The electricity supply industry is undergoing a profound transformation worldwide. Market forces, scarcer natural resources, and an ever-increasing demand for electricity are some of the drivers responsible for such unprecedented change. Against this background of rapid evolution, the expansion programs of many utilities are being thwarted by a variety of well-founded, environment, land-use, and regulatory pressures that prevent the licensing and building of new transmission lines and electricity generating plants.

1. A
SEMINAR REPORT
“FLEXIBLE AC TRANSMISSION SYSTEM”

2. INDEX
SR.NO. CHAPTER NAME PAGE
NO.
1 1.INTRODUCTION 1
2 2.TYPES OF FACTS CONTROLLERS 3
2.1series Controller
2.2 Shunt Controllers
2.3 Combined Series –Series Controllers
2.4 Combined Series-Shunt Controllers
2.5 Benefits Of Facts Controllers
3 3. CONTROL OF POWER SYSTEM 11
4
5
6
3.1 Generation ,Transmission ,Distribution
3.2 Power System Constraints
3.3 Controllability Of Power System
3.4 Benefits Of Control Of Power System
3.5 Benefits Of Utilising Facts Devices
3.6 Classification
3.6.1 First Generation Of Facts
3.6.11 Static Var Compensator
3.6.12 Thyristor Controlled Series Capacitor
3.6.2 Secind Generation Facts
3.6.21static Compensation
3.6.22 Static Synchronous Series Compensator
3.6.23 Unified Power Flow Controller
APPLICATIONS
4.1facts Application To Steady State Power System Problems
4.2facts Application To Optimal Power Flow
CONCLUSION
REFERENCES
19
20
21

3. LIST OF FIGURES
Sr.No. Fig. No. Name Of Figure Page No.
1 Fig.2.1 Single line diagram of series compensator 3
2 Fig.2.2 Diagram of parallel compensator 4
3 Fig .3.1 Block diagram of power system 11
4 Fig. 3.2 Single line diagram and phasordiagram 13
5 Fig.3.3 Unified power flow controller 17

4. CHAPTER 1: INTRODUCTION
The electricity supply industry is undergoing a profound transformation worldwide.
Market forces, scarcer natural resources, and an ever-increasing demand for electricity are some
of the drivers responsible for such unprecedented change. Against this background of rapid
evolution, the expansion programs of many utilities are being thwarted by a variety of well-
founded, environment, land-use, and regulatory pressures that prevent the licensing and building
of new transmission lines and electricity generating plants.
The ability of the transmission system to transmit power becomes impaired by one or
more of the following steady state and dynamic limitations: (a) angular stability, (b) voltage
magnitude, (c) thermal limits, (d) transient stability, and (e) dynamic stability. These limits
define the maximum electrical power to be transmitted without causing damage to transmission
lines and electrical equipment .In principle, limitations on power transfer can always be relieved
by the addition of new transmission lines and generation facilities. Alternatively, flexible
alternating current transmission system (FACTS) controllers can enable the same objectives to
be met with no major alterations to power system layout. FACTS are alternating current
transmission systems incorporating power electronic-based and other static controllers to
enhance controllability and increase power transfer capability
.
The FACTS concept is based on the substantial incorporation of power electronic
devices and methods into the high-voltage side of the network, to make it electronically
controllable. FACTS controllers aim at increasing the control of power flows in the high-
voltage side of the network during both steady state and transient conditions. The concept
of FACTS as a total network control philosophy was introduced in 1988 by Dr. N.
Hingorani .Owing to many economical and technical benefits it promised, FACTS received
the support of electrical equipment manufacturers, utilities, and research organizations
around the world. This interest has led to significant technological developments of
FACTS controllers. Several kinds of FACTS controllers have been commissioned in
various parts of the world. The most popular are: load tap changers, phase-angle regulators,

5. static VAR compensators, thyristor controlled series compensators, interphase power
controllers, static compensators, and unified power flow controllers.
In this paper, the state of the art in the development of FACTS controllers is
presented. The paper presents the objectives, the types, and the benefits of FACTS
controllers. Moreover, various FACTS controllers are described, their control attributes are
presented, and their role in power system operation is analyzed.
Objectives of FACTS controllers
The main objectives of FACTS controllers are the following:
1. Regulation of power flows in prescribed transmission routes.
2. Secure loading of transmission lines nearer to their thermal limits.
3. Prevention of cascading outages by contributing to emergency control.
4. Damping of oscillations that can threaten security or limit the usable line capacity.

6. CHAPTER 2: TYPES OF FACTS CONTROLLERS
2.1 Series controllers
The series controller could be a variable impedance, such as capacitor,
reactor, or a power electronics based variable source of main frequency, sub-synchronous
and harmonic frequencies (or a combination) to serve the desired load. In principle, all
series controllers inject voltage in series with the line. As long as the voltage is in phase
quadrature with the line current, the series controller only supplies or consumes variable
reactive power. Any other phase relationship will involve handling of real power as well.
Series controllers include SSSC, IPFC, TCSC, TSSC, TCSR, and TSSR.
Fig2.1: Single line diagram of series compensator.
2.2 Shunt controllers.
As in the case of series controllers, the shunt controllers may be variable
impedance, variable source, or a combination of these. In principle, all shunt controllers
inject current into the system at the point of connection. Even a variable shunt impedance
connected to the line voltage causes a variable current flow and hence represents injection

7. Even a variable shunt impedance connected to the line voltage causes a variable current
flow and hence represents injection of current into the line. As long as the injected current
is in phase quadrature with the line voltage, the shunt controller only supplies or consumes
reactive power. Any other phase relationship will involve handling of real power as well.
.
Fig2.2: Diagram of parallel compensator
2.3 Combined series-series controllers:-
This could be a combination of separate series controllers, which are
controlled in a coordinated manner, in a multiline transmission system. Or it could be a
unified controller in which series controllers provide independent series reactive
compensation for each line but also transfer real power among the lines via the proper link.
The real power transfer capability of the unified series-series controller, referred to as
IPFC, makes it possible to balance both real and reactive power flow in the lines and
thereby maximize the utilization of the

8. transmission system. The term “unified” here means that the dc terminals of all controller
converters are all connected together for real power transfer.
2.4 Combined series-shunt controllers:-
This could be a combination of separate shunt and series
controllers, which are controlled in a coordinated manner, or a UPFC with series and shunt
elements. In principle, combined shunt and series controllers inject current into the system
with the shunt part of the controller and voltage in series in the line with the series part of
the controller. However, when the shunt and series controllers are unified, there can be a
real power exchange between the series and shunt controllers via the proper link.
Combined series-shunt controllers include UPFC, TCPST, and TCPA
FACTS CONTROLLERS
STATCOM:
STATCOM is a static synchronous generator operated as a shunt-
connected static var compensator whose capacitive or inductive output current can
be controlled independent of the ac system voltage. The use of STATCOM as a
FACTS controller is proposed .
SVC:
SVC is a shunt-connected static var generator or absorber whose output is
adjusted to exchange capacitive or inductive current so as to maintain or control
specific parameters of the electrical power system (typically bus voltage). SVC is
an important FACTS controller already widely in operation. Ratings range from
60 to 600 MVAR , SVC can be considered as a “first generation” FACTS
controller and uses thyristor controllers. It is a shunt reactive compensation
controller consisting of a combination of fixed capacitor or thyristor-switched
capacitor in conjunction with thyristor-controlled reactor.

9. TCR:
TCR is a shunt-connected thyristor-controlled inductor whose effective
reactance is varied in a continuous manner by partial-conduction control of the
thyristor valve. TCR has been used as one of the economical alternatives of
FACTS controllers .
TSC:
TSC is a shunt-connected thyristor-switched capacitor whose effective
reactance is varied in a stepwise manner by full- or zero-conduction operation of
the thyristor valve .
TSR:
TSR is a shunt-connected thyristor-switched inductor whose effective
reactance is varied in a stepwise manner by full- or zero-conduction operation of
the thyristor valve .
TCBR:
TCBR is a shunt-connected thyristor-switched resistor, which is controlled to
aid stabilization of a power system or to minimize power acceleration of a
generating unit during a disturbance
.
SSSC:
SSSC is a static synchronous generator operated without an external electric
energy source as a series compensator whose output voltage is in quadrature with,
and controllable independently of, the line current for the purpose of increasing or
decreasing the overall reactive voltage drop across the line and thereby
controlling the transmitted electric power . The SSSC may include transiently
rated energy storage or energy absorbing devices to enhance the dynamic
behavior of the power system by additional temporary real power compensation,
to increase or decrease momentarily, the overall real (resistive) voltage drop
across the line.

10. TCSC:
TCSC is a capacitive reactance compensator, which consists of a series
capacitor bank shunted by a thyristor-controlled reactor in order to provide a
smoothly variable series capacitive reactance. The description of the first TCSC
installation is given .
TSSC:
TSSC is a capacitive reactance compensator, which consists of a series
capacitor bank shunted by a thyristor-switched reactor to provide a stepwise
control of series capacitive reactance.
TCSR:
TCSR is an inductive reactance compensator, which consists of a series
reactor shunted by a thyristor-controlled reactor to provide a smoothly variable
series inductive reactance .
TSSR:
TSSR is an inductive reactance compensator, which consists of a series
reactor shunted by a thyristor-controlled reactor to provide a stepwise control of
series inductive reactance .
TCPST:
TCPST is a phase-shifting transformer adjusted by thyristor switches to
provide rapidly variable phase angle . This controller is also referred to as
TCPAR.
UPFC:
UPFC is a combination of STATCOM and a SSSC which are coupled via a
common dc link to allow bidirectional flow of real power between the series
output terminals of the SSSC and the shunt output terminals of the STATCOM

11. and are controlled to provide concurrent real and reactive series line
compensation without an external electric energy source. The UPFC, by means of
angularly unconstrained series voltage injection, is able to control, concurrently or
selectively, the transmission line voltage, impedance, and angle or, alternatively,
the real and reactive power flow in the line. The UPFC may also provide
independently controllable shunt reactive compensation. The UPFC proposed by
Gyugyi is the most versatile FACTS controller for the regulation of voltage and
power flow in a transmission line.
GUPFC:
GUPFC can effectively control the power system parameters such as bus
voltage, and real and reactive power flows in the lines . A simple scheme of
GUPFC consists of three converters, one connected in shunt and two connected in
series with two transmission lines terminating at a common bus in a substation. It
can control five quantities, i.e., a bus voltage and independent active and reactive
power flows in the two lines. The real power is exchanged among shunt and series
converters via a common dc link.
IPC:
IPC is a series-connected controller of active and reactive power consisting, in
each phase, of inductive and capacitive branches subjected to separately phase-
shifted voltages. The active and reactive power can be set independently by
adjusting the phase shifts and/or the branch impedances, using mechanical or
electronic switches. In the particular case where the inductive and capacitive
impedance form a conjugate pair, each terminal of the IPC is a passive current
source dependent on the voltage at the other terminal and the practical design
aspects of a 200 MW prototype for the interconnection of the 120 kV networks
were described. However, the original concept proposed has undergone
modifications that are described
.

12. TCVL:
TCVL is a thyristor-switched metal-oxide varistor used to limit the voltage
across its terminals during transient conditions .
TCVR:
TCVR is a thyristor-controlled transformer that can provide variable in-
phase voltage with continuous control.
IPFC:
IPFC is a combination of two or more SSSCs that are coupled via a common
dc link to facilitate bi-directional flow of real power between the ac terminals of
the SSSCs and are controlled to provide independent reactive compensation for
the adjustment of real power flow in each line and maintain the desired
distribution of reactive power flow among the lines. The IPFC structure may also
include a STATCOM, coupled to the IPFC common dc link, to provide shunt
reactive compensation and supply or absorb the overall real power deficit of the
combined SSSCs.
2.5 Benefits Of Facts Controllers:
FACTS controllers enable the transmission owners to obtain, on a case-by-
case basis, one or more of the following benefits:
2.5.1Cost:
Due to high capital cost of transmission plant, cost considerations
frequently Over weigh all other considerations. Compared to alternative methods
of solving transmission loading problems, FACTS technology is often the most
economic alternative.
2.5.2 Convenience:
All FACTS controllers can be retrofitted to existing ac transmission plant with

13. varying degrees of ease. Compared to high voltage direct current or six-phase
transmission schemes, solutions can be provided without wide scale system
disruption and within a reasonable timescale.
2.5.3 .Environmental impact:
In order to provide new transmission routes to supply an ever increasing
worldwide demand for electrical power, it is necessary to acquire the right to
convey electrical energy over a given route. It is common for environmental
opposition to frustrate attempts to establish new transmission routes. FACTS
technology, however, allows greater throughput over existing routes, thus meeting
consumer demand without the construction of new transmission lines. However,
the environmental impact of the FACTS device itself may be considerable.
2.5.4. Control of power flow to follow a contract, meet the utilities own needs,
ensure optimum power flow, minimize the emergency conditions, or a
combination thereof.
2.5.5. Contribute to optimal system operation by reducing power losses and
improving voltage profile.
2.5.6. Increase the loading capability of the lines to their thermal capabilities,
including short term and seasonal.
2.5.7 Increase the system security by raising the transient stability limit, limiting
short-circuit currents and overloads, managing cascading blackouts and damping
electromechanical oscillations of power systems and machines.
2.5.8. Provide secure tie line connections to neighboring utilities and regions
thereby decreasing overall generation reserve requirements on both sides.

14. CHAPTER 3: CONTROL OF POWER SYSTEMS
3.1 Generation, Transmission, Distribution
In any power system, the creation, transmission, and utilization of
electrical power can be separated into three areas, which traditionally determined
the way in which electric utility companies had been organized. These are
illustrated in Figure 1 and are:
• Generation
• Transmission
• Distribution
Fig 3.1 : Block Diagram of power system
Although power electronic based equipment is prevalent in each of these three
areas, such as with static excitation systems for generators and Custom Power
equipment in distribution systems, the focus of this paper and accompanying
presentation is on transmission, i.e. moving the power from where it is generated
to where it is utilized.
3.2 Power SystemConstraints
As noted in the introduction, transmission systems are being pushed closer
to their stability and thermal limits while the focus on the quality of power
delivered is greater than ever. The limitations of the transmission system can take
many forms and may involve power transfer between areas or within a single area
or region and may include one or more of the following characteristics:
• Steady-State Power Transfer Limit

15. CHAPTER 4:APPLICATIONS
4.1 Facts Applications To Steady State Power System Problems
For the sake of completeness of this review, a brief overview of the
FACTS devices applications to different steady state power system problems is presented
in this section. Specifically, applications of FACTS in optimal power flow and deregulated
electricity market will be reviewed.
4.2 FACTS Applications to Optimal Power Flow
In the last two decades, researchers developed new algorithms for solving the
optimal power flow problem incorporating various FACTS devices . Generally in power
flow studies, the thyristor controlled FACTS devices, such as SVC and TCSC, are usually
modeled as controllable impedance controllable sources. The Interline Power Flow
Controller (IPFC) is one of the voltage source converter(VSC) based FACTS Controllers
which can effectively manage the power flow via multi-line Transmission System.
•Better utilization of existing transmission system assets
•Increased transmission system reliability and availability
•Increased dynamic and transient grid stability and reduction of loop
flows
•Increased quality of supply for sensitive industries
• Voltage Stability Limit
• Dynamic Voltage Limit
• Transient Stability Limit
• Power System Oscillation Damping Limit
• Inadvertent Loop Flow Limit
• Thermal Limit
• Short-Circuit Current Limit
• Others

16. Each transmission bottleneck or regional constraint may have one or more of
these system-level problems. The key to solving these problems in the most cost-
effective and coordinated manner is by thorough systems engineering analysis.
3.3 Controllability of Power Systems
To illustrate that the power system only has certain variables that can be
impacted by control, we have considered here the power-angle curve, shown in
Figure 2. Although this is a steady-state curve and the implementation of FACTS
is primarily for dynamic issues, this illustration demonstrates the point that there
are primarily three main variables that can be directly controlled in the power
system to impact its performance. These are:
• Voltage
• Angle
• Impedance
Fig 3.2: Single line diagram with phasor diagram
3.4 Benefits of Control of Power Systems
Once power system constraints are identified and through system
studies viable solutions options are identified, the benefits of the added power
system control must be determined. The following offers a list of such benefits:
• Increased Loading and More Effective Use of Transmission Corridors

17. • Added Power Flow Control
• Improved Power System Stability
• Increased System Security
• Increased System Reliability
• Added Flexibility in Starting New Generation
• Elimination or Deferral of the Need for New Transmission Lines
3.5 Benefits of utilizing FACTS devices
The benefits of utilizing FACTS devices in electrical transmission systems can be
summarized as follows
•Better utilization of existing transmission system assets
•Increased transmission system reliability and availability
•Increased dynamic and transient grid stability and reduction of loop flows
•Increased quality of supply for sensitive industries
3.6 Classification
There are different classifications for the FACTS devices: Depending on the
type of connection to the network FACTS devices can differentiate four
categories
• serial controllers
• derivation controllers
• serial to serial controllers
• serial-derivation controllers
Depending on technological features, the FACTS devices can be divided into two
generations
• first generation: used thyristors with ignition controlled by gate(SCR).
. second generation: semiconductors with ignition and extinction
controlled by gate (GTO´s , MCTS , IGBTS , IGCTS , etc).

18. 3. 6.1 First Generation Of Facts
3.6.2 Static VAR Compensator (SVC)
A static VAR compensator (or SVC) is an electrical device for providing fast-
acting reactive power on high-voltage electricity transmission networks. SVCs are
part of the Flexible AC transmission system device family, regulating voltage and
stabilizing the system. The term "static" refers to the fact that the SVC has no
moving parts (other than circuit breakers and disconnects, which do not move
under normal SVC operation). Prior to the invention of the SVC, power factor
compensation was the preserve of large rotating machines such as synchronous
condensers. The SVC is an automated impedance matching device, designed to
bring the system closer to unity power factor
3. 6.3 .Thyristor-Controlled Series Capacitor (TCSC)
TCSC controllers use thyristor-controlled reactor (TCR) in parallel with
capacitor segments of series capacitor bank. The combination of TCR and
capacitor allow the capacitive reactance to be smoothly controlled over a wide
range and switched upon command to a condition where the bi-directional
thyristor pairs conduct continuously and insert an inductive reactance into the
line. TCSC is an effective and economical means of solving problems of transient
stability, dynamic stability, steady state stability and voltage stability in long
transmission lines. TCSC, the first generation of FACTS, can control the line
impedance through the introduction of a thyristor controlled capacitor in series
with the transmission line
3.6.4 Thyristor-Controlled Phase Shifter (TCPS)
In a TCPS control technique the phase shift angle is determined as a nonlinear
function of rotor angle and speed. However, in real-life power system with a large
number of generators, the rotor angle of a single generator measured with respect
to the system reference will not be very meaningful.
3.6.5 SECOND GENERATION OF FACTS
3.6. 6 Static Compensator (STATCOM)

19. 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 [21] A static synchronous compensator
(STATCOM) is a regulating device used on alternating current electricity
transmission networks. It is based on a power electronics voltage-source converter
and can act as either a source or sink of reactive AC power to an electricity
network. If connected to a source of power it can also provide active AC power. It
is a member of the FACTS family of devices. Usually a STATCOM is installed to
support electricity networks that have a poor power factor and often poor voltage
regulation. There are however, other uses, the most common use is for voltage
stability.
3.6.7 Static Synchronous Series Compensator (SSSC)
This device work the same way as the STATCOM. It has a voltage source
converter serially connected to a transmission line through a transformer. It is
necessary an energy source to provide a continuous voltage through a condenser
and to compensate the losses of the VSC. A SSSC is able to exchange active and
reactive power with the transmission system. But if our only aim is to balance the
reactive power, the energy source could be quite small. The injected voltage can
be controlled in phase and magnitude if we have an energy source that is big
enough for the purpose. With reactive power compensation only the voltage is
controllable, because the voltage vector forms 90º degrees with the line intensity.
In this case the serial injected voltage can delay or advanced the line current. This
means that the SSSC can be uniformly controlled in any value, in the VSC
working slot.
3.6.8 Unified Power Flow Controller (UPFC)
A unified power flow controller (UPFC) is the most promising device in the
FACTS concept. It has the ability to adjust the three control parameters, i.e. the
bus voltage, transmission line reactance, and phase angle between two buses,
either simultaneously or independently. A UPFC performs this through the

20. control of the in-phase voltage, quadrature voltage, and shunt compensation. The
UPFC is the most versatile and complex power electronic equipment that has
emerged for the control and optimization of power flow in electrical power
transmission systems. It offers major potential advantages for the static and
dynamic operation of transmission lines. The UPFC was devised for the real-time
control and dynamic compensation of ac transmission systems, providing
multifunctional flexibility required to solve many of the problems facing the
power industry. Within the framework of traditional power transmission concepts,
the UPFC is able to control, simultaneously or selectively, all the parameters
affecting power flow in the transmission line. Alternatively, it can independently
control both the real and reactive power flow in the line unlike all other
controllers.
Fig 3.3: Unified power flow controller

21. CONCLUSION
This paper has presented various FACTS controllers and analyzed their control attributes
and benefits. The flexible ac transmission system (FACTS), a new technology based on
power electronics, offers an opportunity to enhance controllability, stability, and power
transfer capability of ac transmission systems. The application of FACTS controllers
throws up new challenges for power engineers, not only in hardware implementation, but
also in design of robust control systems, planning and analysis.

22. REFERENCES
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