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FLEXIBLE AC TRANSMISSION SYSTEMS
1.1
INTRODUCTION
An electric power system consists of three principle divisions, the generating stations,
the transmission systems, and the distribution systems. The transmission systems are the
connecting links between the generating stations and the distribution systems and lead to
other power systems over interconnections. The primary function of an electric power system
is to pool power plants and load centers in order to supply the real and reactive powers
demanded by the various loads connected to the system at a required reliability and maximum
efficiency at a lower cost.
In the present day scenario, transmission systems are becoming increasingly stressed,
more difficult to operate, and more insecure with unscheduled power flows and higher losses
because of growing demand and tight restrictions on the construction of new lines. On the
other hand, power flows in some of the transmission lines are well below their thermal limits
due to constraints such as voltage and transient stability limits, while certain lines are
overloaded, which has as an overall effect of deteriorating voltage profiles and decreasing
system stability and security. In addition, existing traditional transmission facilities, in most
cases, are not designed to handle the control requirements of complex, highly interconnected
power systems. This overall situation requires the review of traditional transmission methods
and practices, and the creation of new concepts which would allow the use of existing
generation and transmission lines up to their full capabilities without reduction in system
stability and security. Another reason that is forcing the review of traditional transmission
methods is the tendency of modern power systems to follow the changes in today’s global
economy that are leading to deregulation of electrical power markets in order to stimulate
competition between utilities.
The rapid development of power electronics has made it possible to design power
electronic equipment of high rating for high voltage systems, the voltage stability problem
resulting from transmission system may be, at least partly, improved by use of the equipment
well-known as Flexible AC Transmission Systems (FACTS) controllers.
Power electronics-based FACTS technology can enhance transmission system control
and increase line loading in some cases all the way up to thermal limits thereby without
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Flexible AC Transmission Systems
compromising reliability. These capabilities allow transmission system owners and operators
to maximize asset utilization and execute additional bulk power transfers, with immediate
bottom-line benefits.
1.2
FACTS BACKGROUND AND DEVELOPMENT
Power flow is a function of transmission line impedance, the magnitude of the
sending and receiving end voltages, and the phase angle between the voltages. By controlling
one or a combination of the power flow arguments, it is possible to control the active, as well
as the reactive power flow in the transmission line.
In the past, power systems were simple and designed to be self-sufficient. Active
power exchange of nearby power systems was rare as ac transmission systems cannot be
controlled fast enough to handle dynamic changes in the system and, therefore, dynamic
problems were usually solved by having generous stability margins so that the system could
recover from anticipated operating contingencies.
Today, it is possible to increase the system loadability and hence security by using a
number of different approaches. It is usual practices in power systems to install shunt
capacitors to support the system voltages at satisfactory levels. Series capacitors are used to
reduce transmission line reactance and thereby increase power transfer capability of lines.
Phase shifting transformers are applied to control power flows in transmission lines by
introducing an additional phase shift between the sending and receiving end voltages.
In past days, all these devices were controlled mechanically and were, therefore,
relatively slow. They are very useful in a steady state operation of power systems but from a
dynamical point of view, their time response is too slow to effectively damp transient
oscillations. If mechanically controlled systems were made to respond faster, power system
security would be significantly improved, allowing the full utilization of system capability
while maintaining adequate levels of stability.
However, with the development of power systems, especially the opening of electric
energy markets, it becomes more and more important to control the power flow along the
transmission line thus to meet the needs of power transfer, provided the early incentives in
the late 1970s to introduce power electronics-based control for reactive compensation. This
normal evolutionary process has been greatly accelerated by more recent developments in the
utility industry, which have aggravated the early problems and highlighted the structural
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Flexible AC Transmission Systems
limitations of power systems in a greatly changed socio-economical environment. The desire
to find solutions to these problems and limitations led to focused technological developments
under the Flexible AC Transmission System (FACTS).
FACTS devices are used for the dynamic control of voltage, impedance and phase
angle of high voltage AC lines. FACTS devices provide strategic benefits for improved
transmission system management through: better utilization of existing transmission assets;
increased transmission system reliability and availability; increased dynamic and transient
grid stability; and enabling environmental benefits.
FACTS are the name given to the application of power electronics devices to the
control of flows and other quantities in power system [Hingorani88, 91, books,].
Semiconductor technology enabled the manufacture of powerful thyristors and later of new
elements such as the gate turn-off thyristors (GTO) and insulated gate bipolar transistors
(IGBT) [Edris00]. Development based on the semiconductor devices first established high
voltage DC transmission (HVDC) technology as an alternative to long distance AC
transmission. HVDC technology, in turn has provided the basis for the development of
FACTS equipment, which can solve problem in AC transmission [Hammons97, Povh00,
Gyugy92].
FACTS technology can allow system owners and operators to increase line loading
without compromising reliability, in some cases all the way up to thermal limits. System
studies are mandatory first step for any company considering FACTS applications.FACTS
apparatuses employing many alternative configurations of power-electronic switching
devices are being considered by others for increasing the power transfer capability of ac
transmission lines industrial customers are becoming more sensitive to variations of utility
supply systems due to the growing demand for process controls in automated plants, so, the
existing transmission lines are over loaded and lead to unstable system. One option to
increase reliability and quality of ac power is to provide sensitive customers with access to
two independent power sources. In the present day scenario private power producers are
increasing rapidly to meet the increase demand due to heavily loaded customers, New
transmission lines or FACTS devices on the existing transmission system can eliminate
transmission over loading, but FACTS devices are preferred in the modern power systems
based on its overall performance.
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Flexible AC Transmission Systems
Several energy companies are realizing the benefits of FACTS installations.
Tennessee Valley Authority (TVA) installed the first STATCOM in 1995 to strengthen ties
between its Sullivan substation and the rest of its network while avoiding the more labor and
space-intensive installation of an additional transformer bank. The device provides
instantaneous control of transmission voltage, increasing capacity to provide TVA with
greater flexibility in bulk power transactions. It also increases reliability and damping out
major grid oscillations.
Thereafter, in 1998, installation of the first UPFC was completed at the Inez
Substation owned by American Electric Power (AEP). The device employs dual back-to-back
voltage-sourced converters with a STATCOM and an SSSC coupled to a DC link capacitor,
enabling it to function as an ideal ac-to-ac power converter. It represents the first controller
capable of providing complete control of all three basic transmission system parameters
(voltage, line impedance, and phase angle) simultaneously. AEP installed the UPFC as the
first major element in a regional reinforcement strategy. It is mitigating potential thermal load
and low voltage problems caused by rapid growth in area far from the company's generating
sources. It will also ensure that a new, high- capacity l38-kV line carries its share of regional
load.
Up to now, installed FACTS equipments have been put forward over time due to a
significant study of FACTS concepts, its potential benefits together with the rapid
developments of modern power electronics technology. Consequently, as power
semiconductor devices continue to improve, particularly as FACTS controller concepts
advance, the cost of FACTS controller will continue to decrease. Large-scale use of FACTS
technology is eventually be an assured scenario.
1.3
THE OBJECTIVES OF FACTS
It has been recognized that the steady-state transmittable power can be increased and
the voltage profile along the line controlled by appropriate reactive shunt and/or series
connected compensation. Shunt compensation, fixed or mechanically switched reactors are
applied to minimize line overvoltage under light load conditions, and shunt compensation,
fixed or mechanically switched capacitors are applied to maintain voltage levels under heavy
load conditions. Series connected reactors are applied to limiting AC power transmission
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Flexible AC Transmission Systems
over long lines. Series connected capacitors are applied to cancel a portion of the reactive
transmission line impedance and thereby increase the transmittable power.
The basic transmission challenge of the evolving deregulated power system, whatever
final form it may take, is to provide a network capable of delivering contracted power from
any supplier to any consumer over a large geographic area under market forces-controlled,
and thus continuously varying, patterns of contractual arrangements. The aggravating
constraint to any potential solution is that, due to cost, right-of-way, and environmental
problems, the network must substantially be based on the existing physical line structure.
The Electric Power Research Institute (EPRI), after years of supporting the
development of high power electronics for such applications as High Voltage DC (HVDC)
Transmission and reactive compensation of AC lines, in the late 1980s formalized the broad
concept of Flexible AC Transmission System (FACTS). The acronym FACTS identifies
alternating current transmission systems incorporating power electronics-based controllers to
enhance the controllability and increase power transfer capability. The FACTS initiative was
originally launched to solve the emerging system problems in the late 1980s due to
restrictions on transmission line construction, and to facilitate the growing power
export/import and wheeling transactions among utilities, with two main objectives:
To increase the power transfer capability of transmission systems, and
To keep power flow over designated routes.
The first objective implies that power flow in a given line should be able to be
increased up to the thermal limit by forcing the necessary current through the series line
impedance if, at the same time, stability of the system is maintained via appropriate real-time
control of power flow during and following system faults. This objective of course does not
mean to say that the lines would normally be operated at their thermal limit loading (the
transmission losses would be unacceptable), but this option would be available, if needed, to
handle severe system contingencies. However, by providing the necessary angle and voltage
stability via FACTS controllers, instead of large steady-state margins, the normal power
transfer over the transmission lines is estimated to increase significantly (about 50%,
according to some studies conducted).
The second objective implies that, by being able to control the current in a line (by,
for example, changing the effective line impedance), the power flow can be restricted to
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Flexible AC Transmission Systems
selected (contracted) transmission corridors while parallel and loop-flows can be mitigated. It
is also implicit in this objective that the primary power flow path must be rapidly changeable
to an available secondary path under contingency conditions to maintain the desired overall
power transmission in the system.
It is easy to see that the achievement of the two basic objectives would significantly
increase the utilization of existing (and new) transmission assets, and could play a major role
in facilitating deregulation with minimal requirements for new transmission lines.
The implementation of the above two basic objectives requires the development of
high power compensators and controllers. The technology needed for this is high power
(multi-hundred MVA) electronics with its real-time operating control. However, once a
sufficiently large number of these fast compensators and controllers are deployed over the
system, the coordination and overall control to provide maximum system benefits and
prevent undesirable interactions with different system configurations and objectives, under
normal and contingency conditions, present a different technological challenge. This
cha1lenge is to develop appropriate system optimization control strategies, communication
links, and security protocols. The realization of such an overall system optimization control
can be considered as the third objective of the FACTS initiative.
1.4
FACTS CONTROLLERS
After years of rapid development, many types of FACTS controllers have been put
forward. Some of them are already brought into operation or being constructed. However,
FACTS controllers, a symbol of which is shown in Figure 1.1(a), can be divided into three
categories as:
1.4.1 Series Connected Controllers:
The series controller could be variable impedance, such as capacitor, reactor, etc., or
power electronics based variable source of main frequency, sub-synchronous and harmonic
frequencies to serve the desired need. In principle, all series controllers inject voltage in
series with the line. Even variable impedance multiplied by the current flow through it,
represents an injected series voltage in 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.
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Flexible AC Transmission Systems
1.4.1.1 Static Synchronous Series Compensator (SSSC)
SSSC [Sen98] 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. Without an extra energy source, SSSC can only inject a variable voltage,
which is 90 degrees leading or lagging the current. Simple model of SSSC is shown in Figure
Figure 1.1.
Simple model of SSSC
1.4.1.2 Interline Power Flow controller (IPFC)
IPFC is a possible definition is the combination of two or more Static Synchronous
Series Compensators which 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’s common DC link, to provide shunt
reactive compensation and supply or absorb the overall real power deficit of the combined
SSSCs.
1.4.1.3 Thyristor-Controlled Series Capacitor (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
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Flexible AC Transmission Systems
variable series capacitive reactance. The TCSC is based on thyristor without the gate turn-off
capability. It is an alternative to SSSC and like an SSSC; it is a very important FACTS
controller. A variable reactor such as a Thyristor-Controlled Reactor (TCR) is connected
across a series capacitor. When the TCR firing angle is 180 degrees, the reactor becomes
non-conducting and the series capacitor has its normal impedance. As the firing angle is
advanced from 180 degrees to less than 180 degrees, the capacitive impedance increases. At
the other end, when the TCR firing angle is 90 degrees, the reactor becomes fully conducting,
and the total impedance becomes inductive, because the reactor impedance is designed to be
much lower than the series capacitor impedance. With 90 degrees firing angle, the TCSC
helps in limiting fault current. In the TCSC, each thyristor is fired with phase angle control
once per cycle. High inductance in TCSC is required; typically its reactance at network
frequency is 5-20 % of the capacitor bank reactance. The TCSC may be a single, large unit,
or may consist of several equal or different-sized smaller capacitors in order to achieve a
superior performance. Simple model of TCSC is shown in Figure 1.2 and its valve current
waveform is shown in Fig.1.3.
IL
>
I
v
Iv v
time
Figure 1.2.
Simple model and valve current waveform of TCSC
1.4.1.4 Thyristor-Switched Series Capacitor (TSSC)
TSSC is a capacitive reactance compensator, which uses thyristor-switches in parallel
with a segment of the series capacitor bank to rapidly insert or remove portions of the bank in
stepwise. Instead of continuous control of capacitive impedance, this approach of switching
inductors at firing angle of 90 degrees or 180 degrees but without firing angle control could
reduce cost and losses of the controller. It is reasonable to arrange one of the modules to have
thyristor control, while others could be thyristor switched. Simple model of TSSC is shown in
Figure 1.3.
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Flexible AC Transmission Systems
In the TSSC the thyristor valve is utilized as a switch that inserts or bypasses the
capacitor bank for any number of complete half-cycles of the network frequency. The
inductance in TSSC can be small as the thyristor valve does not switch at high valve voltages.
Typically the reactance of the inductor is 2-5 % of the capacitor bank reactance at rated
network frequency.
IL
>
Iv
Iv v
Time
Figure 1.3.
Simple model and valve current waveform of TSSC
The first series connected FACTS controllers, TCSC, were put into operation about
eight year years ago. TCSC as shown in Fig.1.2 can vary the impedance continuously to
levels below and up to the lines natural impedance. TCSC have been realized also in
commercial projects. The main task for this type of controller is load flow control and the
improvement of stability conditions in the system. A further advantage of TCSC is the ability
to damp sub synchronous resonance.
1.4.1.5 Thyristor-Controlled Series Reactor (TCSR)
TCSR is an inductive reactance compensator, which consists of, a series reactor
shunted by a thyristor-controlled reactor in order to provide a smoothly variable series
inductive reactance. When the firing angle of the thyristor controlled reactor is 180 degrees, it
stops conducting, and the uncontrolled reactor acts as a fault current limiter. As the angle
decreases below 180 degrees, the net inductance decreases until firing angle of 90 degrees,
when the net inductance is the parallel combination of the two reactors. Simple model of
TCSR is shown in Figure 1.3.
1.4.1.6 Thyristor-Switched Series Reactor (TSSR)
TSSR is an inductive reactance compensator, which consists of a series reactor
shunted by a thyristor-controlled switched reactor in order to provide a stepwise control of
series inductive reactance. This is a complement of TCSR, but with thyristor switches fully
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Flexible AC Transmission Systems
on or off to achieve a combination of stepped series inductance. Simple model of TSSR is
shown in Figure 1.4.
Figure 1.4.
Simple model of TCSR and TSSR
1.4.2 Shunt Connected 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 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 variable reactive power. Any other phase relationship
will involve handling of real power as well.
1.4.2.1 Static Synchronous Compensator (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. Figure 1.5. shows a simple one-line diagram of STATCOM based on
a voltage-sourced converter and a current-sourced converter. For the voltage-sourced
converter, its AC output voltage is controlled such that it is just right for the required reactive
current flow for any AC bus voltage DC capacitor voltage is automatically adjusted as
required to serve as a voltage source for the converter. STATCOM can also be designed as an
active filter to absorb system harmonics.
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Flexible AC Transmission Systems
Figure 1.5.
STATCOM based on a voltage-sourced and a current-sourced converter.
1.4.2.2 Static Synchronous Generator (SSG)
SSG is a static self-commutated switching power converter supplied from a static
synchronous electric energy source (such as battery, flywheel, super conducting magnet,
large DC storage capacitor). It is operated to produce a set of adjustable multiphase output
voltages, which may be coupled to an AC power system for the purpose of exchanging
independently controllable real and reactive power. Simple model of SSG is shown in Figure
1.6.
Figure 1.6.
Simple model of SSG
1.4.2.3 Static Var Compensator (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 based on thyristor without the
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Flexible AC Transmission Systems
gate turn-off capability. It includes separate equipment for leading and lagging vars; SVC is
considered by some as a lower cost alternative to STATCOM, although this may not be the
case if the comparison is made based on the required performance and not just the MVA size.
Simple model of SVC is shown in Figure 1.7.
1.4.2.4 Thyristor Controlled Reactor (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 is a
subset of SVC in which conduction time and hence, current in a shunt reactor is controlled by
a thyristor-based AC switch with firing angle control. Simple model of TCR is shown in
Figure 1.7.
1.4.2.5 Thyristor Switched Reactor (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. TSR
is another subset of SVC. TSR is made up of several shunt-connected inductors, which are
switched in and out by thyristor switches without any firing angle controls in order to achieve
the required step changes in the reactive power consumed from the system. Use of thyristor
switches without firing angle control results in lower cost and losses, but without a
continuous control. Simple model of TSR is shown in Figure 1.7.
1.4.2.6 Thyristor Switched Capacitor (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. TSC
is also a subset of SVC in which thyristor based AC switches are used to switch in and out
(without firing angle control) shunt capacitors units, in order to achieve the required step
change in the reactive power supplied to the system. Unlike shunt reactors, shunt capacitors
cannot be switched continuously with variable firing angle control. Simple model of TSC is
shown in Figure 1.7.
1.4.2.7 Static Var Generator or Absorber (SVG)
SVG is a static electrical device, equipment, or system that is capable of drawing
controlled capacitive and/or inductive current from an electrical power system and thereby
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Flexible AC Transmission Systems
generating or absorbing reactive power. Generally considered to consist of shunt connected,
thyristor-controlled reactor(s) and/or thyristor-switched capacitors. The SVG is simply a
reactive power (var) source that, with appropriate controls, can be converted into any specific
or multipurpose reactive shunt compensator. Thus, both the SVC and the STATCOM are
static var generators equipped with appropriate control loops to vary the var output so as to
meet specific compensation objectives. Simple model of SVG is shown in Figure 1.7.
1.4.2.8 Static Var System (SVS)
SVS is a combination of different static and mechanically switched var compensators
whose outputs are coordinated. Simple model of SVS is shown in Figure 1.7.
Figure 1.7.
Simple model of SVC, SVG, SVS, TCR, TSR, and TSC
1.4.2.9 Thyristor Controlled Braking Resistor (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. TCBR involves cycle-by-cycle switching of a resistor (usually a linear resistor)
with a thyristor-based AC switch with firing angle control. For lower cost, TCBR may be
thyristor switched without firing angle control. However, with firing control, half-cycle by
half-cycle firing control can be utilized to selectively damp low frequency oscillations.
Simple model of TCBR is shown in Figure 1.7.
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Flexible AC Transmission Systems
Figure 1.8.
Simple model of TCBR
1.4.3 Combined Series-Shunt Controllers:
This could be a combination of separate series controllers, which are controlled in a
coordinated manner, in a multi-line 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 power link. The real power transfer capability
of the unified series-series controller, referred to as Interline Power Flow Controller, makes
it possible to balance both the real and reactive power flow in the lines and thereby maximize
the utilization of the transmission system. Note that the term "unified" here means that the
DC terminals of all controller converters are all connected together for real power transfer.
1.4.4 Combined Series-Shunt Controllers:
This could be a combination of separate shunt and series controllers, which are
controlled in a coordinated manner, or a Unified Power Flow Controller with series and shunt
elements. In principal, 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 power link.
1.4.4.1 Unified Power Flow Controller (UPFC)
UPFC [18-20, 23, 76] is a combination of static synchronous compensator
(STATCOM) and a static series compensator (SSSC), which are coupled via a common DC
link, to allow bi-directional flow of real power between the series output terminals of the
SSSC and the shunt output terminals of the STATCOM, and are controlled to provide
concurrent real and reactive series line compensation without an external electric energy
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Flexible AC Transmission Systems
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. Additional storage such as a superconducting magnet connected to the DC link via an electronic interface would provide the
means of further enhancing the effectiveness of the UPFC. As mentioned before, the
controlled exchange of real power with an external source, such as storage, is much more
effective in control of system dynamics than modulation of the power transfer within a
system. Simple model of UPFC is shown in Figure 1.8.
Figure 1.9.
Simple model of UPFC
The UPFC consists of shunt (exciting) and series (boosting) transformer, which are
connected by two GTO converters and a DC circuit represented by the capacitor. Inverter 2 is
used to generate a voltage source at the fundamental frequency with variable amplitude (0 
Vs  Vsmax) and phase angle (0   s 2), which is added to the AC transmission line by the
series connected boosting transformer. In this way the inverter output voltage injected in
series with the line can be used for direct voltage control, series compensation, phase shifter
and their combination.
Inverter 1 is used primarily to provide the real power demand of inverter 2 at the
common DC line terminal from the AC power system. Inverter 1 can also generate or absorb
reactive power at its AC terminal, which is independent of the active power it transfers to the
DC terminal. Therefore with proper controls, it can also fulfill the function of independent
advanced static VAR compensator providing reactive power compensation for the
transmission line and thus executing indirect voltage regulation at the input terminal of the
UPFC. Presently there are two UPFCs being constructed in the world, one in Inez substation
of AEP power system in USA, and the other in France. Renz summarizes the major benefits
produced by the UPFC as follows:
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Flexible AC Transmission Systems
 Thermal overload and low voltages are eliminated.
 Adequate power supply will be available for several years of growth.
 Real power system losses will be reduced be more than 24 MW.
By this context, UPFCs are a new generation of power system control devices. It will
play major role in solving technical issues of open power market [43-44, 50, 87].
1.4.4.2 Thyristor-Controlled Phase Shifting Transformer (TCPST)
TCPST is a phase-shifting transformer [39] adjusted by thyristor switches to provide a
rapidly variable phase angle. In general, phase shifting is obtained by adding a perpendicular
voltage vector in series with a phase. This vector is derived from the other two phases via
shunt-connected transformers. The perpendicular series voltage is made variable with a
variety of power electronics topologies. A circuit concept that can handle voltage reversal can
provide phase shift in either direction. This controller is also referred to as ThyristorControlled Phase Angle Regulator (TCPAR). Simple model of TCPST is shown in Figure
1.10.
Figure 1.10. Simple model of TCPST or TCPAR
The thyristor controlled phase angle regulator (TCPAR) which can influence a flat
change of phase angle. The phase shift is accomplished by adding or subtracting a variable
voltage component that is perpendicular to the phase voltage of the line. This perpendicular
voltage component is obtained from a transformer connected between the other two phases.
The attributes of this device are the power control damping of oscillations and transient
stability. A conceptual TCPAR as shown in Figure 1.10 employs an excitation transformer
with three secondary windings with turn ratios 1, 3 and 9. The thyristor switch produces a
voltage VT that is added in series to the line voltage V to produce a phase shift .
1.4.4.3 Interphase Power Controller (IPC)
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Flexible AC Transmission Systems
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.
1.4.4.4 Thyristor-Controlled Voltage Limiter (TCVL)
TCVL is a thyristor-switched metal-oxide varistor (MOV) used to limit the voltage
across its terminals during transient conditions. The thyristor switch can be connected in
series with a gapless arrester, or part of the gapless arrester (10-20%) can be bypassed by a
thyristor switch in order to dynamically lower the voltage limiting level. In general, the MOV
would have to be significantly more powerful than the normal gapless arrester, in order that
TCVL can suppress dynamic overvoltages, which can otherwise last for tens of cycles.
Simple model of TCVL is shown in Figure 1.11.
Figure 1.11. Simple model of TCVL
1.4.4.5 Thyristor-Controlled Voltage Regulator (TCVR)
TCVR is a thyristor-controlled transformer, which can provide variable in-phase
voltage with continuous control. For practical purposes, this may be a regular transformer
with a thyristor-controlled tap changer [Figure 1.12.(a)] or with a thyristor-controlled AC to
AC voltage converter for injection of variable AC voltage of the same phase in series with the
line [Figure 1.12.(b)]. Such a relatively low cost controller can be very effective in
controlling the flow of reactive power between two AC systems.
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Flexible AC Transmission Systems
Figure 1.12.
Simple model of TCVR: (a) TCVR based on tap changing and (b) based on voltage
injection
Table 1.1 shows the summary checklist of control attributes for different FACTS
controllers.
TABLE 1.1
CONTROL ATTRIBUTES FOR VARIOUS FACTS CONTROLLERS
FACTS
controllers
Control Attributes
STATCOM
Voltage control, VAR compensation, damping oscillations and
voltage stability
SVC, TCR, TCS
and TRS
Voltage control, VAR compensation, damping oscillations,
transient and dynamic stability and voltage stability
TCBR
SSSC, TCSC,
TSSC, TCSR and
TSSR
Damping oscillations and transient and dynamic stability
TCPST
Active power control, damping oscillations, transient and
dynamic stability and voltage stability
UPFC
Active and reactive power control, voltage control, VAR
compensation, damping oscillations, transient and dynamic
stability, voltage stability and fault current limiting
TCVL
Transient and dynamic voltage limit
TCVR and IPFC
1.5
Current control, damping oscillations, transient and dynamic
stability, voltage stability and fault current limiting
Reactive power control, voltage control, damping oscillations,
transient and dynamic stability and voltage stability
CONCLUSION
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Flexible AC Transmission Systems
A number of FACTS devices have been described in this chapter and are the result of
10 years or more of development. It seems certain that development of FACTS technology
will continue, aimed at providing flexible control devices to meet deregulated systems needs
whilst continuing to strive for lower costs.
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