FACTS
Flexible AC Transmission System (Facts) is a new integrated
concept based on power electronic switching converters
and dynamic controllers to enhance the system utilization
and power transfer capacity as well as the stability, security,
.
reliability and power quality of AC system interconnections
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INTRODUCTION
 Flexible Alternating Current Transmission System.
 FACTS as they are generally known, are new devices that
improve transmission systems.
 FACTS is a static equipment used for the AC transmission
of electrical energy.
 It is generally a power electronics based device.
 Meant to enhance controllability and increase power
transfer capability.
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BENEFITS OF FACTS DEVICES
 Regulation of power
transmission routes.
flows
in
prescribed
 Reduces the need for construction of new
transmission lines, capacitors and reactors.
 Provides greater ability to transfer power
between controlled areas.
 These devices help to damp the power
oscillations that could damage the equipment.
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
Improves the transient stability of the system.

Controls real and
independently.

Damping of oscillations which can threaten security or limit
the usable line capacity.
reactive
power
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flow
in
the
line
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Better utilization of existing transmission system
assets

 Increased transmission system reliability and
availability
(lower vulnerability to load changes,
UPFC CIRCUIT DIAGRAM
line faults)
 Increased quality of supply for sensitive
industries
(through mitigation of flicker, frequency
variations)
 Environmental benefits
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Basic Types of FACTS Controllers
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Basic Types of FACTS Controllers
FACTS controllers are classified as
 Series Controllers
 Shunt Controllers
 Combined Series-Series Controllers
 Combined Series-Shunt Controllers
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Basic Types of FACTS Controllers
Series Controllers:
 It could be a variable impedance
(capacitor, reactor, etc) or a
power electronic based variable
source of main frequency,
subsynchonous and harmonic
frequencies to serve the desired
need.
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Basic Types of FACTS Controllers
Series Controllers:

Inject a voltage in series with
the line.
 If the voltage is in phase
quadrature with the current,
controller supplies or
consumes reactive power.
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Basic Types of FACTS Controllers
Shunt Controllers:
 It could be a variable impedance
(capacitor, reactor, etc) or a
power electronic based variable
source or combination of both.
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Basic Types of FACTS Controllers
Shunt Controllers:
 Inject a current in the system.
 If the current is in phase
quadrature with the voltage,
controller supplies or consumes
reactive power.
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Basic Types of FACTS Controllers
Combined Series-Series
Controllers:
 It could be a combination of
separate series controllers or
unified controller.
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Basic Types of FACTS Controllers
Combined Series-Series
Controllers:
 Series controllers supply
reactive power for each line
and real power among lines
via power link.
 Interline power flow
controller balance real and
reactive power flow in the
lines.
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Basic Types of FACTS Controllers
Combined Series-Shunt
Controllers:
 It could be a combination
of separate series & shunt
controllers or unified
power flow controller.
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Basic Types of FACTS Controllers
Combined Series-Shunt
Controllers:
 Inject current into the system
with the shunt controller and
voltage in series with the line
with series controller.
 When the controllers are
unified, exchange real power
between series and shunt
controllers via power link.
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Basic Types of FACTS Controllers
Choice of the controller:
 Series controller controls the
current/power flow by controlling the
driving voltage and transmission line
impedance.
 To control current/power flow and
damp oscillations, series controller is
several times more powerful than
shunt controller.
 Shunt controller injects current in the
line
 Thus it is used for more effective
voltage control & damp voltage
oscillations.
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Basic Types of FACTS Controllers
 Injecting the voltage in series
with the line can improve the
voltage profile.
 But shunt controller is more
effective to improve the voltage
profile at substation bus.
 Series controllers should bypass
short circuit currents and
handle dynamic overloads.
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Basic Types of FACTS Controllers
 Controllers with gate turn
off devices are based on dc to
ac converters used for
exchange the active/reactive
power with ac lines.
 This requires energy storage
device.
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CONTROL OF REACTIVE POWER
AND VOLTAGE
Copyright © P. Kundur
This material should not be used without the author's consent
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Reactive Power and Voltage Control
Control objectives contributing to efficient and
reliable operation of power system:

Voltage at terminals of all equipment are within
acceptable limits
 both utility and customer equipment designed to
operate at certain voltage rating
 prolonged operation outside allowable range
could cause them damage

System stability is satisfactory
 voltage levels and reactive power control have
significant impact on stability

The reactive power flow is minimized so as to
reduce I 2R and I 2X losses to a minimum
 ensures transmission system operates efficiently
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Production and Absorption of Reactive
Power (Q)

Synchronous Generators
 can generate or absorb Q depending on excitation
 capability limited by field current, armature current,
limits
 automatic voltage regulator continuously adjusts
excitation to control armature voltage
 primary source of voltage support!

Overhead lines
 at loads below natural or surge impedance load (SIL),
produce Q
 at loads above SIL, absorb Q

Underground cables
 have high SIL due to high capacitance
 always loaded below SIL, and hence generate Q
cont'd
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Production and Absorption of Q (cont'd)

Transformers
 absorb Q due to shunt magnetizing reactance
and series leakage inductance

Loads
 a typical "load bus" is composed of a large
number of devices
 composite characteristics are normally such that
a load bus absorbs Q
 industrial loads usually have shunt capacitors to
improve power factor

As power flow conditions vary, reactive power
requirements of transmission network vary

Since Q cannot be transmitted over long distances,
voltage control has to be effected using special
devices dispersed throughout the system
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Methods of Voltage Control

Control of voltage levels is accomplished by
controlling the production, absorption, and flow of
reactive power at all levels in the system

Generating units provide the basic means of voltage
control

Additional means are usually required to control
voltage throughout the system:
 sources or sinks of reactive power, such as
shunt capacitors, shunt reactors, synchronous
condensers, and static var compensators (SVCs)
 line reactance compensators, such as series
capacitors
 regulating transformers, such as tap-changing
transformers and boosters
cont'd
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Methods of Voltage Control (cont'd)
 Shunt capacitors and reactors, and series
capacitors provide passive compensation
 are either permanently connected to
the transmission and distribution
system, or switched
 contribute to voltage control by
modifying the network
characteristics
 Synchronous condensers and SVCs
provide active compensation; the reactive
power absorbed/ supplied by them are
automatically adjusted so as to maintain
voltages of the buses to which they are
connected
 together with the generating units,
they establish voltages at specific
points in the system
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Objectives of Reactive Power
Compensation
 To control voltage and/or improve
maximum power transfer capability
 Achieved by modifying effective line
L
parameters:
Z 
C
C
 characteristic impedance,
 The voltage profile is determined by ZC
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Shunt Reactors

Used to compensate the undesirable voltage effects
associated with line capacitance
 limit voltage rise on open circuit or light load

Shunt compensation with reactors:
 increases effective ZC
 reduces the effective natural load , i.e., voltage at
which flat voltage profile is achieved

They are connected either:
 directly to the lines at the ends, or
 to transformer tertiary windings; conveniently
switched as var requirements vary

Line reactors assist in limiting switching surges

In very long lines, at least some reactors are
required to be connected to lines
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Shunt Capacitors

Used in transmission systems to compensate
for I 2X losses

Connected either directly to H.V. bus or to
tertiary winding of transformers

Normally distributed throughout the system so
as to minimize losses and voltage drops

Usually switched: a convenient means of
controlling voltage

Shunt capacitor compensation of transmission
lines in effect
 decreases ZC

Advantages: low cost and flexibility of
installation and operating

Disadvantages: Q output is proportional to
square of the voltage; hence Q output reduced
at low voltages

Shunt capacitors are used extensively in
distribution systems for power factor
correction and feeder voltage control
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Series Capacitors

Connected in series with the line

Used to reduce effective inductive reactance
of line
 increases maximum power
 reduces I 2X loss

Series capacitive compensation in effect
reduces
 characteristic impedance ZC

Reactive power produced increases with
increasing power transfer
 Self regulating !

Typical applications
 improve power transfer capablity
 voltage regulation
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Advantages of Series Compensation
1.
Increase in transmission capacity
 The power transfer capacity of a line is
given by
P
E.V
sin 
X
where, E is sending end voltage
V is receiving end voltage
X is reactance of line
δ is phase angle between E and V
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 Power transfer without and with compensation:
P1 
E.V
sin 
XL
P2 
E.V
sin 
(X L  XC )
P2
XL
1
1



P1 ( X L  X C ) (1  X C / X L ) 1  K
where K is degree of compensation.
The economic degree of compensation
lies in the range of 40-70%
(K < 1, i.e. 0.4-0.7)
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2. Improvement of System Stability
 For same amount of power transfer and
same value of E and V, the δ in the case
of series compensated line is less than
that of uncompensated line.
P
E.V
sin 1
XL
E.V
P
sin  2
(X L  XC )
sin  2 ( X L  X C )

sin 1
XL
• A lower δ means better system stability
• Series compensation offers most
economic solution for system stability
as compared to other methods (reducing
generator, transformer
reactance,
bundled conductors, increase no. of
parallel circuits
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3. Less installation Time
 The installation time of the series
capacitor is smaller (2 years
approx.)
as
compared
to
installation time of the parallel
circuit line (5 years approx.)
 This reduces the risk factor.
 Hence used to hit the current
thermal limit.
 The life of x-mission line and
capacitor is generally 20-25
years.
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Disadvantages
1. Increase in fault current
2. Mal operation of distance relayif the degree of compensation
and location is not proper.
3. High recovery voltage of linesacross the circuit breaker
contacts and is harmful.
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Some key consideration in the
application of series capacitor
1). Voltage rise due to reactive current
2). Bypassing and reinsertion
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3. Location of series capacitor
 Factors influencing choice of location of capacitor
bank
 COST
 ACCESSIBILITY
 FAULT LEVEL
 VOLTAGE PROFILE
 EFFECTIVENESS IN IMPROVING POWER
TRANSFER CAPABILITY
 Following are the usual location:
 Midpoint of the line
 Line terminals
 1/3 or 1/4 of a line
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Synchronous Condenser
 A synchronous machine running without
a prime mover or a mechanical load
 Depending on field excitation, it can
either absorb or generate vars
 With a voltage regulator, it can
automatically adjust vars to maintain
constant voltage
 Started as an induction motor and then
synchronized
 Normally connected to tertiary windings
of transformers
 Unlike a SVC, a synchronous condenser
has an internal voltage
 Speed of response not as fast as that of
an SVC
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Basic Types Of FACTS
Compensation
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Static VAR Compensators (SVC)

Shunt connected static var generators and/or
absorbers whose outputs are varied so as to control
specific power system quantities

The term static is used to denote that there are no
moving or rotating components

Basic types of SVCs:

TCR,

FC-TCR,

MSC-TCR,

TSC-TCR,

TSR

A static var system (SVS) is an aggregation of SVCs
and mechanically switched capacitors or reactors
whose outputs are coordinated

When operating at its capacitive limit, an SVC
behaves like a simple capacitor
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Characteristic of realistic SVS:
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Composite SVS –POWER SYSTEM CHARACTERISTIC
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Thyristor controlled reactor (TCR)
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Thyristor controlled reactor (TCR) is controllable
susceptance (B)
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HARMONICS
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Advantages of TCR

The main advantages of the TCR are flexibility of control and
ease in up rating.

Different control strategies can be easily implemented.

The voltage reference and current slope can be controlled in
a simple manner.

A TCR SVC can have its rating extended by the addition of
more TCR banks, as long as the coupling transformer rating
is not exceeded.

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The TCR responds rapidly, typically in duration of one-anda-half to three cycles.
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Disadvantages of TCR

The TCRs do not possess high overload capability
because the air-core design of their reactors.
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Thyristor Switched Capacitors (TSC)
 Switching of capacitors excites transients.
 Thyristor firing controls are designed to
minimize the switching transients.
 Capacitance changed in discrete steps. The
susceptance is adjusted by controlling the no.
of parallel capacitors.
 The capacitors serve as filters for harmonics
when only the reactor is switched.
 Advantage: Dynamic stability is better
 Disadvantages: more no. of TSCs, more cost
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SWITCH OPERATION OF TSC
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Basic TSC (a) and associated waveforms (b)
Operating V-I area of single TSC
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TCR-FC
 The TCR-FC system provides continuously
controllable lagging to leading VArs through
thyristor control of reactor current.
 Leading VArs are supplied by two or more
fixed capacitor banks. The TCR is generally
rated larger than the total of fixed
capacitance so that net lagging VArs can also
be supplied.
 The variation of current through the reactor
is obtained by phase angle control of back to
back pair of thyristors connected in series
with the reactor.
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FIXED-CAPACITOR THYRISTOR
CONTROLLED REACTOR (FC–TCR)
 The
TCR
provides
continuously
controllable reactive power only in the
lagging power-factor range.
 To extend the dynamic controllable
range to the leading power-factor
domain, a fixed-capacitor bank is
connected in shunt with the TCR.
 The TCR MVA is rated larger than
61
the fixed capacitor to compensate the
capacitive MVA.
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FIXED-CAPACITOR THYRISTOR
CONTROLLED REACTOR (FC–TCR)
 The fixed-capacitor banks, usually connected
in a star configuration. Each capacitor
contains a small tuning inductor that is
connected in series and tunes the branch to act
as a filter for a specific harmonic order.
 One capacitor group is tuned to the 5th
harmonic and another to the 7th, whereas yet
another is designed to act as a high-pass filter.
At fundamental frequency, the tuning reactors
62
slightly reduce the net MVA rating of the fixed
capacitors.
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Basic TCR-FC and
its VAr demand vs VAr output
characteristics
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Characteristics of FC-TCR
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Drawbacks of FC-TCR
 A drawback of the FC–TCR SVC is the
circulation of large currents in the FC–TCR
loop needed for cancellation of capacitive
vars. This results in high steady-state losses,
even when the SVC is not exchanging any
reactive power with the power system.
 these losses can be minimized by switching
the fixed capacitors through mechanical
breakers, ensuring that the capacitors are
inserted in the compensator circuit only
65
when leading VARs are needed.
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Mechanically Switched Capacitors (MSC)
 In this scheme MSC’s are also used with TCR’s.
 Uses conventional mechanical or SF6 switches instead
of thyristors to switch the capacitors.
 More economical when there are a large no. of
capacitors to be switched than using TSCs.
 The speed of switching is however longer and this may
affect transient stability.
• This method is suitable for steady load conditions,
where the reactive power requirements are predictable
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Thyristor Switched Capacitor / Thyristor
controlled Reactor (TSC / TCR)
 A combination of TSC and TCR is, in
the majority of cases, the optimum
solution.
 With this combination, continuous
variable reactive power is obtained
throughout the complete control
range as well as full control of both
the inductive and the capacitive parts
of the compensator.
 This is a very advantageous feature
permitting
optimum
performance
during large disturbances in the
power system.
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Operating V-I area of the TSC-TCR type VAr
generator with two thyristor-switched capacitor banks
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TSC-TCR
Basic TSC-TCR type static var generator and its VAr demand vs VAr
output characteristic.
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Figure 11.52 A typical static var system
(b) Voltage-reactive power
characteristic
(a) Voltage-current
characteristic
Figure 11.53 SVS steady-state characteristics
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Static Synchronous Compensator
(STATCOM)
 STATCOM or Static Synchronous Compensator
is a power electronic device using force
commutated devices like IGBT, GTO etc. to
control the reactive power flow through a power
network and thereby increasing the stability of
power network. STATCOM is a shunt device i.e.
it is connected in shunt with the line. A Static
Synchronous Compensator (STATCOM) is also
known as a Static Synchronous Condenser
(STATCON). It is a member of the Flexible AC
Transmission System (FACTS) family of
devices.
 The terms Synchronous in STATCOM mean
that it can either absorb or generate reactive
power in synchronization with the demand to
stabilize the voltage of the power network.
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Working Principle of STATCOM:
To understand the working principle of
STATCOM, we will first have a look at the
reactive power transfer equation. Let us
consider two sources V1 and V2 are connected
through an impedance Z = Ra + jX as shown in
figure below.
In the above reactive power flow equation,
angle δ is the angle between V1 and V2. Thus
if we maintain angle δ = 0 then Reactive
power flow will become
Q = (V2/X)[V1-V2]
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P = V1V2Sinδ / X =0
To summarize, we can say that if the angle
between V1 and V2 is zero, the flow of active
power becomes zero and the flow of reactive
power depends on (V1 – V2). Thus for flow of
reactive power there are two possibilities.
1)
If the magnitude of V1 is more than V2,
then reactive power will flow from source V1 to
V2.
2)
If the magnitude of V2 is more than V1,
reactive power will flow from source V2 to V1.
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Design of STATCOM:
STATCOM has the following components:
1)
A Voltage Source Converter, VSC
The voltage-source converter is used to convert the
DC input voltage to an AC output voltage. Two of the
common VSC types are as below.
2)
a)
Square-wave Inverters using Gate Turn-Off
Thyristors: In this type of VSC, output AC voltage is
controlled by changing the DC capacitor input
voltage, as the fundamental component of the
converter output voltage is proportional to the DC
voltage.
b)
PWM Inverters using Insulated Gate Bipolar
Transistors (IGBT): It uses Pulse Width Modulation
(PWM) technique to create a sinusoidal waveform
from a DC voltage source with a typical chopping
frequency of a few kHz. In contrast to the GTObased type, the IGBT-based VSC utilizes a fixed
DC voltage and varies its output AC voltage by
changing the modulation index of the PWM
modulator.
DC Capacitor
DC Capacitor is used to supply constant DC voltage
to the voltage source converter, VSC.
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3)
Inductive Reactance
A Transformer is connected between the output of
VSC and Power System. Transformer basically
acts as a coupling medium. In addition, Tranformer
neutralize harmonics contained in the square
waves produced by VSC.
4)
Harmonic Filter
Harmonic Filter attenuates the harmonics and
other high frequency components due to the VSC.
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 Source V1 represents the output voltage of the
STATCOM. In case of reactive power demand
increases in the power system, STATCOM
increases its output voltage V1 while maintain
the phase difference between V1 and V2 to
zero (it shall be noted here that there will always
exists small phase angle between V1 and V2 to
cater for the leakage impedance drop in the
interconnecting Transformer ). As V1 > V2,
reactive power will flow from STATCOM to the
power system. Thus STATCOM, supplies
reactive power and acts as reactive power
generator.
 if the voltage of power system increase due to
load throw off, STATCOM will reduce its output
voltage V1 and therefore will absorb reactive
power to stabilize the voltage to normal value.
The above mode of operation of STATCOM is
called Voltage Regulation Mode.
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 As we know every equipment has got their
own limitations, so STATCOM must also have
some limitation of supplying or absorbing
reactive power. Yes, there exists a limitation
and this limitation is imposed by the current
carrying capacity of force commutated devices
like IGBT, GTO etc. Therefore, if the operation
of STATCOM reaches their limitation, it does
not further increases or decreases its output
voltage V1 rather it supplies or absorbs fixed
reactive power equal to its limiting value at a
fixed voltage and current and acts like
constant current source. This mode of
operation of STATCOM is called VAR Control
Mode.
Thus form the above discussion, the operation of
STATCOM can be classified into two modes:
1)
Voltage Regulation Mode
2)
VAR Control Mode
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V-I CHARACTERISTICS OF STATCOM
 The STATCOM can supply both the capacitive
and the inductive compensation and is able to
independently control its output current over the
rated maximum capacitive or inductive range
irrespective of the amount of ac-system voltage.
It is capable of yielding the full output of capacitive
generation almost independently of the system voltage
(constant-current output at lower voltages). This
capability is particularly useful for situations in which the
STATCOM is needed to support the system voltage
during and after faults where voltage collapse would
otherwise be a limiting factor.
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 Figure illustrates that the STATCOM has an
increased transient rating in both the capacitive- and
the Inductive-operating regions.
 In practice, the semiconductor switches of the
converter are not lossless, so the energy stored in the
dc capacitor is eventually used to meet the internal
losses of the converter, and the dc capacitor voltage
diminishes.
 However, when the STATCOM is used for reactivepower generation, the converter itself can keep the
capacitor charged to the required voltage level. This
task is accomplished by making the output voltages
of the converter lag behind the ac-system voltages
by a small angle (usually in the 0.18–0.28 range).
 In this way, the converter absorbs a small amount of
real power from the ac system to meet its internal
losses and keep the capacitor voltage at the desired
level.
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COMPARISON BETWEEN STATCOM AND SVC
 The main difference between a STATCOM and
an SVC is the way they operate: a STATCOM
works as a controllable voltage source while an
SVC works as a dynamically controllable
reactance connected in parallel.
 Compared with an SVC, a STATCOM offers the
possibility of feeding the grid with the maximum
available reactive current even at low voltage
levels, this is possible because in every
equilibrium condition the injected reactive power
varies linearly with the voltage at the Point of
Common Coupling (PCC). In contrast, for an
SVC there is a quadratic dependence of the
reactive power on the voltage at the PCC which
means that to inject the same reactive power it is
necessary to install an SVC with a nominal
capacity higher than that of a STATCOM.
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 With regard to the maximum transient capacitive
current it is observed that in an SVC the
capacitive current is limited by the size of the
capacitor and by the magnitude of the AC
voltage. In the case of a STATCOM the maximum
capacitive current that can be injected is limited
by the maximum current capacity of the
semiconductors used and is independent of the
voltage level at the PCC.
 Another feature of a STATCOM is that the DClink capacitor serves as storage for active power.
Therefore in certain situations, depending on the
capacitor size, it is possible to regulate the
interchange of active power with the grid also.
 STATCOM devices are capable of much faster
dynamic reaction (1/4-1 cycle) than an SVC. In a
STATCOM the speed of response is limited by
the commutation frequency of the IGBT’s
(normally 1 kHz).
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Comparison of
STATCOM and SVC Characteristics
(a) V-I characteristics:
(b) P- δ characteristic with mid-point compensation:
Source: N.G. Hingorani and L. Gyugi, "Understanding FACTS", IEEE Press, 1999
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Comparative Summary of Alternative
Forms of Compensation

Switched shunt capacitor compensation generally
provides the most economical reactive power
source for voltage control
 ideally suited for compensation transmission
lines if reduction of ZC, rather than reduction of
line length θ is the primary consideration
 however, heavy use of shunt capacitor
compensation could result in poor voltage
regulation and may have an adverse effect on
system stability

Series capacitor is self-regulating, i.e., its reactive
power output increases with line loading
 ideally suited for applications where reduction of
line length (θ) is the primary consideration
 improves voltage regulation and system stability

A combination of series and shunt capacitors may
provide the ideal form of compensation in some
cases
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Comparative Summary (cont'd)

A static var compensator (SVC) is ideally suited for
applications requiring direct and rapid control of
voltage
 has advantage over series capacitors where
compensation is required to prevent voltage sag
at a bus involving multiple lines; total cost may
be less than that for series compensation of each
of the lines

When an SVC is used to permit a high power
transfer over a long distance, the possibility of
instability when the SVC is pushed to its reactive
limit must be recognized
 when operating at its capacitive limit, the SVC
becomes a simple capacitor

An SVC has limited overload capability and has
higher losses than series capacitor compensation

STATCOM overcomes some of the limitations of an
SVC
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Tap-Changing Transformers

Transformer with tap-changing facilities constitute
an important means of controlling voltages
throughout the power system

Control of a single transformer will cause changes
in voltages at its terminals
 in turn this influences reactive power flow
 resulting effect on the voltages at other buses
will depend on network configuration and
load/generation distribution

Coordinated control of the tap changers of all
transformers interconnecting the subsystems
required to achieve overall desired effect

During high system load conditions, network
voltages are kept at highest practical level to
 minimize reactive power requirements
 increase effectiveness of shunt capacitors and
line charging
cont'd
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Tap-Changing Transformers (cont'd)

The highest allowable operating voltage of the
transmission network is governed by
 requirement that insulation levels of equipment
not be exceeded
 need to take into consideration possible
switching operations and outage conditions

During light load conditions, it is usually required to
lower network voltages
 reduce line charging
 avoid underexcited operation of generators

Transformers with under-load tap-changers (ULTC)
are used to take care of daily, hourly, and minuteby-minute variations in system conditions

Off-load tap-changing transformers used to take
care of long-term variations due to system
expansion, load growth, or seasonal changes
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Modelling of Transformer ULTC Control
Systems

Functional block diagram of ULTC control system
shown in Fig. 11.79 and block diagram suitable for
system studies

Line drop compensator regulates voltage at a
remote point along the line or feeder

Measuring element consists of adjustable dead
band relay with hysteresis. The output of the
measuring element is Vm; which takes a value of 0,
1, or -1, depending on input Verr

Time delay element prevents unnecessary tap
changes
Figure 11.79 Functional block diagram of control system for automatic
changing of transformer taps
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Figure 11.80 ULTC control system model
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Distribution System Voltage Regulation

Substation bus regulation
 substation transformer equipped with ULTC
facilities to control secondary voltage
 alternatively, substation may have a separate
voltage regulator

Feeder regulation
 feeder regulators control the voltage of each
feeder
 older units are the induction type - provide
accurate and continuous control; however, they
are costly and have been superseded by step
type regulator
 step voltage regulator (SVR) is basically an
autotransformer with taps or steps in the series
winding; however, it is purely a voltage control
device and not used for voltage transformation
cont'd
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Figure 11.75 Schematic of an induction regulator
Figure 11.76 Schematic of a step voltage regulator
Figure 11.77 SVR control mechanism
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Distribution System Voltage Regulation
(cont'd)

Application of voltage regulators and capacitors for
control of voltage profile along a feeder is
illustrated in Fig. 11.78
 curve 1 shows voltage with distributed loads
along the line, without any regulation
 the addition of voltage regulator R1, capacitor C
and voltage regulator R2, brings the voltage
profile along the entire feeder (from the first
consumer to the last) to within max and min
limits
Figure 11.78 Voltage profile of a feeder with a station regulation (R1),
supplementary regulator (R2) and a shunt capacitor bank (C)
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Implementation of Overall Reactive Power
Control

Effect of reactive power control is felt mostly
locally:
 equipment for supplying Q at appropriate points
throughout the system necessary

Coordination of the overall scheme a complex task:
 approach is still largely based on operator
experience and off-line load flow studies
 implementation of automated schemes with
optimum dispatch is feasible and practical
methods are being pursued

EDF and ENEL have used secondary and tertiary
voltage control schemes to provide coordinated
voltage control in HV networks
 CIGRE TF 38.02.23 set up to assess the potential
and provide guidelines
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Appendix to Section on
Control of Reactive Power and Voltage
1. Copy of Section 11.2.9 from the book “Power
System Stability and Control”

- Provides information on Modeling of Reactive
Compensating Devices
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