Air circuit breakers as
clear from the name that these circuit breakers operate at atmospheric
pressure. ACBs are still preferable choice up to voltage 15 KV. Even ACB’s can be used for LT
also . MCCB’s can be used upto 630/ 800A above that limit air circuit breakers
are used.
Working Principle of Air
Circuit Breaker
Main aim for every circuit
breaker is to prevent rebuilt of arcing after electric current goes zero by
creating a situation where in the contact gap will withstand the system
recovery voltage. 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,
- Air circuit
breaker increases 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 electric 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.
OPERATIONAL PROCEDURE OF ACB
Every circuit breaker
objective is usually to increase the arc area this can be achieved by forcing
the arc into contact with as large an area as possible of insulating material.
There are arc chutes in every circuit breaker usually arc is driven into it. Arc
chute is suitably shaped in such a way to 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.
There is further 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 own mini arc chute.
So each of the split arcs has its own cooling and lengthening effect due to its
own mini arc chute and hence individual split arc voltage becomes high. These collectively, make
the overall 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 1 KV, does not require any arc
control device.
During
breaking of a circuit breaker arc is produced which needed to be quenched as
earlier as possible so that arc doesn’t re-strike. Circuit breakers usually
named after the Medium in which circuit breaker breaks or medium used to quench
the arc during breaking of circuit breaker. In Air circuit breaker air serves
the same purpose.
Mainly for heavy fault electric current on low voltages ACBs with appropriate
arc control device, are good choice. These breakers normally have two pairs of
contacts. The main pair of contacts carries the electric 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 electric 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 of operation
of air circuit breaker, and therefore the arc is quenched
finally during the electric current zero.
Air circuit breakers working is shown above which self explanatory.
Although this type of
circuit breakers have become obsolete for medium voltage application, but they are still
preferable choice for high electric current rating in low voltage application.
Advantages of air circuit
breakers
- There is no
chance of fire hazard which usually occur in Oil circuit breaker.
- The breaking
speed of Air circuit breaker is much higher than oil
circuit breaker.
- Arc quenching is much faster in case of air
circuit breaker than oil circuit breaker.
- As the
duration of arc is smaller, so lesser amount of heat realized from arc to electric current carrying contacts hence the
service life of the contacts becomes longer.
- The stability
of the system can be well maintained as it depends on the speed of
operation of circuit breaker.
- Requires much
less maintenance compared to oil circuit breaker.
There are also some
disadvantages of air blast circuit breakers-
- In order to
have frequent operations, it is necessary to have sufficiently high
capacity air compressor.
- Frequent
maintenance of compressor, associated air pipes and automatic control
equipments is also required.
- Due to high
speed electric current interruption there is always
a chance of high rate of rise of re-striking voltage and electric current chopping.
- There also a
chance of air pressure leakage from air pipes junctions.
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