UNIT IV: Reactive Power Control

Overview of Reactive Power Control, Reactive Power

Compensation in Transmission Systems, advantages and
disadvantages of different types of Compensating Equipment
for Transmission Systems, Load Compensation, specifications
of Load Compensator, Uncompensated and Compensated
Transmission Lines, Shunt and Series Compensation. Brief
introduction to role of FACTS devices for Reactive power
Control.
 Introduction
The modern electric utility industry began in the 1880s. On
September 4, 1882, the first commercial power station, located on
Pearl Street in lower Manhattan, known as Pearl Street electricity
generating station went into operation providing light and
electricity power to customers in a one square mile area.

This was a DC power system built by Thomas Alva Edison, which

was comprised of a steam driven DC generator connected through
an electrical cable system at 110V and 59 customers spread over an
approximate area with 1.5 km radius.

But now electrical energy is generated, transmitted and distributed

exclusively in the form of alternating current (AC).
 Introduction
The main drawback of DC generation is that it can neither be
generated in bulk nor stepped up to higher levels for efficient
transmission.

Development of transformers changed the intact scenario of

electrical power systems. Invention of induction motors (single
phase and poly phase) by Nikola Tesla and the massive increase in
load demand called for restructuring of the power systems.

A majority of the loads in the present power system are inductive in

nature, and whenever they are supplied from an AC source, they
draw a current which is lagging by the applied voltage at an angle
and immediately the power factor comes into picture.
 Power Factor
The cosine of the phase angle between two electrical quantities such
as voltage and current in an AC circuit is known as power factor.
For instance, consider a load supplied by a voltage V and drawing a
current of magnitude I , which lags the voltage by an angle . The
phasor diagram of the circuit is given, in Fig. 4.1.

The power factor of the circuit is given by, Power Factor =

Fig. 4.1 Phase diagram, I lags V

 Power Factor
The load current I can be resolved into two components.

1. Horizontal component, I Cos in phase with the applied voltage V

2. Vertical component, I Sin 90° out of phase with the applied voltage V

The horizontal component, I Cos is known as active or wattful

component and the vertical component, I Sin is known as the reactive
or wattless component.

The reactive component is a measure of the power factor.

Greater the reactive component, larger the phase angle between the
applied voltage and the current drawn by the load. Hence the power
factor Cos is low.

Therefore, a circuit having small reactive current I Sin will have high
power factor and vice-versa.
 Power Factor
Power factor is a characteristic of alternating current (AC) circuits.
Its value is always between (0.0) and (1.0), the higher the number,
the greater or better the power factor.

If a purely resistive load is connected to a power supply, the current

and voltage will be in phase with each other, and the angle equals
to zero. Consequently, the power factor will be unity and the
electrical energy flows in a single direction across the network in
each cycle.

Inductive loads such as transformers and motors (any type of

wound coil) generate reactive power with current waveform lagging
the voltage.

Capacitive loads such as capacitor banks or buried cables generate

reactive power with current phase leading the voltage.
 Power Factor
Both types of loads will absorb energy during part of the AC cycle,
and store it in the magnetic or electric field, only to return
this energy back to the source during the rest of the cycle.

The power factor concept can also be analyzed in terms of power

drawn by the circuit, from the power triangle shown in Fig. 4.2

Fig. 4.2 Power Traingle

 Power Factor
The right angled triangle ABC shown in Fig. 4.2 is known as the power triangle
and is directly drawn from the triangle OAB in the phasor diagram shown in
Fig. 4.1 Each side of the current triangle OAB of Fig. 4.1 is multiplied by voltage
V , to get the power triangle ABC shown in Fig. 8.2.

To understand the power factor, we will first start with the definition of
some basic terms:

From the power triangle ABC, AB = V I Cos which represents Active

Power (also called Actual Power or Working Power or Real Power) and is
measured in Watts or Kilowatts i.e., W or KW. It is the power that actually
enables the equipment and performs useful work.

BC = V I Sin which represents Reactive Power and is measured in Volt

Ampere Reactive i.e. or . It is the power that a magnetic
equipment (transformer, motor etc.,) needs to produce the magnetizing
flux.
 Power Factor
AC = V I , which represents Apparent Power and is measured in
Volt Amperes or Kilo Volt Amperes i.e. or . It is the
"vectorial summation" of KVAR and KW.

The power factor can also be defined in terms of power by inferring

some points from the power triangle.

The apparent power V I has two components. One is V I Cos i.e.,

Active Power and the other one is V I Sin i.e., Reactive Power at
right angles to each other.

From the power triangle which is a right angled one, we can write,
 Power Factor

So, the power factor can also be defined as the ratio of active power
to apparent power.

It is clear from the power triangle that the lagging reactive power
due to the lagging current is responsible for the low power factor.

The smaller the reactive power, higher the power factor and vice
versa.
 Reactive Power
In a purely resistive AC circuit, voltage and current waveforms are
in phase and reverse their polarity at the same instant in each cycle
(illustrated in Fig. 4.3).

Fig. 4.3
 Reactive Power
When reactive loads are present (such as capacitors or inductors),
energy is stored in the form of electric or magnetic fields respectively in
the loads during part of the AC cycle and results in a time difference
between the current and voltage waveforms (for a capacitor, current
leads voltage; for an inductor, current lags voltage).

This stored energy returns to the source during the rest of the cycle and
is not available to do work at the load. This energy, continuously
flowing back and forth (to and fro), is known as reactive power while
the active power flows from one point of the network to another.

For a complete cycle, the net reactive power flow is zero as the energy
flowing in one direction for half a cycle is equal to the energy flowing in
the opposite direction in the next half of the cycle.
 Sources of Reactive Power
The main sources of reactive power are given below.
Overhead Lines:
When current flows through a line it produces a magnetic field that
absorbs reactive power, given by I2X where I is the current flowing
through the line and X is the reactance of the line in ohms per phase.
A lightly loaded overhead line is a net generator of reactive power,
whereas a heavily loaded line is a net absorber of reactive power.
However, underground cables have a small inductance and relatively
large capacitances due to closeness, large size of conductors and high
relative permitivity of the dielectric material used. Hence, they generate
reactive power.
Transformers and Motors:
Transformers produce magnetic fields and therefore absorb reactive
power. Inductive loads such as motors (any type of wound coil) absorb
reactive power with their current waveform lagging the voltage.
 Causes of Low Power Factor
Circuits containing purely resistive loads such as filament lamps,
strip heaters, cooking stoves, etc., operate with a power factor of
unity. When the power factor is 1, all the energy supplied by the
source is consumed by the load.

Circuits containing inductive or capacitive elements usually have a

power factor below 1. When the power factor is equal to 0, the
energy flow is entirely reactive, and stored energy in the load
returns to the source at the end of each cycle.

Good power factor is considered to be greater than 0.85 or 85%.

 Causes of Low Power Factor
Most A.C motors (any type of wound coil) are of the induction type
i.e., single-phase or three-phase induction motors generate reactive
power with the current waveform lagging the voltage, and
consequently operating at a poor power factor.

Lighting loads such as arc lamps and electric discharge lamps

operate at a low lagging power factor.

Industrial heating furnaces such as arc furnaces usually operate on

the principle of striking an arc, and operate at a low lagging power
factor.

Normal power factor (NPF) ballasts typically have a value of (0.4)

(0.6). Ballasts with a power factor greater than (0.9) are considered
to be high power factor ballasts (HPF).
 Disadvantages of Low Power Factor
Power factor plays a vital role in A.C circuits since the power
consumed depends upon the operating power factor.

It is evident that Real power P = V I I = P /V

From the above expression, for a fixed power and voltage the load
current is inversely proportional to the power factor. Lower the
power factor, higher will be the load current and vice versa.

Low power factor results in the following disadvantages:

1. Increase in copper losses When a load has a power factor lower

than 1, more current is required to deliver the same amount of
useful energy. Copper losses (I2R losses) increase with increase in
current. This results in poor efficiency with increase in losses.
 Disadvantages of Low Power Factor
2. Large conductor size
To transmit a given power at a constant voltage, the conductor will
have to carry relatively more current at a lower power factor. This
obliges greater conductor size i.e., the cross-sectional area of
transmission lines, cables and motor conductors has to be designed
based on the increased current and entails more cost.
3. Large KVA rating
Electric machinery like alternators, transformers, motors and
switchgear are always rated in KVA. And KVA = KW / Cos . From
this relation it is clear that KVA rating of the equipment is inversely
proportional to the power factor. The lower the power factor, the larger
will be the KVA rating. Therefore at low power factor, the KVA rating
of the equipment should be made more, making the equipment size
larger and expensive
 Disadvantages of Low Power Factor
4. Poor voltage regulation
A large current with a low lagging power factor results in greater
voltage drops (IZ) in power system components like alternators,
transformers, transmission and distribution lines. This increased
voltage drop results in poor voltage regulation and decreased voltage at
the receiving end, and impairs the performance of consumer loads. In
order to maintain receiving end or consumer end voltages within
permissible limits, additional equipment like voltage regulators, booster
transformers and on-load tap changing transformers are required,
which are expensive in nature.
5. Reduced handling capacity
Since an increase in the reactive component of the current prevents full
utilization of installed capacity, the lagging power factor reduces the
handling capacity of all the elements of the system.
 Disadvantages of Low Power Factor
6. Increase generation and transmission costs

The significance of the power factor lies in the fact that utility
companies supply customers with Volt-Amperes, but bill them for
Watts. Power factors below 1.0 require a utility to generate more
than the minimum Volt-Amperes necessary to supply the working
power i.e., the real power (Watts). This increases generation and
transmission costs. Utilities may charge additional costs or penalize
the customers (not domestic loads but large consumers like
industries) who have a power factor below some limit.
Example 4.1 Consider a 10 KW, single phase A.C motor having an
efficiency of 90 percent, operating at a terminal voltage of 230 V.
Calculate the current drawn by the motor when (i) it is operating at
a power factor 1 (unity p.f) and (ii) it is operating at a 0.7 lagging
power factor
Example 4.2 An alternator is supplying a load of 500 KW at unity
power factor. If the same machine is operated at a power factor 0.7
lagging, calculate the KW capacity of the supplying load.
Example 4.3 Assume a 4 KV single-phase circuit, which feeds a load
of 450 kW and operates at a lagging power factor of 0.75. If it is
desired to improve the power factor, determine the following: a)
The reactive power consumption. b) The new corrected power factor
after installing a shunt capacitor bank with a rating of 400 KVAR
 Load Compensation
Load compensation is the management of reactive power to
improve the power factor and the quality of supply in an ac power
system.
The main objectives in load compensation are:
Improvement of voltage profile
Power factor improvement
Load Balancing
Improved voltage profile
It is important to maintain the voltage profile within +-5% of the
rated value. The main reason for voltage variation is unbalanced
parameters in the generation side and consumption side. If the
reactive power that is being consumed is greater than what is being
generated then there is a definite chance of increased voltage levels.
 Load Compensation
But if both of them are equal then the voltage levels become flat.
Hence in order to maintain a flat voltage profile we have to
determine the active power transfer capability of the system and the
necessary reactive power to be compensated has to be carried out
using shunt compensating elements i.e either a capacitor or an
inductor
Power factor correction
An unity power factor is desirable for better economic and technical
operation of the system. Usually p.f correction means to generate
reactive power as close as possible to the load which requires it
rather than generate it at a distance and transmit it to the load, as
this results not only in large conductor size but also in increased
losses
 Load Compensation
Load balancing

A very important concept of load compensation is load balancing. It

is desirable to operate the three phase system under balanced
condition as unbalanced operation results in flow of negative
sequence current in the system and is highly dangerous especially
for rotating machines.

An ideal load compensator would perform the following functions,

It would provide controllable and variable reactive power almost

instantaneously as required by the load.

It should operate independently in all three phases.

It should maintain constant voltage at its terminal.

 Load Compensation
Ideal Compensator
Load Compensation
Load Compensation
Load Compensation
Load Compensation
Load Compensation
Load Compensation
Load Compensation
Load Compensation
Reactive Power Flow in an Uncompensated Transmission Line
Let us consider a transmission line connecting two busses (a simple
model) as shown in Fig 4.4. Let the R value of the transmission line
be zero. Hence the losses in the line is zero. Let the load be such that
it draws | I| amps at a terminal voltage of | V | volts.

We know that a transmission line consists of series inductance and

shunt capacitance.

Now, the reactive power absorbed by the line will be

QL =| I2 | XL

Fig 4.4 Uncompensated transmission line

Reactive Power Flow in an Uncompensated Transmission Line
Case I: If the reactive power generated by the line given by

QC = | V 2 |/XC equals the reactive power absorbed, then

Therefore, the natural power transmitted in the line is given by

Reactive Power Flow in an Uncompensated Transmission Line
The above expression indicates that the reactive power generated is
equal to the reactive power absorbed, this occurs when the line is
terminated by a resistance equal to its loading surge impedance.
This condition introduces a flat voltage profile over the complete
distance of the transmission line as shown in Fig . 4.5(a)

Case II: If the reactive power generated in the line is more than the
reactive power absorbed i.e., QC > QL then,
Reactive Power Flow in an Uncompensated Transmission Line
This implies that Z > Zn , where Zn is the natural impedance of the
line. This condition introduces a voltage rise over the complete
distance of the transmission line as shown in Fig. 4.5(b)
Case III: If the reactive power absorbed by the line is more than the
reactive power generated (i.e) QL > QC then

Which implies that Z < Zn , where Zn is the natural impedance of

the line. This condition introduces a voltage, say over the complete
distance of the transmission line as shown in Fig. 4.5(c)
Reactive Power Flow in an Uncompensated Transmission Line

Fig 4.5 Voltage variation over a transmission line.

Need for reactive power compensation
We know that there is a back and forth movement of reactive power
from supply to the reactor in a way that in the first quarter cycle of the
AC signal, a capacitor stores the power while in the second quarter
cycle, the stored power gets back to the AC source. This to and fro
movement of the reactive power between the source and load must be
controlled.
Also, the loads in industrial equipment like induction motors, induction
furnaces, arc, etc. are the ones that operate at poor power factor while
fluorescent tubes, fans, etc. that operate at low power factor requires
quite a large amount of reactive power hence the level of voltage at the
load terminals get reduced.
Due to this reason, the power factor of the system must be necessarily
improved using some specific methods
 Need for reactive power compensation
With reactive power compensation, transmission efficiency is
increased. Along with this, the steady-state and temporary over-
voltages can be regulated that resultantly avoids blackouts.
The demand for this reactive power is mainly originated from the
inductive load connected to the system. These inductive loads are
generally electromagnetic circuits of electric motors, electrical
transformers, the inductance of transmission and distribution
networks, induction furnaces, fluorescent lightings, etc.
This reactive power should be properly compensated otherwise, the
ratio of actual power consumed by the load, to the total power i.e.
vector sum of active and reactive power, of the system becomes
quite less.
Need for reactive power compensation
This ratio is alternatively known as the electrical power factor, and a
lower ratio indicates a poor power factor of the system.

If the power factor of the system is poor, the ampere burden of the
transmission, distribution network, transformers, alternators and
other types of equipment connected to the system, becomes high for
required active power. And hence reactive power compensation
becomes so important. This is commonly done by a capacitor bank.
Methods of Reactive Power Compensation
A low value of power factor requires large reactive power and this
affects the voltage level. Hence in order to compensate for the
reactive power, the power factor of the system must be improved.
Thus, the methods for reactive power compensation are nothing but
the methods by which poor power factors can be improved. The
methods are as follows:

Using capacitor banks

Using synchronous condensers

Using static VAr compensators

Methods of Reactive Power Compensation
Synchronous Machine Control
This can be treated as the principal source of reactive power control.
Synchronous machines can be made to generate or absorb reactive
power depending upon their level of excitation.
This control is based on the principle that an over-excited synchronous
machine generates reactive power and an under-excited synchronous
machine absorbs reactive power.
The principal advantage of this control is that it offers flexibility for all
load conditions, since it supplies reactive power when overexcited
(peak load period) and consumes reactive power when it is under
excited (off-peak load period). There is smooth variation in generation
of reactive power by this method, when compared to the step-by-step
variation by the capacitor control method
Methods of Reactive Power Compensation
Capacitor Control
Capacitors can also be treated as chief sources of reactive power
available at the load end or receiving end.
They are again classified into two types by the virtue of their mode of
connection:
(a) series capacitors and
(b) shunt capacitors.
The objective of using a capacitor either in series or parallel is to
compensate reactive power and to consequently improve the power
factor and voltage profile of the system.
Methods of Reactive Power Compensation
a) Series Capacitors:

The series capacitors (capacitors which are connected in series with

the lines) directly neutralize the inductive reactance of the system to
which it is connected.

Since the effect of the series capacitor can be considered as a

negative reactance in series, the net impedance will be = R + j(XL
Xc). Therefore, the voltage drop IZ is reduced.

Application of a series capacitor to a feeder and its vector diagram

representation is shown in Fig. 4.6 (a) and Fig. 4.6 (b) Voltage drop
of the feeder without series capacitor is expressed as

VD = I R Cos I XL Sin
Methods of Reactive Power Compensation

Fig 4.6 (a) Without series capacitor (b) With series capacitor
Methods of Reactive Power Compensation
Where,

R = Resistance of the feeder

XL = Inductive reactance of the feeder

= Angle between the receiving end voltage Vr and current I in the

feeder

Resultant voltage drop of the feeder with the application of series

capacitor is V D = I R Cos + I (XL - XC) Sin

Where, XC = Capacitive reactance of the series capacitor After placing a

capacitor in series with a feeder, the resultant impedance will be

= R + j (XL XC). Therefore, I XC is the reduction in voltage drop after

series compensation.
Methods of Reactive Power Compensation
If XL = XC then V D = I R i.e., the line has only resistive drop owing
to the resistance of the line and zero inductive drop.

If = 1 then = 0 and therefore, I(XL - XC = 0 and V D =

IR. In such unity power factor applications, series capacitors have
practically no value.

The power factor should be lagging if the voltage drop is to decrease

considerably between the sending end and the receiving end by
applying a series capacitor.

For long transmission lines where the net reactance is high, series
capacitors are effective for enhancing system stability
Methods of Reactive Power Compensation
b) Shunt capacitors:

Shunt capacitors (capacitors connected in parallel with the lines) are

widely used in distribution systems for compensating reactive
power and to improve the power factor.

Shunt capacitors draw a leading current which counteracts the

lagging component of the inductive load current (some or the entire
part) at the point of installation. T

Thus, it modifies the characteristics of inductive load by drawing

leading current. A shunt capacitor has a similar effect as an over-
excited synchronous generator or a motor. The application of a
shunt capacitor to a feeder and its vector diagram representation is
shown in Fig. 4.7(a) and Fig. 4.7(b)
Methods of Reactive Power Compensation
Voltage drop of the feeder without shunt capacitor is expressed as
VD = IR R + IxXL
where, R = Resistance of the feeder
XL = Inductive reactance of the feeder
IR = Real power component of the current (in phase)
Ix = Reactive component of current (out of phase, lagging the
voltage by 90°)
Resultant voltage drop of the feeder with the application of shunt
capacitor is V D = IR R + IxXL ICXL
Where, IC = reactive component of the current leading the voltage
by 90° (out of phase) The voltage rise due to the installation of shunt
capacitor can be expressed from the difference between the above
two expressions VR = ICXL Volts.
Methods of Reactive Power Compensation

Fig 4.7 (a) Without shunt capacitor (b) With shunt capacitor
 FACTS
FACTS stands for Flexible AC Transmission System. It is a power
electronics-based system where static devices are used to enhance
and increase the power transfer capability and controllability.

FACTS is a short form of Flexible AC Transmission System. These

devices are used in a power system network to increase the power
transfer capability of transmission lines and will increase the voltage
stability, transient stability, voltage regulation, reliability, and
thermal limits of the transmission network.
 Need of FACTS Devices
Before the invention of power electronics switches, these problems
were solved by connecting a capacitor, reactor, or synchronous
generator with the help of mechanical switches. But there is a lot of
problems to use the mechanical switches. It has a very slow
response, and there is a problem with the wear and tear of
mechanical switches.

After the invention of power electronics switches like the thyristor

that can be used for high voltage applications, power electronics-
based FACTS controllers were developed.
 Types of FACTS Controller

Series Controller

Shunt Controller

Combined Series-Series Controller

Combined Series-Shunt Controller

 Shunt Controller
This type of device is used to inject current into the power system at
the end of connection. Similar to the series controller, it also consists
of variable impedance like capacitor & inductor.

When a capacitor is used to connect in parallel with the power

system, the method is known as shunt capacitive compensation.

Example:- STATCOM, Static VAR compensator (SVC), Static

synchronous compensator (SSC) etc.
Combined Series-Series Controller
In this controller, combined the series-series controller with the line.
It can transfer real power with lines a via DC power link.

It can be connected to unified controllers so that the DC terminals of

converters are interlinked.

Example:- Interlink power flow controller (IPFC)

Combined Series-Shunt Controller
In this controller, combined the series-shunt controller with the
transmission line.

This type of controller is used to introduce voltage in parallel using

the shunt controller, and along with it, to introduce current in series
using the series controller.

Example:- Unified power flow controller (UPFC)

 Static Var Compensator
It is a shunt type controller which controls the power flow in
transmission system and improves the transient stability of power
grids. This controller regulates the voltage at its terminals by
controlling the amount of reactive power injected into or absorbed
from the power system.
 Static Var Compensator
When the system voltage is low, SVC generates the reactive power and

when the voltage is high it absorbs the reactive power. The reactive power

is varied by switching the three phase inductor and capacitor banks. SVCs

are basically thyristor controlled reactive power devices and common types

of SVC are given below.

Thyristor controlled Reactor (TCR)

It is a shunt connected static var absorber or generator. It consists of a fixed

reactor in series with bidirectional thyristor switches. The impedance of this

device varied in a continuous manner by varying the conduction angles of

thyristors.
 Static Var Compensator
The output of this device is adjusted to exchange either inductive
or capacitive current. It maintains and controls the parameters
(typically a bus voltage) of the power system. It is an alternative to
STATCOM in terms of cost.
Thyristor Switched Capacitor (TSC)
It consists of a shunt connected capacitor which is connected in series
with bidirectional thyristor switches. The impedance or reactance of this
device is varied in a stepwise manner by controlling the thyristors
either in a zero or full conduction operation. This controller offers
no harmonics, no transients, and low losses.
 Static Var Compensator
Thyristor Switched Reactor (TSR)

It is a special case of a TCR where phase control of the current is not

exercised, instead the reactor is switched such that thyristors are
either fully ON or OFF as in case of TSC. The advantage of TSR over
TCR is that no harmonics current generation. Also, this controller
use thyristors without firing control and hence lower cost and
losses.
The reactive compensation control in electric power system
use the above stated SVC types in different configuration,
such as combination of TCR and TSC, combination of TCR
and TSC with filter circuit and TCR with filter circuit as
shown in figure.
 STATCOM
STATCOM means static synchronous compensator and it has the
similar characteristics to that of synchronous condenser but it has no
inertia as it is an electronic device.

It consists of a solid state voltage source inverter coupled with a

transformer and this arrangement is tied to a transmission line. This
arrangement supplies or draws reactive power at a faster rate
compared with synchronous motor condenser.

This controller injects the current almost in quadrature with the line
voltage, so that it matches a capacitive or an inductive reactance at
the point where it is connected. STATCOM can be either voltage
source or current source based controller but mostly voltage source
is preferred.
STATCOM
Static Series Synchronous Compensator (SSSC)
It is a series version of STATCOM and it is an advanced kind of
control series compensation. It produces the output voltage in
quadrature with the line current such that the overall reactive
voltage drop across the line is increased or decreased.

Although it is like a STATCOM, the output voltage is in series with

the line and hence it controls the voltage across the line, so its
impendence. It has a capability to induce both inductive and
capacitive voltage in series with the line and hence the power
control.
Unified Power Flow Controller (UPFC)
UPFC is the combination of STATCOM and SSSC which are coupled by
via a common DC link. It can exhibit the characteristics of both SSSC
with series voltage injection and STATCOM with shunt current
injection, with added features.

It has a unique ability to perform independent control of real and

reactive power flow. Also, these can be controlled to provide concurrent
reactive and real power series line compensation without use of an
external energy source.

In the UPFC, SSSC (or converter-2) injects a voltage with controllable

magnitude and phase angle in series with the line though a series
transformer. The function of STATCOM (or converter-1) is to absorb or
supply the reactive power demanded by SSSC at the common DC link.
Unified Power Flow Controller (UPFC)
It can also supply or absorb the controllable reactive power to the
transmission line to provide independent shunt reactive
compensation.
Thyristor Controlled Series Capacitor (TCSC)
It is a capacitive reactance compensator. It consists of a series
capacitor bank which connected in parallel with a thyristor
controlled reactor that provides a smooth variable series capacitive
reactance.

The total impedance of the system can be varied by changing the

conduction angle of the thyristors and hence the circuit becomes
either inductive or capacitive. If the total circuit impedance is
inductive, the fault current is limited by this controller. A simple
model of TCSC is shown in figure below.
 Thyristor Switched Series Capacitor (TSSC)

Similar to TCSC, it is also a capacitive reactance compensator

consisting of thyristor switched reactor in parallel with a series
capacitor. It provides the stepwise control of series capacitive
reactance.

Instead of controlling in continuous manner, it switches the reactor

such that the thyristors are fired at 900 and 1800. This controller can
be implemented without firing angle control to reduce the cost and
losses.
 Thyristor Controlled Series Reactor (TCSR)
It is an inductive reactance compensator which consists of a series
reactor in parallel with thyristor switched reactor. This controller
provides a smooth variable inductive reactance.

When the thyristors firing angle is 1800, the reactor stops conducting
and hence the uncontrolled reactor only is in series with the line that
acts as a fault current limiter. If the firing angle is below 1800, the net
(or overall) inductance decreases, thereby voltage is controlled in
the network.
Thyristor Switched Series Reactor (TSSR)

Similar to TCSR, TSSR is also an inductive reactance compensator

but it provides the stepwise control. This controller switches
thyristors such that they are either fully ON or fully OFF in order to
achieve stepped series inductance.

Thyristor Controlled Phase Shifting Transformer (TCPST)

It is a variable phase angle controller, which consists of thyristors

and phase shifting transformer. The variable phase angle control is
achieved by switching the thyristor for different conduction angles.
Interline Power Flow Controller (IPFC)

It is the new technique for effective power flow and compensation

management of multiline transmission systems. It consists of a
number of converters which are connected with a common DC link
and each converter is provided for series compensation for a
selected transmission line.

In addition to the reactive power compensation, this controller can

able to transfer real power among the transmission lines due to a
common DC link. So it is possible to equalize both real and reactive
power between the lines.
Interline Power Flow Controller (IPFC)