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132/33 KV SUBSTATION EQUIPMENTS MINI PROJECT

SREE CHAITANYA COLLEGE OF ENGINEERING 1

CHAPTER-1

INTRODUCTION

The present day electrical power system is a. c. i.e. electric power is generated, transmitted

and distributed in the form of Alternating current. The electric power is produce at the

power station, which are located at favorable places, generally quite away from the

consumers. It is delivered to the consumer through a large network of transmission and

distribution. At many place in the line of power system, it may be desirable and necessary to

change some characteristic of electric supply. This is accomplished by suitable apparatus

called sub-station for example, generation voltage (11KV or 6.6KV) at the power station is

stepped up to high voltage (Say 220KV to 132KV) for transmission of electric power.

Similarly near the consumers localities, the voltage may have to be stepped down to

utilization level. This job is again accomplished by suitable apparatus called As substation

About the substation 132/33 KV

The substation near court chowrasta, in karimnagar is one of the power grids in the state of

Andhra Pradesh .The main source of this substation is coming out from the durshed 220kv

substation .132/33kv substation, karimnagar charged on date 27-08-1974.

The 132kv feeding to the substation is from 220kv /132kv durshed substation. This

substation under the control of superintending Engineer / TL &SS/ Karimnagar &Divisional

Engineer / TL& SS/karimnagar.

The following feeders are available in the substation 132/33 kv are

1. 33KV DHARMARAM

2. 33KV VAVILALAPALLY

3. 33KV GANGADHARA

4. 33 KV BHAGATH NAGAR

5. 33KV BOINPALLY

6. 33KV THOTAPALLY

7. 33KV PADMANAGAR

8. 33 KV WATER WORKS

9. 33 KV CHOPPADANDI

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SINGLE LINE DIAGRAM OF 132/33 KV SUBSTATION

a one-line diagram or single-line diagram is a simplified notation for representing a three-phase power system. The one-line diagram has its largest application in power flow studies.

Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and

conductors are shown by standardized schematic symbols. Instead of representing each of

three phases with a separate line or terminal, only one conductor is represented. It is a form

of block diagram graphically depicting the paths for power flow between entities of the

system. Elements on the diagram do not represent the physical size or location of the

electrical equipment, but it is a common convention to organize the diagram with the same

left-to-right, top-to-bottom sequence as the switchgear or other apparatus represented.

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SYMBOLS OF EQUIPMENTS USED IN A SUBSTATION

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CHAPTER-2

SUBSTATION

1.INTRODUCTION

A substation may be defined as an assembly of apparatus, which transforms the

characteristics of electrical energy from one form to another form.

At generation stations the electric power is produced at low voltages. The electric power

should not be transmitted at these low voltages due to large amount of line losses andeconomical reasons. To minimize the losses and for economical transmission the low

voltages are stepped up to high voltages and transmitted to far off place. The

consumers do not use such higher voltages directly and so they must be transformed to

low voltages for distribution purpose and done in these substations .So the substation

may be called as link between generation stations and consumers. The transmission

voltages are 66KV,110KV,132KV,220KV,400KV etc. The distribution voltages generally

used are 6.6KV,11KV and 33KV.

Substations usually contain transformers in order to change voltage levels; they are

connected to a "bus" via a circuit breaker. Specifically, substations are used for some or

all of the following purposes: connection of generators, transmission or distribution

lines, and loads to each other; transformation of power from one voltage level to

another; interconnection of alternate sources of power; switching for alternate

connections and isolation of failed or overloaded lines and equipment; controlling

system voltage and power flow; reactive power compensation; suppression of

overvoltage; and detection of faults, monitoring, recording of information, power

measurements, and remote communications. Minor distribution or transmission

equipment installation is not referred to as a substation.

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Substations present in the power system performs various operations depends on the

application such as stepping up the voltage, stepping down the voltage, high voltage

transmission and switching stations to route the power to desired load center.

Substations are installed to perform any of the following operations.

1. To switch ON and OFF the power lines, known as switching operation.

2. To transform voltage from higher to lower or vice versa, known as voltage

transformation operation.

3. To convert A.C, into D.C. or vice versa, known as power converting operation.

4. To convert frequency from higher to lower or vice-versa ,known as frequency

converting operation.

5. To improve the power factor by installing synchronous condenser at the end of

the line, known as power factor correction operation.

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1.1 classification of sub-stations

substations are classified according to service requirement and constructional

features .

1.According to service requirement: A substation may be called upon to change

voltage or improve power factor or convert a. c. power into d. c. power etc.

According to the service requirement, substations may be classified into

a. Transformer substation:

Those substations which change the voltage level of electric supply are called

transformer substations. These substations receive power at some voltage and deliver it at

some other voltage. Obviously, transformer will be the main component in such

substations. Most of the sub stations in the power system are of this type.

b. switching sub-station : These substations do not change the voltagelevel i.e. incoming and outgoing lines have the same voltage. However ,they

simply perform the switching operations of power lines. A switching

substation is a substation which does not contain transformers and operates

only at a single voltage level. Switching substations are sometimes used as

collector and distribution stations. Sometimes they are used for switching the

current to back-up lines or for parallelizing circuits in case of failure

c. Power factor correction substationThese sub-station which improve the power factor of the system are

called power factor correction substation. These are generally located at

receiving end of transmission lines. These substations generally use

synchronous condensers as the power improvement equipment

d. Frequency changer sub-station :

Those sub-stations, which change the supply frequency, are known as

frequency change substations . Such sub-station may be required for industrial utilization.

e. Converting sub-station :

Those sub-station which change a. c. power into d. c. power are called converting

s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding

etc.

f. Industrial sub-station:

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Those sub-stations, which supply power to individual industrial concerns are known

as industrial sub-stations.

2.According to constructional features : A substation has many components whichmust be housed properly to ensure continuous and reliable service. According to

constructional features the substations are classified as :

Indoor substation

Outdoor substation

Underground substation

Pole-mounted substation

1. Indoor Sub-station :-

For voltage up to 11KV, the equipment of the s/s is installed indoor because of

economic consideration. However, when the atmosphere is contaminated with impurities,

these sub-stations can be erected for voltage up to 66KV.

2. Outdoor Sub-Station :-

Outdoor Substations are used for all voltage levels from 52 to 765 kV. They are built

outside cities, usually at points along the cross-country lines of bulk transmission systems.

They comprise switchgear like circuit breakers, disconnectors, instrument transformers,

power transformers, surge arrestors and bus bars. The control and protection equipment is

housed in central buildings or in small switching bay oriented containers in the

switchyard .

For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such

Voltage the clearances between conductor and the space required for switches, C.B. and

other equipment becomes so great that it is not economical to installed the equipment

indoor.

3. Under ground sub-station :-

In thickly populated areas, the space available for equipment and building is limited

and the cost of the land is high. Under such situations, the sub-station is created

underground. The design of underground s/s requires more careful consideration.

The size of the s/s should be as minimum as possible.

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There should be reasonable access for both equipment & personal.

There should be provision for emergency lighting and protection against fire.

There should be good ventilation

4. Pole-mounted sub-station :-

This is an outdoor sub-station with equipment installed overhead on double pole or 4-pole

structure. It is the cheapest from of s/s for voltage not exceeding 11KV (or 33KV in some

cases). Electric power is almost distributed in localities through such sub-station. The 11KV

line is connected to the T/F through gang isolator and fuses. The lighting arresters are

installed on the H.T. Side to protect the sub-station from lighting strokes. The T/F step

down voltage to 400 V, 3 phase, 4 wire supply. The voltage between any two lines is 400 V

& between line & neutral is 230V. The oil circuit breaker installed on the L.T. side

automatically Isolates the mounted sub-station.T/F are generally in the event of faultgenerally 200 KVA T/F is used.

FUNCTIONS OF SUBSTATION:

1. Supply of required electrical power.

2. Maximum possible coverage of the supply network.

3. Maximum security of supply.

4. Shortest possible fault-duration

5. Optimum efficiency of plants and the network.

6. Supply of electrical power within targeted frequency limits, (49.5 Hz and50.5 Hz).

7. Supply of electrical power within specified voltage limits.

8. Supply of electrical energy to the consumers at the lowest cost

CHAPTER-3

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EQUIPMENTS IN A SUB-STATION

The main equipments in a sub-station are:

1. BUS BARS

2. INSULATORS

3. CIRCUIT BREAKER

4. INSTRUMENT TRANSFORMER

5. POWER TRANSFORMER

6. WAVE TRAP

7. SWITCH GEAR

8. ISOLATORS

9. MISCELLONOUS EQUIPMENTS

10.PROTECTIVE RELAY

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1. BUS BARS

When a no. of lines operating at the same voltage have to be directly connected electrically,

bus-bars are used, it is made up of copper or aluminum bars (generally of rectangular X-

Section) and operate at constant voltage.Duplicate bus-bar, generally it consist of two bus-bars a main bus-bar and spare bus-

bar. The incoming and outgoing lines can be connected to either b/b. With the help of a bus-

bar coupler, which consist of a circuit breaker and isolators. However, in case of repair of

main bus-bar or fault accusing on it, the continuity of supply to the circuit can be maintain

by transforming it to the spare bus-bar for voltage exceeding 33KV, Duplicate bus-bar is

frequently used.

Bus bars , or buses, are conductors. Or group of conductors, that serve as a common

connection for two or more circuits. The bus bars experience forces when currents flow in

them. These forces can be great when short-circuit currents flow. A bus bar must be able towithstand the forces caused by the flow of fault currents. Bur bars are the important

elements in the electrical substation. Bus bars acts as nodal point in the substation, which

connects different incoming and outgoing circuits.

Bus bars used in the substations are generally rectangular or circular cross section bars.

These bus bars can be either solid or hollow structures. Hollow circular cross section bus

bars are employs in EHV substations to reduce the corona effect.

An aluminum or copper conductor supported by insulators that interconnects the loads and

the sources of electric power in an electric power system. A typical application is the

interconnection of the incoming and outgoing transmission lines and transformers at an

electrical substation. Bus-bars also interconnect the generator and the main transformers ina power plant. In an industrial plant such as an aluminum smelter, large bus-bars supply

several tens of thousands of amperes to the electrolytic process. See also Electric power

substation.

The size of the bus bar determines its application and the amount of current that it can carry

safely. They can be tubular, solid or flat depending on the application and to serve different

needs.

Various incoming and outgoing circuits are connected to bus bars. Bus bars receive power

from incoming circuits and deliver power to outgoing circuits.

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Bus bars are usually made up of Aluminum or copper and they are able to conduct

electricity to transmit power from the source of electric power to the load. They are usually

supported by insulators and conduct electricity within switchboards, substations or other

electric apparatus. Some typical applications of these devices can be to form the

interconnectedness of the incoming and outgoing electrical transmission lines and

transformers at an electrical substation; supplying huge amounts of amperes to the

electrolytic process in an aluminum smelter by using large bus bars and also interconnecting

generators to the main transformers in a power plant.

Different types of bus bar arrangements are employed based on the voltage, reliability of

the supply, flexibility in transmitting power and cost. The other aspects considering in

designing the bus bars arrangements are:

1. Simplicity in the design

2. Maintenance of different elements without interruption in the power supply

3. Future expansion feasibility

4. Economical in cost of installation and operation

Different bus bar arrangements:

Some of the switching schemes are bus bar arrangements employed in the substations are

listed below:

1. Single Bus-bar arrangement

2. Double Main Bus-bar scheme

3. Main and Transfer bus-bar scheme

4. One and half breaker scheme

5. Ring Main arrangement scheme

1) Single Bus-bar scheme:

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This is the simplest bus bar scheme available which consists of single set of bus bars

connected to the generators, transformers and load feeders. All the feeders are connected

by circuit breaker and set of isolators. This arrangement helps to remove the connecting

elements (Generators, transformers, etc ) for maintenance by opening the circuit breaker

contacts and further opening the isolators. The entire Sub Station is lost in case of a fault on

the bus bar or on any bus bar isolator and also in case of maintenance of the bus bar.

Another disadvantage of this switching scheme is that in case of maintenance of circuit

breaker, the associated feeder has also to be shutdown.

Advantages:

1. This bus bar arrangement enjoys less cost of installation

2. Less maintenance

3. simple operation

Disadvantages:

1. Fault on the bus bar all the feeders connected to the bus bars should be disconnected

2. when Bus bar is under maintenance total supply and all feeders should be

disconnected

3. Least flexibility and reliability

2) DOUBLE BUS BAR ARRANGEMENT:

In this double bus bar arrangement , Each circuit can be connected to either one of these

bus bars through respective bus bar isolator. Bus coupler breaker is also provided so that

the circuits can be switched on from one bus to the other on load. This scheme suffers from

the disadvantage that when any circuit breaker is taken out for maintenance, the associated

feeder has to be shutdown.

This Bus bar arrangement was generally used in earlier 220 kV sub stations

3) Main and Transfer Bus bar Scheme:

Main and Transfer bus bar scheme is similar to single bus bar arrangement with

additional transfer bus connected. Tie circuit breaker is provided to tie both the main and

transfer bus. During normal operation all the circuits are connected to the main bus. When

circuit breaker connected to the circuit (transmission line) is required to trip for

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maintenance, tie circuit breaker connecting the main and transfer bus is closed. The relay

protection for the circuits connected to the transfer bus is taken care by the tie circuit

breaker.

Advantages:

1. Low initial cost

2. Any breaker can be taken of circuit for maintenance

Disadvantages:

1. Requires one extra breaker for bus tie

2. Switching is somewhat complicated when breaker is under maintenance

4) One and Half breaker Bus bar scheme:

In One and half breaker scheme, two circuits are connected between the three

circuit breakers. Hence One and Half breaker name was coined for this type of arrangement.

Under normal operating conditions all the breakers are closed and both the bus bars are

energized. Any Circuit fault will trip two circuit breakers and no other circuit will be affected

in this arrangement. When a bus bar fault occur only breakers adjacent to bus bars trips and

no circuit will loose power. Two bus bars can also be taken out of service without affectingthe power flow if the power source circuit ( alternator circuit) and receiving circuit

(transmission line) available in the same bay.

Advantages:

1. Most flexible operation possible

2. High reliability

3. Bus failure will not remove any circuit from service

Disadvantages:

1. High cost

2. Relaying is somewhat complicated since the middle breaker must responsible for both

the circuits on either direction and should operate

5) Ring bus bar scheme:

In this ring main bus bar scheme arrangement, breakers are connected in ring and

circuits are connected between the breakers. There will be same number of circuits as the

number of breakers in the arrangement. During normal operation all the breakers are

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closed. During circuit fault two breakers connecting the circuit trips. During breaker

maintenance the ring is broken but all the lines remain in service.

Advantages:

1. Low cost

2. Flexible operation for breaker maintenance

3. Any breaker can be taken out of service without interrupting load

4. Power can be fed from both the direction

Disadvantages:

1. Fault occur during maintenance will break the ring

2. Relaying is complex

3. Breaker failure during fault will trip one additional circuit

Major Type of Bus bar

The major types are

(1) Rigid bus-bars, used at low, medium, and high voltage

The rigid bus-bar is an aluminum or copper bar, which is supported by porcelain insulators.

The rigid bus-bar provides a quick and qualified installation, the necessary compensation of

linear thermal deformations of buses and minor errors in the installation of bus-barsupports

(2) Strain bus-bars, used mainly for high voltage

The strain bus-bar is a flexible, stranded conductor which is strung between substation

metal structures and held by suspension-type insulators.

(3) Insulated-phase bus-bars, used at medium voltage

The insulated-phase bus-bar is a rigid bar supported by insulators and covered by agrounded metal shield. The main advantage of this system is the elimination of short circuits

between adjacent phases.

(4) Sulfur hexafluoride (SF6)-insulated bus-bars, used in medium- and high-

voltage systems

The sulfur hexafluoride-insulated bus-bar is a rigid aluminum tube, supported by insulators

and installed in a larger metal tube, which is filled with high-pressure sulfur hexafluoride

gas.

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Advantages of Bus barView

Fitting of any modular device with the same mounting height Components interchangeable at any time

Compact energy distribution up to max. 150A

Full scope for the future

Completely touch proof
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Circuit breaker

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, by interrupting continuity, to immediately discontinue electrical

flow. Unlike a fuse, which operates once and then has to 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.

Fig 1.1.1 An air circuit breaker forlow voltage (less than 1000 volts) power distribution switchgear

Operation
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All circuit breakers have common features in their operation, although details vary

substantially depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is

usually done within the breaker enclosure. Circuit breakers for large currents or high voltages

are usually arranged with pilot devices to sense a fault current and to operate the trip opening

mechanism. The trip solenoid that releases the latch is usually energized by a separate

battery, although some high-voltage circuit breakers are self-contained with current

transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt

the circuit; some mechanically-stored energy (using something such as springs or compressed

air) contained within the breaker is used to separate the contacts, although some of the energy

required may be obtained from the fault current itself. Small circuit breakers may be

manually operated; larger units have solenoids to trip the mechanism, and electric motors to

restore energy to the springs.The circuit breaker contacts must carry the load current without

excessive heating, and must also withstand the heat of the arc produced when interrupting

(opening) the circuit. Contacts are made of copper or copper alloys, silver alloys, and other

highly conductive materials. Service life of the contacts is limited by the erosion of contact

material due to arcing while interrupting the current. Miniature and molded case circuit

breakers are usually discarded when the contacts have worn, but power circuit breakers and

high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled,

and extinguished in a controlled way, so that the gap between the contacts can again

withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas,

or oil as the medium in which the arc forms. Different techniques are used to extinguish the

arc including:

Lengthening / deflection of the arc

Intensive cooling (in jet chambers)

Division into partial arcs

Zero point quenching (Contacts open at the zero current time crossing of the AC

waveform, effectively breaking no load current at the time of opening. The zero crossing

occurs at twice the line frequency i.e. 100 times per second for 50Hz and 120 times per

second for 60Hz AC)
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Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore

power to the interrupted circuit.

Arc interruption

Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger

ratings will have metal plates or non-metallic arc chutes to divide and cool the arc. Magnetic

blowout coils or permanent magnets deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of

oil through the arc. Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the

arc using a magnetic field, and then rely upon the dielectric strength of the sulfur

hexafluoride (SF6) to quench the stretched arc. Vacuum circuit breakers have minimal arcing

(as there is nothing to ionize other than the contact material), so the arc quenches when it is

stretched a very small amount (

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installed in. Therefore, circuit breakers must incorporate various features to divide and

extinguish the arc.

In air-insulated and miniature breakers an arc chute structure consisting (often)

of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc

into the arc chute. Larger circuit breakers such as those used in electrical power distribution

may use vacuum, an inert gas such as sulphur hexafluoride or have contacts immersed in oil

to suppress the arc.

The maximum short-circuit current that a breaker can interrupt is determined by

testing. Application of a breaker in a circuit with a prospective short-circuit current higher

than fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to

explode when reset. Miniature circuit breakers used to protect control circuits or small

appliances may not have sufficient interrupting capacity to use at a panelboard; these circuit

breakers are called "supplemental circuit protectors" to distinguish them from distribution-

type circuit breakers.

CLASSIFICATION OF CIRCUIT BREAKERS:

1.Air-break circuit breakers

2.Oil circuit breakers

3.Air-blast circuit breakers

4.SF6 circuit breakers

5.vacuum circuit breakers

3.1 SF6 gas Circuit breakers

In this circuit breaker, sulphurhexaflouride ( SF6 ) gas is used as the arc

quenching medium. The SF6 gas is an electro negative gas and has a strong tendency to

absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6

gas and an arc is struck between them. The conducting free electrons in the arc are rapidly

captured by the gas to form relatively immobile negative ions. This loss of conducting

electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The

SF6 circuit breakers are very effective for high power and high voltage service.

Construction: Fig 15 shows the parts of a typical SF6 circuit breaker. It consists of fixed

and moving contacts enclosed in a chamber called arc interruption chamber containing SF 6
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gas. This chamber is connected to SF6 gas reservoir. When the contacts of breaker are opened

the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the

arc interruption chamber. The fixed contact is a hollow cylindrical current carrying contact

fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in

the sides to permit the SF6 gas to let out through these holes after flowing along and across

the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper-

tungsten arc resistant material. Since SF6 gas is costly, its reconditioned and reclaimed by a

suitable auxiliary system after each operation of the breaker.

Working:

In t closed position of the breaker the contacts remained surrounded by SF6 gas at a pressure

heof about 2.8 kg/cm2. When the breaker operates the moving contact is pulled apart and an

arc is struck between the contacts. The movement of the moving contact is synchronized with

the opening of a valve which permits SF6 gas at 14 kg/cm2

pressure from the reservoir to the

arc interruption chamber.

The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to

form immobile negative ions which are ineffective as charge a carriers. The result is that the

medium between the contacts quickly builds up high dielectric strength and causes the

extinction of the arc. After the breaker operation the valve is closed by the action of a set of

springs.

Advantages over oil and air circuit breakers:

a.Due to superior arc quenching property of SF6 , such breakers have very short arcing time

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b.Dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much

larger currents.

c.Gives noiseless operation due to its closed gas circuit

d.Closed gas enclosure keeps the interior dry so that there is no moisture problem

e.There is no risk of fire as SF6 is non inflammable

f.There are no carbon deposits

g.Low maintenance cost, light foundation requirements and minimum auxiliary equipment

h.SF6 breakers are totally enclosed and sealed from atmosphere, they are particularly su h.

SF6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable

where explosion hazard exists

Disadvantages:

SF6 breakers are costly due to high cost of SF6

B. SF6 gas has to be reconditioned after every operation of the breaker, additional

equipment is required for this purpose

Applications:

SF6 breakers have been used for voltages 115kV to 230 kV, power ratings 10 MVAto 20 MVA and interrupting time less than 3 cycles

SF6 gas circuit breakers are the most widely used ones today. SF6 is an

electronegative gas and is excellent for use as an arc quenching medium. The principle is

similar to that of ABCBs. SF6 gas breakers have many advantages such as compactness, less

maintenance, efficient arc interruption and low cost. Owing to the above advantages it has

now replaced all other breakers in Medium Voltage, High Voltage, Extra High Voltage and

Ultra High Voltage ranges. The operating voltage range is from 52kV to 765kV

Sulfur hexafluoride (SF6) is an excellent gaseous dielectric for high voltage power

applications. It is used extensively in high voltage circuit breakers and other switchgear

employed by the power industry.

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The combined electrical, physical, chemical and thermal properties offer many

advantages when used in power switchgear. Some of the properties of SF6 which make its

use in power applications desirable are:

High dielectric strength

Unique arc-quenching ability

High thermal stability

Good thermal conductivity

Electro negative gas

Vacuum circuit breaker: Vacuum circuit breaker:

In this breaker, vacuum is being used as the arc quenching medium. Vacuum offers

highest insulating strength, it has far superior arc quenching properties than any othermedium. When contacts of a breaker are opened in vacuum, the interruption occurs at first

current zero with dielectric strength between the contacts building up at a rate thousands of

times that obtained with other circuit breakers.

Principle:

When the contacts of the breaker are opened in vacuum (10 -7 to 10 -5 torr), an arc is

produced between the contacts by the ionization of metal vapours of contacts. The arc is

quickly extinguished because the metallic vapours, electrons, and ions produced during arccondense quickly on the surfaces of the circuit breaker contacts, resulting in quick recovery

of dielectric strength. As soon as the arc is produced in vacuum, it is quickly extinguished

due to the fast rate of recovery of dielectric strength in vacuum.

Construction:

Fig 16 shows the parts of a typical vacuum circuit breaker. It consists of fixed

contact, moving contact and arc shield mounted inside a vacuum chamber. The movable

member is connected to the control mechanism by stainless steel bellows .This enables thepermanent sealing of the vacuum chamber so as to eliminate the possibility of leak .A glass

vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the

deterioration of the internal dielectric strength by preventing metallic vapours falling on the

inside surface of the outer insulating cover.

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Working: Working:

When the breaker operates the moving contacts separates from the fixed contacts and

an arc is struck between the contacts. The production of arc is due to the ionization of metal

ions and depends very much upon the material of contacts. The arc is quickly extinguished

because the metallic vapours, electrons and ions produced during arc are diffused in short

time and seized by the surfaces of moving and fixed members and shields.

Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in

a vacuum breaker occurs with a short contact separation.

Advantages:

a. They are compact, reliable and have longer life.

b. There are no fire hazards

c. There is no generation of gas during and after operation

d. They can interrupt any fault current. The outstanding feature of a VCB is that it can break

any heavy fault current perfectly just before the contacts reach the definite open position.

e. They require little maintenance and are quiet in operation

f. Can withstand lightning surges

g. Low arc energy

h. Low inertia and hence require smaller power for control mechanism.

4.2Applications:

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For outdoor applications ranging from 22 kV to 66 kV. Suitable for majority of

applications in rural area.

Fig 4.2 vacuum circuit breaker

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3.CIRCUIT BREAKERS3 Oil circuit breaker

Oil circuit breakers are further classified as

Bulk oil Circuit Breakers (BOCB)

Minimum oil Circuit breakers (MOCB)

In BOCBs the interrupting Chambers is placed in tank of oil at earth potential and the

incoming and outgoing conductors are connected through insulator bushings and in MOCBs

the interrupting units are placed in insulating chambers at live potential.

In oil circuit breaker the arc drawn across the contacts is contained inside the interrupting

pot and thus the hydrogen bubble, formed by the vaporised oil (gas) is also contained inside

the chamber. As the contacts continue to move and whenever the moving contact rod

separates itself from the orifice at the bottom of the chamber, an exit similar to a nozzle

becomes available for exhausting the hydrogen that is trapped inside the interrupting

chamber. The operating voltage range is up to 132kV[1]

.

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The oil in OCBs serves two purposes. It insulates between

the phases and between the phases and the ground, and it provides the medium for the

extinguishing of the arc. When electric arc is drawn under oil, the arc vaporizes the oil and

creates a large bubble that surrounds the arc. The gas inside the bubble is around 80%

hydrogen, which impairs ionization. The decomposition of oil into gas requires energy that

comes from the heat generated by the arc. The oil surrounding the bubble conducts the heat

away from the arc and thus also contributes to deionization of the arc. Main disadvantage of

the oil circuit breakers is the flammability of the oil, and the maintenance necessary to keep

the oil in good condition (i.e. changing and purifying the oil)

4.3.1 Bulk Oil Circuit Breakers

Bulk oil circuit breakers are enclosed in metal-grounded weatherproof tanks that are

referred to as dead tanks. The original design of bulk OCBs was very simple and inexpensive.Example of such a breaker, called plain break oil circuit breaker, is in

The arc was drawn directly inside of the container tank without any additional arc

extinguishing but the one provided by the gas bubble surrounding the arc. Plain break

breakers were superceded by arc controlled oil breakers.

The arc controlled oil breakers have an arc control device surrounding the breaker

contacts. The purpose of the arc control devices is to improve operating capacity, speed up

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the extinction of arc, and decrease pressure on the tank. The arc control devices can be

classified into two groups: cross-blast and axial blast interrupters.

In cross blast interrupters, the arc is drawn in front of several lateral vents. The gas

formed by the arc causes high pressure inside the arc control device. The arc is forced to bow

into the lateral vents in the pot, which increases the length of the arc and shortens the

interruption time. The axial blast interrupters use similar principle as the cross blast

interrupters. However, the axial design has a better dispersion of the gas from the interrupter.

Figure 12 illustrates a typical 69 kV breaker of 2500 MVA breaking capacity. All

three phases are installed in the same tank. The tank is made of steel and is grounded. This

type of breaker arrangement is called the dead tank construction. The moving contact of each

phase of the circuit breaker is mounted on a lift rod of insulating material. There are two

breaks per phase during the breaker opening. The arc control pots are fitted over the fixed

contacts. Resistors parallel to the breaker contacts may be installed alongside the arc control

pots. It is customary and convenient for this type of breakers to mount current transformers

in the breaker bushings.

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The oil in OCBs serves two purposes. It insulates between the phases and

between the phases and the ground, and it provides the medium for the extinguishing of the

arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubblethat surrounds the arc. The gas inside the bubble is around 80% hydrogen, which impairs

ionization. The decomposition of oil into gas requires energy that comes from the heat

generated by the arc. The oil surrounding the bubble conducts the heat away from the arc and

thus also contributes to deionization of the arc. Main disadvantage of the oil circuit breakers

is the flammability of the oil, and the maintenance necessary to keep the oil in good condition

(i.e. changing and purifying the oil)

4.3.1 Bulk Oil Circuit Breakers

Bulk oil circuit breakers are enclosed in metal-grounded weatherproof tanks that are

referred to as dead tanks. The original design of bulk OCBs was very simple and inexpensive.

Example of such a breaker, called plain break oil circuit breaker, is in

The arc was drawn directly inside of the container tank without any additional arc

extinguishing but the one provided by the gas bubble surrounding the arc. Plain break

breakers were superceded by arc controlled oil breakers.

The arc controlled oil breakers have an arc control device surrounding the breaker

contacts. The purpose of the arc control devices is to improve operating capacity, speed up

the extinction of arc, and decrease pressure on the tank. The arc control devices can be

classified into two groups: cross-blast and axial blast interrupters.

In cross blast interrupters, the arc is drawn in front of several lateral vents. The gas

formed by the arc causes high pressure inside the arc control device. The arc is forced to bow

into the lateral vents in the pot, which increases the length of the arc and shortens the

interruption time. The axial blast interrupters use similar principle as the cross blast

interrupters. However, the axial design has a better dispersion of the gas from the interrupter.

Figure 12 illustrates a typical 69 kV breaker of 2500 MVA breaking capacity. All

three phases are installed in the same tank. The tank is made of steel and is grounded. This

type of breaker arrangement is called the dead tank construction. The moving contact of each

phase of the circuit breaker is mounted on a lift rod of insulating material. There are two

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breaks per phase during the breaker opening. The arc control pots are fitted over the fixed

contacts. Resistors parallel to the breaker contacts may be installed alongside the arc control

pots. It is customary and convenient for this type of breakers to mount current transformers

in the breaker bushings.

1 bushing 6 plunger guide

2 oil level indicator 7 arc control device3 vent 8 resistor

4 current transformer 9 plunger bar

5 dashpot

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At voltages higher than 115 kV, it is customary to use separate tanks for each phase.

The practical limit for the bulk oil breakers is 275 kV. Figure 13 shows 220 kV one phase

dead tank circuit breaker.

1 bushing 7 arc control unit

2 oil level indicator 8 parallel contact

3 vent 9 resistor

4 linear linkage 10 plunger bar

5 dashpot 11 impulse cushion

6 guide block

The oil circuit breakers are usually installed on concrete foundations at the ground

level. During interruption of heavy fault currents the breakers tend to move. To minimize the

damage to the breakers, breakers with very high interrupting capacity have an impulse

cushion is provided at the bottom of the breakers. The cushion is filled with an inert gas, for

example nitrogen.

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1

bushing 7 arc control unit

2 oil level indicator 8 parallel contact3 vent 9 resistor

4 linear linkage 10 plunger bar

5 dashpot 11 impulse cushion

6 guide block

The oil circuit breakers are usually installed on concrete foundations at the ground

level. During interruption of heavy fault currents the breakers tend to move. To minimize the

damage to the breakers, breakers with very high interrupting capacity have an impulse

cushion is provided at the bottom of the breakers. The cushion is filled with an inert gas, for

example nitrogen.

4.3.2 Minimum Oil Breakers

In the bulk oil breakers, the oil serves as both arcs extinguishing medium and main

insulation. The minimum oil breakers were developed to reduce the oil volume only to

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amount needed for extinguishing of the arc about 10% of the bulk- oil amount. The arc

control for the minimum oil breakers is based on the same principle as the arc control devices

of the bulk oil breakers. To improve breaker performance, oil is injected into the arc.

The interrupter containers of the minimum oil breakers are made of insulating

material and are insulated from the ground. This is usually referred to as live tank

construction. For high voltages (above 132 kV), the interrupters are arranged in series. It is

essential to ensure that each interrupter carries its share of the duty. Care must be taken that

all breaks occur simultaneously, and that the restriking voltage is divided equally across the

breaks during the interrupting process. The natural voltage division depends on stray

capacitances between the contacts and to the ground, and therefore is in very uneven. This is

corrected by connecting capacitances or resistors in parallel with the interrupting heads.

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5.1 Oil circuit breaker

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5.1 Representation of electrical arc in oil circuit breaker

In oil circuit breakers, the arc is drawn in oil inside a special compartment of the interrupting

chamber called the explosion pot. The intense heat of the arc decomposes the oil and

produces gases, mainly composed of hydrogen, generating high pressure that produces a fluid

flow through the arc and out of the

explosion pot through vents situated on its walls. Thus extending the arcs column and

carrying its energy away until its total extension see Fig 3.

At transmission voltages below 345 kV, oil breakers used to be popular. They are

increasingly losing ground to gas-blast circuit breakers such as air-blast breakers and SF6

circuit breakers.

5.2 Air-blast circuit breaker

In air-blast circuit breakers, compressed air at a pressure of 20-30kg/cm2 is employed as an

arc quenching medalhemicium.These breakers are suitable for operating voltage of 132kv and

above. When the contacts part, a blast valve is opened to discharge the high-pressure air to

the ambient, thus creating a very-high-velocity flow near the arc to dissipate the energy.

ADVANTAGES:

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i.cheapness and free availability of the interrupting medium,chemical stability and inertness

of air

ii.high speed operation

iii.elimination of fire hazard

iv.less maintanence

v.suitability for frequent operation

vi.facility for high speed reclosure

DISADVANTAGES:

i.An air compressor plant has to be installed and maintained

ii.problem of current chopping

iii.problem of restriking voltage

This breaker is classified into 2 types

1.Cross blast circuit breakers

2.Axial blast circuit breakers

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, by interrupting continuity, to immediately discontinue

electrical flow. Unlike a fuse, which operates once and then must be replaced, a circuit

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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.

Circuit-breakers are used to make or break electric currents in circuits during normal

operation of the system, during system faults and during system disturbances. Most high

voltage circuit breakers are physically located in the switchyard but are operated from

panels provided in the control room.

Acircuit-breaker is expected to have the following characteristics.

It must be capable of closing on to and carrying full load currents.

It must have an appropriate mechanism to automatically disconnect the

Load under prescribed conditions.

It must be able to successfully interrupt short-circuit currents flowing

through the lines controlled by it.

The gaps between its contacts must not flash-over when the circuit breaker

is open.

The circuit breaker, when closed on to a circuit in which a fault exists,

must be able to reopen to isolate the faulted section without being

damaged.

It must be capable of withstanding the flow of short-circuit currents until

they are interrupted by an adjoining circuit breaker. It must be capable of withstanding the electro-magnetic forces and thermal stresses

caused by the flow of short-circuit currents.

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Circuit breaker

Different types of circuit breakers are used in the substations which depends upon

maximum voltage level, maximum continuous current carrying capacity and maximum

interrupting capacity.

Low Voltage Circuit Breaker:

These breakers are made for direct current (DC) applications and are commonly used in

domestic, commercial, and industrial fields. They can be installed in multi-tiers in LV

switchboards or switchgear cabinets. Low voltage circuit breakers are usually placed in

draw-out enclosures that permit removal and interchange without dismantling the

switchgear. Miniature circuit breakers (MCB) and molded case circuit breakers (MCCB) are

some common types of low voltage circuit breakers.

Medium Voltage Circuit Breakers:

These breakers can be assembled into metal enclosed switchgear line used for indoor

applications, or as individual components for outdoor applications like substations. Medium

voltage circuit breakers use discrete current sensors and protection relays, and can be

attached into the circuit by bolted connections to bus bars or wires. Vacuum circuit

breakers, air circuit breakers and SF6 circuit breakers are some examples of medium voltage

circuit breakers.

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High Voltage Circuit Breakers:

These breakers help in protecting and controlling electrical power transmission networks.

They are solenoid operated and are employed with current sensing protective relays that

function through current transformers.

Magnetic Circuit Breakers :

These breakers use a three dimensional electromagnetic coil whose pulling force increases

with the current. The contacts are held closed by a latch so that when the current in the coil

goes beyond the rating of the circuit breaker, the coil's pull releases the latch which allows

the contacts to open with a spring action.

Thermal Circuit Breakers:

These breakers employ heat to break the circuit current flow and consist of a bimetallic

strip, made of two types of materials welded together. At high heat levels, this strip bends

at an angle that pulls the lever down and breaks the connection between the circuit

breaker's contact plate and the stationary contact plate.

Rated circuit breakers, common trip breakers, Earth leakage circuit breakers are another

type. One of the most important difference between circuit breakers and fuses is that circuit

breakers can be reset either manually or automatically to resume normal operation,

whereas fuses once used, have to be replaced.

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4.INSTRUMENT TRANSFORMER

A current transformer (CT) is used for measurement of electric currents. Current

transformers, together with voltage transformers (VT) (potential transformers (PT)), areknown as instrument transformers.

Main information on the state of the power system and the substation equipment is

acquired by measuring various parameters. The measuring, monitoring, control and

protection devices measure and use parameters such as current. voltage. power factor.

frequency. active power, reactive power. direction of power flow. load balance. and phase

angles. These parameters are measured by using two types of analog sensors. current

transformers (CT) and voltage transformers (VT). These transducers provide the

instantaneous values of currents and voltages. The remaining parameters are derived from

these measurements. Older substations have instruments that are operated by analog

signals from the transducers. In modem substations, currents and voltages are acquired inthe form of quantized samples using analog to digital converters. The samples are then

processed by digital signal processors to estimate the desired parameters.

Instrument transformers are used for measuring voltage and current in electrical power

systems, and for power system protection and control. Where a voltage or current is too

large to be conveniently used by an instrument, it can be scaled down to a standardized low

value. Instrument transformers isolate measurement, protection and control circuitry from

the high currents or voltages present on the circuits being measured or controlled.

Current TransformerA current transformer is essentially a step-down transformer which steps-down the current

in a known ratio, the primary of this transformer consist of one or more turn of thick wire

connected in series with the line, the secondary consist of thick wire connected in series

with line having large number of turn of fine wire and provides for measuring instrument,

and relay a current which is a constant faction of the current in the line. The current

transformer (CT) is often treated as a black box. It is a transformer that is governed by the

laws of electromagnetic induction: = k AcNf

Where

= Induced voltage

= Flux density

Ac = Core cross-sectional area

N = Turns

f = Frequency

k = Constant of proportionality

Current transformers are basically used to take the readings of the currents entering the

substation. This transformer steps down the current from 800 amps to 1 amp. This is

done because we have no instrument for measuring of such a large current. The main use of

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this transformer is (a) distance protection; (b) backup protection; (c) measurement.

A current transformer is a transformer designed to provide a current in its secondary coil

proportional to the current flowing in its primary coil.

When current in a circuit is too high to directly apply 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 also isolates the 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.

Current transformers are used extensively for measuring current and monitoring the

operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical

utility's watt-hour meter on virtually every building with three-phase service and single-

phase services greater than 200 amps.

The CT is typically described by its current ratio from primary to secondary. Often, multiple

CTs are installed as a "stack" for various uses. For example, protection devices and revenue

metering may use separate CTs to provide isolation between metering and protection

circuits, and allows current transformers with different characteristics (accuracy, overload

performance) to be used for the devices.

Current transformer

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Voltage Transformer:

It is essentially a step - down transformer and step down the voltage in known ratio. The

primary of these transformer consist of a large number of turn of fine wire connected across

the line. These secondary winding consist of a few turns and provides for measuring

instruments and relay a voltage which is known fraction of the line voltage.Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are designed

to have an accurately known transformation ratio in both magnitude and phase, over a

range of measuring circuit impedances. A voltage transformer is intended to present a

negligible load to the supply being measured. The low secondary voltage allows protective

relay equipment and measuring instruments to be operated at a lower voltages.

Both current and voltage instrument transformers are designed to have predictable

characteristics on overloads. Proper operation of over-current protective relays requires

that current transformers provide a predictable transformation ratio even during a short-

circuit.

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT)

is a transformer used in power systems to step down extra high voltage signals and provide

a low voltage signal, for measurement or to operate a protective 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 formsa carrier communication network throughout the transmission network.

The standards define a voltage transformer as one in which "the secondary voltage is

substantially proportional to the primary voltage and differs in phase from it by an angle

which is approximately zero for an appropriate direction of the connections.

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Voltage transformer

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5.WAVE TRAP

Line traps prevent the transmission of high frequency carrier signals on high voltage

transmission lines to unwanted directions without a loss of energy at power frequency.

Line traps are a key component in PLC (Power Line Carrier) systems used for remote control

signals, voice communication, remote metering and control between substations in the

electrical T&D network. The Alstom Grid line trap is reliable and lightweight and requires

little or no maintenance. Line trap also is known as Wave trap.

Wave traps are provided in the substation because, there are multiple reasons for this,

depending on the configuration. If the wave traps are on different lines, they are likely

tuned to different carrier frequencies and are used to filter out the carrier for the line they

are installed on. If they are installed on the same line, there may be multiple carrier

frequencies used, or carrier is applied to multiple phases if on different phases.

Line Trap consists of Inductive coil usually connected in the outdoor yard incoming line. Line

traps are usually mounted above Capacitor Voltage Transformer (CVT) or on separate

structure.

30 kHz to 500 kHz frequency range

Operational up to 800 kV, the line trap can be used within the 30 kHz to 500 kHz frequency

range. It complies with IEC, ANSI or the equivalent standards.

Lightweight, reliable, and maintenance-free

Line traps are air core, dry types, with mounting flexibility and can withstand high, short-

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circuits. They are lightweight, can be used outdoors, and are doted with a reliable, open-

style and maintenance-free design. The line traps provide excellent cooling, are equipped

with extremely reliable tuning devices and have a self-resonance frequency greater than 50

kHz.

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5. POWER TRANSFORMER

Power transformer is a static electrical device, involving no continuously moving parts, used

in electric power systems to transfer power between circuits through the use of

electromagnetic induction. The term power transformer is used to refer to those

transformers used between the generator and the distribution circuits, and these

are usually rated at 500 kVA and above. Power systems typically consist of a large number of

generation locations, distribution points, and interconnections within the system or with

nearby systems, such as a neighboring utility. The complexity of the system leads to a

variety of transmission and distribution voltages. Power transformers must be used at each

of these points where there is a transition between voltage levels. Power transformers

are selected based on the application, with the emphasis toward custom design being

more apparent the larger the unit.

Power transformers are available for step-up operation, primarily used at the generator and

referred to as generator step-up (GSU)transformers, and for step-down operation, mainly

used to feed distribution circuits. Power transformers are available as single-phase or three-

phase apparatus. Transformer is a vital link in a power system which has made possible the

power generated at low voltages (6600 to 22000 volts) to be stepped up to extra high

voltages for transmission over long distances and then transformed to low voltages for

utilization at proper load centers. This flux induces an electro-motive force in the secondary

winding too. When load is connected across this winding, current flows in the secondary

circuit. This produces a demagnetizing effect, to counter balance this the primary winding

draws more current from the supply so that

IP NP=IS NS

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Electrical Power Transformer is a static device which transforms electrical energy from one

circuit to another without any direct electrical connection and with the help of mutual

induction between to windings. It transforms power from one circuit to another without

changing its frequency but may be in different voltage level.

Power transformer takes the AC mains (wall) supply voltage and converts it into one or

more AC voltages that are more convenient for our needs. For a valve amp this usually

means a low voltage for the heaters and a high voltage for the anode supply, at the veryleast.

Working Principle of transform

The working principle of transformer is very simple. It depends upon Faraday's laws of

Electromagnetic Induction. Actually mutual induction between two or more winding is

responsible for transformation action in an electrical transformer.

er

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Faraday's laws of Electromagnetic Induction

According to these Faraday's laws, Rate of change of flux linkage with respect to time is

directly proportional to the induced EMF in a conductor or coil".

An Ideal Transformer is an imaginary transformer which does not have any loss in it,

means no core losses, copper losses and any other losses in transformer. Efficiency of this

transformer is considered as 100%.

Types of Transformer

Transformers can be categorized in different ways, depending upon their purpose, use,

construction etc. The types of transformer are as follows,

Step Up Transformer & Step Down Transformer - Generally used for stepping up and

down the voltage level of power in transmission and distribution power network.

Transformer & Single Phase Transformer - Former is generally used in three phase power

system as it is cost effective than later but when size matters it is preferable to use bank

of three Single Phase Transformer as it is easier to transport three single phase unit

separately than one single three phase unit.

Electrical Power Transformer, Distribution Transformer & Instrument Transformer -

Transformer generally used in transmission network is normally known as PowerTransformer, distribution transformer is used in distribution network and this is lower

rating transformer and current transformer & potential transformer, we use for relay and

protection purpose in electrical power system and in different instruments in industries

are called Instrument Transformer.

Two Winding Transformer & Auto Transformer - Former is generally used where ratio

between High Voltage and Low Voltage is greater than 2. It is cost effective to use later

where the ratio between High Voltage and Low Voltage is less than 2.

Outdoor Transformer & Indoor Transformer - Transformers designed for installing atoutdoor is Outdoor Transformer and Transformers designed for installing at indoor is

Indoor Transformer.

EHV power transformers are usually oil immersed with all three phases in one tank. Auto

transformers can offer advantage of smaller physical size and reduced losses. The different

classes of power transformers are:

Oil immersed, natural cooling

Oil immersed, air blast cooling

Oil immersed, oil circulation forced

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Oil immersed, oil circulation forced, air blast cooling

Power transformers are usually the largest single item in a substation. For economy of

service roads, transformers are located on one side of a substation, and the connection to

switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take

precaution against the spread of fire. Hence, the transformer is usually located around asump used to collect the excess oil.

Transformers that are located and a cell should be enclosed in a blast proof room.

Power transformers

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7. SWITCH GEAR

One of the basic functions of switchgear is protection, which is interruption of short-circuit

and overload fault currents while maintaining service to unaffected circuits. Switchgear also

provides isolation of circuits from power supplies. Switchgear is also used to enhance

system availability by allowing more than one source to feed a load.

In an electric power system, switchgear is the combination of electrical disconnect

switches, fuses or circuit breakers used to control, protect and isolate electrical equipment.

Switchgear is used both to de-energize equipment to allow work to be done and to clear

faults downstream. This type of equipment is important because it is directly linked to the

reliability of the electricity supply.

The very earliest central power stations used simple open knife switches, mounted on

insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making

opening manually operated switches too dangerous for anything other than isolation of a

de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely

controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed

structure with electrically operated switching elements, using oil circuit breakers. Today, oil-

filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing

large currents and power levels to be safely controlled by automatic equipment

incorporating digital controls, protection, metering and communications.High voltage switchgear was invented at the end of the 19th century for operating motors

and other electric machines. The technology has been improved over time and can be used

with voltages up to 1,100 kV.

Typically, the switchgear in substations is located on both the high voltage and the low

voltage side of large power transformers. The switchgear on the low voltage side of the

transformers may be located in a building, with medium-voltage circuit breakers for

distribution circuits, along with metering, control, and protection equipment. For industrial

applications, a transformer and switchgear line-up may be combined in one housing, called

a unitized substation or USS.

There are many different types of switch gears. To the common types of switchgears are

included vacuum switch gears, oil insulated switch gears ,and gas insulated switchgears.

Also there are simple open air switchgears.

The vacuum circuit Switch gear is the type of Switchgears that has minimal arcing. when the

arc is stretched to less than 2 to 3 mm ,it quenches. They are frequently used in modern

medium voltage switch gear of up to 35000 volts.

The oil insulated switchgear depends on the oil vaporization blast through its arc.

The gas insulated switchgear stretches the arc with a magnetic field. it also depends on the

dielectric strength of the gas to quench the stretched arc.

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The open air switch gear circuit breakers use compressed air to blow out the arc.whe the

displaced air is trying to escape, it blows out the arc.

Switch gear

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8.ISOLATORS

Isolating switches are also called isolators. disconnects, disconnecting switches.

Isolators are provided for isolation from live parts for the purpose of maintenance. Isolatorsare located at either side of the circuit breaker. Isolators are operated under no load.

Isolator does not have any rating for current breaking or current making. Isolators are

interlocked with circuit breakers

Disconnector or isolator switch is used to make sure that an electrical circuit can be

completely de-energized for service or maintenance. Such switches are often found

in electrical distribution and industrial applications where machinery must have its source of

driving power removed for adjustment or repair. High-voltage isolation switches are used in

electrical substations to allow isolation of apparatus such as circuit

breakers and transformers, and transmission lines, for maintenance.

Function of Isolators :

Isolating switches are used to disconnect circuit-breakers. sections of bus bars and

parts of the system.

An isolator is used to open a circuit only after the flow of current has been

interrupted by another device.

These switches are slow moving devices but are inexpensive compared to loadswitches and circuit breakers.

These switches are also used to transfer circuits from one bus bar to another and to

provide flexibility during system operation

These switches must be able to:

carry normal load currents continuously,

carry fault currents until they are cleared by an interrupting device and,

make and break small currents when the voltage difference across their

terminals is not significant.

The open and closed status of an isolator can be visually verified. Often, videocameras are

placed at strategic locations in a switchyard to visually inspect the operation of an

isolator

without leaving the control room. These cameras are remotely controlled and can be

pointed towards selected equipment in the substation.

In substation , it is often desired to disconnect a part of the system for general

maintenance and repairs. This is accomplished by an isolating switch or isolator. An

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isolator is a switch and is design to often open circuit under no load, in other words,

isolator switches are operate only when the line is which they are connected carry

no load.

The major difference between an isolator and a circuit breaker is that an isolator is

an off-loaddevice intended to be opened only after current has been interrupted by

some other control device. Safety regulations of the utility must prevent any attempt

to open the disconnector while it supplies a circuit.

For example, consider that the isolator are connected on both side of a cut breaker, if the

isolators are to be opened, the C.B. must be opened first.

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frequently used in different Electrical Isolators (Outdoor Isolators) are used to protect from

the direct contact of electricity. They are the very essential part of electrical equipments.

We offer different types of electrical Isolators which include Electric Fence Isolators and

Electric Current Isolators. All these types are of supreme quality. They are made shock proof

by special technology. They are being industries with total comfort. We are among the

leading Electric Current Isolators Suppliers in India who supply all the famous brands of

electric switches.

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MISCELLONOUS EQUIPMENTS

Metering and Indicating Instrument:

There are several metering and indicating Instrument (e.g.Ammeters, Volt-meters, energymeter etc.) installed in a Sub-Station tomaintain which over the ckt quantities. The

instrument transformer areinvariably used with them for satisfactory operation.

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Surge Arrestors or Lightning Arrester:

A lightning arrester is a device used on electrical power systems to protect the insulation on

the system from the damaging effect of lightning. The typical lightning arrester also known

as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge

or switching surge travels down the power system to the arrester, the current from thesurge is diverted around the protected insulation in most cases to earth.

Surge Arresters or Lightning Arresters discharge the over voltage surges to earth and

protect the equipment insulation from switching surges and lightning surges. Surge

arresters are generally connected between phase conductor and ground. In a Substation

surge arrester is located at the starting of the substation as seen from incoming

transmission lines and is the first equipment of the substation. Surge arresters are also

provided near the transformer terminals phase to ground. Two type of surge arresters are

available 1) Gapped Arresters 2) Gapless Zinc Oxide arresters.

Lightning Arrester or Surge Arreseter

Air-Break Switch

an electric switch in which the opening and closing of contacts and extinguishing of the

electric arc are accomplished by means of compressed air. An air-break switch consists of

three basic structural elements: a reservoir with a supply of compressed air, an arc

extinguisher, and an electro pneumatic actuator.

The principal advantages of air-break switches lie in the fact that they are fireproof and

explosion proof, have rapid connect and disconnect operation, and are relatively simple in

design. The presence of equipment for the production and storage of compressed-air

supplies is a disadvantage. Air-break switches at currents up to 750 kV, which are generally

used at high-voltage power plants and substations, are manufactured in the USSR.

Modern air-break switches are provided with an enclosed isolating switch whose contacts

are housed in an insulated casing that fills with compressed air upon disconnection . Air-

break switches at currents of 110 kV and higher (up to 750 kV) are manufactured with air-

filled isolating switches..

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An air break switch comprising one or more fixed contacts and one or more movable

contacts which are movable between open and closed positions, and a coil which,

preferably in conjunction with

The function of a switch is to open or close a path for electricity.

Low voltage kife switch

A knife switch has a simple handle that swings back and forth to open or close a circuit. The

switch itself is not covered with insulation. It is easy to see if the switch is open (on) or

closed (off). The knife switch in the photos below could be used to turn off a power tool or

other equipment.

A simple knife switch is good for doing experiments with electricity of low voltage. Dry cells

have low voltage and are often used for these experiments. If you touch the metal of the

switch, you should not get hurt when using very low voltage with dry cells!

Earth Switch:

Station Earthing System includes Earth Mat and Earth electrodes placed below ground level.

These Earth Mat and Earth electrode is connected to the equipment structures, neutral

points for the purpose of Equipment earthing and neutral point earthing.

Function earthing system is to provide low resistance earthing for

1. Discharging currents from the surge arresters, overhead shielding, earthing switches

2. For equipment body earthing

3. For safe touch potential and step potential in substation.

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Earth Switch is used to discharge the voltage on the circuit to the earth for safety. Earth

switch is mounted on the frame of the isolators. Earth Switch is located for each incomer

transmission line and each side of the bus bar section.

BatteryA battery is a device that converts the chemical energy contained in its active materials

directly into electric energy by means of an electrochemical oxidation-reduction

(redox)reaction. In the case of a rechargeable system, the battery is recharged by a reversal

of the process.

Batteries RoomBatteries are very important part of the grid. It works as a standby storage device, that

provides D.C power to the grids dc supply equipment in case of failure of A.C supply.Different protection devices i.e relays, circuit breakers and other control equipment of relay

room, 11KVcontrol room, 132KV control room and yard operates on 110 D.C volt supply that

is normally supplied by a rectifier. In case of failure of A.C power batteries works as a

standby source of 110 D.C supply. No. of cells installed = 552 Volt/cell, 150 AH Total Output

Voltage = 110 Volt. Recommended Float Voltage = 202 Volt/cell at 25 CRecommended Boost

Voltage = 2.4 Volt/cell Minimum2.8 Volt/cell Maximum Total Float Voltage = 121 Volt

Capacitor

Capacitors are used in substation to improve power factor.

Series Reactors

Series reactors are used to limit short circuit current and to limit current surges associated

with fluctuating loads. Series reactors are located at the strategic locations such that the

fault levels are reduced.

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Series Reactors

Lightning Protection:

Ligh

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