ANESI

FLEXIBLE AC TRANSMISSION SYSTEM

(FACTS)&POWER QyALITY
Dr. Krishna Sarker
Department of Electrical Engineering
Lecture Series on Electrical Power System
Lecture On FACTS & PQ
 2 MARKS

1. What is the necessity of compensation?

significantly improve the
The reactive power through the system can
power angle
pertormance / parameters of the power system such as voltage profile,
characteristics, stability margin and Damping to power oscillations
2. What are the objectives of line compensation?
() To increase the power transmission capacity of the line
acceptable
(ii) To keep the voltage profile of the line along its length within
as well
bounds to ensure the quality of supply to the connected customer
as to minimize the line insulation costs

FACTS devices?
3. How isthe reactive power controlled, using
voltage at its
The SVC is a shunt device of the FACTS group, regulates
in to or absorbed from
terminals by controlling the amount of reactive power injected
SVCgenerates reactive power
the power system. When a system voltage is low, the
reactive power (SVC
(SVCCapacitive). When a system voltage is high, it absorbs
inductive)
network?
4. How isreactive power controlled in electrical
mechanically
Traditionally, rotating synchronous condensers and fixed or
compensation.
switched capacitors or inductors have been used for reactive power
provide or absorb the
However, in recent years static VAR compensators are used to
required reactive power have been developed. Dr. Krishna sarker
 network.
5.
Explainthe objectives of FACTScontrollers in the power system
in transmission
() Better the control of power flow (Real and Reactive)
lines.
(i) Limits Short circuit current and increase the loadability
(iii) Increase dynamic and transient stability of power system
(iv) Load compensation
(v) Power quality improvement
6. What are the advantages of FACTS controllers?
the
(i) The flow of power is ordered. It may be as per the contract or as per
requirements of the utilities
(ii) Itincreases the loading capability of the lines to the thermal capability
(i11) It improves the stability of the system and thus make the system secure
(1V) Provides secure Tie Line connection to the neighboring utilities and
regions, thereby decreasing overall generation reserve requirements on
both sides
7. List the disadvantage of fixed series compensation.
(i) It is effective only during heavy loads
(i) Whenever an outage occurs on a line, with series compensation, the
other
series compensation is removed. This may cause overloading of
parallel lines
(iii) If series compensation is added to an existing system, it is generally
necessary to have it on all the lines in parallel.
(iv) One major drawback in the series capacitance compensation is that
special productive devices are required to protect the capacitors and
bypass the high current produced when a short circuit occurs.
8 What is meant by Thyristor Controlled Series Capacitor?
TCSC is a capacitive reactance compensator, which consists of series capacitor
bank shunted by a thyristor-controlled reactor.

6|Page
Dr. Krishna Sarker
9.
Define the term Static VAR
The SvC is a shunt device of
compensator.
FACTSgroup using power electronics to conto
POwer tlow and improve transient stability on power
grids. The SVC reguiates vonta
at s terminals by controlling the amount of
reactive power injected into or absorbed
from the power
system.
10,
What are the different types of compensation
schemes?
Mainly two types of compensation are carried out,
() Load compensation
(i) Line compensation
11, Define the term FACTS.
Alternating current transmission system incorporating power electronics based
and other static controllers to enhance controllability and
increase power transter
capability
12, What is best location for SVC?
In general the best location is at a point where voltage swings are
greatest.
Normally, the midpoint of atransmission line between the two areas is a good
location.
13. What are the main areas of application of FACTS devices?
FACTS mainly find application in following areas,
i) Power transmission
(ii) Power Quality
(i) Wind power grid Connection
(1) Cable Systems
14. What is load compensation?
Load compensation is a management of reactive power to improve the quality
of supply especially the voltage and power factor levels
Three main objectives of the load compensation are
(1) Better voltage profile

Dr. Krishna Sarker 7| Page

 () Power factor correction
(ii) Load balancing
15,
Define VAR compensation.
Is defined as the management of reactive power to improve the pertormance
OT AC Power systems: Maximizing stability by increasing flow of active power.
16.
What are various categories of FACTS controllers?
(i) Series FACTS controllers
(iii) Shunt FACTS controllers
(iV) Combined Series- Series FACTS controllers
(v) Combined Series- Shunt FACTS controllers
17. What is IPFC?
Interline power Flow Controller is a combination of two or more independently
controllable static synchronous series compensator (SSSC) which are solid state
Voltage source converters which inject an almost sinusoidal voltage at variable
magnitude and couples via a common DC link.
18. What are reactive power compensation and Compensators?
Reactive power control for a transmission line is often called reactive power
compensation. External devices or sub systems that control reactive power on
transmission line are known as compensators.

19. What is shunt compensation?

The Shunt compensators are connected parallel to the transmission lines with
the help of Circuit breakers. Shunt reactors compensate for the line capacitance, and
they control over voltages at no loads and light load conditions. The shunt
compensators need careful system design because of high charging in-rush currents.
20. What is series compensation?
The Series compensators are connected series with the transmission lines.
Series compensators are used to partially offset the effects of the series inductances of
transmission lines. It provides automatic adjustment of reactive power compensation.
8|Page
Dr. Krishna Sarker
21,
What are the methods used for compensating the
uncompensated
transmission lines?
() Load compensation- One capacitor is connected parallel across the load
(ii) System compensation - In addition with the parallel capacitor the power
utility devices are also connected.
22. List the
advantages of SVC
() In high-power networks, SVCs are used for voltage control and for
attaining several other objectives such as damping and stability control.
(ii) Increase in Steady-State Power-Transfer Capacity
(iii) Enhancement of Transient Stability

16 MARKS
I ) Give the complete analysis of lossless distributed parameter transmission
lines and derive power equations for symmetrical case (12)
(ii) Write a brief note on IPFC (4)

1. ()Give the complete analysis of lossless distributed parameter transmission

lines and derive power equations for symmetrical case (12)

Most power-transmission lines are characterized by distributed parameters:

series resistance, r, series inductance, I; shunt conductance, g and shunt
capacitance, c--all per-unit (pu) length.
& These parameters all depend on the conductors' size, spacing, clearance above
the ground, and frequency and temperature of operation.
In addition, these parameters depend on the bundling arrangement of the line
conductors and the nearness to other parallel lines.

Dr. Krishna Sarker 9|Page

 Ihe characteristic behavior of atransmission line is dominated by its and c
Parameters. Parameters r and g account for the transmission losses. The
undamental cquations governing the propagation of energy along a line are the
following wave equations:
dyzzy (1)

dyzzy+ (2)

where zy =(r t jol)(g +joC).

For a lossless line, the general solutions are given as
V(*) = V,cosßx -jZ,1,sin Bx (3)

T() =I,cosßx -isin fx (4)

* These equations are used to calculate voltage and current anywhere on line, at a
distance xfrom the sending end, in terms of the sending-end voltage and current
and the line parameters. From the above equations we get
V,cosß, -7,
I,= (5)
jZ,sinßa
Where

Z, = |Q- the surge impedance or characteristic impedance

B= wVlc rad/km = the wave number

Bo wvlca = the electrical length of an a-km line

Iis the line inductance in henries per kilometer (H km), c is the line shunt
capacitance in farads per kilometer (F/ km), and 1/Wlc is the propagation
velocity of electromagnetic effects on the transmission line.

If ,=V,0 and , V, 2- 8=, (cos 8-jsin 8), then

Dr. Krishna Sarker 10 | Page

 V,sin& + j(V,cos6-- V,cosßa)
(6)
Zosinßa
Therefore, the power at the sending end is given as
S, = P,+jQ, = V,; (7)

S, = V,V,sins +j V;cosßa-V,V,cos8
Zosinßa (8)
Zosinßa
Likewise, power at the receiving end is given as

S, = P, +jO. StnoVcosßa-V,V,coss
+j (9)
Zosinßa Zosinßa
P+jQs P,+jQ,

V,

aal2
aa

Figure 1.1: The power on a lossless distributed line.

Comparing Equations (8) and (9) and taking the directional notation of Fig, 1.1
into account, it is concluded that for a lossless line, P, =-P, as expected.
However, 0, #0, ,because of thereactive-power absorption/ generation in the
line. From Equations (8) and (9), the power flow from the sending end to the
receiving end is expressed as

P=: X sin& (10)

Dr. Krishna sarker 11 | Page

 AcCordingly, the maximum power transfer is scen to depend on the line length
Wheh the power-transfer requirement for a gi ven length of a line increases
higher transmission voltages of V, and V, must be selected.
(i) Write a brief noteon IPFC (4)
In its general form the Interline Power Flow Controller employs a number of
dc-to-ac converters each are providing series compensation for a different line.
*n other words, the IPFC comprises a number of Static Synchronous Series
Compensators. However, within the general concept of the IPFC, the
Compensating converters are linked together at their dc terminals.
3 With this scheme, in addition to providing series reactive compensation, any
converter can be controlled to supply real power to the common de link frorn its
own transmission line. Thus, an overall surplus power can be made available
from the underutilized lines which then can be used by other lines for real
power compensation.

* In this way, some of the converterS, compensating overloaded lines or lines

with a heavy burden of reactive power flow, can be equipped with full two
dimensional, reactive and real power control capability, similar to that offered
by the UPFC.

Evidently, this arrangement mandates the rigorous maintenance of the overall

power balance at the common de terminal by appropriate control action, using
the general principle that the under loaded lines are to provide help, in the form
of appropriate real power transfer, for the overloaded lines.
2. What are the objectives of line compensation? Explain the effect of shunt
and series compensation on power transmission capacity of a short symmetrical
transmission line (Or) Explain the effect of shunt and series compensation on
power transmission capacity (Or) Explain the detail about effect of shunt and
series compensation on transmission line. (16)

Dr. Krishna sarker 12 | Page

ObËectives of line compensation
Reduces line voltage drops
* Limits load-dependent voltage drops
Influences load flow in parallel transmission lines
Increases transfer capability
Reduces transmission angle
§ Increases system stability
Eitect of shunt and series compensation on
transmission line
Shunt compensation

* Shunt reactors compensate for the line capacitance, and because they control
over voltages at no loads and light loads, they are often connected permanently
to the line, not to the bus.
Shunt capacitors are used to increase the power-transfer capacity and to
compensate for the reactive-voltage drop in the line. The application of shunt
capacitorS requires careful system design.
Effect of Shunt compensation on Transmission Line

V,= VZò
X,/2 X,i2
Vat

AB

82
V,= VL0'

The midpoint-capacitor compensation of a short, symmetrical line

Dr. Krishna Sarker

13 | Page
 Reconsider the short. symmetrical line: apply a shunt capacitor at the midpoint
0 ne line So that a shunt susceptance is incrementally added (AB), as shown in
igure. For the system in this figure, the power transfer in terms of the midpoint
voltage on the line is
P= VWm
-sinz (1)

*The differential change in power, DP, as aresult of adifferential change, AVm.

Is given as
2V
AP =sinzVm (2)

AI, = VmABe (3)

3 The current A, in the midline shunt capacitor modifies the line currents in the

sending and receiving ends of the line to the following:

I,= I,- 2 and I, = l+ 2
(4)

As Vm=V,+jli,X/2,

AVm = 4 4
(5)
Substituting the results of Eq. (5) in Eq. (2), we get
Wm
AP =
2 sin,AB: (6)
If the midpoint voltage of the line is approximately equal to Vcos ô/2, then the
Incremental rating of the shunt-capacitor compensation will besQ:h = VAB.,

AP 1
(7)
AQsh
§ By comparing the above, we deduce that for an equivalent power transfer on a
short electrical line,

Dr. Krishna Sarker 14 | Page

 2
DQse (8)
AQ sh -(n)
Series compensation
series inductances
oeleS capacitors are used to partiallyoffset the effects of the
maximum
Ol mes. Series compensation results in the improvement of the
power-transmission capacity of the line.
The net effect is alower load angle for a given power-transmission level and,
therefore, ahigher-stability margin.
* The reactive-power absorption of a line depends on the transmission current, so
when series capacitors are employed, automatically the resulting reactive-power
compensation is adjusted proportionately.
* Also, because the series compensation effectively reduces the overall line
reactance, it is expected that the net line-voltage drop would become less
susceptible to the loading conditions.

Effect of Series compensation on Transmission Line

The consideration of series compensation invariably raises the issue of its

comparison with shunt compensation. A simple system analysis can be
performed to develop a basic understanding of the effect of shunt and series
compensation on power-transmissioncapacity.
Consider a short, symmetrical electrical line as shown in figure for an
uncompensated line, and assuming V, = V,=V, the power equation becomes

X-sinQ =2sinz
P= (9)
cosz
From the voltage-phasor equations and the phasor diagram in Figure,
2V

I|-sinz (10)

Dr. Krishna Sarker 15 | Page

 the effective reactance of aline is controlled by inserting a series
capacitor,
and if the line terminal voltages are held unchanged, then a AX,change in the
Iine reactance will result in a Al change in the
current, where
2V AX,
Al,=sin;X= V, =V28
(11)

X,12 X/2

V,20
(a)

X,/2 -1X X,2 jU,+ ,)

(X+4X,)

(b

Series compensation ofa short symmetrical transmission line

Therefore, from Eq. (2.21), the corresponding change in the power transfer will be
AP =
2sin,cos; 4x; (12)
Using Eqs. (11) and (12), Eq. (13) may be written as
1
AP = -Ax) (13)
2tanz
As

-AXl is the reactance added by series capacitors, -AX; = A0,, represents the
incremental var rating of the series capacitor. Therefore
AP 1
(14)
AQse 2tan~
Dr. KrishnaSarker
16 | Page
| 3. (8)
Explain the concept and need for reactive power.
rated
(i) Discuss the possible control actions tomaintain the voltage at
value in transmission line
(8)

3. (i) Explain the concept and need for reactive power. (8)

Concept
The portion of electricity establishes and sustains the electric and magnetic
ields of alternating-current equipment. Reactive power must be supplied to most types
of magnetic equipment, such as motors and transforners. It also must supply the
reactive losses on transmission facilities. Reactive power is provided by generators,
synchronous condensers, or electrostatic equipment such as capacitors and drectly
influences electric system voltage. It is usually expressed in Kilovars (KVAR) or
Megavars (MVAR).
Need for reactive power

Voltage control in an electrical power system is important for proper operation

for electrical power equipment to prevent damage such as overheating of
generators and motors, to reduce transmission losses and to maintain the ability
of the system to withstand and prevent voltage collapse.
* Decreasing reactive power causing voltage to fall while increasing it causing
voltage to rise. A voltage collapse may be occurs when the system try to serve
much more load than the voltage can support.
* When reactive power supply lower voltage, as voltage drops current must
increase to m¡intain power supplied, causing system to consume more reactive
power and the voltage drops further If the current increase too much,
transmission lines go off line, overloading other lines and potentially causing
cascading failures.

Dr. Krishna Sarker 17 |Page

 he voltage drops too low. some generators willdisconnect automatically to
protect themselves. Voltage collapse occurs when an increase in load or less
Beneration or transmission facilities causes dropping voltage, which causes a
Turther reduction in reactive power from capacitor and line charging. and still
there further voltage reductions.
*T voltage reduction continues, these will cause additional elements to trip,
leading further reduction in voltage and loss of the load. The result in these
entire progressive and uncontrollable declines in voltage is that the system
unable to provide the reactive power required supplying the reactive power
demands

(ii) Discuss the possible control actions to maintain the voltage at rated value
in transmission line (8)

3 Voltage stability is concerned with the ability of a power system to maintain

acceptable voltage at allbuses in the system under normal conditions and after
being subjected to a disturbance
Tomaintain security of such systems, it is desirable to plan suitable measures to
improve power system security and increase voltage stability margins.
The only way to counteractthis problem is by reducing the reactive power load
in the system or by adding new reactive power generation systems in the
weakest points of the system, thereby, increasing the voltage at those points.
* The flexible AC transmission system (FACTS) controllers are capable of
supplying or absorbing reactive power at faster rates. The introduction of
FACTS controllers are increasingly used to provide voltage and power flow
controls.
Insertion of FACTS devices is found to be highly effective in preventing
voltage instability and minimize the active or real power loss on transmission
lines.

Dr. Krishna Sarker 18 | Page

 DeneS and shunt compensating devices are used to enhance the static voltage
Stability margin and reduce the real power loss appreciably
When a series-connected compensating voltage is used to control the
ransmission line voltage, the compensating voltage is also at any phase angie
with the prevailing line current.
n the process., it emulates in serjes with the line a capacitor that
increases the
Power flow of the line or an inductor that decreases the power flow of the Iine,
and a positive resistor that absorbs active power from the line or a negative
resistor that delivers active power to the line. Therefore, the desired
Compensating voltage is actually an impedance emulato.
A Shunt-connected compensating voltage can also modify the transmission
Iine's sending- end voltage. In certain special cases for point-to-point transfer of
power between two isolated networks or interconnection of two transmisSIon
lines with different voltages or phase angles (or frequencies), this scheme is a
preferred choice.

4. Explain the basic construction, working and characteristics of any one type
of SVC
(16)

SVC
Static var compensators (SVCs) are used primarily in power systems for voltage
control as either an end in itself or a means of achieving other objectives, such as
system stabilization. The performance of svC voltage control is critically dependent
on several factors, including the influence of network resonances, transformer
saturation, geomagneticeffects, and voltage distortion.
Types ofSVC

Saturable reactor
Thyristor controlled reactor
Dr. Krishna Sarker 19 Page
 * Thyristor switched capacitor
Thyristor switched reactor
Thyristor controlled transformer
(1). The Thyristor-Controlled Reactor (TCR)
Construction
A TCR is one of the most important building blocks of Thyristor-based SvCS.
Although it can be used alone, it is more often employed in conjunction with fixed or
thyristor-switched capacitors to provide rapid, continuous control of reactive power
over the entire selected lagging-to-leading range.

ITCA
+

Schematic diagram ofa TCR

High-Voltage
Compensator Bus

Potential
Translormer
Medium-Voltage
Compensator Bus

Voltage-Regulator
and -Conttol
Fxed
System Capacitors LC Filter

TCR

One line diagram ofa TCR compensator with fixed-shunt capacitors

Dr. Krishna Sarker 20 | Page

 *A bSic single-phasc TCR comprises an
anti-parallel-connected pair o
thyristor valves, T; and I;, in series with alinear air-core reactor, as illustrated
in Figures 1&2.
*The anti-parallel-connected thyristor pair acts like a
bidirectional switch, with
Ihyristor valve T; conducting in positive hlf-cycles and thyristor valve 12
Conducting in negative half-cycles of the supplyvoltage.
The firing angle of the thyristors is measured from the zero crossing of the
voltage appearing across its terminals. The controllable range of the TCR firing
angle, a, extends from 90 to 180.
A fiing angle of 90 results in full thyristor
conduction with acontinuous
Sinusoidal current flow in the TCR. As the firing angle is varied from 90 to
Close to 180", the current flows in the form of
discontinuous pulses
symmetrically located in the positive and negative half-cycles.
Once the thyristor valves are fired. the cessation of current
occurs at its natural
Zero croSSing, aprocess known as the line commutation. The current reduces to
zero n for a firing angle of 180".
$ Thyristor firing at angles below 90 introduces dc
components in the current,
disturbing the symmetrical operation of the two anti parallel valve branches.
A characteristic of the line-commutation process with
which the TCR operates
is that once the valve conduction has
commenced, any change in the firing
angle can only be implemented in the next half-cycle, leading to the so-called
thyristor dead time.
Operating Characteristics of a TCR
Operating Characteristics without Voltage Control
The sinusoidal current flowing in this reactor is equal to the fundamental
component of the non sinusoidal current flowing in the TCR.

Dr. Krishna Sarker

21 | Page
 ror a general SVC. this can be considered as a black box with an
unknown but
purely reactive circuit inside, the overall compensator susceptance Bsvc can be
defined with the following equation:
I_vc= VjBsvc (1)

In the simple case of a TCR, the

compensator susceptance is
Bsyc = BrcR (2)
140
a=

Vsvc 4
Operating Range
Lirnit
Production
mtion

0Bsvc

svc
The voltage-current (V-I) characteristics without voltage control

This shows the SVC current as a function of the system voltage for different
firing angles, as depicted in Figure.
This V-I characteristic is given in a very general sense. No control system is
assumed to vary the firing angle, and any operating point within the two limits
is possible depending on the system voltage and the setting of the firing angle.
This characteristic clearly illustrates the limits of the operating range, and it
may include the steady-state characteristics of the various possible controls.
This characteristic is the usual way in which the system engineers prefer to look
at the compensator, because the characteristic shows the steady-state
performance of the SVC plant.

Dr. Krishna Sarker 22 | Page

Operating Characteristic with Voltage Control
* Two system characteristics such as system 1and system 2 are
depicted in
Figure, that illustrate the decline in system node voltage when the
node is
loaded inductively and reactive power is
absorbed.
The Corresponding operating points for the two system
conditions are A, and Az.
T the system voltage of system 2 rises will creates new
Current
Limit characteristic system 2.

Vsvc
Lint
Production
Jndervoltage
Vret Absorptiun
Limit
System 2"
System 2
System 1

'svc Isvo
Production Control Range Absorption
Overtoad Range
The voltage-current (V-I) characteristics with voltage control

Operating point A then moves to the right and reaches the absorption limit of
the compensator.
Any further increase in system voltage cannot be compensated for by the
control system, because the TCR reactor is already fully conducting.
The operating point A Will, therefore, move upward on the characteristic,
corresponding to the fully on reactor connected to the system.
3 The compensator then operates in the overload range, beyond which a current
limit is imposed by the firing control to prevent damage to the thyristor valve
from an over current.

At the left-hand side, the compensator will reach the production limit if the
system voltage drops excessively; the operating point will then lie on the
characteristic of the under voltage range.

Dr. Krishna Sarker

23 | Page
 (2) AThyristor switched capacitor (TSC)
*A thyristor switched capacitor (TSC) is a type of equipment used tor
compensating reactive power in electrical power systems.
* ltconsists of a power capacitor connected in series with a bidirectional thyristor
valve and, usually, a current limiting reactor (inductor).
* The thyristor switched capacitor is an important component of a Static VAR
Compensator (SVC), where it is often used in conjunction with a thyristor
controlled reactor (TCR).
$ Static VAR compensators are a member of the Flexible AC transmission
system (FACTS) family
*A TSC is usually a three-phase assembly, connected either in a delta or a star
arrangement.
Ünlike the TCR, a TSC generates no harmonics and sO requires no filtering. For
this reason, some SVCs have been built with only TSCs.
This can lead to a relatively cost-effective solution where the SVConly requires

capacitive reactive power, although a disadvantage is that the reactive power

output can only be varied in steps.
Configuration
A basic single-phase TSC consists of an anti-parallel-connected thyristor-valve
pair that acts as a bidirectional switch in series with a capacitor and a current
limiting small reactor, as shown in Fig.(a).
The thyristor switch allows the conduction for integral number of half-cycles.
The capacitor is notphase controlled, as is a TCR.
The small-series inductor is installed to limit current transients during
overvoltage conditions and planned switching operations, as well as when
switching at incorrect instants or at the inappropriate voltage polarity.
The inductor magnitude is chosen to give a natural resonant frequency of four
to five times the system nominal frequency, which ensures that the
inductance
24 | Page
 neither creates a harmonic-resonant circuit with the network nor hampers the
TSCcontrol system.
* Another function of this series inductor is to act in
combination with the
capacitor as a filter for harmonics generated by the associated TCR.
*In some cases, discharge circuits are provided with the capacitors to rapidly
dissipate the remnant charge on the capacitor after a switch-oft.

(a)

Turn-Oft v)

Dschauing
No Discharging
Vel)

Vrt)

Voltages after turn-off to the TSC: (a) a circuit diagram and (b) the
Voltage -Current waveforms

A practical TSC compensator involves n 3-phase TSC banks of equal rating

connected in shunt.
The overall TSC susceptance at any given instant is the
sum of conducting
TSC. In some cases, the ratings of different constituent TSC steps may be
chosen based on a binary system.

Dr. Krishna Sarker 25 | Page

 In this
scheme, capacitors are rated for susceptance Band one capacitor is
rated for
3 Thus thesusceptance
B/ 2.
total number
of possible TSC steps gets extended to 2n. The TSC
provides a fast
response--typically between one-half to one cycle.
However, this response time may be extended because of any delays in the
measurement and control systems. The TSCs provide
switching operations, in stark contrast to MSCs. virtually unlimited
Operating Characteristic
4Vsvc

AV2 System Load Line V

C,G.c C,

IT8C
Capacitive -4 Inductve
The operating characteristic of a TSC

The TSC has a discrete

voltage-current operating characteristic, as shown in
Figure. The shape of this characteristic a function of the
number of TSC
units, their individual ratings, and a hysteresis
voltage AV, which is built in to
avoid undesirable frequent switching of
capacitors.
3 In a closed-loop voltage control
operation, the TSC regulates the bus voltage
within the range V,ref t AV/ 2.

Dr. Krishna Sarker

26 | Page
S Explain in detail about the
classification of diferent FACTS controllers. (16)

* Flexible AC Transmission System (FACTS) is defined as

'Alternating current
Tansmission systems incorporating power electronic-based and other static
Controllers toenhance controllability and increase power transfer capability.
The FACTS controller is defined as a power
electronic based system and other
Static equipment that provide control of one or more AC
transmission system
parameters.
The FACTS controllers can be
classified as
1. Shunt connected
controllers
2. Series connected controllers
3. Combined series-series controllers
4. Combined shunt-series controllers
Series Controllers

H
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 (or a combination) 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.

Dr. Krishna Sarker

27 | Page
 *AS long as the voltage is in phase
quadrature with the line current, the series
Controlleronly supplies or consumes variable reactive power. Any other phase
relationship will involve handling of real power as well.
Shunt Controllers

Line

As in the case of series

Controllers, the shunt Controllers may be variable
impedance, variable source, or a combination of these. In
Controllers inject current into the system at the point ofprinciple, all shunt
Even variable shunt impedance connection.
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.
Combined series-series Controllers

ines
ac

dc power
link

Dr. Krishna Sarker

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