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Abstract— This paper presents a new method to enhance the
transient stability of multi-machine power system including wind
farms, when a severe network disturbance occurs in the power
system. For this purpose, the energy capacitor system (ECS)
composed of power electronic devices and electric double layer
capacitor (EDLC) is proposed. The control scheme of ECS is
based on a sinusoidal PWM voltage source converter (VSC) and
fuzzy logic controlled dc-dc buck/boost converter using insulated
gate bipolar transistors (IGBT). Two wind farms are considered to
be connected to the power system. Two-mass drive train model of
wind turbine generator system (WTGS) is used in the analyses as
the drive train modeling has great influence on the dynamic
characteristics of WTGS. Real wind speed data is used in the
analyses to obtain realistic responses. Different types of
symmetrical and unsymmetrical faults are considered as the
network disturbance. Simulation results clearly show that the
proposed ECS can enhance the transient stability of wind
generators in multi-machine power system as well as their low
voltage ride through (LVRT) capability.
Index Terms— dc-dc buck/boost converter, electric double layer
capacitor (EDLC), energy capacitor system (ECS), pitch
controller, transient stability, voltage source converter (VSC),
wind energy.
I. INTRODUCTION
ECENTLY due to the environmental problem such as
global warming and the exhaustion of fossil fuel, the
renewable energy sources like wind energy, solar energy,
biomass etc are considered as clean and prospective energy
sources of the future world. Among them, 12% of the world’s
electricity is expected to be generated from wind power in 2020
[1]. Therefore, it can be easily comprehended that a huge
number of wind farms are going to be connected with the
existing power systems in the near future. Two types of wind
generator topologies are available in the market. These are fixed
speed and variable speed wind generators. Induction generators
are used widely, in general, as wind generator due to their
superior characteristics such as brushless and rugged
construction, low cost, maintenance free, and operational
Manuscript received May 28, 2007.
S. M. Muyeen, R. Takahashi, T. Murata, and J. Tamura are with the
Electrical and Electronic Engineering Department, Kitami Institute of
Technology, 165 Koen-Cho, Kitami, Hokkaido, 090-8507, Japan (e-mail:
muyeen@pullout.elec.kitami-it.ac.jp).
M. H. Ali is with Changwon National University, 9 Sarim-Dong, Changwon,
Gyeongnam, 641-773, South Korea.
simplicity. In 2004, the world-wide market share of variable
speed WTGS was around 60% [2]. Among those, doubly fed
induction generator (DFIG) is mostly used as variable speed
wind generator. Wound field synchronous generator, and
permanent magnet synchronous generator (PMSG) are also
currently used as variable speed wind generators. However,
DFIG, would field synchronous generator, and PMSG have
better fault ride through capabilities as frequency converter is
closely connected to the machine [3-7]. On the other hand, the
fixed speed wind generator that uses the squirrel cage induction
generator needs additional tool to enhance the fault ride through
capability. This is because it requires large reactive power to
recover the air gap flux when a short circuit fault occurs in the
power system [8]. If sufficient reactive power is not supplied,
then the electromagnetic torque of wind generator decreases
significantly. Then the difference between mechanical and
electromagnetic torques becomes large and the wind generator
and turbine speeds increase rapidly. As a result, the induction
generator becomes unstable and it requires to be disconnected
from the power system. However, the recent trend is to decrease
the shut down operation because a shut down of large wind farm
can have a serious effect on the power system operation. As for
example, in Germany the wind generator shut down
phenomenon has been reduced by adopting the low voltage ride
through (LVRT) requirement from German grid operator
named E.ON Netz. The E.ON Netz standard requires that the
machine remains connected to the grid if the terminal voltage is
over 0.15pu for approximately 0.6s [9]. American Wind Energy
Association (AWEA) is also recommending to adopt the LVRT
requirement developed by E.ON Netz. In general, all generators
connecting to the transmission system are required to comply
with the grid code. The grid codes were originally decided with
synchronous generators in mind. But due to the recent addition
of huge amount of wind power to the grid, in many countries
[10-11] the new grid codes have been developed to ensure
secure power system operation. Therefore, it is important to
investigate a suitable method to enhance the LVRT capability of
fixed speed wind generators and also augment the transient
stability of the entire power system.
A braking resistor has been recognized and used as a
powerful tool for transient stability enhancement of
multi-machine power system for a long time [12-13]. But as a
braking resistor has only a control ability of active power
consumption, it is not suitable for the transient stability
Transient Stability Augmentation of Power
System Including Wind Farms by Using ECS
S. M. Muyeen, Student Member, IEEE, R. Takahashi, M. H. Ali, Member, IEEE, T. Murata,
and J. Tamura, Senior Member, IEEE
R
This is the author's version of an article published in IEEE Transactions on Power Systems. Changes were made to this version by the
publisher prior to publication. DOI: 10.1109/TPWRS.2008.920082
2
improvement of power system including wind farms. With the
recent development of FACTS devices, SVC and STATCOM
have been used for transient stability augmentation of power
system [14-15]. The application of STATCOM for stabilizing
wind generator is reported in [16-18]. However, since the
traditional STATCOM has only an ability of reactive power
control, its application is limited to reactive power support in
the power system. To overcome this problem,
STATCOM/BESS, i.e., STATCOM with battery energy storage
system has been emerged as more promising devices for power
system applications [19-21], as it has both real and reactive
power control abilities. But BESS is based on chemical process
and thus it has some problems such as low response speed and
short service life.
Alternative technology recently used in power system
applications is an energy capacitor system (ECS) composed of
power electronic devices and electric double layer capacitor
(EDLC) [22-27]. This system has a feature of "clean energy"
from an environmental viewpoint compared to batteries, as it
doesn’t contain heavy metals or toxic materials like Ni, Cd, Pb.
Over-charging and over-discharging in EDLC do not have a
negative effect on its lifetime, though they have in the case of
batteries. EDLC can be cycled millions of time, i.e., it has a
virtually unlimited cycle life. Its standby loss is very low within
the range of 0.2% of its power rating. As it has both real and
reactive power control abilities, ECS can be applied to load
leveling, peak saving, sub-synchronous oscillations, transient
and dynamic stability enhancement of power system. Moreover,
it can also be applied for output power smoothing of and
terminal voltage regulation of both fixed and variable speed
wind generators.
In this study, ECS is proposed for enhancing the transient
stability of multi-machine power system including fixed speed
wind farms. The control scheme of ECS is based on a sinusoidal
PWM (pulse width modulation) voltage source converter (VSC)
and dc-dc buck/boost converter using insulated gate bipolar
transistors (IGBT). Fuzzy logic controller (FLC) is proposed as
the control methodology of dc-dc buck/boost converter since it
can be applied to a system with uncertainties. This is one of the
salient features of this work. The proposed modeling, control
strategy, and rating of ECS is also suitable for both fixed and
variable speed wind generators output power smoothing, though
the output power leveling is not discussed in this paper. The
two-mass shaft model of wind turbine generator system
(WTGS) is considered as the shaft system modeling has a great
influence on the transient stability characteristics of wind
generators [28]. Several types of symmetrical and
unsymmetrical faults are considered as a network disturbance.
In order to evaluate the transient performance of ECS, the
transient stability index is calculated in terms of the total kinetic
energy of synchronous generators. From the simulation results,
it can be concluded that the proposed ECS can enhance the
transient stability of multi-machine power system including
wind farms.
II. MODEL SYSTEM
A. Model System for Simulation Analyses
Fig. 1 shows the model system composed of 9-bus main
system and two wind farms. Synchronous generators 1 and 2
(SG1 and SG2) are a steam turbine and hydro turbine generators
respectively. The IEEE generic turbine model and approximate
mechanical-hydraulic speed governing system [29] is used for
synchronous generator 1 (SG1). The IEEE “non-elastic water
column without surge tank” turbine model and “PID control
including pilot and servo dynamics” speed-governing system
[30] is used for synchronous generator 2 (SG2). IEEE alternator
supplied rectifier excitation system (AC1A) [31] is used in the
exciter model of both synchronous generators. Wind farms 1
and 2 are considered to be connected to the main system through
a long and short transmission lines respectively. In the both
wind farm, induction generators are used as wind generators.
Each wind farm has a power capacity of 50 MVA and consists
of five fixed-speed wind generators of 10 MVA power rating. It
Fig. 1. Model system
SG2
SG1
j
0.0625
(P/V=1.2/1.01)
0.0085+j0.072
(j0.0745)
0.0119+j0.1008
(j0.1045)
(P/Q=1.5/0.2)
(P/Q=1.75/0.2) (P/Q=0.9/0.3)
0.010+j0.085
(j0.088) 0.017+j0.092
(j0.079)
0.032+j0.16
(j0.153)
0.039+j0.170
(j0.179)
j
0.0586
j
0.0576
Infinite bus
Tr.1 Tr.2
Fault
50Hz, 100MVA BASE
F3
Tr.3
1.04
o
0.0
Load A Load B
1
23
4
56
78
9
CB
F2
F1
(P/V=1.9/1.02)
Load C
j
0.2
j
0.2
18
0.05+j0.1
17
Hydro Turbine
Steam Turbine
ECS-2
10
11
0.1+j0.2
ECS-1
WF1
C
IG6
j1.0
IG7
IG8
IG9
IG10
10(MW)
j1.0
j1.0
j1.0
j1.0
C
C
C
C
P=0.1(pu)
V=1(pu)
P=0.1(pu)
P=0.1(pu)
P=0.1(pu)
P=0.1(pu)
19
20
21
22
23
V=1(pu)
V=1(pu)
V=1(pu)
V=1(pu)
C
IG1
j1.0
IG2
IG3
IG4
IG5
10(MW)
j1.0
j1.0
j1.0
j1.0
C
C
C
C
P=0.1(pu)
V=1(pu)
P=0.1(pu)
P=0.1(pu)
P=0.1(pu)
P=0.1(pu)
12
13
14
15
16
V=1(pu)
V=1(pu)
V=1(pu)
V=1(pu)
WF2
3
is assumed that several fixed-speed wind generators are lumped
together to obtain the 10 MVA fixed-speed wind generator
[28,32]. A capacitor bank, C, has been used for reactive power
compensation of IG at steady state. The value of capacitor C is
chosen so that power factor of the wind power station during the
rated operation becomes unity [18]. Generator parameters are
shown in Table I. The system power base is 100MVA.
TABLE I
GENERATOR PARAMETERS
Synchronous Generators
SG1 SG2 Induction Generators
MVA 200 130 MVA 10
ra (pu) 0.003 0.003 r1 (pu) 0.01
xa (pu) 0.102 0.130 x1 (pu) 0.1
Xd (pu) 1.651 1.200 Xmu (pu) 3.5
Xq (pu) 1.590 0.700 r21 (pu) 0.035
X/d (pu) 0.232 0.300 x21 (pu) 0.030
X/q (pu) 0.380 r22 (pu) 0.014
X//d (pu) 0.171 0.220 x22 (pu) 0.098
X//q (pu) 0.171 0.250 Hg (pu) 0.3
T/do (sec) 5.900 5.000 Hwt (pu) 3.0
T/qo (sec) 0.535 Kw (pu) 90
T//do (sec) 0.033 0.040
T//qo (sec) 0.078 0.050
H (sec) 9.000 2.500
B. Wind Turbine Modeling
Mathematical relation for mechanical power extraction from
the wind can be expressed as follows [33].
Where, Pw is extracted power from the wind, is the air density
[kg/m3], R is blade radius [m], Vw is wind speed [m/s] and Cp is
the power coefficient which is a function of both tip speed ratio,
, and blade pitch angle, [deg]. The wind turbine
characteristic used in this study is shown in Fig. 2 for different
values of . The two-mass drive train parameters of wind
generators are shown in Table I, where, Hg and Hwt are the
generator and wind turbine inertia constants respectively, and
Kw is the shaft stiffness between the two masses.
In this study, the conventional pitch controller shown in Fig.
3 is used. The purpose of using the pitch controller is to
maintain the output power of wind generator at rated level by
controlling the blade pitch angle of turbine blade when wind
speed is over the rated speed.
C. Modeling of Energy Capacitor System (ECS)
Energy capacitor system (ECS) consists of EDLC and power
electronic devices. Schematic diagram of ECS is shown in Fig. 4,
where the EDLC bank is shown by a rectangular box, the PWM
voltage source converter (VSC) is shown inside the box of doted
line and the dc-dc buck/boost converter is shown inside the box
of dashed line. The PWM VSC controls the DC-link voltage and
the reactive power flowing into the ECS, while the real power is
controlled by the FLC controlled dc-dc buck/boost converter.
The MVA rating of VSC and the MW rating of EDLC bank
need not to be equal. Moreover, for transient stability
enhancement or power system oscillation damping the MW
rating of stored device does not have to be large enough [19].
1) Modeling of EDLC
The detailed model of the EDLC cell shown in Fig. 5 can
represent the terminal characteristics of the EDLC cell precisely
as described in [27]. In this study, the rated EDLC bank voltage
is chosen 4.0KV. The rated capacity of the EDLC bank is
25MW, 0.22MWh. The parameters of the EDLC bank are
shown in Table II.
TABLE II
DETAILED MODEL PARAMETERS OF EDLC BANK
Capacitance Internal Resistance
Cb1 2.0F Rb1 0.0106
Cb2 50.0F Rb2 0.265
Cb3 48.0F Rb3 0.254
3
2
P 0.5 R V C ( , ) (1)
w w P
=
Fig. 2. CP- curves for different pitch angles
04812 16 20
0.0
0.1
0.2
0.3
0.4
0.5
Cp
MOD2 Wind Turbine
in degree
=0
=6
=12
=18
=24
Rb3 Rb2 Rb3
Cb3 Cb2 Cb1
Fig. 5. Detailed model of EDLC bank
Fig. 3. Pitch controller
1
1+5s
10
0
/s
0
90
1.0
PIG +
eKp=200
Ti=0.3
PI Controller
Vd
Cd
Ld=0.005H
Vbank
DC-DC Buck/Boost Converter
Pe
g1
g2
a
b
c
Fig. 4. Schematic diagram of energy capacitor system (ECS)
EDLCbank
PWM VSC
6-Pulse
IGBT
Bridge
4
2) PWM Voltage Source Converter Modeling
For the PWM VSC, the well-known vector control scheme is
used as shown in Fig. 6. The VSC converts the dc voltage across
the storage device into a set of three-phase ac output voltages.
These voltages are supplied to the ac system through the
impedance of the coupling transformer. The dq quantities and
three-phase electrical quantities are related to each other by
reference frame transformation. The angle of the transformation
is detected from the three phase voltages (va,vb,vc) at each
connection point of ECS (Bus 11 & Bus 18) by using phase
locked loop (PLL) system. Suitable adjustment of phase and
magnitude of the VSC output voltage allows effective control of
power exchange between the ECS and the ac system. In this
paper, ECS is used also to regulate the wind farm terminal
voltages. Therefore, the aim of the control is to maintain the
magnitude of voltage at the wind farm terminals (Bus 11 & Bus
18) to be constant. The vector control scheme which generates
the three-phase reference signals are compared with the
triangular carrier wave signal in order to generate the switching
signals for the IGBT switched VSC. High switching frequencies
can be used to improve the efficiency of the converter, without
incurring significant switching losses. In the simulation
analyses, the switching frequency is chosen 1000Hz. The
snubber circuit resistance and capacitance values of the IGBT
devices shown in Fig. 4 are 5000 and 0.05 µF respectively.
The VSC rating is considered to be the same of wind farm rating.
The rated DC link voltage is 4.0kV. The ECS is connected to the
66kV line by a single step down transformer (66kV/2.18kV)
with 0.2p.u leakage reactance (base value 100MVA). The
DC-link capacitor value is 50000 µF.
3) Modeling of dc-dc Buck/Boost Converter
The dc-dc buck/boost converter shown inside the dashed line of
Fig. 4 operates alternately by controlling switches g1 and g2 to
be ON or OFF. When the line power, PL (power on the line from
bus 11 or bus 18 as shown in Fig. 1), is less than the reference
power, the EDLC discharges, working in boost converter mode
and vice versa. The error signal between the line power and the
reference power is progressed through a fuzzy logic controller
and produce the dc-dc buck/boost converter duty cycle. Then
the duty cycle is compared with the saw tooth carrier wave to
generate the gate signals for buck/boost converter as shown in
Fig.7. The frequency of the saw tooth carrier signal is chosen
250Hz. The proposed fuzzy logic controller for dc-dc
buck/boost converter works well in both steady state and
transient conditions under small and large variation of its
operational variables. The fuzzy logic controller design phase is
given in the appendix.
III. SIMULATION RESULTS
Wind farm compatible grid code is more or less similar. Wind
farm terminal voltage has to return to 90 percent of the nominal
voltage within 3 seconds after the starting of voltage drop
[10-11]. Otherwise, the plant has to be shutdown. This paper
shows a control methodology to overcome the voltage dip of
wind farm during a network disturbance in power system. When
a network fault occurs, the ECS absorbs the transient energy by
switching off the switch, g2, of the dc-dc buck/boost converter.
Therefore, the real power can be controlled. On the other hand,
the reactive power demand of wind farm is supplied according
to the error signal between the wind farm terminal voltage and
the reference voltage. To obtain the realistic responses, the
two-mass shaft model of WTGS is considered in this paper. The
symmetrical three-line-to-ground fault, 3LG, and the
unsymmetrical double-line-to-ground fault, 2LG (phase B, C &
ground), double-line fault, 2LL (between phase B & C), and
single-line to ground fault, 1LG (phase C & ground) are
considered as a network disturbance, which occurs at fault point
PI-1
Vdc
*
PI-2
2/3
VSC
PLL
e
3/2
Vdc
Ia,b,c
Va,b,c
I
*
d
V
*
a,b,c
V
*
cq
+
+
V
*
k
Vk
+
Id
PI-3 PI-4
+
Iq
I
*
qV
*
cd
Fig. 6. Control block diagram of PWM based VSC
Carrier Wave
Wind Farm
Connection
Point
(Bus 11
or Bus 18)
EDLC Bank
2-Level
VSC
ECS
G1
1+sT1
1+s T2
G2
1+s T1
1+s T2
Carrier
Wave
Comparator
g1
g2
Fig. 7. Control block of DC-DC buck/boost converter
e
e
Z
-
1
+
Fuzzy
Logic
Controller
n
PL
-
+
+
Z
-
1
+-
PRef
5
F1, F2, and F3 in Fig. 1. The fault is considered to occur at
100.1 sec. The circuit breakers (CB) on the faulted lines are
opened at 100.2 sec, and finally, at 101.0 sec the circuit breakers
are reclosed. All types of dampings are disregarded to obtain the
worse scenario. Real wind speed data, obtained in Hokkaido
Island, Japan, shown in Fig. 8 and Fig. 9 respectively, are used
in wind farms 1 and 2. Time step is 0.00001 sec and simulation
results during 10 sec from 100 sec to 110 sec will be shown.
Simulations have been done by using PSCAD/EMTDC [34].
The terminal voltage responses of wind farms 1 and 2 for the
3LG fault at point F2 in Fig. 1 with and without considering
ECS are shown in Fig. 10 and Fig. 11 respectively. When the
ECS is not considered, the voltage drop occurs at the wind farm
and also wind generator terminals. Then, the electromagnetic
torques of induction generators drop suddenly as the
electromagnetic torque is proportional to the square of the
terminal voltage. But the mechanical torques of wind turbines
do not change rapidly during the short time interval. As a result,
the turbine hubs and generator rotors accelerate due to the large
difference between the mechanical and electromagnetic torques
and the wind generators become unstable. But when the ECS is
used, since the necessary reactive power is supplied from the
ECS properly according to the error signals, the terminal
voltages of wind farms can be return back to the pre-fault level.
Thus the electromagnetic torques can be restored quickly and
the wind generators become stable. The generator rotor and
turbine hub speeds for WTGS-1 and WTGS-6 are shown in Fig.
12 and Fig. 13 respectively. As WTGS-6 is far from the fault
point, the generator rotor and hub speeds become stable with
and without ECS. Fig. 14, Fig. 15, and Fig. 16 show the voltage
responses at bus 5, 6, and 8 respectively, from which it can be
seen that the ECSs maintain the bus voltages constant after the
severe 3LG fault. The real and reactive power of ECS 1 and 2
are shown in Fig. 17 and Fig. 18 respectively. The load angle
responses of SG1 and SG2 are shown in Fig. 19. As the ECSs
can absorb the transient energy of the power system during the
0 50 100 150 200 250 300
8
1 0
1 2
1 4
1 6 W i n d F ar m -1
Wind Speeds [m/sec]
T im e [ sec ]
W in d S p ee d f or I G 1
W in d S p ee d f or I G 2
W in d S p ee d f or I G 3
W in d S p ee d f or I G 4
W in d S p ee d f or I G 5
Fig. 8. Wind Speeds in wind farm-1
0 50 100 150 200 250 300
6
8
1 0
1 2
1 4
1 6 W i nd F a rm -2
Wind Speeds [m/sec]
T im e [ sec]
W in d S p ee d f or I G 1
W in d S p ee d f or I G 2
W in d S p ee d f or I G 3
W in d S p ee d f or I G 4
W in d S p ee d f or I G 5
Fig. 9. Wind Speeds in wind farm-2
Fig. 10. Terminal voltage (Bus 11) of wind farm-1(3LG)
Fig. 11. Terminal voltage (Bus 18) of wind farm-2(3LG)
Fig. 12. Induction generator rotor and turbine hub speed of WTGS-1 (3LG)
Fig. 13. Induction generator rotor and turbine hub speed of WTGS-6 (3LG)
6
Fig. 17. Real and reactive power of ECS-1(3LG)
Fig. 18. Real and reactive power of ECS-2(3LG)
Fig. 20. Stored energy of ECS 1 and 2(3LG)
Fig. 21. EDLC bank voltage of ECS 1 and 2(3LG)
Fig. 22. DC-link voltage of ECS-1(3LG)
Fig. 23. DC-link voltage of ECS-2(3LG)
Fig. 14. Voltage at Bus 5(3LG)
Fig. 15. Voltage at Bus 6(3LG)
Fig. 16. Voltage at Bus 8(3LG)
Fig. 19. Load angle of synchronous generators (3LG)
7
network disturbance, the transient stability of the synchronous
generators can also be enhanced significantly. Figs. 20 - 23
show the responses of the stored energy of EDLC capacitor
banks, the EDLC bank voltages, and the DC-link voltages in
ECS 1 and 2.
Fig. 24 and Fig. 25 show the terminal voltage responses of wind
farms 1 and 2 respectively for the unsymmetrical 2LG fault
occurred at fault point F2 in Fig. 1. The real and reactive power
responses of both ECS are shown in Fig. 26 and Fig. 27
respectively. The load angle responses of SG1 and SG2 are
shown in Fig. 28. From these results it is clear that the proposed
ECS can enhance the transient stability of entire power system
also for unsymmetrical faults.
IV. TRANSIENT STABILITY EVALUATION
For the evaluation of transient stability, we used the stability
index, Wc, [35] as described below.
where, T is the simulation time of 10.0 sec, and Wtotal is the total
kinetic energy calculated by using the rotor speed of each
synchronous generator as follows:
where N is the number of synchronous generators, and Ji and
i denote inertia moment and rotor speed of each generator. The
smaller the value of Wc, the better the system transient stability.
The transient stability index against 1LG, 2LL, 2LG, and 3LG
faults with and without considering the ECS are shown in Table
III, Table IV, Table V, and Table VI respectively, for different
fault points of the model system. From these results it can also
be understood that the proposed ECS can improve the transient
stability of entire power system.
TABLE III
TRANSIENT STABILITY INDEX [WC(SEC)] FOR 1LG FAULT
TABLE IV
TRANSIENT STABILITY INDEX [WC(SEC)] FOR 2LL FAULT
TABLE V
TRANSIENT STABILITY INDEX [WC(SEC)] FOR 2LG FAULT
Fault Location Without Controller With ECS
F1 2.40 1.94
F2 1.82 1.30
F3 2.42 1.77
Fault Location Without Controller With ECS
F1 3.04 2.52
F2 2.49 1.78
F3 3.06 2.24
Fault Location Without Controller With ECS
F1 3.59 3.16
F2 2.73 1.99
F3 3.72 2.67
(2A)powerbasesystem
T
dt
total
W
dt
d
(sec)
c
W
=
0
(2B)(J)
N
1i i
W
total
Wዊ
=
=
(2C)(J)
2
mi
i
J
2
1
i
W=
Fig. 26. Real and reactive power of ECS-1(2LG)
Fig. 27. Real and reactive power of ECS-2(2LG)
Fig. 24. Terminal voltage (Bus 11) of wind farm-1(2LG)
Fig. 25. Terminal voltage (Bus 18) of wind farm-2(2LG)
Fig. 28. Load angle of synchronous generators (2LG)
8
TABLE VI
TRANSIENT STABILITY INDEX [WC(SEC)] FOR 3LG FAULT
V. CONCLUSIONS
In this paper, energy capacitor system (ECS) is proposed to
enhance the transient stability of multi-machine power system
including wind farms. It is also shown that the ECS can enhance
the LVRT capability of wind farms according to the grid code.
Detailed modeling of each component and suitable control
strategy of ECS are presented. The control scheme of ECS is
based on a sinusoidal PWM voltage source converter and fuzzy
logic controlled dc-dc buck/boost converter. Real wind speed
data is used in each wind farm model to obtain the realistic
responses. The effectiveness of the proposed control system is
verified with different types of fault conditions at different
location in the power system model. Finally, it is concluded that
the proposed FLC controlled ECS system can be applied
effectively to enhance the transient stability of multi-machine
power system including wind farms.
VI. APPENDIX
The triangular membership functions with overlap used for
the input and output fuzzy set are shown in Fig. 29, in which the
linguistic variables are represented by NB (Negative Big), NS
(Negative Small), Z (Zero), PS (Positive Small), and PB
(Positive Big). The entire rule base is given in Table VII.
TABLE VII
FUZZY RULE TABLE
e
n
NB NS ZO PS PB
NB NB NB NS NS ZO
NS NB NS NS ZO PS
ZO NS NS ZO PS PS
PS NS ZO PS PS PB
e
PB ZO PS PS PB PB
In this work, for the inference mechanism, Mamdani’s
max-min (or sum-product) [36] method is used. The center of
gravity method [36] is used for defuzzification to obtain n.
REFERENCES
[1] The European Wind Energy Association, EWEA Publications, 2005,
“Wind Force 12- A Blueprint to Achieve 12% of the World’s Electricity
From Wind power by 2020,” 2005, [Online], http://www.ewea.org/
[2] F. V. hulle, “Large Scale Integration of Wind Energy in the European
Power Supply Analysis, Issue and Recommendations,” EWEA, Tech. Rep.,
December 2005.
[3] P. Ledesma and J. Usaola, “Doubly fed induction generator model for
transient stability analysis,” IEEE Trans. on Energy Conversion, Vol.20,
No.2, pp.388-397, 2005.
[4] P. La Seta and P. Schegner, “Comparison of stabilizing methods for
doubly-fed induction generators for wind turbines,” International
Conference on Future Power System, Conference CDROM, 2005.
[5] T. Sun, Z. Chen, and F. Blaabjerg, “Transient stability of DFIG wind
turbines at an external short-circuit fault,” Wind Energy, Vol.8, No.3,
pp.345-360, 2005.
[6] A. Yazdani and R. Iravani, “A Neutral-Point Clamped Converter System
for Direct-Drive Variable-Speed Wind Power Unit,” IEEE Trans. on
Energy Conversion, Vol.21, No.2, pp.596-607, 2006.
[7] S. M. Muyeen, R. Takahashi, T.Murata, J.Tamura, and M. H. Ali,
“Transient Stability Analysis of Permanent Magnet Variable Speed
Synchronous Wind Generator,” Paper accepted to present at International
Conference on Electrical Machines and Systems 2007 (ICEMS 2007),
Paper No. D20070409-221, Seoul, Korea, October 2007.
[8] Claudio L.Souza et. al., “Power System Transient Stability Analysis
Including Synchronous and Induction Generator,” IEEE Porto Power Tech
Proceedings, Vol.2, pp.6, 2001.
[9] IEEE Power & Energy Magazine, Vol.3, No.6, pp.30-32, 2005.
[10] Federal Energy Regulatory Commission (FERC), United States of
America, Docket No. RM05-4-000 – Order No. 661, “Interconnection for
Wind Energy,” Issued June 2, 2005.
[11] Ireland National Grid, Grid Code Version 2, January, 2007, “Wind Farm
Power Station Grid Code Provisions, WFPS1,” pp.213-216.
[12] A. Rubaai, A. R. Ofoli, D. Cobbinah, M. D. Kankam, “Two-layer
supervisory controller-based thyristor-controlled braking resistor for
transient stability crisis,” IEEE Trans. on Industry Applications, Vol. 41,
Issue. 6, pp. 1539 – 1547, 2005
[13] M. H. Ali, T. Murata, and J. Tamura, “Transient stability augmentation by
fuzzy logic controlled braking resistor in multi-machine power system,”
Transmission and Distribution Conference and Exhibition 2002, Asia
Pacific, IEEE/PES, Vol. 2, pp.1332-1337, 2002.
[14] S. Mohagheghi, G. K. Venayagamoorthy, and R. G. Harley, “Adaptive
Critic Design Based Neuro-Fuzzy Controller for a Static Compensator in a
Multimachine Power System,” IEEE Trans. on Power Systems, Vol. 21,
No. 4, pp. 1744-1754, 2006.
[15] M. H. Haque, “Improvement of first swing stability limit by utilizing full
benefit of shunt FACTS devices,” IEEE Trans. on Power System, Vol. 19,
No. 4, pp. 1894 – 1902, 2004.
[16] Z. Daad-Saoud, “Application of STATCOMs to Wind Farms,” IEE Proc.
Gener. Transm. Distrib. Vol.145, No.5, pp.511-517, 1998.
[17] Z. Chen, F. Blaabjerg, Y Hu, “ Voltage Recovery of Dynamic Slip Control
Wind Turbines with a STATCOM,” International Power Electronic
Conference (IPEC05), S29-5, pp.1093-1100, 2005.
[18] S. M. Muyeen, Mohammad Abdul Mannan, Mohd. Hasan Ali, Rion
Takahashi, Toshiaki Murata, and Junji Tamura, “Stabilization of Wind
Turbine Generator System by STATCOM,” IEEJ Trans.PE., Vol.126-B,
No.10, pp.1073-1082, 2006.
[19] L. Zhang, C. Shen, M. L. Crow, L. Dong, S. Pekarek, and S. Atcitty,
“Performance Indices for the Dynamic Performance of FACTS and
FACTS with Energy Storage,” Electric Power Component and System,
vol.33, no.3, pp.299-314, March 2005.
[20] Z. Yang, C. Shen, L. Zhang, M.L. Crow, and S. Atcitty, “Integration of a
STATCOM and battery energy storage,” IEEE Trans. Power Syst., vol.16,
no.2,pp.254-260, May 2001.
[21] Y. Cheng, C. Qian, M. L. Crow, S. Pekarek, S. Atcitty, “A Comparison of
Diode-Clamped and Cascaded Multilevel Converters for a STATCOM
with Energy Storage,” IEEE Trans. on Industrial Electronics, Vol. 53, No.
5, pp.1512-1521, 2006.
[22] Russell L. Spyker, and R. Mark Nelms, “Optimization of Double-Layer
Capacitor Arrays,” IEEE Trans. on Industry Applications, Vol.36, No.1,
pp.194-198, 2000.
Fault Location Without Controller With ECS
F1 4.89 3.44
F2 4.01 2.67
F3 4.88 3.55
Fig. 29. Fuzzy sets and their corresponding membership functions.
Output (n)
PB
NB PS
Z
00.25
1.0
-0.25
-0.5
NS
Input (e, e)
0.5
PB
NB PS
Z
0.5 0.75
1.0
0.25
0
NS
1.0
9
[23] Y. Nozaki, K. Akiyama, H. Kawaguchi, and K. Kurokawa, “An Improved
Method for Controlling an EDLC-Battery Hybrid Stand-alone
Photovoltaic Power System,” Applied Power Electronics Conference and
Exposition (APEC 2000), Vol.2, pp.781 – 786, 2000.
[24] Y. Jia, R. Shibata, N. Yamamura, and M. Ishida, “A Control Method of
Prolonging the Service Life of Battery in Stand-alone Renewable Energy
System using Electric Double Layer Capacitor (EDLC),” Power
Electronics and Drives Systems, (PEDS 2005). Vol.1, pp.228 – 233, 2005.
[25] G. Ariyoshi, K. Murata, K. Harada, and K. Yamasaki, “Load Levelling
using EDLCs under PLL control,” IEICE Trans. Fund. , Vol. E83-A, No.6,
pp. 1014-1022, 2000.
[26] L. Zubieta, and R. Bonert, “Characterization of Double-Layer Capacitors
for Power Electronics Applications,” IEEE Trans. on Industry
Applications, Vol. 36, No.1, pp.199-205, Jan. 2000.
[27] T. Kinjo, T. Senjyu, N. Urasaki, and H. Fujita, “Output levelling of
renewable energy by electric double-layer capacitor applied for energy
storage system,” IEEE Trans. on Energy Conversion, Vol. 21, Issue.1, pp.
221-227, 2006.
[28] S.M.Muyeen, Mohd. Hasan Ali, R. Takahashi, T. Murata, J.Tamura, Y.
Tomaki, A. Sakahara, and E. Sasano, “Transient Stability Analysis of Grid
Connected Wind Turbine Generator System Considering Multi-Mass
Shaft Modeling,” Electric Power Components & Systems, Vol.34, No.10,
pp.1121-1138, 2006.
[29] Working Group on Prime Mover and Energy Supply Models for System
Dynamic Performance Studies, “Dynamic Models for Fossil Fuelled Steam
Units on Power System Studies,” IEEE Trans. on Power Systems, Vol. 6,
No. 2, pp.753-761, 1991.
[30] Working Group on Prime Mover and Energy Supply Models for System
Dynamic Performance Studies, “Hydraulic Turbine And Turbine Control
Models for System Dynamic Studies,” IEEE Trans. on Power Systems,
Vol. 7, No. 12, pp.167-179, 1992.
[31] IEEE Recommended Practice for Excitation System Models for Power
System Stability Studies, IEEE Std. 421.5-1992.
[32] I. Zubia, X. Ostolaza, G. Tapia, A. Tapia, J. R. saenz, “Electrical Fault
Simulation and Dynamic Response of a Wind Farm,” Proceedings of the
IASTED International Conference Power and Energy Systems, pp.595-600,
2001.
[33] Heier, Grid Integration of Wind Energy Conversion System, Chicester,
UK, John Wiley & Sons Ltd., 1998.
[34] PSCAD/EMTDC Manual, Manitoba HVDC Research Center,
1994.
[35] M. Yagami, S. Shibata, T. Murata, and J. Tamura, “An Analysis of
Superconducting Fault Current Limiter for Stabilization of Synchronous
Generator in Multimachine System:A Two-Machine Infinite Bus System,”
IEEJ Trans. P.E., Vol.123, No.2, pp.133-142, 2003.
[36] D. Driankov, H. Hellendoorn and M. Reinfrank, An Introduction to Fuzzy
Control. Springer-Verlag, 1993.
S. M. Muyeen was born in Khulna, Bangladesh on September 08, 1975. He
received his B.Sc. Eng. Degree from Rajshahi University of
Engineering and Technology (RUET), Bangladesh and
M.Sc. Eng. Degree from Kitami Institute of Technology,
Japan, in 2000 and 2005 respectively, all in Electrical and
Electronic Engineering. Presently he is working towards his
Ph.D. degree at the Kitami Institute of Technology, Japan.
His research interests are power system, electrical machine,
FACTS, energy capacitor system (ECS), wind generator stabilization and
multi-mass drive train of wind turbine.
Rion Takahashi received the B.Sc. Eng. and Dr. Eng. Degrees from Kitami
Institute of Technology, Japan, in 1998 and 2006
respectively, all in Electrical and Electronic Engineering.
Now he is working as research assistant in Department of
Electrical and Electronic Engineering, Kitami Institute of
Technology. His major research interests include analysis of
power system transient, FACTS and wind energy conversion
system.
Mohd. Hasan Ali was born in Rajshahi, Bangladesh on June 01, 1971. He
received his B.Sc. Eng. Degree from Rajshahi University of
Engineering and Technology (RUET), Bangladesh in 1995
and M. Sc. Eng. and Dr. Eng. Degrees from Kitami Institute
of Technology, Japan, in 2001 and 2004 respectively, all in
Electrical and Electronic Engineering. Dr. Ali was a
Postdoctoral Fellow under the JSPS (Japan Society for the
Promotion of Science) Program at the Kitami Institute of
Technology, Japan since November 2004 to November 2006. Currently he is a
research professor in Electrical Engineering Department of Changwon
National University, South Korea. He is a member of the IEB, the IEE of Japan,
and the IEEE Power Engineering Society. His main field of interest includes
power system, electrical machine, intelligent control, FACTS, superconducting
magnetic energy storage (SMES) and wind energy.
Toshiaki Murata completed his Electrical Engineering Curriculum of the
Teacher Training School from Hokkaido University, Japan.
Since 1969, he had been a Research Assistant at the Kitami
Institute of Technology, Japan. He received Dr. Eng. degree
from Hokkaido University in 1991. Presently he is an
associate professor at the Kitami Institute of Technology.
Junji Tamura received his B. Sc. Eng. Degree from Muroran Institute of
Technology, Japan, in 1979 and M.Sc. Eng. and Dr. Eng.
degrees from Hokkaido University, Japan, in 1981 and 1984
respectively, all in electrical engineering. In 1984, he became
a lecturer and in 1986, an associate professor at the Kitami
Institute of Technology, Japan. Currently he is a professor at
the Kitami Institute of Technology. He is the senior member
of IEEE.
