trainning report of ntpc,bongaigaon

8/4/2019 Trainning Report of Ntpc,Bongaigaon

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NTPC LTDBGTPP

PROJECT: SWITCHYARD

SUBMITTED BY

JAYANTA KAR

RTU,KOTA

GUIDED BY

MR, ARUP BHATTACHARYA


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ACKNOLEDGEMENT

i owe a huge thanks to a large number of people without whom thispractical training of mine would not have been possible.. i express mysincere gratitude to the training and management wing of NTPCLTD, bongaigaon for giving me the opportunity to get a first handtechnical knowledge.

i am also extremely grateful to MR, ARUP BHATTACHARYA (DGMELECTRICAL) for permitting me to take training at their prestigiousorganization. i am extremely thankful for his valuable guidance andgiving me a part of his precious time.i would like to express my sincere thanks to all other for their valuableguidance and scholarly suggestions, prudent admonition, effectivemanagement which made my training process smoother....

PROJECT GUIDE SUBMITTED BY

MR A.BHATTACHARYA JAYANTA KAR


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CONTENTS

SL.NO DESCRIPTION

(A)

(B)

(C)

(i)

(ii)

(iii)

(D)

SWITCHYARD

TRANSFORMER

BASIC ASPECTS OF PROTECTION

CIRCUIT BREAKER

RELAY

ISOLATOR

CONCLUSION


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SWITCHYARD

A switchyard is essentially a hub for electrical power sources. For instance, aswitchyard will exist at a generating station to coordinate the exchange of power

between the generators and the transmission lines in the area. A switchyard willalso exist when high voltage lines need to be converted to lower voltage fordistribution to consumers.

Therefore a switchyard will contain; current carrying conductors, grounding wiresand switches, transformers, disconnects, remotely controlled arc snuffingbreakers, metering devices, etc.

SINGLE LINE DIAAGRAM OF NTPC BONGAIGAON SWITCHYARD


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Transformer

A transformeris a device that transfers electrical energy from one circuit to

another through inductively coupled conductorsthe transformer's coils. A

varying current in the first orprimarywinding creates a varying magnetic flux inthe transformer's core and thus a varying magnetic field through

the secondarywinding. This varying magnetic field induces a

varying electromotive force (EMF), or "voltage", in the secondary winding. This

effect is called mutual induction.

If a load is connected to the secondary, an electric current will flow in the

secondary winding and electrical energy will be transferred from the primary

circuit through the transformer to the load. In an ideal transformer, the induced

voltage in the secondary winding (Vs) is in proportion to the primary voltage (Vp),
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and is given by the ratio of the number of turns in the secondary ( Ns) to the

number of turns in the primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows

an alternating current (AC) voltage to be "stepped up" by making Ns greater

than Np, or "stepped down" by making Ns less than Np.

In the vast majority of transformers, the windings are coils wound around

a ferromagnetic core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden

inside a stage microphone to huge units weighing hundreds of tons used to

interconnect portions ofpower grids. All operate with the same basic principles,although the range of designs is wide. While new technologies have eliminated

the need for transformers in some electronic circuits, transformers are still found

in nearly all electronic devices designed forhousehold ("mains") voltage.

Transformers are essential for high-voltage electric power transmission, which

makes long-distance transmission economically practical.

Basic principles

The transformer is based on two principles: first, that an electric current can

produce a magnetic field (electromagnetism), and, second that a changing

magnetic field within a coil of wire induces a voltage across the ends of the coil

(electromagnetic induction). Changing the current in the primary coil changes the

magnetic flux that is developed. The changing magnetic flux induces a voltage in

the secondary coil.
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An ideal transformer is shown in the adjacent figure. Current passing through the

primary coil creates a magnetic field. The primary and secondary coils are

wrapped around a core of very high magnetic permeability, such as iron, so that

most of the magnetic flux passes through both the primary and secondary coils.

Different parts of Transformer are:

1) CONSERVATOR - conservator is a type of tank , used to help oil filling this issituated upper portion of the power transformer . mainly these are cylindricallyshapped...

it is used to provide adequate space for the expansion of oil when transformer isloaded or when ambient temprature changes.

2) BREATHER - Breather is a device used for absorb the moisture content of aoil and sucked air

3)SILICA GEL BREATHER: it sucks the moisture from the air which is taken bytransformer so that dry air is taken by transformer.

4)RADIATORS: these are used for cooling of the transformer oil.
http://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_corehttp://en.wikipedia.org/wiki/Permeability_(electromagnetism)http://en.wikipedia.org/wiki/Iron


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5)BUCHHOLZ RELAY: it is avery sensitive gas and oiloperated instrument which safely detect the formation of gas or sudden prssureinside the oil transformer.

this is a protecting device used to protect our transformer windings . this is adouble ended device one end is conneced to conservator other is connected totank. there are two windings inside the relayone for detecting oil level goin to empty and other is connected to a alarm circuitfor warning

6) TANK - basically this is a container used to keep windings(both) and coolingoil.

7) PRIMARY WINDING - in the case of power transmission primary windingsare the main element external connection from the power is connected to thewinding

8) SECONDARY WINDING - this is a another windin for redusing power(in thecase of step down purpos)

Coolant
http://en.wikipedia.org/wiki/File:Drehstromtransformater_im_Schnitt_Hochspannung.jpg


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High temperatures will damage the winding insulation. Small transformers do not

generate significant heat and are cooled by air circulation and radiation of heat.

Power transformers rated up to several hundred kVA can be adequately cooled

by natural convective air-cooling, sometimes assisted by fans. In larger

transformers, part of the design problem is removal of heat. Some power

transformers are immersed in transformer oil that both cools and insulates the

windings.The oil is a highly refined mineral oil that remains stable at

transformeroperating temperature. Indoor liquid-filled transformers are required

by building regulations in many jurisdictions to use a non-flammable liquid, or to

be located in fire-resistant rooms.Air-cooled dry transformers are preferred for

indoor applications even at capacity ratings where oil-cooled construction would

be more economical, because their cost is offset by the reduced building

construction cost.The oil-filled tank often has radiators through which the oil circulates by natural

convection; some large transformers employ forced circulation of the oil by

electric pumps, aided by external fans or water-cooled heat exchangers. Oil-filled

transformers undergo prolonged drying processes to ensure that the transformer

is completely free ofwater vaporbefore the cooling oil is introduced. This helps

prevent electrical breakdown under load. Oil-filled transformers may be equipped

withBuchholz relays, which detect gas evolved during internal arcing and rapidly

de-energize the transformer to avert catastrophic failure. Oil-filled transformers

may fail, rupture, and burn, causing power outages and losses. Installations of

oil-filled transformers usually includes fire protection measures such as walls, oil

containment, and fire-suppression sprinkler systems.

Polychlorinated biphenyls have properties that once favored their use as

a coolant, though concerns over theirenvironmental persistence led to a

widespread ban on their use. Today, non-toxic, stable silicone-based oils,

orfluorinated hydrocarbons may be used where the expense of a fire-resistant

liquid offsets additional building cost for a transformer vault. Before 1977, even

transformers that were nominally filled only with mineral oils may also have been

contaminated with polychlorinated biphenyls at 10-20 ppm. Since mineral oil and

PCB fluid mix, maintenance equipment used for both PCB and oil-filled

transformers could carry over small amounts of PCB, contaminating oil-filled

transformers.
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Some "dry" transformers (containing no liquid) are enclosed in sealed,

pressurized tanks and cooled by nitrogen orsulfur hexafluoride gas.

Experimental power transformers in the 2 MVA range have been built

with superconducting windings which eliminates the copper losses, but not the

core steel loss. These are cooled by liquid nitrogen orhelium.

TYPES

Current transformers

A current transformer (CT) is a measurement device designed to provide a

current in its secondary coil proportional to the current flowing in its primary.

Current transformers are commonly used in metering and protective relays in

the electrical power industry where they allow safe measurement of large

currents, often in the presence ofhigh voltages. The current transformer safely

isolates measurement and control circuitry from the high voltages typically

present on the circuit being measured.

Current transformers are often constructed by passing a single primary turn

(either an insulated cable or an uninsulated bus bar) through a well-

insulatedtoroidal core wrapped with many turns of wire. The CT is typically

described by its current ratio from primary to secondary. 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 have

severaltap points to provide a range of ratios. Care must be taken that the

secondary winding is not disconnected from its load while current flows in the

primary, as this will produce a dangerously high voltage across the open

secondary and may permanently affect the accuracy of the transformer.

Specially constructed wideband CTs are also used, usually with an oscilloscope,

to measure high frequencywaveforms or pulsed currents within pulsed

powersystems. One type provides a voltage output that is proportional to themeasured current; another, called a Rogowski coil, requires an

external integratorin order to provide a proportional output.
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Voltage transformers

Voltage transformers (VT) or potential transformers (PT) are another type of

instrument transformer, used for metering and protection in high-voltage circuits.

They are designed to present negligible load to the supply being measured and

to have a precise voltage ratio to accurately step down high voltages so that

metering and protective relay equipment can be operated at a lower potential. Typically

the secondary of a voltage transformer is rated for 69 V or 120 V at rated primary

voltage, to match the input ratings of protective relays.

The transformer winding high-voltage connection points are typically labeled as

H1, H2 (sometimes H0 if it is internally grounded) and X1, X2 and sometimes an

X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may

also be available on the same voltage transformer. The high side (primary) may

be connected phase to ground or phase to phase. The low side (secondary) is

usually phase to ground.

The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This

applies to current transformers as well. At any instant terminals with the same

suffix numeral have the same polarity and phase. Correct identification of

terminals and wiring is essential for proper operation of metering and protective

relays.

Some meters operate directly on the secondary service voltages at or below 600

V. VTs are typically used for higher voltages (for example, 765 kV for power

transmission) , or where isolation is desired between the meter and the

measured circuit.
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ISOLATOR

A device or assembly of devices which isolates or disconnects an on-hookstation or CTS from all wires which exit the PCZ and which has been acceptedas effective for security purposes by the Telephone Security Panel.

An isolator is not the same as a switch. It should only be opened when notcarrying current, and has the purpose of ensuring that a circuit cannot becomelive whilst it is out of service for maintenance or cleaning. The isolator must breakall live supply conductors; thus both phase and neutral conductors must beisolated. It must, however, be remembered that switching off for mechanical

maintenance is likely to be carried out by non-electrically skilled persons andthat they may therefore unwisely use isolators as on-load switches. To preventan isolator, which is part of a circuit where a circuit breaker is used for switching,from being used to break load current, it must be interlocked to ensure operationonly after the circuit breaker is already open. In many cases an isolator can beused to make safe a particular piece of apparatus whilst those around it are stilloperating normally.

HORIZONTAL DOUBLE BREAK ISOLATOR

This type of construction has three insulator stacks per pole. The two one eachside is fixed and one at the center is rotating type. The central insulator stack canswing about its vertical axis through about 900C. The fixed contacts are providedon the top of each of the insulator stacks on the side. The contact bar is fixedhorizontally on the central insulator stack. In closed position, the contact shaftconnects the two fixed contacts. While opening, the central stack rotates through900C, and the contact shaft swings horizontally giving a double break.


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The isolators are mounted on a galvanized rolled steel frame. The three polesare interlocked by means of steel shaft. A common operating mechanism isprovided for all the three poles. One pole of a triple pole isolator is closed

position.

PANTOGRAPH ISOLATOR

Illustrates the construction of a typical pantograph isolator. While closing, thelinkages of pantograph are brought nearer by rotating the insulator column. Inclosed position the upper two arms of the pantograph close on the overheadstation bus bar giving a grip. The current is carried by the upper bus bar to thelower bus bar through the conducting arms of the pantograph. While opening, therotating insulator column is rotated about its axis. Thereby the pantograph bladescollapse in vertical plane and vertical isolation is obtained between the lineterminal and pantograph upper terminal.

Pantograph isolators cover less floor area. Each pole can be located at a suitablepoint and the three poles need not be in one line, can be located in a line atdesired angle with the bus axis.


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OPERATION

It is a device meant for load operation hence not to operate while it carries loadcurrent. It cannot be operated with less pressure i.e. hesitatingly with small jugsi.e. to operate firmly in one stroke only. Also put on hand gloves while operatingthe switch. The operation of three poles is obtained by mechanical interlocking ofthe three poles. Further for all three poles there is a common operatingmechanism.


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Circuit breaker

A circuit breakeris an automatically operated electricalswitch designed to

protect an electrical circuit from damage caused by overload orshort 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 switchgeardesigned to protect high voltage circuits feeding an entire city.

Operation

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
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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

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

materials. Service life of the contacts is limited by the erosion due to interrupting

the arc. Miniature and molded case circuit breakers are usually discarded when

the contacts are worn, but power circuit breakers and high-voltage circuit

breakers have replaceable contacts.

When a current is being 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, oroil 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 secondfor 50Hz and 120 times per second for 60Hz AC)

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.
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Types of circuit breaker

Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and

industrial application, and include:

MCB (Miniature Circuit Breaker)rated current not more than 100 A. Trip

characteristics normally not adjustable. Thermal or thermal-magnetic

operation. Breakers illustrated above are in this category.

MCCB (Molded Case Circuit Breaker)rated current up to 2500 A.

Thermal or thermal-magnetic operation. Trip current may be adjustable in

larger ratings.

Low voltage power circuit breakers can be mounted in multi-tiers in low-

voltage switchboards orswitchgearcabinets.

The characteristics of Low Voltage circuit breakers are given by international

standards such as IEC 947. These circuit breakers are often installed in draw-out

enclosures that allow removal and interchange without dismantling the

switchgear.

Large low-voltage molded case and power circuit breakers may have electrical

motor operators, allowing them to be tripped (opened) and closed under remote

control. These may form part of an automatic transfer switch system for standby

power.

Low-voltage circuit breakers are also made for direct-current (DC) applications,

for example DC supplied for subway lines. Special breakers are required for

direct current because the arc does not have a natural tendency to go out on

each half cycle as for alternating current. A direct current circuit breaker will haveblow-out coils which generate a magnetic field that rapidly stretches the arc when

interrupting direct current.

Small circuit breakers are either installed directly in equipment, or are arranged in

a breaker panel.
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The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is

the most common style in modern domestic consumer units and commercial

electrical distribution boards throughout Europe. The design includes the

following components:

1. Actuatorlever- used to manually trip and reset the circuit breaker.

Also indicates the status of the circuit breaker (On or Off/tripped). Most

breakers are designed so they can still trip even if the lever is held or

locked in the "on" position. This is sometimes referred to as "free trip" or

"positive trip" operation.

2. Actuator mechanism - forces the contacts together or apart.3. Contacts - Allow current when touching and break the current when

moved apart.

4. Terminals

5. Bimetallic strip.

6. Calibration screw - allows the manufacturerto precisely adjust the

trip current of the device after assembly.

7. Solenoid

8. Arc divider/extinguisher
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Magnetic circuit breaker

Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force

increases with the current. Certain designs utilize electromagnetic forces in

addition to those of the solenoid. The circuit breaker contacts are held closed by

a latch. As the current in the solenoid increases beyond the rating of the circuit

breaker, the solenoid's pull releases the latch which then allows the contacts to

open by spring action. Some types of magnetic breakers incorporate a hydraulic

time delay feature using a viscous fluid. The core is restrained by a spring until

the current exceeds the breaker rating. During an overload, the speed of the

solenoid motion is restricted by the fluid. The delay permits brief current surges

beyond normal running current for motor starting, energizing equipment, etc.

Short circuit currents provide sufficient solenoid force to release the latch

regardless of core position thus bypassing the delay feature. Ambient

temperature affects the time delay but does not affect the current rating of a

magnetic breaker

Thermal magnetic circuit breaker

Thermal magnetic circuit breakers, which are the type found in most distribution

boards, incorporate both techniques with the electromagnet responding

instantaneously to large surges in current (short circuits) and the bimetallic strip

responding to less extreme but longer-term over-current conditions. The thermal

portion of the circuit breaker provides an "inverse time" response feature which

provides faster or slower response for larger or smaller over currents

respectively.
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Common trip breakers

When supplying a branch circuit with more than one live conductor, each live

conductor must be protected by a breaker pole. To ensure that all live conductors

are interrupted when any pole trips, a "common trip" breaker must be used.

These may either contain two or three tripping mechanisms within one case, or

for small breakers, may externally tie the poles together via their operating

handles. Two pole common trip breakers are common on 120/240 volt systems

where 240 volt loads (including major appliances or further distribution boards)

span the two live wires. Three-pole common trip breakers are typically used to

supply three-phase electric powerto large motors or further distribution boards.

Two and four pole breakers are used when there is a need to disconnect the

neutral wire, to be sure that no current can flow back through the neutral wire

from other loads connected to the same network when people need to touch the

wires for maintenance. Separate circuit breakers must never be used for

disconnecting live and neutral, because if the neutral gets disconnected while the

live conductor stays connected, a dangerous condition arises: the circuit will

appear de-energized (appliances will not work), but wires will stay live

and RCDs will not trip if someone touches the live wire (because RCDs needpower to trip). This is why only common trip breakers must be used when

switching of the neutral wire is needed
http://en.wikipedia.org/wiki/Major_appliancehttp://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Residual-current_devicehttp://en.wikipedia.org/wiki/File:Breaker3phase2a_proc.jpghttp://en.wikipedia.org/wiki/Major_appliancehttp://en.wikipedia.org/wiki/Three-phase_electric_powerhttp://en.wikipedia.org/wiki/Residual-current_device


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Medium-voltage circuit breakers

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembledinto metal-enclosed switchgear line ups for indoor use, or may be individual

components installed outdoors in a substation. Air-break circuit breakers

replaced oil-filled units for indoor applications, but are now themselves being

replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage

circuit breakers described below, these are also operated by current sensing

protective relays operated through current transformers. The characteristics of

MV breakers are given by international standards such as IEC 62271. Medium-

voltage circuit breakers nearly always use separate current sensors

and protective relays, instead of relying on built-in thermal or magnetic

overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to

extinguish the arc:

Vacuum circuit breakerWith rated current up to 3000 A, these breakers

interrupt the current by creating and extinguishing the arc in a vacuum

container. These are generally applied for voltages up to about 35,000

V, which corresponds roughly to the medium-voltage range of power systems.Vacuum circuit breakers tend to have longer life expectancies between

overhaul than do air circuit breakers.

Air circuit breakerRated current up to 10,000 A. Trip characteristics are

often fully adjustable including configurable trip thresholds and delays.

Usually electronically controlled, though some models

are microprocessorcontrolled via an integral electronic trip unit. Often used

for main power distribution in large industrial plant, where the breakers are

arranged in draw-out enclosures for ease of maintenance.

SF6 circuit breakers extinguish the arc in a chamber filled with sulfur

hexafluoride gas.
http://en.wikipedia.org/wiki/Electrical_substationhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Microprocessorhttp://en.wikipedia.org/wiki/Electrical_substationhttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Microprocessor


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High-voltage circuit breakers

Electrical power transmission networks are protected and controlled by high-

voltage breakers. The definition ofhigh voltage varies but in power transmission

work is usually thought to be 72.5 kV or higher, according to a recent definition

by the International Electrotechnical Commission (IEC). High-voltage breakers

are nearly alwayssolenoid-operated, with current sensing protective

relays operated through current transformers. In substations the protective relay

scheme can be complex, protecting equipment and buses from various types of

overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish

the arc.

Bulk oil

Minimum oil

Air blast

Vacuum

SF6

Due to environmental and cost concerns over insulating oil spills, most newbreakers use SF6 gas to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains

the breaking mechanism is at line potential, ordead tankwith the enclosure at

earth potential. High-voltage AC circuit breakers are routinely available with

ratings up to 765 kV. 1200KV breakers are likely to come into market very soon

High-voltage circuit breakers used on transmission systems may be arranged to

allow a single pole of a three-phase line to trip, instead of tripping all three poles;

for some classes of faults this improves the system stability and availability.
http://en.wikipedia.org/wiki/Power_transmissionhttp://en.wikipedia.org/wiki/International_Electrotechnical_Commissionhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Electrical_substationhttp://en.wikipedia.org/wiki/Sulfur_hexafluoridehttp://en.wikipedia.org/wiki/Sulfur_hexafluoridehttp://en.wikipedia.org/wiki/Power_transmissionhttp://en.wikipedia.org/wiki/International_Electrotechnical_Commissionhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Electrical_substationhttp://en.wikipedia.org/wiki/Sulfur_hexafluoride


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Relay

A relay is an electrically operated switch. Many relays use an electromagnet to

operate a switching mechanism mechanically, but other operating principles are

also used. Relays are used where it is necessary to control a circuit by a low-

power signal (with complete electrical isolation between control and controlled

circuits), or where several circuits must be controlled by one signal. The first

relays were used in long distance telegraph circuits, repeating the signal coming

in from one circuit and re-transmitting it to another. Relays were used extensively

in telephone exchanges and early computers to perform logical operations.

A type of relay that can handle the high power required to directly control an

electric motor is called a contactor. Solid-state relays control power circuits with

nomoving parts, instead using a semiconductor device to perform switching.

Relays with calibrated operating characteristics and sometimes multiple

operating coils are used to protect electrical circuits from overload or faults; in

modern electric power systems these functions are performed by digital

instruments still called "protective relays".

Applications

Relays are used to and for:

Control a high-voltage circuit with a low-voltage signal, as in some types

ofmodems or audio amplifiers,

Control a high-current circuit with a low-current signal, as inthe startersolenoid of an automobile,

Detect and isolate faults on transmission and distribution lines by opening

and closing circuit breakers (protection relays),
http://en.wikipedia.org/wiki/Electrichttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Contactorhttp://en.wikipedia.org/wiki/Solid-state_relayshttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Modemshttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Starter_motorhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Automobilehttp://en.wikipedia.org/wiki/Circuit_breakershttp://en.wikipedia.org/wiki/Electrichttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Contactorhttp://en.wikipedia.org/wiki/Solid-state_relayshttp://en.wikipedia.org/wiki/Moving_partshttp://en.wikipedia.org/wiki/Protective_relayhttp://en.wikipedia.org/wiki/Modemshttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Starter_motorhttp://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Automobilehttp://en.wikipedia.org/wiki/Circuit_breakers


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Conclusion:

I am very glad after completing my vocational training as now I am

familiar with many electrical systems specially the switchyard and its parts

about which I had been unknown in the institute. During the vocational

training I had been exposed to the practical electrical equipments which are

totally different from those we are familiar with in the institute. I had been

made familiar with cooling system and many working system of transformer

which play very important role in AC systems.

In making the vocational training a successful one my guide DGM Mr. A.

Bhattachrya had always been helpful in all related matters.all staffs of

Electrical Erection Department had been so nice and helpful to me

regarding technical matters during the vocational training.

trainning report of ntpc,bongaigaon

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