109327924 basic operation of air circuit breaker

8/13/2019 109327924 Basic Operation of Air Circuit Breaker

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Prepared by:

Lim K.C.

EOP/124

23-02-2010


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What is a circuit-breaker?It is a switching device for making and interrupting current.

This function applies to any devices from the largest circuit-breaker down to

the smallest relay.

GeneralThe flow of electric current in a circuit is a form of kinetic energy, and it cannot

be stopped instantaneously. The energy must be taken out of the system for allcurrent to cease, and this takes a finite time, no matter how small it may be.


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The breaking of AC circuit, and the behavior of arc


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Another example

Back


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When the contacts of a switch separate while carrying current, thatcurrent cannot be stopped there , but will continue to flow between theopening contacts in the form of an arc

The arc is a column of white-hot gas which is ionized by its heat and

provides a conducting path for the current

This path has appreciable resistance which increases as the contactscontinue to separate and the arc lengthens

Eventually the resistance becomes so high that the current falls to thepoint where it can no longer maintain the arc temperature, ionizationceases, the air ceases to conduct and the arc goes out, finally stoppingthe current.

Theory of the circuit breaker arc


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V

I

V I

VI

V

I

90

90

90

90

Resistive load (I in-phase with V)

Inductive load (I lagging V by 90 )

Capacitive load (I leading V by 90 )

Resistive and Inductive load (I laggingwith V between 0 and 90)


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When short circuit occurs with an inductiveresistive circuit, the fault

current is 100% asymmetrical if the fault occurs at the instant of

voltage zero.

The figure below shows where the asymmetrical is partial between 0%

and 100%.

Back


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Circuit breakercontactsDC generator

Load

DC interruption

G

Load

Generator and cableresistance


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Circuit breakercontactsDC generator

LoadG

Load

V I

G

Load

When breaker contacts separate,the current cannot stopped there

and will continue to flow in theform of an arc.

After the contacts separate farenough, the arc does not restrikeand finally extinguished


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G

Load

Circuit breakercontactsACgenerator

Assume that the circuit is purely resistive

Load

AC interruption


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G

Load

Direct short circuit

Ohms lawI = V/R

R=?R = 0.01

V = 254 volts

I = 25,400A

Type of faults: to fault

to to earth fault to earth fault

3 faultCurve


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Protection relay will detect the fault.And activate the circuit breaker

Contacts open, and the current will flow in the form of an arc

G

Load


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V

I

G

Load

-When contacts of circuit-breaker

separate while carrying current at certaininstantaneous values, the current flowbetween the opening contacts will be inform of arc

Continues


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V

I

G

Load

G

Load

v

G

Load

When instantaneous current goes to

natural zero, the arc is extinguishedmomentarily.

But when the instantaneous voltagegoes high enough to restrike the arcagain.

This action will continue for

few cycles until the circuit-breaker contacts separatedfar enough so that the

voltage cannot break itdown. The current finallyinterrupted

When voltagehigh enough


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V

I

G

Load

G

Load

G

Load

After few cycles,

the arc goes out

The gap

becomes toogreat for the

voltage to

break it down

Example:AC current breaking


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G

Load

Cable also including inductance

Cable has resistance


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G

Load

V

I

90

Assume that the breaker is open

at certain instantaneous currentas shown in the figure

Continue


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V

I

90

G

Load

G

Load

V

G

Load

Restriking voltageThe arc restriking after current zerois the voltage which immediately

reappears across the gap


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G

Load

G

Load

V

I

V

V

With inductive circuit, thearc restriking immediatelyafter current at the naturalzero, because the voltage ishigh when current at zero


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Main fix contact

Main moving contact

Contacts will be pitted afteroperating many timesespecially high load current

Breaker full open

The arc would take some

time to extinguish.


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Example

-To avoid the main contactsdamage, the arcing contacts are

built

Arcing contacts

With arcing contacts, the arcalso take some time toextinguish

Breaker fully open

The arcing contacts will open afterthe main contacts opening

During breaker closing, the arcingcontacts will close first before themain contacts

Close


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Circuit-breaker arcing contacts closed first, then follow bymain contacts. So that the main contacts more lasting andless carbon deposit on the contacts surfaces


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Arc chute

Close

Breaker fully open

With arc chute, the arcextinguished before the breakerfully open.

Metal pieces

When the arc, driven upwards by the electromagneticforces, enters the bottom of the chute, it is split into manysections by the barriers, but the metal pieces ensureelectrical continuity between the arcs in each section; theseveral arcs are thus in series. The electromagnetic forces

within each section of the chute cause the arc in that sectionto take up the form of a helix


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But when breaking with the small loadcurrent, the electromagnetic force due tocurrent loop is not strong, so that the arcdriven by the electromagnetic force is

weak, Because of this, it would also takesome time to extinguish


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Air Puffer

Air is compressed in a

cylinder operated by thetripping mechanism andblows any reluctant arcupwards into the chute


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M TrippingCoil

Closing coil

Charging motor

Opening spring

Closing spring

Main breakercontacts

(Discharged)

(Discharged)

Charge


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M TrippingCoil

Closing coil

Charging motor

Opening spring

Closing spring


MM M MMM

(Charged)

(Uncharged)

Tripping coil

Close


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Closing coil

Charging motor

Opening spring

Closing spring


TrippingCoil

MM

(Charged)

(Uncharged)

MM

(Charged)

(Discharged)

Trippingcoil

Open


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Closing coil

Charging motor

Opening spring

Closing spring

TrippingCoil

(Discharge)

M

(Charged)


(Discharged)

Repeat


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The

END

109327924 basic operation of air circuit breaker

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