbine is the stability
of the dc- link voltage of variable frequency ac-dc-ac
converter (VGC) in the rotor circuit. Fig. 19 and Fig. 20
show that there is drop in the overshoot of the dc-link
voltage from 1252 V to 1245 V during L-L fault after
STATCOM is installed. Decrease in the overshoot of the
dc-link voltage means minimizing the risk of damage to the
GSC and also helping RSC able to restart when the fault is
cleared.
After installing 3 MVA STATCOM at bus B3,the
voltage at PCC (bus B440) has improved to 0.6 pu as
shown in Fig. 22. The STATCOM provides the required
reactive power to maintain the wind warm in service during
and after the grid disturbance.
Time (s)
Fig.22 PCC (Bus B440) voltage during a voltage sag of 30% at Bus
120kV with STATCOM
Time (s)
Fig.19 The overshoot of the DC link voltage during L-L fault without
STATCOM
132
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[4]
Table 5
Comparison Of Pcc (Bus440) Voltage During Different Faults With
And Without Statcom
PCC (Bus440) Voltage
Type of Fault
Single line to
ground (SLG) fault
Line to line (L-L)
fault
30% voltage sag
Without
STATCOM (pu)
With
STATCOM (pu)
0.54
0.68
0.53
0.67
0.45
0.6
[5]
[6]
[7]
VI. CONCLUSION
In this paper we have analyzed the role of STATCOM to
maintain a DFIG based wind farm in service during grid
faults which include single line to ground fault, line to line
fault and voltage sag due to a remote fault. The SATCOM
provided the required reactive power for the operation of
the wind farm during and after the faults. The STATCOM
was connected at the point of common coupling at bus B3
in the system. Simulink tool in MATLAB is used for the
simulation of the system. The developed model in Simulink
is similar to Indian distribution and transmission system.
The simulation results show that using faults the PCC
voltage (bus B440) drops which may lead to overcurrent in
the system. But after installing STATCOM, the PCC
voltage has increased in all the three cases and voltages reestablish quickly after the fault is cleared. Without
STATCOM
the bus voltages cannot re-establish so
quickly and cause the wind turbine to trip from the grid
thus causing interrupted operation. Thus STATCOM
increases the voltage stability, giving fast and smooth
response.
[8]
[9]
[10]
[11]
[12]
[13]
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