Switchyard design overview

All the basic substation / switch-yard equipment s their installation, working etc is described in this pdf file.

1. SWITCHYARD DESIGN
A Seminar Report Submitted
In partial fulfilment of the requirements
For the award of the degree
BACHELOR OF TECHNOLOGY
IN
Electrical Engineering
By
MILIND PUNJ
(Roll. No. 1104220026)
Under the Guidance
Of
Santosh kumari


Department Of Electrical Engineering
Madan Mohan Malaviya University of Technology,
Gorakhpur, U.P.

2. Department Of Electrical Engineering
Madan Mohan Malaviya University of
Technology, Gorakhpur

Certificate
This is to certify that the report work entitled “SWITCHYARD
DESIGN” submitted in partial fulfillment of the requirement for the
degree of Bachelor of Technology in “ELCTRICAL ENGINEERING”,
is a bonafide seminar work carried out by MR. MILIND PUNJ under my
supervision and guidance.




Date: ___________






___________________
___________________
Electrical
Engineering Department,
M.M.M.U.T., Gorakhpur

3. CANDIDATE’S DECLARATION


I declare that the work presented in this report titled “SWITCHYRD
DESIGN”, submitted to the Electrical Engineering Department, Madan
Mohan Malaviya University of Technology, Gorakhpur for the
award of the Bachelor of Technology degree in Electrical Engineering,
is my original work. I have not plagiarized or submitted the same work
for the award of any other degree.




MILIND PUNJ
Roll No. 1104220026

4. Acknowledgement

I wish to express sense of gratitude to my guide _________________,
______________, Electrical Engineering Department, Madan Mohan
Malaviya University of Technology, Gorakhpur, to give me guidance at
every moment during my entire thesis and giving valuable suggestions.
He gave me unfailing inspiration and whole hearted cooperation in
caring out my seminar work. His continuous encouragement in the work
and effort to push the work through is gratefully acknowledged.
I also very grateful to _________________, ______________, Electrical
Engineering Department, Madan Mohan Malaviya University of
Technology, Gorakhpur, for his huge cooperation and valuable
suggestions from time to time during my entire seminar work. I also
extend my gratitude to all the members of the department without whose
support this report would not have been materialized.
Last but not the least, I wish to thank all my friends and seniors who also
helped me in the successful completion of this work.

5. CONTENT
 INTRODUCTION TO SWITHCYARD
 DIFFRENCE BETWEEN SWITCHYARD AND SUBSTATION
 FUNCTIONS OF SWITCHYRD
 LAYOUT OF SWITCHYARD
 BAY EQUIPMENTS
 BUSBARS
(1) SINGLE BUS BAR ARRANGEMENT
(2) SINGLE SECTIONIZED BUS BAR ARRANGEMENT
(3) DOUBLE BUS BAR ARRANGEMENT
(4) DOUBLE BUS BAR WITH TRANSFER BUS ARRANGEMENT
(5) ONE AND HALF BUS BAR ARRANGEMENT

 CIRCUIT BREAKER
(1) AIR BLAST CIRCUIT BREAKER
(2) OIL CIRCUIT BREAKER
(3) VACUUM CIRCUIT BREAKER
(4) SF6 CIRCUIT BREAKER

 ISOLATORS
(1)PANTOGRAPH ISOLATOR
(2)BREAK ISOLATOR

 INSTRUMENT TRANSFORMER
(1)CURRENT TRANSFORMER
(2)VOLTAGE TRANSFORMER

 ARRESTORS
 VARIABLE SHUNT REACTOR
 CAPACITIVE VOLTAGE TRANSFORMER
 WAVE TRAP

 SWITCHYARD CONTROL ROOM
 CONCLUSION
 REFRENCES

6. Introduction:
Switchyard is a part of power plant , where generated voltage comes from generator
transformer. Switchyard system transform voltage from high to low, or the reverse, or perform
any of several other important functions. Between the generating station and consumer, electric
power may flow through several substations at different voltage levels. It include transformers to
change voltage levels between high transmission voltages and lower distribution voltages, or at
the interconnection of two different transmission voltages. The yard is the places from where the
electricity is send outside. A switchyard does not accommodate a transformer. This type of
substation is remotely operated to re-route power supplies where there is an immediate or critical
need. A switching substation may also be known as a switchyard, and these are commonly
located directly adjacent to or nearby a power station. In this case the generators from the power
station supply their power into the yard onto the Generator Bus on one side of the yard, and the
transmission lines take their power from a Feeder Bus on the other side of the yard. A
switchyard is essentially a hub for electrical power sources. For instance, a switchyard will exist
at a generating station to coordinate the exchange of power between the generators and the
transmission lines in the area. A switchyard will also exist when high voltage lines need to be
converted to lower voltage for distribution to consumers. Therefore a switchyard will contain;
current carrying conductors, grounding wires and switches, transformers, disconnects, remotely
controlled arc snuffing breakers, metering devices, etc. Some substation, such as power plant
switchyard are simply switching stations where different connections can be made between
various transmission lines. The whole switchyard divided into bay area and a control room as
shown in fig 1. A switchyard bay have following main equipments capacitive voltage
transformer(c.v.t), shunt reactor, bus bay, isolators, breakers, current transformer and voltage
transformer. Now the switchyard control room consists of various panels to control the bay
equipments. All these equipments are controlled & monitored automatically by SCADA system
present in control room.

7. Fig-1 switchyard

Switchyard vs. Substation
A switchyard connects and disconnects lines on the grid for various reasons. The operations and
equipment are essentially all at the same voltage level. The switchyard is a junction connecting
the Transmission & Distribution system to the power plant. Tie line is a connection point
between two or more power plants or simply a power grid while a transmission line is a high
voltage line which transfers electrical power from the power generating plants to substations.

A substation changes the voltage level. It has transformers. It changes the voltage up or down for
transmission or distribution. For example a local distribution substation for domestic supply may
receive voltage at around 10kV and transform this down to 220 or 120 volts for distribution to
consumers. An electrical substation is a subsidiary station of an electricity generation,
transmission and distribution system where voltage is transformed from high to low or the
reverse using transformers. A substation contains a transformer, which steps-up or steps-down
power voltages, according to the end-use purpose and destination. These transformers emit a low
humming sound, and in built-up residential areas. Transformers are primarily contained within a
cement sound enclosure to minimize noise.




Bays
Control room

8. Functions of switchyard
 Protection of transmission lines: A circuit breaker is an automatically operated
electrical switch designed to protect an electrical circuit from damage caused by overload
or short circuit. Its basic function is to detect a fault condition and interrupt current flow.
Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be
reset (either manually or automatically) to resume normal operation. Circuit breakers are
made in varying sizes, from small devices that protect an individual household appliance
up to large switchgear designed to protect high-voltage circuits feeding an entire city.
Isolators is an easily removed piece of the actual conductor of a circuit. The purpose of
disconnects is to isolate equipment. Disconnects are not used to interrupt circuits; they
are no-load devices. A typical use of disconnects is to isolate a circuit breaker by
installing one disconnect on either side of the circuit breaker (in series with the breaker).
Operation of disconnects is one of the most important and responsible jobs of a power
plant operator. One error in isolation of equipment, or the accidental grounding of line
equipment, can be a fatal mistake. A lightning arrester, also known as lightning
conductor, is a device used on electrical power systems and telecommunications systems
to protect the insulation and conductors of the system from the damaging effects
of lightning. The typical lightning arrester has a voltage terminal and a ground terminal.
When a lightning surge (or switching surge, which is very similar) travels along the
power line to the arrester, the current from the surge is diverted through the arrestor, in
most cases to earth. A surge arrester is a product installed near the end of any conductor
which is long enough before the conductor lands on its intended electrical component.
The purpose is to divert damaging lightning-induced transients safely to ground through
property changes to its varistor in parallel arrangement to the conductor inside the unit.
Also called a surge protection device (SPD) or transient voltage surge suppressor
(TVSS), they are only designed to protect against electrical transients resulting from the
lightning flash, not a direct lightning termination to the conductors.


 Determination & control of exchange of power: Current transformer are used with
ammeters, watt meters, power-factor meters, watt-hour meters,compensators, protective
and regulating relays and the trip coil of circuit breakers. One current transformer can be
used to operate several instruments, provided that the combined burden does not exceed
that for which the transformer is designed and compensated. The current transformer is
connected directly in series with the line. Voltage transformer also know as potential
transformer, are used with volt-meters, wattmeters, watt-hour meters, power-factor
meters, frequency meters, synchroscopes and synchronizing apparatus, protective and
regulating relays and the no-voltage and over-voltage trip coils of automatic circuit
breakers. One transformer can be used for a number of instruments at the same time if the
total current taken by the instrument does not exceed that for which the transformer is
designed and compensated. The ordinary voltage transformer is connected across the line,
and the magnetic flux in the core depends upon the primary voltage

9.  Maintain system frequency with target levels: System frequency is a continuously
changing variable that is determined and controlled by the second-by-second (real
time) balance between system demand and total generation. If demand is greater than
generation, the frequency falls while if generation is greater than demand, the
frequency rises.
National Grid has a licence obligation to control frequency within the limits
specified in the 'Electricity Supply Regulations', i.e. ±1% of nominal system
frequency (50.00Hz) save in abnormal or exceptional circumstances. National Grid
must therefore ensure that sufficient generation and / or demand is held in automatic
readiness to manage all credible circumstances that might result in frequency
variations.
There are two types of Frequency Response Dynamic and Non Dynamic Response.
Dynamic Frequency Response is a continuously provided service used to manage the
normal second by second changes on the system. While Non Dynamic Frequency
Response is usually a discrete service triggered at a defined frequency deviation .

Fig 2.
Real time frequency monitoring system by national grid on 14
th
april 2014 at 1215hr
 Fault analysis & correction: In an electric power system, a fault is any abnormal electric
current. For example, a short circuit is a fault in which current bypasses the normal load.
An open-circuit fault occurs if a circuit is interrupted by some failure. In three-
phase systems, a fault may involve one or more phases and ground, or may occur only
between phases. In a "ground fault" or "earth fault", charge flows into the earth.
The prospective short circuit current of a fault can be calculated for power systems. In
power systems, protective devices detect fault conditions and operate circuit breakers and
other devices to limit the loss of service due to a failure. First, some simplifying

10. assumptions are made. It is assumed that all electrical generators in the system are in
phase, and operating at the nominal voltage of the system. Electric motors can also be
considered to be generators, because when a fault occurs, they usually supply rather than
draw power. The voltages and currents are then calculated for this base case.
Next, the location of the fault is considered to be supplied with a negative voltage source,
equal to the voltage at that location in the base case, while all other sources are set to
zero. This method makes use of the principle of superposition.
To obtain a more accurate result, these calculations should be performed separately for
three separate time ranges:
1. subtransient is first, and is associated with the largest currents
2. transient comes between subtransient and steady-state
3. steady-state occurs after all the transients have had time to settle

Overhead power lines are easiest to diagnose since the problem is usually obvious, e.g., a
tree has fallen across the line, or a utility pole is broken and the conductors are lying on
the ground.Locating faults in a cable system can be done either with the circuit de-
energized, or in some cases, with the circuit under power. Fault location techniques can
be broadly divided into terminal methods, which use voltages and currents measured at
the ends of the cable, and tracer methods, which require inspection along the length of the
cable. Terminal methods can be used to locate the general area of the fault, to expedite
tracing on a long or buried cable.
[3]
In very simple wiring systems, the fault location is
often found through inspection of the wires. In complex wiring systems (for example,
aircraft wiring) where the wires may be hidden, wiring faults are located with a Time-
domain reflectometer.
[4]
The time domain reflectometer sends a pulse down the wire and
then analyzes the returning reflected pulse to identify faults within the electrical wire.
In historic submarine telegraph cables, sensitive galvanometers were used to measure
fault currents; by testing at both ends of a faulted cable, the fault location could be
isolated to within a few miles, which allowed the cable to be grappled up and repaired.
The Murray loop and the Varley loop were two types of connections for locating faults in
cables
Sometimes an insulation fault in a power cable will not show up at lower voltages. A
"thumper" test set applies a high-energy, high-voltage pulse to the cable. Fault location is
done by listening for the sound of the discharge at the fault. While this test contributes to
damage at the cable site, it is practical because the faulted location would have to be re-
insulated when found in any case.
[5]

In a high resistance grounded distribution system, a feeder may develop a fault to ground
but the system continues in operation. The faulted, but energized, feeder can be found
with a ring-type current transformer collecting all the phase wires of the circuit; only the
circuit containing a fault to ground will show a net unbalanced current. To make the
ground fault current easier to detect, the grounding resistor of the system may be
switched between two values so that the fault current pulses. A relay is
an electrically operated switch. Many relays use an electromagnet to mechanically
operate a switch, but other operating principles are also used, such as solid-state relays.

11. Relays are used where it is necessary to control a circuit by a low-power signal (with
complete electrical isolation between control and controlled circuits), or where several
circuits must be controlled by one signal. The first relays were used in long
distance telegraph circuits as amplifiers: they repeated the signal coming in from one
circuit and re-transmitted it on another circuit. Relays were used extensively in telephone
exchanges and early computers to perform logical operations. In electrical engineering,
a protective relay(fig.3) is a device designed to trip a circuit breaker when a fault is
detected. The first protective relays were electromagnetic devices, relying on coils
operating on moving parts to provide detection of abnormal operating conditions such as
over-current, over-voltage, reverse power flow, over- and under- frequency.
Microprocessor-based digital protection relays now emulate the original devices, as well
as providing types of protection and supervision impractical with electromechanical
relays. In many cases a single microprocessor relay provides functions that would take
two or more electromechanical devices. By combining several functions in one case,
numerical relays also save capital cost and maintenance cost over electromechanical
relays. However, due to their very long life span, tens of thousands of these "silent
sentinels" are still protecting transmission lines and electrical apparatus all over the
world. An important transmission line or generator unit will have cubicles dedicated to
protection, with many individual electromechanical devices, or one or two
microprocessor relays

Fig. 3 Protection relay

 Monitoring of power supply: A Power Monitoring System is a network of meters
connected to the Internet to provide real time data on the power system in your
facility. The meters feed an on line software system that allows the owners and service
providers to identify potential problems with the electrical systems. All electrical
equipment in your facility is hooked to a meter. The meter is essentially a monitoring tool
that is connected to the Internet. Like a computer, the tool has tremendous storage
capacity and continually monitors the power , even events up to one millisecond can be
captured. The information is fed directly to a software management system that is made

12. available in real time. Alarms can be set and email notifications sent out if your power
systems functions outside of its normal parameters.A Power Monitoring system allows
you to see what you can‟t see. This creates a proactive approach to energy management
rather than waiting for the bill to arrive at the end of the month. With out a way to
monitor and measure you electrical system, there is no guidance there to help you
determine where your energy resources are going.


Fig. 4 Real time monitoring of power system of power plant

 Communications: Power-line communication (PLC) carries data on a conductor that is
also used simultaneously for AC electric power transmission or electric power
distribution to consumers. It is also known as power-line carrier, power-line digital

13. subscriber line (PDSL), mains communication, power-line telecommunications,
or power-line networking(PLN).
A wide range of power-line communication technologies are needed for different
applications, ranging from home automation to Internet access which is often
called broadband over power lines (BPL). Most PLC technologies limit themselves to
one type of wires (such as premises wiring within a single building), but some can cross
between two levels (for example, both the distribution network and premises wiring).
Typically transformers prevent propagating the signal, which requires multiple
technologies to form very large networks. Various data rates and frequencies are used in
different situations. Following are characteristics of PLC:
1. Power line Communication is the technology that enables the transmission of
data over power line that carries and supplies electric power.
2. Replacing the slow data transmission rate with only one-way communication,
the emerging PLC technology has brought wider bandwidth with two-way
communications.
3. PLC is a kind of communication technology, which uses Medium
Voltage(MV) & Low Voltage(LV) distribution network as the
communication media to implement transmission of data, voice and real time
image.
Line trap is also known as Wave Trap(fig.5). Its function is to trap high frequency
communication signals sent on the line from the remote substation and diverting them to
the telecom/teleprotection panel in the substation control room (through coupling
capicitors & LMU). This is relevant in Power Line Carrier Communication (PLCC)
system for communication among various substations. The carrier can communicate
voice and data by superimposing an analog signal over the standard 50 or 60 Hz
alternating current (AC).

14. Fig.5 wavetrap
Basic Layout of Switchyard

While designing a switchyard the following aspects must be taken into consideration:
 Low capital cost.
 Reliability of the supply power.
 Low operating cost
 High efficiency
 Low cost of energy generation.
 Simplicity of design.
 Reserve capacity to meet future requirements

15. The following table gives an approximate idea of atmospheric condition for setting up the
switchyard or any substation.

The working of switchyard as shown in fig.6, the switchyard is connected to generator via
generating transformer(GT) in the Generating bay. Here the voltage produced by generator
is stepped up (in 400kv s/w 21/400kv) and send for transmission. During the process it is
passed through circuit breakers(CB) and isolators for purpose of protection. In case faults CB
and isolators plays an important role because a very large amount of current will flow which
is undesirable and consequently trip the power plant. There are various bays which provides
different combinations of connection required during overhauling & faults in equipments or
grid failure. Starting with first generator bay with is already discussed above. Next is Bus
coupler bay as the name suggest used for interconnecting of main buses. It comes under
operation when one of the buses gets faulty then whole power is taken onto another bus by
using bus coupler(a type of interconnecting transformer) but under normal conditions the
circuit remain open . During the normal operation a bay called Feeder bay is connected to
main buses directly, to transmit the power to the various substations. Further another type of
bay called Transfer bus bay, it is very special type of bay because it is used for special
purpose i.e. whenever there is fault in generating bay CB or isolator then whole supply
transferring to the main bus instead transferred to transfer bus from where it is directly
passed to the feeder circuit in the feeder bay. The whole purpose of setting up this circuit is
to bypass the generating bay incase of faults otherwise the demand and generation frequency
will mismatch and whole grid will shut down.

16. Fig. 6 Layout of 400kv switchyard by Single Line Diagram(SLD)

Bay Equipments

 Bus Bar: The Bus scheme is the arrangement of overhead bus bar and associated
switching equipments in a substation. The operational flexibility and reliability of the
substation greatly depends upon the bus scheme .

Here I reiterate that the electric substation is a junction point where usually more than
two transmission lines terminate. Actually in most of EHV and HV substations more

17. than half a dozen of lines terminate. In many large transmission substations the total
numbers of lines terminating exceeds one or two dozen. In this scenario obviously the
first requirement is avoidance of total shutdown of the substation for the purpose of
maintenance of some equipment(s) or due to fault somewhere. Total shutdown of
substation means complete shutdown of all the lines connected to this particular
substation. So the switching scheme is adopted depending upon the importance of the
substation, reliability requirement, flexibility and future expansion etc.. Of course
substation construction and operational cost is also to be considered. Clearly a EHV or
UHV transmission substation where large numbers of important lines terminate is
extremely important and the substation should be designed to avoid total failure and
interruption of minimum numbers of circuit.
The Main Criteria‟s To be Considered During Selection of one Particular Bus Bar
Arrangement Scheme Among Others
(i) Simplicity of system.
(ii) Easy maintenance of different equipments.
(iii) Minimizing the outage during maintenance.
(iv) Future provision of extension with growth of demand.
(v) Optimizing the selection of bus bar arrangement scheme so that it gives maximum
return from the system.
Some very commonly used bus bar arrangement are discussed below-
 Single Bus System: Single Bus System is simplest and cheapest one. In this
scheme all the feeders and transformer bay are connected to only one single bus
as show.
Advantages of Single Bus System:
1) This is very simple in design.
2) This is very cost effective scheme.
3) This is very convenient to operate.
Disadvantages of Single Bus System:
1. One but major difficulty of these type of arrangement is that, maintenance of
equipment of any bay cannot be possible without interrupting the feeder or
transformer connected to that bay.
2. The indoor 11KV switchboards have quite often single bus bar arrangement.

18.  Single Bus System with Bus Sectionalizer: Some advantages are realized if a
single bus bar is sectionalized with circuit breaker. If there are more than one
incoming and the incoming sources and outgoing feeders are evenly distributed
on the sections as shown in the figure, interruption of system can be reduced to a
good extent.
Advantages of Single Bus System with Bus :
If any of the sources is out of system, still all loads can be fed by switching on
the sectional circuit breaker or bus coupler breaker. If one section of the bus bar
system is under maintenance, part load of the substation can be fed by energizing
the other section of bus bar.

Disadvantages of Single Bus System with Bus Sectionalizer:
1) As in the case of single bus system, maintenance of equipment of any bay
cannot be possible without interrupting the feeder or transformer connected to that
bay.
2) The use of isolator for bus sectionalizing does not fulfill the purpose. The
isolators have to be operated „off circuit‟ and which is not possible without total
interruption of bus – bar. So investment for bus-coupler breaker is required.


 Double Bus System
1) In double bus bar system two identical bus bars are used in such a way that any
outgoing or incoming feeder can be taken from any of the bus.
2)Actually every feeder is connected to both of the buses in parallel through
individual isolator as shown in the figure.

19. By closing any of the isolators one can put the feeder to associated bus. Both of
the buses are energized and total feeders are divided into two groups, one group is
fed from one bus and other from other bus. But any feeder at any time can be
transferred from one bus to other. There is one bus coupler breaker which should
be kept close during bus transfer operation. For transfer operation, one should first
close the bus coupler circuit breaker then close the isolator associated with the bus
to where the feeder would be transferred and then open the isolator associated
with the bus from where feeder is transferred. Lastly after this transfer operation
he or she should open the bus coupler breaker.

Advantages of Double Bus System:
Double Bus Bar Arrangement increases the flexibility of system.
Disadvantages of Double Bus System:
The arrangement does not permit breaker maintenance without interruption.


 One and A Half Breaker Bus System
This is an improvement on the double breaker scheme to effect saving in the
number of circuit breakers. For every two circuits only one spare breaker is
provided. The protection is however complicated since it must associate the
central breaker with the feeder whose own breaker is taken out for maintenance.
For the reasons given under double breaker scheme and because of the prohibitory
costs of equipment even this scheme is not much popular. As shown in the figure
that it is a simple design, two feeders are fed from two different buses through
their associated breakers and these two feeders are coupled by a third breaker
which is called tie breaker. Normally all the three breakers are closed and power

20. is fed to both the circuits from two buses which are operated in parallel. The tie
breaker acts as coupler for the two feeder circuits.
During failure of any feeder breaker, the power is fed through the breaker of the
second feeder and tie breaker, therefore each feeder breaker has to be rated to feed
both the feeders, coupled by tie breaker.

Advantages of One and A Half Breaker Bus System:
During any fault on any one of the buses, that faulty bus will be cleared instantly
without interrupting any feeders in the system since all feeders will continue to
feed from other healthy bus.
Disadvantages of One and A Half Breaker Bus System:
This scheme is much expensive due to investment for third breaker.


 Main and Transfer Bus System

This is an alternative of double bus system. The main conception of Main and
Transfer Bus System is, here every feeder line is directly connected through an
isolator to a second bus called transfer bus. The said isolator in between transfer
bus and feeder line is generally called bypass isolator. The main bus is as usual
connected to each feeder through a bay consists of circuit breaker and associated
isolators at both side of the breaker. There is one bus coupler bay which couples
transfer bus and main bus through a circuit breaker and associated isolators at

21. both sides of the breaker. If necessary the transfer bus can be energized by main
bus power by closing the transfer bus coupler isolators and then breaker. Then the
power in transfer bus can directly be fed to the feeder line by closing the bypass
isolator. If the main circuit breaker associated with feeder is switched off or
isolated from system, the feeder can still be fed in this way by transferring it to
transfer bus.

Switching Operation for Transferring a Feeder to Transfer Bus from Main Bus
without Interruption of Power
(i) First close the isolators at both side of the bus coupler breaker.
(ii) Then close the bypass isolator of the feeder which is to be transferred to
transfer bus.
(iii) Now energized the transfer bus by closing the bus coupler circuit breaker
from remote.
(iv) After bus coupler breaker is closed, now the power from main bus flows to
the feeder line through its main
breaker as well as bus coupler breaker via transfer bus.
(v) Now if main breaker of the feeder is switched off, total power flow will
instantaneously shift to the bus coupler breaker and hence this breaker will serve
the purpose of protection for the feeder.
(vi) At last the operating personnel open the isolators at both sides of the main
circuit breaker to make it isolated from rest of the live system.
So it can be concluded that in Main & Transfer Bus System the maintenance of
circuit breaker is possible without any interruption of power. Because of this
advantage the scheme is very popular for 33KV and 13KV system.

22.  Ring Bus System
The schematic diagram of the system is given in the figure. It provides a double
feed to each feeder circuit, opening one breaker under maintenance or otherwise
does not affect supply to any feeder. But this system has two major disadvantages.
One as it is closed circuit system it is next to impossible to extend in future and
hence it is unsuitable for developing system. Secondly, during maintenance or any
other reason if any one of the circuit breaker in ring loop is switch of reliability of
system becomes very poor as because closed loop becomes opened. Since, at that
moment for any tripping of any breaker in the open loop causes interruption in all
the feeders between tripped breaker and open end of the loop.

23.  Circuit Breaker: It is used to interrupt circuits while current is flowing through them.
The making and breaking of contacts in a Oil type circuit breaker are done under oil, this
oil serves to quench the arc when the circuit is opened. The operation of the breaker is
very rapid when opening. As with the transformer, the high voltage connections are made
through bushings. Circuit breakers of this type are usually arranged for remote electrical
control from a suitably located switchboard.
Some recently developed circuit breakers have no oil, but put out the arc
by a blast of compressed air; these are called air circuit breakers. Another type encloses
the contacts in a vacuum or a gas (sulfur hexafluoride, SF6) which tends to self maintain
the arc.
There are some important circuit breakers with their brief characterstics:
 Air blast circuit breaker: In Air Blast Circuit Breaker, air at high pressure is
blast upon the arc formed between the contacts. The air blast blows away the
ionized air between the contacts.

See the Sketches (Figs-A and B) illustrating the arc extinction process of the axial
blast type breaker . The contacts are in closed position by spring pressure. For
opening the contacts. Air at high pressure from the air receiver (Fig-C) is blasted
to the interruption chamber. This pressure exceeds the spring pressure and pushes
the moving contact away from the fixed contact. This opens the contacts and air at
high pressure passes through the nozzle and port to the atmosphere. This axial
flow of air at high speed extinguishes the arc within 2 or 3 cycles of current wave
and ionized gas is blown away. Then the port is closed by the moving contact
arm(Fig-B) and the space between the contacts is filled with fresh air at high
pressure. This enables the breaker to withstand high Transient recovery
Voltage (TRV). Compare Fig-A with Fig-B. In Fig-B the arc is extinguished and

24. spring is in compressed state. To close the contacts, a valve arrangement lets the
air from the chamber to pass to the outside atmosphere. This makes the spring
pressure to close the fixed and moving contacts.

25. Some main advantages of the Air Blast Circuit Breaker(ABCB) are:
 Arc extinction is very fast. Hence it is suitable for frequent opening and
closing operation.
 Due to refilling of separated contacts space by fresh air at high pressure, the
separation requirement between the contacts is quite less in comparison to
OCB. This makes the size of the breaker smaller.
 The ionized gas flushed out to the atmosphere. Hence unlike OCB here the arc
quenching medium does not deteriorate with time. This eliminates some
maintenance burden.
 It is non-inflammable.
 Finally one important advantage is that in ABCB the arc quenching depends
on the high pressure air which is obtained from a compressor, an external
source. So in case of ABCB the arc extinction or arcing time does not depends
upon the arc current. (In case of OCB the arcing time depends on the current
to be interrupted).
 The breaker breaking capacity depends upon the external source, the high
pressure air.
The Air Blast Circuit Breakers has some disadvantages. The important one is
that Air Blast Circuit Breakers require a compressor plant (not shown in Fig-C)
which requires regular maintenance. Hence ABCB is not economical for low
voltage applications. There are other issues like current chopping and restriking
voltage which requires to be handled by proper design and damping mechanism.

 Oil circuit breaker: We already discussed Vacuum Circuit Breaker . In
modern power systems these two types of circuit breakers are mainly used for
high voltage application. While vacuum breakers are mainly used for voltage upto
38 kV, SF6 breakers are used starting from distribution voltage at 11 kV upto 765
kV and 1200 kV level. Although the use of oil breaker has reduced very much
one can still find oil CB in many installations. So I liked to write a little about oil
circuit breaker in one article.

Oil Circuit Breakers (OCB) can be categorized into two types. One is Bulk Oil
Circuit Breaker (BOCB) and the other type is Minimum Oil Circuit Breaker
(MOCB). MOCB type is also called as Low Oil Circuit Breaker.

Bulk Oil Circuit Breaker (BOCB):The Bulk Oil CB design is very simple (Fig-
A). In Fig-A the arc control device between the fixed and moving contacts is not
shown, so making the sketch even simpler. This type of circuit breaker uses a
steel tank containing oil and the contacts are immersed in the oil. The steel tank is
earthed (dead tank type). In this type construction the oil requirement is more as

26. the oil is required to provide insulation to the contacts from the steel tank and
insulation between the contacts(in open state). The oil also serves as the medium
for extinguishing the arc formed when the moving contact separates from fixed
contact. When the contacts separate, arc is formed between the contacts. The arc
gives rise to formation of gas in the oil which initiates oil circulation. This
phenomena helps in extinguishing the arc so breaking the circuit. For higher
voltage this very simple principle cannot be much effective. So an arc control
device is usually used to facilitate arc extinction process.
The BOCB is available as single tank type or three tank type. Usually for lower
voltage use, below 38 kV, single tank type is adopted with barrier between the
phases. For higher voltage application three separate tanks are used.




Fig-a oil c.b.

Minimum Oil Circuit Breaker
If you visit an old substation, having BOCB installed, you immediately recognise
the oversize Circuit Breaker. As explained above, BOCB is large in size and
requires more space.

27. Minimum Oil Circuit Breakers (MOCB) require less oil as the purpose here is
only to extinguish the arc and not for providing insulation to the contact. Arc
interruption takes place inside the Interrupter. The whole system is placed inside
the porcelain housing. Because of this insulating porcelain the insulation
requirement of contacts is reduced very much. This is the reason of its smaller
size. As such the MOCBs are of live tank outdoor type design and mainly used
for voltage levels above 38 kV.

The oil circuit breakers have some severe disadvantages. The main disadvantage
is that the OCB can explode causing harm to the personnel and other equipment
of the system. The tank type design is very bulky so making it difficult for
transportation and handling and requires more space. The OCB requires more
maintenance in comparison to vacuum and SF6 breakers. Irrespective of these few
disadvantages the OCBs are not going to vanish within few years.

 Sulphur Hexafloride(SF6) Circuit Breaker : At this point we are aware that the
medium in which arc extinction of the circuit breaker takes place greatly
influences the important characteristics and life of the circuit breaker. In the last
article the working of a vacuum circuit breaker was illustrated. We already know
that the use of vacuum circuit breaker is mainly restricted to system voltage
below 38 kV. The characteristics of vacuum as medium and cost of the vacuum
CB does not makes it suitable for voltage exceeding 38 kV. In the past for higher
transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit Breaker
(ABCB) were used. These days for higher transmission voltage
levels SF6 Circuit Breakers are largely used. OCB and ABCB have almost
become obsolete. In fact in many installations SF6 CB is used for lower
voltages like 11 kV, 6 kV etc..

Sulphur Hexafluoride symbolically written as SF6 is a gas which satisfy the
requirements of an ideal arc interrupting medium. So SF6 is extensively used
these days as an arc interrupting medium in circuit breakers ranging from 3
kv upto 765 kv class. In addition to this SF6 is used in many electrical
equipments for insulation. Here first we discuss in brief, some of the essential
properties of SF6 which is the reason of it's extensive use in circuit breakers

o SF6 gas has high dielectric strength which is the most important quality of
a material for use in electrical equipments and in particular for breaker it is
one of the most desired properties. Moreover it has high Rate of Rise of
dielectric strength after arc extinction. This characteristics is very much
sought for a circuit breaker to avoid restriking.
o SF6 is colour less, odour less and non toxic gas.SF6 is an inert gas. So in
normal operating condition the metallic parts in contact with the gas are
not corroded. This ensures the life of the breaker and reduces the need for
maintenance.

28. o SF6 has high thermal conductivity which means the heat dissipation
capacity is more. This implies greater current carrying capacity when
surrounded by SF6 .
o The gas is quite stable. However it disintegrates to other fluorides of
Sulphur in the presence of arc. but after the extinction of the arc
the SF6 gas is reformed from the decomposition.SF6 being non-flammable
so there is no risk of fire hazard and explosion.
The construction and working principles of SF6 circuit breaker varies from
manufacturer to manufacturer. In the past double pressure type
of SF6 breakers were used. Now these are obsolete. Another type
of SF6 breaker design is the self blast type, which is usually used for medium
transmission voltage. The Puffer type SF6 breakers of single pressure type are
the most favoured types prevalent in power industry. Here the working
principle of Puffer type breaker is illustrated (Fig-A).illustrated in the figure
the breaker has a cylinder and piston arrangement. Here the piston is fixed but
the cylinder is movable. The cylinder is tied to the moving contact so that for
opening the breaker the cylinder along with the moving contact moves away
from the fixed contact (Fig-A(b)). But due to the presence of fixed piston
the SF6 gas inside the cylinder is compressed. The compressed SF6 gas flows
through the nozzle and over the electric arc in axial direction. Due to heat
convection and radiation the arc radius reduces gradually and the arc is finally
extinguished at current zero. The dielectric strength of the medium between
the separated contacts increases rapidly and restored quickly as fresh SF6 gas
fills the space. While arc quenching, small quantity of SF6 gas is broken down
to some other fluorides of sulphur which mostly recombine to
form SF6 again. A filter is also suitably placed in the interrupter to absorb the
remaining decomposed byproduct.

The gas pressure inside the cylinder is maintained at around 5 kgf per sq. cm.
At higher pressure the dielectric strength of the gas increases. But at higher
pressure the SF6 gas liquify at higher temperature which is undesired. So
heater is required to be arranged for automatic control of the temperature for
circuit breakers where higher pressure is utilised. If the SF6 gas will liquify
then it loses the ability to quench the arc.

Like vacuum breaker, SF6 breakers are also available in modular design form
so that two modules connected in series can be used for higher voltage
levels. SF6 breakers are available as both live tank and dead tank types. In Fig-
B above a live tank outdoor type 400 kV SF6 breaker is shown.

29.  Isolators(disconectors): Circuit breaker always trip the circuit but open contacts of
breaker cannot be visible physically from outside of the breaker and that is why it is
recommended not to touch any electrical circuit just by switching off the circuit breaker.
So for better safety there must be some arrangement so that one can see open condition of
the section of the circuit before touching it. Isolator is a mechanical switch which isolates
a part of circuit from system as when required. Electrical isolators separate a part of the
system from rest for safe maintenance works.So definition of isolator can be rewritten as
Isolator is a manually operated mechanical switch which separates a part of the electrical
power system normally at off load condition.

Types of Electrical Isolators:There are different types of isolators available depending
upon system requirement such as

1) Double Break Isolator
2) Single Break Isolator
3) Pantograph type Isolator.
Depending upon the position in power system, the isolators can be categorized as
1) Bus side isolator – the isolator is directly connected with main bus
2) Line side isolator – the isolator is situated at line side of any feeder
3) Transfer bus
side isolator – the
isolator is directly
connected with
transfer bus.

Constructional Features of Double Break Isolators: Lets have a discussion on
constructional features of Double Break Isolators. These have three stacks of post
insulators as shown in the figure. The central post insulator carries a tubular or flat male
contact which can be rotated horizontally with rotation of central post insulator. This rod
type contact is also called moving contact.

The female type contacts are fixed on the top of the other post insulators which fitted at

30. both sides of the central post insulator. The female contacts are generally in the form of
spring loaded figure contacts. The rotational movement of male contact causes to come
itself into female contacts and isolators becomes closed. The rotation of male contact in
opposite direction make to it out from female contacts and isolators becomes open.
Rotation of the central post insulator is done by a driving lever mechanism at the base of
the post insulator and it connected to operating handle (in case of hand operation) or
motor (in case of motorized operation) of the isolator through a mechanical tie rod.

Constructional features of Single Break Isolators: The contact arm is divided into two
parts one carries male contact and other female contact. The contact arm moves due to
rotation of the post insulator upon which the contact arms are fitted. Rotation of both post
insulators stacks in opposite to each other causes to close the isolator by closing the
contact arm. Counter rotation of both post insulators stacks open the contact arm and
isolator becomes in off condition. This motorized form of this type of isolators is
generally used but emergency hand driven mechanism is also provided.

 Earthing Switches: Earthing switches are mounted on the base of mainly line side
isolator. Earthing switches are normally vertically break switches. Earthing arms (contact
arm of earthing switch) are normally aligned horizontally at off condition. during
switching on operation, these earthing arms rotate and move to vertical position and make
contact with earth female contacts fitted at the top of the post insulator stack of isolator at
its outgoing side. The erarthing arms are so interlocked with main isolator moving
contacts that it can be closed only when the main contacts of isolator are in open position.
Similarly the main isolator contacts can be closed only when the earthing arms are in
open position.

Operation of Electrical Isolator: As no arc quenching technique is provided in isolator
it must be operated when there is no chance current flowing through the circuit. No live
circuit should be closed or open by isolator operation. A complete live closed circuit must
not be opened by isolator operation and also a live circuit must not be closed and
completed by isolator operation to avoid huge arcing in between isolator contacts. That is
why isolators must be open after circuit breaker is open and these must be closed
before circuit breaker is closed. Isolator can be operated by hand locally as well as by
motorized mechanism from remote position. Motorized operation arrangement costs
more compared to hand operation; hence decision must be taken before choosing an
isolator for system whether hand operated or motor operated economically optimum for
the system. For voltages up to 145KV system hand operated isolators are used whereas
for higher voltage systems like 245 KV or 420 KV and above motorized isolators
areused.

31. Fig.7 pantograph isolator

Fig. 8 double break isolator

 Current Transformer: A current
transformer (CT) is used for measurement of
alternating electric currents. Current transformers,
together with voltage transformers (VT)
(potential transformers (PT)), are known
as instrument transformers. When current in a
circuit is too high to apply directly to measuring
instruments, a current transformer produces a
reduced current accurately proportional to the
current in the circuit, which can be conveniently
connected to measuring and recording
instruments. A current transformer isolates the
Fig. 11 Current transformer

32. measuring instruments from what may be very high voltage in the monitored circuit.
Current transformers are commonly used in metering and protective relays in
the electrical power industry.

Design of C.T.- Like any other transformer, a current transformer has a primary
winding, a magnetic core and a secondary winding. The alternating currentin the primary
produces an alternating magnetic field in the core, which then induces an alternating
current in the secondary winding circuit. An essential objective of current transformer
design is to ensure the primary and secondary circuits are efficiently coupled, so the
secondary current is linearly proportional to the primary current.
The most common design of CT consists of a length of wire wrapped many times around
a silicon steel ring passed 'around' the circuit being measured. The CT's primary circuit
therefore consists of a single 'turn' of conductor, with a secondary of many tens or
hundreds of turns. The primary winding may be a permanent part of the current
transformer, with a heavy copper bar to carry current through the magnetic core.
Window-type current transformers (aka zero sequence current transformers, or ZSCT) are
also common, which can have circuit cables run through the middle of an opening in the
core to provide a single-turn primary winding. When conductors passing through a CT
are not centered in the circular (or oval) opening, slight inaccuracies may occur.
Shapes and sizes can vary depending on the end user or switchgear manufacturer. Typical
examples of low-voltage single ratio metering current transformers are either ring type or
plastic molded case. High-voltage current transformers are mounted on porcelain
insulators to isolate them from ground. Some CT configurations slip around the bushing
of a high-voltage transformer or circuit breaker, which automatically centers the
conductor inside the CT window.Current transformers can be mounted on the low voltage
or high voltage leads of a power transformer; sometimes a section of bus bar is arranged
to be easily removed for exchange of current transformers.


Fig. 12 principle of operation of instrument transformer

33.  Potential transformer: Voltage transformers
(VT) (also called potential transformers (PT)) are
a parallel connected type of instrument
transformer, used for metering and protection in
high-voltage circuits or phasor phase shift
isolation. They are designed to present negligible
load to the supply being measured and to have an
accurate voltage ratio to enable accurate
metering.
Some transformer winding primary (usually high-
voltage) connection points may be labelled as H1,
H2 (sometimes H0 if it is internally designed to be
grounded) and X1, X2 and sometimes an X3 tap
may be present. Sometimes a second isolated
winding (Y1, Y2, Y3) may also be available on the
same voltage transformer. The primary may be connected phase to ground or phase to
phase. The secondary is usually grounded on one terminal.
There are three primary types of voltage transformers(VT): electromagnetic, capacitor,
and optical. The electromagnetic voltage transformer is a wire-wound transformer. The
capacitor voltage transformer uses a capacitance potential divider and is used at higher
voltages due to a lower cost than an electromagnetic VT. An optical voltage transformer
exploits the electrical properties of optical materials.
[11]
measurement of high voltages is
possible by the potential transformers.

 Variable shunt reactor- Variable Shunt Reactors Variable Shunt Reactors are used in
high voltage energy transmission systems to stabilize the voltage during load variations.
A traditional shunt reactor has a fixed rating and is either connected to the power line all
the time or switched in and out depending on the load. Recently Variable Shunt Reactors
(VSR) have been developed and introduced on the market. The rating of a VSR can be
changed in steps, The maximum regulation range typically is a factor of two, e.g. from
100-200 Mvar. The regulation speed is normally in the order seconds per step and around
a minute from max to min rating. VSRs are today available for voltages up to 550 kV.
The largest three-phase VSRs in operation have a rating of 120-200 Mvar at 420 kV and
single-phase variable shunt reactors banks rated 200-285 Mvar at 420 kV have been
installed in Italy. The variability brings several benefits compared to a traditional fixed
shunt reactors. The VSR can continuously compensate reactive power as the load varies
and thereby securing voltage stability. Other important benefits are: reduced voltage
jumps resulting from switching in and out of traditional fixed reactors flexibility for
future (today unknown) load and generation patterns improved interaction with other
transmission equipment and/or systems such as coarse tuning of SVC equipment limiting
the foot print of a substation if parallel fixed shunt reactors can be replaced with one VSR

34. a VSR can be used as a flexible spare unit and be moved to other locations in the power
grid if needed.

 Wavetrap : TRAP Line trap also is known as Wave trap. What it does is trapping the
high frequency communication signals sent on the line from the remote substation and
diverting them to the telecom/teleprotection panel in the substation control room (through
coupling capacitor and LMU). This is relevant in Power Line Carrier Communication
(PLCC) systems for communication among various substations without dependence on
the telecom company network. The signals are primarily teleprotection signals and in
addition, voice and data communication signals.Line trap also is known as Wave trap.
What it does is trapping the high frequency communication signals sent on the line from
the remote substation and diverting them to the telecom/teleprotection panel in the
substation control room (through coupling capacitor and LMU). This is relevant in Power
Line Carrier Communication (PLCC) systems for communication among various
substations without dependence on the telecom company network. The signals are
primarily teleprotection signals and in addition, voice and data communication signals.
The Line trap offers high impedance to the high frequency communication signals thus
obstructs the flow of these signals in to the substation busbars. If there were not to be
there, then signal loss is more and communication will be ineffective/probably
impossible.

 Lightning arrestors: A lightning arrester is a device used on electrical power systems
and telecommunications systems to protect the insulation and conductors of the system
from the damaging effects of lightning. The typical lightning arrester has a high voltage
terminal and a ground terminal. When a lightning surge (or switching surge, which is
very similar) travels along the power line to the arrester, the current from the surge is
diverted through the arrestor, in most cases to earth. In telegraphy and telephony, a
lightning arrestor is placed where wires enter a structure, preventing damage to electronic
instruments within and ensuring the safety of individuals near them. Smaller versions of
lightning arresters, also called surge protectors, are devices that are connected between
each electrical conductor in power and communications systems and the Earth. These
prevent the flow of the normal power or signal currents to ground, but provide a path
over which high-voltage lightning current flows, bypassing the connected equipment.
Their purpose is to limit the rise in voltage when a communications or power line is
struck by lightning or is near to a lightning strike. If protection fails or is absent, lightning
that strikes the electrical system introduces thousands of kilovolts that may damage the
transmission lines, and can also cause severe damage to transformers and other electrical
or electronic devices.
Lightning- produced extreme voltage spikes in incoming power lines can damage
electrical home appliances COMPONENT A potential target for a lightning strike, such as a
television antenna, is attached to the terminal labeled A in the photograph. Terminal E is
attached to a long rod buried in the ground. Ordinarily no current will flow between the
antenna and the ground because there is extremely high resistance between B and C, and also
between C and D. The voltage of a lightning strike, however, is many times higher than that

35. needed to move electrons through the two air gaps. The result is that electrons go through the
lightning arrester rather than traveling on to the television set and destroying it. A lightning
arrester may be a spark gap or may have a block of a semiconducting material such as silicon
carbide or zinc oxide. Some spark gaps are open to the air, but most modern varieties are
filled with a precision gas mixture, and have a small amount of radioactive material to
encourage the gas to ionize when the voltage across the gap reaches a specified level. Other
designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp)
connected between the protected conductor and ground, or voltage-activated solid-state
switches called varistors or MOVs.
Lightning arresters built for power substation use are impressive devices,
consisting of a porcelain tube several feet long and several inches in diameter, typically
filled with disks of zinc oxide. A safety port on the side of the device vents the occasional
internal explosion without shattering the porcelain cylinder. Lightning arresters are rated
by the peak current they can withstand, the amount of energy they can absorb, and the
breakover voltage that they require to begin conduction. They are applied as part of a
lightning protection system, in combination with air terminals and bonding. BUSBAR In
electrical power distribution, a busbar (also spelled bus bar, buss bar or bussbar, with the
term bus being a contraction of the Latin omnibus - meaning for all) is a strip or bar of
copper, brass or aluminium that conducts electricity within a switchboard,distribution
board, substation, battery bank or other electrical apparatus. Its main purpose is to
conduct electricity, not to function as a structural member. The cross-sectional size of the
busbar determines the maximum amount of current that can be safely carried. Busbars
can have a cross-sectional area of as little as 10 mm2 but electrical substations may use
metal tubes of 50 mm in diameter (20 cm2) or more as busbars. An aluminium smelter
will have very large busbars used to carry tens of thousands of amperes to the
electrochemical cells that produce aluminium from molten salts. DESIGN AND
PLACEMENT Busbars are typically either flat strips or hollow tubes as these shapes
allow heat to dissipate more efficiently due to their highsurface area to cross-sectional
area ratio. The skin effect makes 50–60 Hz AC busbars more than about 8 mm (1/3 in)
thickness inefficient, so hollow or flat shapes are prevalent in higher current applications.
A hollow section has higher stiffness than a solid rod of equivalent current-carrying
capacity, which allows a greater span between busbar supports in outdoor switchyards. A
busbar may either be supported on insulators, or else insulation may completely surround
it. Busbars are protected from accidental contact either by a metal earthed enclosure or by
elevation out of normal reach. Power Neutral busbars may also be insulated. Earth (safety
grounding) busbars are typically bare and bolted directly onto any metal chassis of their
enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or
busway, segregated-phase bus, or isolated-phase bus. Busbars may be connected to each
other and to electrical apparatus by bolted, clamp, or welded connections. Often joints
between high-current bus sections have matching surfaces that are silver-plated to reduce
the contact resistance. At extra-high voltages (more than 300 kV) in outdoor buses,
corona around the connections becomes a source of radio-frequency interference and
power loss, so connection fittings designed for these voltages are used. Busbars are
typically contained inside switchgear, panel boards, or busway. Distribution boards split
the electrical supply into separate circuits at one location. Busways, or bus ducts, are long
busbars with a protective cover. Rather than branching the main supply at one location,

36. they allow new circuits to branch off anywhere along the route of the busway. INTER
CONNETTING TRANSFORMER The function of the inter-connecting transformer is -
as the name suggests - to inter-connect two systems at different voltages. Normally, they
will be either 400kV/132kV or 220kV/110kV, of say about 100 MVA rating. They are bi-
directional. During the plant start-up, they "import" power from the grid either at 400kV
or 220kV and step down to 132kV or 110kV to supply the station auxiliaries. Once the
plant is started and synchronized to the grid, the same transformer can now be used to
"export" power to the grid. They are normally auto-transformers and they will have a
delta connected tertiary winding of about 33kV voltage rating, for providing a circulating
path for the zero-sequence currents. The spec would read: 400/132/33kV, 100MVA
 Capicitor voltage transformer: A capacitor voltage transformer (CVT), or capacitance
coupled voltage transformer (CCVT) is a transformer used inpower systems to step down
extra high voltage signals and provide a low voltage signal, for measurement or to
operate aprotective relay. In its most basic form the device consists of three parts: two
capacitors across which the transmission line signal is split, an inductive element to tune
the device to the line frequency, and a transformer to isolate and further step down the
voltage for the instrumentation or protective relay. The tuning of the divider to the line
frequency makes the overall division ratio less sensitive to changes in the burden of the
connected metering or protection devices. The device has at least four terminals: a
terminal for connection to the high voltage signal, a ground terminal, and two secondary
terminals which connect to the instrumentation or protective relay. CVTs are typically
single-phase devices used for measuring voltages in excess of one hundred kilovolts
where the use of wound primary voltage transformers would be uneconomical. In
practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in
series. This provides a large voltage drop across C1 and a relatively small voltage drop
across C2. The CVT is also useful in communication systems. CVTs in combination with
wave traps are used for filtering high frequency communication signals from power
frequency. This forms a carrier communication network throughout the transmission
network.


Switchyard control room:
This room is the brain of switchyard which consists of various panels digital devices to control
the equipments automatically though SCADA system. It has prime duty to prepare load variation
of its region on daily basis and send it to load dispatch center. Also it instruct power plant the
amount power required for consumption.
Load dispatch centers are made to Ensure Integrated Operation of Regional and National Power
Systems to facilitate transfer of electric power within and across the regions and trans-national
exchange of power with Reliability, Security and Economy. The Load Despatch Department is
the nerve centre for the operation , planning , monitoring and control of the power system .
Electricity cannot be stored and has to be produced when it is needed. It is therefore essential that
power system is planned and operated optimally & economically. This is the main objective of
Load Despatch Centre. The objective of Load Despatch Department are:

37. 1. Matching the power demand with system integrity , reliability and security of
generation and transmission facilities
2. Regulating the system frequency .
3. Optimum utilisation of resources.
4. Quick restoration of normalcy after system disturbances.
Thus the objectives of Load Despatch Department is to co-ordinate generation , transmission
and distribution of electricity from moment to moment to achieve maximum security and
efficiency. The functions of Load Despatch Department are Dynamic in nature. While
performing the functions the policies laid down by management are strictly followed.


The main responsibilities of Regional Load Dispatch Centers are:
1. To ensure the integrated operation of the power system in the Region.
2. Monitoring of system parameters and system security.
3. Daily scheduling and operational planning.
4. Facilitating bilateral and inter-
regional exchanges of power.
5. Analysis of tripping/disturbances
and facilitating immediate
remedial measures.
6. System studies,planning and
contingency analysis.
7. Augmentation of telemetry,
computing and communication
facilities.
8. Computation of energy despatch
and drawal values using SEMs.
Functions of switchyard:
1. Controlling of bay equipments remotely.
2. Control & monitors each system parameters through SCADA.
3. Regional Load Dispatch Centers instructs & informs the plant through control room
on amount of power to be generated.
4. Control room prepares the load variation of their respective regions on daily basis &
send to their L.D.C‟s.
5. Control room communicates with all the substations of the region through hot
lines(PLCC) to get information of system parameters.




Switchyard control room with control panels

38. REFRENCE
 “Switchyard design ppt” NTPC VINDHYACHAL
 “Substation equipments” Wikipedia
 “220kv switchyard design” www.authorstream.com
 “Bus bar arrangement” www.skm-eleksys.com
 “Load dispatch centre” www.posco.in
 “substation design manual.pdf” www.ergonpower.com