International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
Miltimachine Power System Stability Enhancement using
Different FACTS Devices
Mr.M.M.Khan#1, Mr.Tanveer Husain#2, Mr.M.M.Ansari#3,
1,2
M.E (EPS) (Pursuing),3Assistant Professor 1,2,3Department of Electrical Engineering,
1,2,3
S.S.B.T‘s COET Bambhori, Jalgaon, India,
Abstract.- Modern Power Transmission networks are
becoming increasingly stressed due to increasing
demand of electricity and restrictions on building new
transmission system. Decreasing power system
stability is one of the major problems of such a
stressed system following a disturbance. Flexible ac
transmission system (FACTS) devices are found to be
very effective in a transmission network for better
utilization of its existing facilities without loss of the
desired stability. The static synchronous compensator
(STATCOM) and Static Var Compensator (SVC) are
the shunt devices of the flexible AC transmission
systems (FACTS) family. When system voltage is low,
STATCOM generates reactive power and system
voltage is high that’s time itsabsorb reactive power
while Static Var Compensation is recover the loss of
stability at the time of fault.
Keywords: Flexible AC Transmission Systems
(FACTS), FACTS Controllers, SVC, TCSC, SSSC,
STATCOM, Power Systems, Power System Stability,
Oscillatory Stability, Small Signal Stability, Transient
Stability, Voltage Stability.
I.
INTRODUCTION
The available power generating plants are often
located at distant locations for economic,
environmental and safety reasons. For instance, it
becomes cheaper to install a thermal power station at
pit-head instead of transporting coal to load centres.
Hydro power is generally available in remote areas
and a nuclear plant may be located at a place away
from urban areas. Additionally, modern power
systems are highly interconnected. Sharing of
generation reserves, exploiting load diversity and
economy gained from the use of large efficient units
without sacrificing reliability are the advantages of
interconnection. Thus power must consequently be
transmitted over long distances. To meet the load and
electric market demands, new lines should be added to
the system, butdue to environmental reasons, the
installation of electric power transmission lines are
often restricted.
The power system may be thought of as a nonlinear
system with many lightly damped electromechanical
modes of oscillation. The three modes of
electromechanical oscillations are:
Local plant mode oscillations
Inter-area mode oscillations
Torsional modes between rotating plant
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In local mode, one generator swings against the rest of
the system at 1.0 to 2.0 Hz. The impact of the
oscillation is localized to the generator and the line
connecting it to the grid.
Inter-area mode of oscillations is observed over a
large part of the network. It involves two coherent
groups of generators swinging against each other at
1Hz or less. This complex phenomenon involves
many parts of the system with highly non-linear
dynamic behavior.
Torsional mode oscillations is associated with a
turbine generator shaft system in the frequency range
of 10-45 Hz. Usually these modes are excited when a
multi-stage turbine generator is joint to the grid
system through a series compensated line.
If the damping of these modes becomes too short, it
can impose severe constraints on the system ‟ s
operation. It is thus important to be able to determine
the nature of those modes, find stability limits and in
many cases use controls to prevent instability. The
poorly damped low frequency electromechanical
oscillations come due to inadequate damping torque in
some generators, causing both local-mode oscillations
and inter-area oscillations (0.2 Hz to 2.5 Hz) [1], [2].
The traditional approach employs power system
stabilizers (PSS) on generator excitation control
systems in order to damp those oscillations. PSSs are
effective but they are usually designed for damping
local modes. In large power systems, they may not
provide enough damping for inter-area modes. So,
more efficient substitutes are needed other than PSS.
In late 1980s, the Electric Power Research Institute
(EPRI) had introduced a new technology program
known as Flexible AC Transmission System (FACTS)
[3]. The main idea behind this program is to increase
controllability and optimize the utilization of the
existing power system capacities by reliable and highspeed power electronic devices. Thelatest generation
of FACTS controllers is based on the concept of the
solid state synchronous voltage sources (SVSs)
introduced by L. Gyugyi in the late 1980s [4].
II.
REVIEW OF LITERATURE
FACTS Controllers
FACTS are defined as “a power electronic based
system and other static equipment that provide control
of one or more AC transmission system parameters to
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
enhance controllability and increase power transfer
capability. Basically, FACTS controllers can be
divided into four categories [7].
1) Series Controller
2) Shunt Controller
3) Combined series-series Controller
4) Combined series-shunt Controller
Table 1: Comparison among FACTS Controllers
Controller
Name
Type
Purpose
Used
Voltage
Shunt
Thyristor
SVC
Control
Power
Series
GTO
Flow
SSSC
Control
Voltage
Shunt
GTO
STATCOM
Control
Voltage
Shunt
and Power
and
GTO
UPFC
Flow
Series
Control
Power
Series
Thyristor
Flow
TCSC
Control
Shunt
Power
and
Thyristor
Flow
TCPAR
series
Control
In the late 1980s, the Electric Power Research
Institute (EPRI) formulated the vision of the FACTS
in which various power-electronics based controllers
regulate power flow and transmission voltage, and
they mitigate dynamic disturbances. Generally, the
main objectives of FACTS are to increase the useable
transmission capacity of lines and control power flow
over designated transmission routes.
A. First generation FACTS
First generation FACTS employs capacitor and
reactor banks with fast solid-state switches in
traditional shunt or series circuit arrangements. The
thyristor switches control the on and off periods of the
fixed capacitor and reactor banks and thereby realize a
variable reactive impedance. Except for losses, they
cannot exchange real power with the system.
Static VAR Compensator (SVC)
The SVC is a reactive shunt device that uses its
reactive capability to alter the bus voltage. It enables a
regulated voltage support. An SVCfor continuous
control contains a thyristor switched capacitor bank in
parallel with a bank of phase angle controlled reactors
and is connected to the transmission voltage level via
a transformer. The SVCinfluences electro-mechanical
oscillations like the PSS: itchanges the line transfer
(by controlling V)as well as modulates voltage
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sensitive loads. Depending on which of these effects
dominate, the SVC is placed either at the midpoint of
a long transmission line or near the load centre.
It is known that the SVCs with an auxiliary
injection of a suitable signal can considerably improve
the dynamic stability performance of a power system
[5]. In the literature, SVCs have been applied
successfully to increase the transient stability of a
synchronous machine [5]. Hammad [6] presented a
fundamental analysis of the application of SVC for
enhancing the power systems stability.
Thyristor-Swiched Bank of three
individiul
Reactorcapacitor
Fig.1:SVC model
B. Second generation of FACTS
The technologies described above are in operation
today, but new power electronic devices with a
potential for damping of electro mechanical
oscillations are constantly suggested. The voltage
source converter (VSC) type FACTS controller group
10 employs self-commutated DC to AC converters,
using GTO thyristors, which can internally generate
capacitive and inductive reactive power for
transmission line compensation, without the use of
capacitor or reactor banks. The converter with energy
storage device can also exchange real power with the
system in addition to the independently controllable
reactive power. The VSC can be used uniformly to
control transmission line voltage, impedance, and
angle by providing reactive shunt compensation, series
compensation, and phase shifting, or to control
directly the real and reactive power flow in the line
[8].
Static Synchronous Compensator (STATCOM)
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The emergence of FACTS devices and in particular
GTO thyristor-based STATCOM has enabled such
technology to be proposed as serious competitive
alternatives to conventional SVC [9]. From the
viewpoint of power system dynamic stability, the
STATCOM provides better damping characteristics
than the SVC as it is able to transiently exchange
active power with the system. The effectiveness of the
STATCOM to control the power system voltage was
presented [10]. However, the effectiveness of the
STATCOM to enhance the angle stability has not been
addressed. Abido [11] presented a singular value
decomposition (SVD) based approach to assess and
measure the controllability of the poorly damped
electromechanical modes by STATCOM different
control channels. It was observed that the
electromechanical modes are more controllable via
phase modulation channel. It was also concluded that
the STATCOM-based damping stabilizers extend the
critical clearing time and enhance greatly the power
system transient stability. Haque [12] demonstrated
that by the use of energy function, a STATCOM can
to provide additional damping torque to the low
frequency oscillations in a power system.
1990-94
1995-99
2000-04
2005-09
160
140
120
100
80
60
40
20
0
SVC
TCSC
STATCOM
SSSC
Fig.3Statistics for FACTS applications to power
systemstability
Table.2 Performance Analysis of FACTS devices
[13]
III.
POWER SYSTEM STABILITY
Stability is nothing but ability of power system to
remain in synchronism when different fault occur in
power system network such as symmetrical and
unsymmetrical fault of power system.
Fig.2:STATCOM Model
Table.2 shows the performance analysis of FACTS
devices. First column of Table .2 shows that series
compensator is good for load flow control. Second
column of the same table shows shunt compensator is
good for voltage stability. Third column of Table.2
shows that all FACTS devices are good enough for the
case of transient stability. A combination of shunt and
series can better perform load flow control, voltage
stability and transient stability. From the above
analysis it is clear that UPFC is one of the most
promising devices in FACTS concept.
ISSN: 2231-5381
Power system stability is classified into three
categories
A. Rotor Angle Stability
B. Voltage Stability
C. Frequency Stability
A.
Rotor Angle Stability
Rotor angle stability is the ability of
synchronous machines of an interconnected
power system to remain in synchronism after
being subjected to a disturbance. It depends on
the ability to maintain/restore equilibrium
between
electromagnetic
torque
and
mechanical torque of each synchronous
machine in the system. Instability that may
result occurs in the form of increasing angular
swings of some generators leading to their loss
of synchronism with other generators.
The rotor angle stability problem involves the
study of the electromechanical oscillations
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UPFC
International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
inherent in power systems. A fundamental
factor in this problem is the manner in which
the power outputs of synchronous machines
vary as their rotor angles change. Under
steady-state conditions, there is equilibrium
between the input mechanical torque and the
outputelectromagnetic torque of each generator,
and the speed remains constant.
For convenience in analysis and for gaining
useful insight into the nature of stability
problems, it is useful to characterize rotor
angle stability in terms of the following two
subcategories: • Small-disturbance (or smallsignal) rotor angle stability is concerned with
the ability of the power system tomaintain
synchronism under small disturbances. The
disturbances are considered to be sufficiently
small that linearization of
system equations is permissible for purposes of
analysis [14].
For convenience in analysis and for gaining useful
insight into the nature of stability problems, it is useful
to characterize rotor angle stability in terms of the
following two subcategories:
Small-disturbance (or small-signal) rotor
angle stability is concerned with the ability of
the power system to maintainsynchronism
under
small
disturbances.
The
disturbancesare considered to be sufficiently
small that linearizationof system equations is
permissible for purposesof analysis.
Large-disturbance rotor angle stability or
transient stability, as it is commonly referred
to, is concerned with the ability of the power
system to maintain synchronism when
subjected to a severe disturbance, such as a
short circuit on a transmission line. The
resulting system response involves large
excursions of generator rotor angles and is
influenced by the nonlinear power-angle
relationship.
B
Voltage Stability
Voltage stability is the ability of a power system to
maintain steady voltages at all buses in the system
after being subjected to a disturbance from a given
initial operating condition.
It depends on the ability to maintain/restore
equilibrium between load demand and load supply
from the power system. Instability that may result
occurs in the form of a progressive fall or rise of
voltages of some buses. A possible outcome of voltage
instability is loss of load in an area, or tripping of
transmission lines and other elements by their
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protective systems leadingto cascading outages. Loss
of synchronism of some generatorsmay result from
these outages or from operating conditions thatviolate
field current limit [15].
C. Frequency Stability
Frequency stability is the ability of a power system to
maintain steady frequency following a severe system
upset resulting in a significant imbalance between
generation and load. It depends on the ability to
maintain/restore
equilibrium
between
system
generation and load, with minimum unintentional loss
of load. Instability that may result occurs in the form
of sustained frequency swings leading to tripping of
generating units and/or loads.
FACTS Applications to Deregulated Electricity
Market
Nowadays, electricity demand is rapidly increasing
without major reinforcement projects to enhance
power transmission networks. Also, the electricity
market is going toward open market and deregulation
creating an environment for forces of competition and
bargaining. Electricity utilities are in need to serve
more loads through their networks and also maintain
the system security. FACTS devices can be an
alternative to reduce the flows in heavily loaded lines,
resulting in increased load ability, low system loss,
improved stability of the network, reduced cost of
production, and fulfilled contractual requirements by
controlling the power flows in the network. Generally,
the changing nature of the electricity supply industry
is introducing many new subjects into power system
operation relatedto trading in a deregulated
competitive market
IV.
CONCLUSION
In this review, the present condition of power system
stability enhancement using FACTS controllers was
discussed and summarized. The essential features of
FACTS controllers and their potential to enhance
system stability was addressed. The location and
feedback signals used for design of FACTS-based
damping controllers were discussed.
The coordination problem among different control
schemes and damping oscillation of system was also
considered. Performance comparison of different
FACTS controllers has been reviewed. The likely
future direction of FACTS technology, especially in
restructured power systems, was discussed as well.
Inaddition, utility experience and major realworldinstallations
and
semiconductor
technologydevelopment have been summarized. A
more review ofFACTS applications to optimal power
flow andderegulated electricity market has been
presented.
ACKNOWLEDGMENT
It takes for me as a matter of pleasure to express my
sincere thanks to Mr.M.M.ANSARIfor his guidance
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International Conference on Global Trends in Engineering, Technology and Management (ICGTETM-2016)
and suggestions during the completion of this paper.I
don’t have words to express my feelings for his timely
guidance and the help for material collection, books
for the seminar-III report.
I am also thankful to Dr. P. J. SHAH
(ASSOCIATE PROF. & HEAD of Dept. Electrical
Engg.) for his instructions provided to me during the
completion of the paper. Last, but not the least, those
are parents and friends who are being a source of
constant inspiration and encouragement for me to
carry out any work.
[6]
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enhancement by static VAR compensators”, IEEE
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1986.
[7]
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