1. IJETSE INTERNATIONAL JOURNAL OF EMERGING TECHNOLOGIES IN SCIENCES AND ENGINEERING, VOL.5, NO.3, MARCH 2012 72
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A Review of Enhancement of Transient
Stability by FACTS Devices
Rahul Somalwar and Manish Khemariya
Department of Electrical & Electronics Engg.
Datta Meghe Institute of Engg. Technology & Research, LNCT
Wardh, Bhopal
Abstract : In recent years, power demand has increased substantially while the expansion of power generation and
transmission has been severely limited due to limited resources and environmental restrictions. As a consequence, some
transmission lines are heavily loaded and the system stability becomes a power transfer-limiting factor. Flexible AC
transmission systems (FACTS) controllers have been mainly used for solving various power system steady state control
problems. However, recent studies reveal that FACTS controllers could be employed to enhance power system stability in
addition to their main function of power flow control. The literature shows an increasing interest in this subject for the
last three decades, where the enhancement of system stability using FACTS controllers has been extensively investigated.
This paper presents a comprehensive review on the research and developments in the power system stability enhancement
using FACTS Devices. In addition, some of the utility experience, real-world installations, and semiconductor technology
development have been reviewed and summarized & suggested a new technology Based on the advancement in
Semiconductor device .
or to control directly the real and reactive power flow in
I Introduction the line [6].
A unified power flow controller (UPFC) is the most
Development of effective ways to utilize promising device in the FACTS concept. It has the
transmission system to the maximum thermal ability to adjust the three control parameters, i.e. the bus
capabilities has caught much research attention in voltage, transmission line reactance, and phase angle
resent year. This is one direct outcome of the concept of between two buses, either simultaneously or
flexible A.C. transmission system (FACTS) aspects of independently. A UPFC performs this through the
which have become possible due to advances in power control of the in-phase voltage, quadrature voltage, and
electronics. shunt compensation.
Generally, the main objectives of FACTS are to The basic components of the UPFC are two voltage
increase the useable transmission capacity of lines and source inverters (VSIs) sharing a common dc storage
control power flow over designated transmission routes. capacitor, and connected to the power system through
There are two generations for realization of coupling transformers. One VSI is connected in shunt
power electronics-based FACTS controllers: the first to the transmission system via a shunt transformer,
generation employs conventional thyristor-switched while the other one is connected in series through a
capacitors and reactors, and quadrature tap-changing series transformer. A basic UPFC functional scheme is
transformers, the second generation employs gate turn- shown in fig 1
off (GTO) thyristor-switched converters as voltage
source converters (VSCs).
The first generation has resulted in the Static Var
Compensator (SVC), the Thyristor- Controlled Series
Capacitor (TCSC), and the Thyristor-Controlled Phase
Shifter (TCPS) [1;2]. The second generation has
produced the Static Synchronous Compensator
(STATCOM), the Static Synchronous Series
Compensator (SSSC), the Unified Power Flow
Controller (UPFC), and the Interline Power Flow
Controller (IPFC) [3 -5].
The two groups of FACTS controllers have distinctly
different operating and performance characteristics. For the maximum effectiveness of the controllers, the
The Voltage source converter (VSC) can be used selection of installing locations and feedback signals of
uniformly to control transmission line voltage, FACTS-based stabilizers must be investigated. On the
impedance, and angle by providing reactive shunt other hand, the robustness of the stabilizers to the
compensation, series compensation, and phase shifting, variations of power system operating conditions is

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equally important factor to be considered. Also, the contribute even further to the growth of FACTS
coordination among different stabilizers is a vital issue technology. After that A device called the Insulated
to avoid the adverse effects. Additionally, performance Gate Bipolar Transistor (IGBT) has been developed
comparison is an important factor that helps in selection with small gate consumption and small turn-on and
of a specific FACTS device.[24] turn-off times.
S.V.Ravi kumar [25] find the effect of UPFC with The IGBT has bi-directional current carrying
location capabilities. More effective use of pulse width
• The effect of UPFC is more pronounced when modulation techniques for control of output magnitude
the controller is placed near heavily disturbed and harmonic distortion can be achieved by increasing
generator. the switching frequencies to the low kHz range.
• The effect of UPFC is more pronounced when However, IGBT has until recently been restricted to
the controller is placed near faulted bus rather voltages and currents in the medium power range.
than placed at remote locations. Larger devices are now becoming available with typical
• UPFC helps in improving transient stability by ratings on the market being 3.3 kV/1.2 kA (Eupec), 4.5
improving critical clearing time. kV/2 kA (Fuji), and 5.2 kV/2 kA (ABB) [9,10].
• The transient stability is improved by The Integrated Gate Commutated thyristor (IGCT)
decreasing first swing with UPFC and SVC. combines the excellent forward characteristics of the
• SVC helps in improving transient stability by thyristors and the switching performance of a bipolar
improving critical clearing time transistor. In addition, IGCT does not require snubber
circuits and it has better turn-off characteristics, lower
Control systems for FACTS controllers may have to be conducting and switching loss, and simpler gate control
designed by using intelligent, adaptive digital compared with GTO and IGBT [11]. The ratings of
controllers based on information obtained from wide- IGCT reach 5.5 kV/1.8 kA for reverse conducting
area measurement networks. For systems using FACTS IGCTs and 4.5 kV/4 kA for asymmetrical IGCTs [12].
controllers, aiming for high levels of damping may not Currently, typical ratings of IGCTs on the market are
be a safe design goal for wide-area control. Adequate 10 kV/2.3 kA (ABB) and 6 kV/6 kA (Mitsubishi)
damping over the largest realistic range of operating The SSSC is a power electronic -based synchronous
conditions may be a more desirable criterion to fulfill Voltage source that generates three phase ac voltages of
[7]. The coordination of multiple FACTS controllers in controllable magnitude and phase angle. This voltage,
the same system as well as in the adjacent systems must which is injected in series with the transmission line, is
be investigated extensively and implemented to ensure almost in quadrature with the line current and hence
the security of power-system operation. emulates an equivalent inductive or capacitive
Nelson et al. [8] considered four FACTS controllers to reactance in series with the transmission
be evaluated and compared: the SVC, the STATCOM, line.
the TCSC, and the UPFC. The effects of different
controllers are expressed in terms of the critical clearing
time (CCT). The controller parameters are selected with
only consideration of maximizing the CCT. The CCT
obtained for the different controllers are compared.
Among the shunt controllers, the STATCOM performs
better than SVC. The TCSC is more effective than the
shunt controllers, as it offers greater controllability of
the power flow in the line.
The UPFC is by far the best controller, as it provides
independent control over the bus voltage and the line
real and reactive power flows.
II Development in FACTS Devices
The technology behind thyristor-based FACTS Fig 2 Static synchronous series compensator
controllers has been present from several decades and is
therefore considered mature. More utilities are likely to When the series injected voltage leads the line current,
adopt this technology in the future as more promising it emulates an inductive reactance causing the power
GTO-based FACTS technology is fast emerging. flow and the line current to decrease. When the line
Further advances in silicon power-switching devices current leads the injected voltage it emulates a
that significantly increase their power ratings will capacitive reactance thereby enhancing the power flow

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over the line. The basic schematic diagram of the static The ideal power switch would switch like an IGBT and
synchronous series compensator with its test system is conduct like a GTO thyristor, and it would have the low
shown in Figure 2. [21] fabrication costs and high yields of the GTO thyristors.
High performance and cost effective high power This is exactly what the IGCT achieves The IGCT has
voltage source inverters (VSI) are a prerequisite for the become the power semiconductor of choice in Medium
realization of SSSC. Since conventional two-pulse Voltage Industrial Applications. Also in the Energy
inverters are not available with higher ratings, multi- Management and the Traction market the versatility of
pulse inverters with higher operating range are used to this power switch has enabled performance
cater the need in SSSC. [22] These multi-pulse improvements and cost savings in a variety of
inverters can be operated at lower switching applications.[20] IGCT are currently being applied to
frequencies, generating symmetrical output voltages such devices as: Medium Voltage Drives (current and
having very low harmonic components. 48-Pulse voltage source), Circuit Breakers, Super-conducting
inverter can be used in high power FACTS controllers, Magnetic Energy Storage Systems (SMES), Dynamic
without AC filters due to its high performance and low Voltage Restorers , STATCOMs, Dynamic
harmonic rate on the AC side. 48-Pulse inverters are Uninterruptible Power Supplies, Power Conditioners,
obtained by combining eight 6-Pulse VSIs with an Induction Heaters, Traction inverters and choppers.
adequate phase shifts between them. Each of the VSI
needs a coupling transformer of which four of them IV Design Rules
require a Y-Y transformer with a turns ratio of 1:1 and
the remaining four require a Delta - Star with a turns A New proposed FACTS control device under
ratio of 1:√3. The output of the phase shifting testing process the, the most important design steps are
transformers is connected in series to cancel out the • The diode turn-off di/dt capability mostly
lower order harmonics.[23,26] determines the size of the di/dt choke.Li ≥
(Vdcmax/(di/dtmax))
III Advance FACTS controller A bigger choke might be chosen in order to
limit switching losses of the diode or to limit
From the very beginning, the development of the surge current stress during shoot-through
power semiconductors was nothing more than a search in a phase leg
for the ideal switch. The lowest on-state and • The clamp circuit parameters, Ccl, and Rs, can
commutation losses, the highest possible commutation be determined after solving the 2nd order
frequency and a simple drive circuit. Power silicon differential equation for the damped parallel
switches have increased steadily in complexity and resonance circuit Li, Ccl, Rs and optimising
capability.[13,14] The first silicon-controlled rectifiers the damping factor to force the diode clamping
could switch power off only at the end of an AC cycle. current to zero as fast as possible without
From the transistor and Darlington to the IGBT, low- succeeding oscillations of significant
voltage applications have benefited all the way along amplitudes, Thereby the influence of the stray
while the medium-voltage user could only look on — inductance, Ls2, of the loop Rs – Ccl– Cdc on
GTO’s and more GTO’s, nothing else . The the voltage overshoot has to be accounted for.
introduction of IGBTs brought faster switching, but at • Safe operating area and turn-off switching
present their switching losses are acceptable only at low losses are valid for a stray inductance value
voltage levels. GTO thyristors consist of thousands of equal to or less than the data sheet value, Lcl
individual switching elements fabricated on a silicon .For applications with higher Lcl values, safe
wafer. Losses occur in all four conditions of operation operating area and switching losses must be
(on, off, switching on, switching off). At medium rechecked.
voltage, GTO’s exhibit very low on-state losses and
reasonable turn-off losses. However, due to switching
V Conclusion
being non-homogeneous, external snubber circuits are
necessary for the switching operation. These snubber
circuits take up more than half the volume of the final In this review, the current status of power system
equipment and account for much of the design stability enhancement using FACTS controllers was
complexity, costs and losses. [19] discussed and scrutinized. The essential features of
The tendency over the years has been for the designers FACTS controllers and their potential to enhance
of all these devices to concentrate mainly on the power system stability was addressed [15 –18]. The location
switching itself, so that little attention has been paid to and feedback signals used for design of FACTS-based
the complexities involved in real-world applications. damping controllers were discussed.

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The coordination problem among different control 450
O n e - m a c h i n e s y s t e m s w i n g c u r ve . F a u l t c l e a r e d a t 0 . 1 3 s
schemes was also considered. Performance comparison 400
of different FACTS controllers has been reviewed. The 350
likely future direction of FACTS technology, especially 300
Delta, degree
in restructured power systems, was discussed as well. In 250
addition, utility experience and major real-world 200
installations and semiconductor technology 150
development have been summarized.
100
50
To investigate the effectiveness of the FACTS 0
controller under different fault condition the equal area 0 0 .2 0 .4 0 .6
t, s e c
0 .8 1 1 .2 1 .4
criteria is applied and simulation is conducted. System is unstable at Pm = 0.91
The result of control algorithm is simple and straight 120
O n e - m a c h in e s y s t e m s w in g c u r ve . F a u lt c le a r e d a t 0 . 1 3
forward that 110
• Without FACTS controller the power system 100
loses synchronism at particular shaft power Ps 90
• With FACTS controller the power system is
Delta, degree
80
stabilized at particular shaft power Ps. 70
Utility of FACTS controller for transmission stability
60
50
enhancement and damping of large oscillation has been 40
elaborated. 30
The stability investigation has been already carried out
0 0.2 0.4 0 .6 0.8 1 1 .2 1.4
t, s ec
and the system is found to be stable after introduction System stable by introducing angle Alpha by Static
of phase shift by FACTS controller . The investigation Phase shifter on same critical time.
has been done by analytical ,graphical & numerical
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