Power Grid, Lucknow

7/28/2019 Power Grid, Lucknow

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DEPARTMENT OF ELECTRICAL ENGINEERING

BABU BANARASI DAS NORTHERN INDIA INSTITUTE OF TECHNOLOGY,

LUCKNOW.

SUMMER TRAINING REPORT

TRANSMISSION AND PROTECTION OF POWER AT

POWERGRID CORPORATION OF INDIA LTD.

LUCKNOW

Submitted By:-

Name: Saurabh Pal

Roll No: 1005620088

(10.06.2013 22.07.2013)


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Acknowledgement

It is a pleasure to present this report which I had compiled during my training at

POWER GRID, Lucknow. There are many persons whom I would like to thank. Following in

the list are who provided me with valuable knowledge during my training period.

1). Devashish shukla.

2).Irfan sir.

I am also thankful to Mr. Irfan sir, our training in charge for his co-operationthroughout my presence in the organisation. He was very helpful and supportive throughout

my training and also helped me in preparing this report.

Without the help of these people I would have never been able to complete this report

successfully.

SAURABH PAL

B.Tech 3rd year

(Electrical Engg.)

BBDNIIT,

Lucknow.


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TABLE OF CONTENT

1). Electrical Circuit Breaker. 01

2). Electrical Isolator or Electrical Isolation Switch. 14

3). Wave trap. 17

4). Current transformer. 18

5). Capacitor voltage transformer. 21

6). Lightning arrester. 22


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Electrical Circuit Breaker

What is Circuit Breaker ?

Definition of Circuit Breaker : Electrical Circuit Breaker is a switching device which canbe operated manually as well as automatically for controlling and protection of electrical power

system respectively. As the modern power system deals with huge currents, the spacial attention

should be given during designing ofcircuit breaker to safe interruption of arc produced during

the operation of circuit breaker. This was the basic definition of circuit breaker.

Introduction to Circuit Breaker

The modern power system deals with huge power network and huge numbers of associated

electrical equipment. During short circuit fault or any other types of electrical fault these

equipment as well as the power network suffer a high stress of fault current in them which may

damage the equipment and networks permanently. For saving these equipments and the power

networks the fault current should be cleared from the system as quickly as possible. Again after

the fault is cleared, the system must come to its normal working condition as soon as possible for

supplying reliable quality power to the receiving ends. In addition to that for proper controlling

of power system, different switching operations are required to be performed. So for timely

disconnecting and reconnecting different parts of power system network for protection and

control, there must be some special type of switching devices which can be operated safely under

huge current carrying condition. During interruption of huge current, there would be large arcing

in between switching contacts, so care should be taken to quench these arcs in safe manner. Thecircuit breaker is the special device which does all the required switching operations during

current carrying condition. This was the basic introduction to circuit breaker

Working Principle of Circuit Breaker

The circuit breaker mainly consists of fixed contacts and moving contacts. In normal on

condition of circuit breaker, these two contacts are physically connected to each other due to

applied mechanical pressure on the moving contacts. There is an arrangement stored potential

energy in the operating mechanism of circuit breaker which is realized if switching signal given

to the breaker. The potential energy can be stored in the circuit breaker by different ways like bydeforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of

potential energy, it must be released during operation. Release of potential energy makes sliding

of the moving contact at extremely fast manner. All circuit breaker have operating coils (tripping

coils and close coil), whenever these coils are energized by switching pulse, the plunger inside

them displaced. This operating coil plunger is typically attached to the operating mechanism of

circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is


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released in forms of kinetic energy, which makes the moving contact to move as these moving

contacts mechanically attached through a gear lever arrangement with the operating mechanism.

After a cycle ofoperation of circuit breaker the total stored energy is released and hence the

potential energy again stored in the operating mechanism of circuit breaker by means of spring

charging motor or air compressor or by any other means. Till now we have discussed about

mechanical working principle of circuit breaker. But there are electrical characteristics of a

circuit breaker which also should be consider in this discussion of operation of circuit breaker.

Lets have a discussion on electrical principle of circuit breaker

The circuit breaker has to carry large rated or fault power. Due to this large power there is

always dangerously high arcing between moving contacts and fixed contact during operation of

circuit breaker. Again as we discussed earlier the arc in circuit breaker can be quenching safely if

the dielectric strength between the current carrying contacts of circuit breaker increases rapidly

during every current zero crossing of the alternating current. The dielectric strength of the media

in between contacts can be increased in numbers of ways, like by compressing the ionized arcing

media since compressing accelerates the deionization process of the media, by cooling the arcing

media since cooling increase the resistance of arcing path or by replacing the ionized arcing

media by fresh gasses. Hence a numbers of arc quenching processes should be involved in

operation of circuit breaker.

Types of Circuit Breaker

According different criteria there are different types of circuit breaker

According to their arc quenching media the circuit breaker can be divided as

1). Air circuit breaker.

2). Oil circuit breaker.

3) SF6 Circuit Breaker.

4) Vacuum Circuit Breaker.

According to their services the circuit breaker can be divided as

1) Outdoor Circuit Breaker

2) Indoor Breaker

According to the operating mechanism of circuit breaker they can be divided as


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1) Spring operated Circuit Breaker

2) Pneumatic Circuit Breaker

3) Hydrolic Circuit Breaker

According to the voltage level of installation types of circuit breaker are referred as

1) High Voltage Circuit Breaker

2) Medium Voltage Circuit Breaker

3) Low Voltage Circuit Breaker


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Air Circuit Breaker Air Blast Circuit Breaker

This type of circuit breakers, is those kind of circuit breaker which operates in air at atmosphericpressure. After development of oil breaker, the medium voltage air circuit breaker (ACB) is

replaced completely by oil circuit breaker in different countries. But in countries like France and

Italy, ACBs are still preferable choice up to voltage 15 KV. It is also good choice to avoid the

risk of oil fire, in case of oil circuit breaker. In America ACBs were exclusively used for the

system up to 15 KV until the development of new vacuum and SF6 circuit breakers.

Working principle of Air Circuit Breaker

The working principle of this breaker is rather different from those in any other types of circuit

breakers. The main aim of all kind of circuit breaker is to prevent the reestablishment of arcingafter current zero by creating a situation where in the contact gap will withstand the system

recovery voltage. The air circuit breaker does the same but in different manner. For

interrupting arc it creates an arc voltage in excess of the supply voltage. Arc voltage is defined as

the minimum voltage required maintaining the arc. This circuit breaker increases the arc voltage

by mainly three different ways,

It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma is

decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is

required to maintain the arc.

It may increase the arc voltage by lengthening the arc path. As the length of arc path is increased,

the resistance of the path is increased, and hence to maintain the same arc current more voltage is

required to be applied across the arc path. That means arc voltage is increased.

Splitting up the arc into a number of series arcs also increases the arc voltage.

Types of ACB

There are mainly two types of ACB are available.

1) Plain air circuit breaker

2) Air blast Circuit Breaker.


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Operation of ACB

The first objective is usually achieved by forcing the arc into contact with as large an area as

possible of insulating material. Every air circuit breaker is fitted with a chamber surrounding the

contact. This chamber is called arc chute. The arc is driven into it. If inside of the arc chute is

suitably shaped, and if the arc can be made conform to the shape, the arc chute wall will help to

achieve cooling. This type of arc chute should be made from some kind of refractory material.

High temperature plastics reinforced with glass fiber and ceramics are preferable materials for

making arc chute.

The second objective that is lengthening the arc path, is achieved concurrently with fist

objective. If the inner walls of the arc chute is shaped in such a way that the arc is not only

forced into close proximity with it but also driven into a serpentine channel projected on the arc

chute wall. The lengthening of the arc path increases the arc resistance.

The third technique is achieved by using metal arc slitter inside the arc chute. The main arc chute

is divided into numbers of small compartments by using metallic separation plates. These

metallic separation plates are actually the arc splitters and each of the small compartments

behaves as individual mini arc chute. In this system the initial arc is split into a number of series

arcs, each of which will have its won mini arc chute. So each of the split arcs has its won cooling

and lengthening effect due to its won mini arc chute and hence individual split arc voltage

becomes high. These collectively, make the over all arc voltage, much higher than the system

voltage. This was working principle of air circuit breaker now we will discuss in details the

operation of ACB in practice.

The air circuit breaker, operated within the voltage level 1KV, does not require any arc control

device. Mainly for heavy fault current on low voltages (low voltage level above 1 KV) ABCs

with appropriate arc control device, are good choice. These breakers normally have two pairs of

contacts. The main pair of contacts carries the current at normal load and these contacts are made

of copper. The additional pair is the arcing contact and is made of carbon. When circuit breaker

is being opened, the main contacts open first and during opening of main contacts the arcing

contacts are still in touch with each other. As the current gets, a parallel low resistive path

through the arcing contact during opening of main contacts, there will not be any arcing in the

main contact. The arcing is only initiated when finally the arcing contacts are separated. The

each of the arc contacts is fitted with an arc runner which helps, the arc discharge to move

upward due to both thermal and electromagnetic effects as shown in the figure. As the arc is

driven upward it enters in the arc chute, consisting of splitters. The arc in chute will become

colder, lengthen and split hence arc voltage becomes much larger than system voltage at the time

ofoperation of air circuit breaker, and therefore the arc is quenched finally during the current

zero.


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SF6 Circuit Breaker

A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6

gas is known as an SF6 Circuit Breaker.

SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has high

affinity of absorbing free electron. Whenever a free electron collides with the SF6 gas molecule,

it is absorbed by that gas molecule and forms a negative ion.

The attachment of electron with SF6 gas molecules may occur in tow different ways,

1) SF6 + e = SF6

2) SF6 + e = SF5

+ F

These negative ions obviously much heavier than a free electron and therefore over all mobility

of the charged particle in the SF6 gas is much less as compared other common gases. We know

that mobility of charged particle is majorly responsible for conducting current through a gas.

Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high

dielectric strength. Not only the gas has a good dielectric strength but also it has the unique

property of fast recombination after the source energizing the spark is removed. The gas has also

very good heat transfer property. Due to its low gaseous viscosity (because of less molecular

mobility) SF6 gas can efficiently transfer heat by convection. So due to its high dielectric

strength and high cooling effect SF6 gas is approximately 100 times more effective arc


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quenching media than air. Due to these unique properties of this gas SF6 Circuit Breaker is

used in complete range of medium voltage and high voltage electrical power system. These

circuit breakers are available for the voltage ranges from 33KV to 800KV and even more.

Disadvantages of SF6 CB

The SF6 gas is identified as a greenhouse gas, safety regulation are being introduced in

many countries in order to prevent its release into atmosphere.Puffer type design of SF6 CB

needs a high mechanical energy which is almost five times greater than that of oil circuit breaker.

Types of SF6 Circuit Breaker

There are mainly three types of SF6 CB depending upon the voltage level of application

1) Single Interrupter SF6 CB applied for up to 245KV(220KV) system

2) Two Interrupter SF6 CB applied for up to 420KV(400KV) system

3) Four Interrupter SF6 CB applied for up to 800KV(715KV) system


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Working of SF6 Circuit Breaker

The working of SF6 CB of first generation was quite simple it is some extent similar to air blast

circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. During

operation of SF6 circuit breaker this highly compressed gas is released through the arc and

collected to relatively low pressure reservoir and then it pumped back to the high pressure

reservoir for reutilize.

The working of SF6 circuit breaker is little bit different in moder time. Innovation of puffer

type design makes operation of SF6 CB much easier. In buffer type design, the arc energy is

utilized to develop pressure in the arcing chamber for arc quenching.


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Here the breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted with a

specific contact gap. A sliding cylinder bridges these to fixed contacts. The cylinder can axially

slide upward and downward along the contacts. There is one stationary piston inside the cylinder

which is fixed with other stationary parts of the SF6 circuit breaker, in such a way that it can not

change its position during the movement of the cylinder. As the piston is fixed and cylinder is

movable or sliding, the internal volume of the cylinder changes when the cylinder slides.

During opening of the breaker the cylinder moves downwards against position of the fixed piston

hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the

cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact

body during closed position. As the cylinder move further downwards, these vent openings cross

the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder

will come out through this vents in high speed towards the arc and passes through the axial hole

of the both fixed contacts. The arc is quenched during this flow of SF6 gas.

During closing of the SF6 circuit breaker, the sliding cylinder moves upwards and as the position

of piston remains at fixed height, the volume of the cylinder increases which introduces low

pressure inside the cylinder compared to the surrounding. Due to this pressure difference SF6 gas

from surrounding will try to enter in the cylinder. The higher pressure gas will come through the

axial hole of both fixed contact and enters into cylinder via vent and during this flow; the gas

will quench the arc.


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Oil Circuit Breaker Bulk and Minimum Oil

Circuit Breaker

Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and

moving contact are immerged inside the insulating oil. Whenever there is a separation of current

carrying contacts in the oil, the arc is initialized at the moment of separation of contacts, and due

to this arc the oil is vaporized and decomposed in mostly hydrogen gas and ultimately creates a

hydrogen bubble around the arc. This highly compressed gas bubble around the arc prevents re-

striking of the arc after current reaches zero crossing of the cycle. The Oil Circuit Breaker is

the one of the oldest type of circuit breakers.

Operation of Oil Circuit Breaker

The operation of oil circuit breaker is quite simple lets have a discussion. When the current

carrying contacts in the oil are separated an arc is established in between the separated contacts.

Actually, when separation of contacts has just started, distance between the current contacts is

small as a result the voltage gradient between contacts becomes high. This high voltage gradient

between the contacts ionized the oil and consequently initiates arcing between the contacts. This

arc will produce a large amount of heat in surrounding oil and vaporizes the oil and decomposes

the oil in mostly hydrogen and a small amount of methane, ethylene and acetylene. The

hydrogen gas can not remain in molecular form and its is broken into its atomic form releasing

lot of heat. The arc temperature may reach up to 5000oK. Due to this high temperature the gas is

liberated surround the arc very rapidly and forms an excessively fast growing gas bubble aroundthe arc. It is found that the mixture of gases occupies a volume about one thousand times that of

the oil decomposed. From this figure we can assume how fast the gas bubble around the arc will

grow in size. If this growing gas bubble around the arc is compressed by any means then rate of

de ionization process of ionized gaseous media in between the contacts will accelerate which

rapidly increase the dielectric strength between the contacts and consequently the arc will be

quenched at zero crossing of the current cycle. This is the basic operation of oil circuit breaker.

In addition to that cooling effect of hydrogen gas surround the arc path also helps, the quick arc

quenching in oil circuit breaker.

Types of oil circuit breakers

There are mainly two types of oil circuit breakers available


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Bulk Oil Circuit Breaker or BOCB

Bulk Oil Circuit Breaker orBOCB is such types of circuit breakers where oil is used as arc

quenching media as well as insulating media between current carrying contacts and earthed parts

of the breaker. The oil used here is same as transformer insulating oil.

Minimum Oil Circuit Breaker or MOCB

These types of circuit breakers utilize oil as the interrupting media. However, unlike bulk oil

circuit breaker, a minimum oil circuit breaker places the interrupting unit in insulating

chamber at live potential. The insulating oil is available only in interrupting chamber. The

features of designing MOCB is to reduce requirement of oil, and hence these breaker are called

minimum oil circuit breaker.

Arc quenching in bulk oil circuit breaker

When the current carrying contacts in the oil are separated an arc is established in between the

separated contacts.

This arc will produce rapidly growing gas bubble around the arc. As the moving contact move

away from fixed contact the length of arc is increased as a result the resistance of the arc

increases. The increased resistance causes lowering the temperature and hence reducing the

formation of gasses surround the arc. The arc quenching in bulk oil circuit breaker takes place

when current passes through zero crossing. If we go through the arc quenching phenomenon

more thoroughly we will find many other factors effects the arc quenching in bulk oil circuitbreaker.


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Vacuum Circuit Breaker or VCB and

Vacuum Interrupter

A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in

vacuum. The technology is suitable for mainly medium voltage application. For higher voltage

Vacuum technology has been developed but not commercially viable. The operation of opening

and closing of current carrying contacts and associated arc interruption take place in a vacuum

chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a

steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure

inside a vacuum interrupter is normally maintained at 10 6 bar.

The material used for current carrying contacts plays an important role in the performance of the

vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. Vacuum

interrupter technology was first introduced in the year of 1960. But still it is a developingtechnology. As time goes on, the size of the vacuum interrupter is being reducing from its early

1960s size due to different technical developments in this field of engineering. The contact

geometry is also improving with time, from butt contact of early days it gradually changes to

spiral shape, cup shape and axial magnetic field contact. The vacuum circuit breaker is today

recognized as most reliable current interruption technology for medium voltage system. It

requires minimum maintenance compared to other circuit breaker technologies.

Advantages of vacuum circuit breaker or VCB

Service life of Vacuum Circuit Breaker is much longer than other types of circuit breakers. Thereis no chance of fire hazard as oil circuit breaker. It is much environment friendly than SF6

Circuit breaker. Beside of that contraction of VCB is much user friendly. Replacement of

Vacuum Interrupter (VI) is much convenient.

Operation of Vacuum Circuit Breaker

The main aim of any circuit breaker is to quench arc during current zero crossing, by establishing

high dielectric strength in between the contacts so that reestablishment of arc after current zero

becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and

four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench

a vacuum arc within very small contact gap. For short contact gap, low contact mass and no

compression of medium the drive energy required in vacuum circuit breaker is minimum. When

two face to face contact areas are just being separated to each other, they do not be separated

instantly, contact area on the contact face is being reduced and ultimately comes to a point and

then they are finally de-touched. Although this happens in a fraction of micro second but it is the

fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts


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concentrated on that last contact point on the contact surface and makes a hot spot. As it is

vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a

conducting media for arc path. Then the arc will be initiated and continued until the next current

zero. At current zero this vacuum arc is extinguished and the conducting metal vapour is re-

condensed on the contact surface. At this point, the contacts are already separated hence there is

no question of re-vaporization of contact surface, for next cycle of current. That means, the arc

cannot be reestablished again. In this way vacuum circuit breaker prevents the reestablishment of

arc by producing high dielectric strength in the contact gap after current zero.

There are two types of arc shapes. For interrupting current up to 10kA, the arc remains diffusedand the form of vapour discharge and cover the entire contact surface. Above 10kA the diffused

arc is constricted considerably by its own magnetic field and it contracts. The phenomenon gives

rise over heating of contact at its center. In order to prevent this, the design of the contacts should

be such that the arc does not remain stationary but keeps travelling by its own magnetic field.

Specially designed contact shape of vacuum circuit breaker make the constricted stationary arc

travel along the surface of the contacts, thereby causing minimum and uniform contact erosion.


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Electrical Isolator or Electrical Isolation

Switch

Definition of Isolator

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.

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 are used.

Types of Electrical Isolators

There are different types of isolators available depending upon system requirement such as

1).Double Break Isolator

1).Single Break Isolator

3).Pantograph type Isolator


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Depending upon the position in power system, the isolators can be categorized as

Bus side isolator the isolator is directly connected with main bus

Line side isolator the isolator is situated at line side of any feeder

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.


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


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Wave 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/teleportation panel in the substation control room (through coupling capacitor andLMU).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 teleportation signals and in addition, voice and data

communication.

Transmission line may carry some disturbance or noise signals having high frequency, it also

carries the high frequency signals from the PLCC. These high frequency signals should not be

coming on the buses as these may damage the equipment on the bays. The Wave trap offers high

impedance to the high frequency communication signals thus obstructs the flow of these signals

in to the substation bus bars. If wave trap were not to be there, then signal loss is more and

communication will be ineffective/probably impossible.


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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 directly apply to measuringinstruments, 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 for protection, measurement and control in high voltage electrical

substations and the electrical grid. Current transformers may be installed inside switchgear or in

apparatus bushings, but very often free-standing outdoor current transformers are used. In a

switchyard, live tankcurrent transformers have a substantial part of their enclosure energized atthe line voltage and must be mounted on insulators. Dead tankcurrent transformers isolate the

measured circuit from the enclosure. Live tankCTs are useful because the primary conductor is

short, which gives better stability and a higher short-circuit current withstand rating. The primary

of the winding can be evenly distributed around the magnetic core, which gives better

performance for overloads and transients. Since the major insulation of a live-tank current

transformer is not exposed to the heat of the primary conductors, insulation life and thermal

stability is improved.

Design

Like any other transformer, a current transformer has a primary winding, a magnetic core, and a

secondary winding. The alternating current flowing in the primary produces an alternating


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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 that the primary and

secondary circuits are efficiently coupled, so that the secondary current bears an accurate

relationship to the primary current.

The most common design of CT consists of a length of wire wrapped many times around asilicon 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. Typicalexamples of low voltage single ratio metering current transformers are either ring type or plastic

moulded case. High-voltage current transformers are mounted on porcelain bushings to insulate

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.

The primary circuit is largely unaffected by the insertion of the CT. The rated secondary current

is commonly standardized at 1 or 5 amperes. For example, a 4000:5 CT would provide an output

current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can

be single ratio or multi ratio, with five taps being common for multi ratio CTs. The load, or

burden, of the CT should be of low resistance. If the voltage time integral area is higher than thecore's design rating, the core goes into saturation towards the end of each cycle, distorting the

waveform and affecting accuracy.

Usage

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 and different characteristics (accuracy, overload performance) to be used for

the devices.


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Safety precautions

Care must be taken that the secondary of a current transformer is not disconnected from its load

while current is flowing in the primary, as the transformer secondary will attempt to continue

driving current across the effectively infinite impedance up to its core saturation voltage. This

may produce a high voltage across the open secondary into the range of several kilovolts,

causing arcing, compromising operator with equipment safety, or permanently affect the

accuracy of the transformer.

Burden

The secondary load of a current transformer is usually called the "burden" to distinguish it from

the load of the circuit whose current is being measured.

The burden, in a CT metering circuit is the (largely resistive) impedance presented to its

secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15VA, 20 VA, 30 VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5,

B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2

of impedance in the metering circuit before its secondary accuracy falls outside of an accuracy

specification. These specification diagrams show accuracy parallelograms on a grid

incorporating magnitude and phase angle error scales at the CT's rated burden. Items that

contribute to the burden of a current measurement circuit are switch-blocks, meters and

intermediate conductors. The most common source of excess burden is the conductor between

the meter and the CT. When substation meters are located far from the meter cabinets, the

excessive length of wire creates a large resistance. This problem can be reduced by using CTs

with 1 ampere secondaries, which will produce less voltage drop between a CT and its metering

devices.

Knee-point core-saturation voltage

The knee-point voltage of a current transformer is the magnitude of the secondary voltage after

which the output current ceases to linearly follow the input current within declared accuracy. In

testing, if a voltage is applied across the secondary terminals the magnetizing current will

increase in proportion to the applied voltage, up until the knee point.( The knee point is defined

as the voltage at which a 10% increase in applied voltage increases the magnetizing current by

50%). From the knee point upwards, the magnetizing current increases abruptly even with smallincrements in voltage across the secondary terminals. The knee-point voltage is less applicable

for metering current transformers as their accuracy is generally much tighter but constrained

within a very small bandwidth of the current transformer rating, typically 1.2 to 1.5 times rated

current. However, the concept of knee point voltage is very pertinent to protection current

transformers, since they are necessarily exposed to currents of 20 or 30 times rated current

during faults.


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Capacitor voltage transformer

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: aterminal 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.


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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.

A capacitive voltage transformer works on the principle of voltage division, by using two

capacitances, one many times larger than the other. So for example, if the capacitances are C1and C2 (C2>C1), and we need to measure a voltage V, the voltage across the larger capacitance

C2 will be V2 = V*C1/(C1+C2) ~. Thus, we have effectively reduced the voltage level to a

fraction of the original without using step down transformers (which for larger voltages will be

bulky and expensive). CVTs on the other hand, are cheaper and smaller. This V2 can be further

isolated and stepped down using a step-down transformer.

Lightning arrester

A lightning arrester (in Europe: surge arrester) is a device used on electrical power systems andtelecommunications 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 theEarth. 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.


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Working of lightning arrester

The lightning arrestor protects the structure from damage by intercepting flashes of lightning and

transmitting their current to the ground. Since lightning strikes tends to strike the highest objectin the vicinity, the rod is placed at the apex of a tall structure. It is connected to the ground by

low-resistance cables. In the case of a building, the soil is used as the ground, and on a ship,

water is used. A lightning rod provides a cone of protection, which has a ground radius

approximately, equal to its height above the ground.


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LA Characteristics

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Power Grid, Lucknow