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8/8/2019 cirkt brkr-AURO

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CIRCUIT BREAKERS

Presented by:

AUROSISH PANI,

Electronics and ElectricalEngineering,

7th Sem. ,Roll no-07452


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CIRCUIT BREAKERSCircuit Breakers are mechanical

devices to close or open contact members,

thus closing or opening of an electric circuitsunder normal or abnormal condition.

An automatic circuit breakers areusually employed for the protection of electric

circuits are equipped with a trip coilconnected to a relay or other means,designed to open the circuit breakerautomatically under abnormal condition.


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Functions of a Circuit Breaker It carries the full load current continuously

without overheating and damage

It opens and closes the circuit on no load

It makes and break the normal operatingcurrent

It makes and breaks the short circuit currentsup to which it designed for.


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Operating PrincipleUnder the normal condition ,i.e. when the circuit

breaker is closed, the current carrying contacts ,

called the electrodes , engage each other under thepressure of spring.

whenever a fault occurs in any parts of a powersystem, the trip coils of the breaker get energizedand the moving contacts are pulled apart by somemechanism, thus opening of circuits. This separationof conductors produces an arc.


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Basic Construction

The basic construction of a circuit breakerrequires the separation of contacts in aninsulating fluid which serves two mainfunctions:

1. Extinguishes the arc drawn between thecontacts when the circuit breaker opens.

2. Provides insulation between the contactsand from each contact to earth.


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Fixedcontact

Movingcontact

ARC

Fixedcontact

Movingcontact

ARC

ARC ISQUENCHED BY

MEDIUM

IN A CIRCUIT BREAKER

OPERATINGPRINCIPLE OF

BREAKER


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The ARCThe electric arc constitute a basic ,indispensable and active

element in the process of current interruption.

1.Basic theory of electric discharge

The conduction of electricity is through the gases or vaporswhich contain positive and negative charge carriers and all typesof discharge involve the very fundamental process of production,movement & absorption of these carriers which is the mode ofcarrying the current between the electrodes. The gas discharge

phenomena can broadly classified as:

a. The non-self sustained dischargeb. The self sustaining discharges


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Initiation of an Arc By high voltage gradient

at the cathode resulting

into field emission.

By increase oftemperature resultinginto thermo ionicemission


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Maintenance of Arc High temperature of the medium around the contacts

caused by high current densities, with high temp the

kinetic energy gained by moving electrons increased. The field strength or volt. gradient which increases

the kinetic energy of the moving electrons andincreases the chances of detaching electrons fromneutral molecule.

An increase in mean free path-the distance throughwhich the electron moves freely.


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Methods of Arc Extinction High resistance method

a. cooling of arc

b. increasing the arc lengthc. reducing the cross section of arc

d. splitting of arc

Low resistance or current zero interruption

a. Lengthening of the gapb.increasing the pressure in the vicinity of the arc

c. Cooling

d. Blast Effect


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Phenomenon of arc extinction Energy Balance or Cassie Theory

This theory states that if the rate of heat

dissipation between the contacts is greaterthen the rate at which heat is generated ,thearc will be extinguished ,otherwise it willrestrike.

Recovery rate or Slepians TheoryThis theory states that if the rate at whichthe ions and electrons combine to form orreplaced by neutral molecules.


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Restriking Voltage & Recovery Voltage

The transient voltage which appears across the breaker contactsat the instant of arc being extinguished is known as restriking

voltage. The power frequency rms voltage ,which appears across the

breaker contacts after the arc is finally extinguished andtransient oscillation die out is called recovery voltage.


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Classification ofCircuit

Breaker The intended voltage application

The location of installation Their external design characteristics

The method and the medium used tointerrupt the current


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Present Trends in Choice ofCBsRated voltage Preferred type

Below 1kv

(low voltage)

-Air circuit breaker

3.6kv to 12kv -Vacuum circuit breaker

-SF6 C.B.

36kv -Minimum oil C.B.

-SF6 C.B.

145kv and 245kv -MOCB outdoor

-SF6 Outdoor Puffer type

420kv -SF6 Outdoor Puffer type


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Air-Break Circuit Breaker The air at atmosphere

pressure is used as arcextinguishing medium

These CBs employ thehigh resistanceinterruption principle

The arc is rapidly

lengthened by means ofarc runner and resistanceof arc increases by coolingand splitting of arc.


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Construction of Air-Break CB1. Main contacts

2. Arcing contacts

3. Arc rising in thedirection ofarrow

4. Arc getting split

5.

Arc splitterplates

6. Current carryingterminals

7. Arc runners


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Arc Extinction in Air-Break CBs Use of Arc Runners and Blow outCoils

As soon as the arc leaves the vicinity to the

contacts it commutes a pair of run out horns. Indoing so the outer blow out system is switched on.These blow out coil provides a magnetic field suchthat the arc footing is subjected to this strong field.

By virtue of this the arc travels upward and its lengthincreases .At a particular length the volt is notsufficient enough to maintain the arc and the arc isextinguished.


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Air-BlastCircuit Breaker In these CBs high

pressure air is forced onthe arc nozzle at theinstant of contactseparation

The ionized medium isblown away by highpressure air, whichprevents restrike

These are used before1980s for 11 to 1100kv.


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Construction of Air-BlastCBs

10. Compressed spring, 7. Fixed contact ,8.moving contact ,13.resistance

switching unit 9.connection for current ,1.Tank ,2. Hollow insulator assembly


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Resistance Switching in ABCB Post zero resistance is

high in ABCB because ofcontact clearance spaceis filled up with highpres air with highdielectric strength.sohigh restriking voltappearing across

contact. To prevent this a

resistance is connectedin shunt with the arc.


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Voltage Distribution in Multibreak CBs Voltage dist should be even in the gaps in series. if

not breaking capacity shared by diff gaps will beunequal.

The unequality of the voltage mainly occurs at theinstant of recovery voltage, when the potential acrossthe contact space is determined by capacitancebetween contact members and between contact andearth.

K1 K2 K3

C1 C2 C3 C4

Vc1Vc3>Vc4


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Equalization by means of external capacitance

External appearance

Voltage distribution


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Oil Circuit Breaker In Minimum Oil CBs the

oil is used as an arcquenching medium and

dielectric medium In Bulk oil CBs

transformer oil is usedboth for arc extinctionmedium as well asinsulating medium

Bulk oil CBs are now adays obsolete


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Construction ofMOCBGas vent

Fixed

contact

Moving

contact

Drain valve

Opening rod

Supporting

chamber

Arc Extinction drive

Circuit Breaking

chamber

Breather

Top Chamber

Terminal

Terminal


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Construction of BOCBConductor

Bushing (porcelain)

Conductor (Cupper)

Tension rod

Fixed Contacts

Arc control device (resin cast)

Gas formed by decomposition

Moving contacts

Dielectric oil (Mineral oil)

Tank (Boiler plates)


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Vacuum Circuit Breaker In this type ofCBs , the

arc bums in the metal

vapour generated byarc when the contactparts.

Due to vacuum ,thistype of interrupter can

have extremely highrate of recovery ofdielectric strength


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Electrical breakdown in Vacuum A pres below 10-5 mm of mercury are consider as

high vacuum.

The charged particles from one electrode movingtowards other electrode at such a press are unlikelyto cause collision with residual gas molecules .Henceionization is less relative to other gas

The secondary emission takes place by bombardment

of high energy on surface of electrodes. Then theelectron emission takes place by virtue of intenseheat. The arc consists of thin column of plasma. Thecore has a temp about 6000K to 15000K. At suchtemp Thermoionic emission takes place from surfaceof electrodes.


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Construction of Vacuum CBsFixed terminal

End shield

Insulating

Envelope

Bellow shield

Movable

terminal

Contacts

Shield support

Flange

Metal Vapor

condenser

shield


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Contact material in Vacuum CBs The arc quenching in the vacuum is solely depends

on contact shape and material

For breaking current up to 4kA plain butt contacts isused.

For more than 4kA the shape should be spiral orcontrate (segmented) .because it provides radial(additional) magnetic field which drives the round the

arcing rings.

Contrate (segmented) contacts Spinning of arc Petal contact


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Properties of contact material High electrical conductivity

Low contact resistance

High thermal conductivity Low current chopping level

High arc withstand stability

High melting point

Low tendency to weld

Easy to manufacture and economical

Generally Copper-Bismuth Alloy, or Copper-Chromium Alloy ,Copper-Beryllium Alloy is used


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Degree of vacuum in interrupter

Highvacuum

Interrupter

vacuum

Absolute Pressure ,TORR

BreakdownvoltagekV,10mmgap

100

10

1

10-6 10-5 10-4 10-3 10-2 10-1 1


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Arcing time in Vacuum Arc duration is usually much less than 1sec

Air circuit breaker

Oil Circuit breaker

Vacuumcircuit breaker

0 100 1000 10000

0.070.06

0.05

0.04

0.03

0.02

0.01

Interrupting current in amp

Timein sec


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Advantages of Vacuum CBs Freedom from oil ,ionized gases eliminates the arcing

and fire hazards Low arc energy and arcing time results in very long

contact life Reduced number of components impart simplicity to

the operating mechanism Contact can not oxidized in vacuum so mechanically

clean after years of operation

No change in contact resistance because of highcontact pressure applied practically maintenance free Switching in vacuums does not produce byproducts,

so a stable dielectric medium


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SF6 Circuit Breaker These type of circuit

breaker use Sulfur

Hexafluoride as thequenching medium

In modern powersystems SF6 circuit

breaker are highestin use.


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Electrical Properties of SF6 Gas High dielectric strength

Excellent arc quenching ability

Excellent thermal stability

Good thermal conductivity


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Physical & Chemical properties Chemically inert

Non-toxic

Non-corrosive

Non-flammable

High density

High electro negativity


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Dielectric properties of SF6 gasDS of sf6 is 2.5 times

of air

DS of sf6 is 30% lessthan oil

DS of sf6 is equal to

oil at 63700 N/m2

And 15% higher at

122500 N/m2


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Operation of SF6 C.B.s


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SF6 DECOMPOSITION PRODUCTSSF6

SF4 + 2F

SOF2 + 2HF MFn

SO2 + HFSiF4 + 2 H2O

SPARK

H2O

H2O SiO2

M


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REMOVAL OF SF6 BYPRODUCTSByproducts are corrosive & likely to affect the

organic materials.

Absorbent materials used in circuit breakers

activated alumina( effective forSOF2,H2S,SF2)

molecular sieves( soda lime - CaO.NaOH )- removesstable gases such as SF4,SiF4 & S2F2

So suggested mix is 50/50 of soda lime &alumina. The suggested weight of absorbent is10% of the weight of the gas


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ADVANTAGES OF SF6 C .B. s Very short arcing period

Can interrupt much larger

currents as compared toother breakers

No risk of fire

Low maintenance, light

foundation, minimum

auxiliary equipments

No over voltageproblem


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Circuit Breaker Time fault clearing =Relay + Circuitbreaker

time time time

Relay = Instant to to Closure oftime fault trip circuit

Circuit breaker = closure of to final arc

time trip ckt extinction

= Opening + Arcing

time time


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Auto ReclosureFault Occurs

Breaker Trips

Breaker Recloses

If Fault

persists

If fault is cleared

Trips openRemains closed


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An Overview of Substation


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DifferentManufactures ofCBs Siemens India Ltd.

ABB, Sweden

Bharat Heavy Electricals Ltd. ,India

General Electric ,USA

Crompton Greaves


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THANKYOU

HAVE A GOOD DAYF

OLKS!!

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