cirkt brkr-AURO
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Transcript of 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!!