Air Circuit Breaker
This
type of circuit breakers, is those kind of circuit breaker which operates in
air at atmospheric pressure. After development of oil breaker, the medium
voltage air circuit breaker (ACB) is replaced completely by oil circuit breaker
in different countries. But in countries like France and Italy, ACBs are still
preferable choice up to voltage 15 KV. It is also good choice to avoid the risk
of oil fire, in case of oil circuit breaker. In America ACBs were exclusively
used for the system up to 15 KV until the development of new vacuum and SF6
circuit breakers.
Working principle of Air Circuit Breaker
The
working principle of this breaker is rather different from those in any other
types of circuit breakers. The main aim of all kind of circuit breaker is to
prevent the reestablishment of arcing after current zero by creating a
situation where in the contact gap will withstand the system recovery voltage.
The air circuit breaker does the same but in different manner. 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,
It may
increase 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 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.
Types of ACB
There
are mainly two types of ACB are available.
-(1) Plain air circuit breaker
(2)- Air blast Circuit Breaker.
Operation of ACB
The
first objective is usually achieved by forcing the arc into contact with as
large an area as possible of insulating material. Every air circuit breaker is
fitted with a chamber surrounding the contact. This chamber is called 'arc
chute'. The arc is driven into it. If inside of the arc chute is suitably
shaped, and if the arc can be made conform to the shape, the arc chute wall
will 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.
The
second objective that is lengthening the arc path, is achieved concurrently
with fist objective. If the inner walls of the arc chute is shaped in such a
way that the arc is not only forced into close proximity with it but also
driven into a serpentine channel projected on the arc chute wall. The
lengthening of the arc path increases the arc resistance.
The
third technique is achieved by using metal 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 won mini arc chute. So each of the split arcs has
its won cooling and lengthening effect due to its won mini arc chute and hence
individual split arc voltage becomes high. These collectively, make the over
all 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 1KV, does not require
any arc control device. Mainly for heavy fault current on low voltages (low
voltage level above 1 KV) ABCs with appropriate arc control device, are good
choice. These breakers normally have two pairs of contacts. The main pair of
contacts carries the 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 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 current zero.
air circuit breaker
Although
this type of circuit breakers have become obsolete for medium voltage
application, but they are still preferable choice for high current rating in
low voltage application.
Air Blast Circuit Breaker
These
types of air circuit breaker were used for the system voltage of 245KV, 420KV
and even more, especially where faster breaker operation was required. Air
Blast Circuit Breaker has some specific advantages over oil circuit breaker
which are listed as follows,
(1) There is no chance of fire hazard caused by oil.
(2)The
breaking speed of circuit breaker is much higher during operation of air blast
circuit breaker.
(3) Arc quenching is much faster during operation of air
blast circuit breaker.
(4) The duration of arc is same for all values of small as
well as high currents interruptions.
(5) As the duration of arc is smaller, so lesser amount of
heat realized from arc to current carrying contacts hence the service life of
the contacts becomes longer.
(6) The stability of the system can be well maintained as it
depends on the speed of operation of circuit breaker.
(7) Requires much less maintenance compared to oil circuit
breaker.
There are also some disadvantages of air blast
circuit breakers
(1)In
order to have frequent operations, it is necessary to have sufficiently high
capacity air compressor.
(2) Frequent maintenance of compressor, associated air pipes
and automatic control equipments is also required.
(3) Due to high speed current interruption there is always a
chance of high rate of rise of re-striking voltage and current chopping.
(4) There also a chance of air pressure leakage from air
pipes junctions.
As we
said earlier that there are mainly two types of ACB, plain air circuit breaker
and air blast circuit breaker. But the later can be sub divided further into
three different categories.
a)
Axial Blast ACB.
b)
Axial Blast ACB with side moving contact.
c)
Cross Blast ACB.
axial blast air circuit breaker
In
Axial Blast ACB the moving contact is in contact with fixed contact with the
help of a spring pressure as shown in the figure. There is a nozzle orifice in
the fixed contact which is blocked by tip of the moving contact at normal
closed condition of the breaker. When fault occurs, the high pressure air is
introduced into the arcing chamber. The air pressure will counter the spring
pressure and deforms the spring hence the moving contact is withdrawn from the
fixed contact and nozzle hole becomes open. At the same time the high pressure
air starts flowing along the arc through the fixed contact nozzle orifice. This
axial flow of air along the arc through the nozzle orifice will make the arc
lengthen and colder hence arc voltage become much higher than system voltage
that means system voltage is insufficient to sustain the arc consequently the
arc is quenched.
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axial blast air circuit breaker
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Axial Blast ACB with side moving contact
In
this type of axial blast air circuit breaker the moving contact is fitted over
a piston supported over a spring. In order to open the circuit breaker the air
is admitted into the arcing chamber when pressure reaches to a predetermined
value, it presses down the moving contact; an arc is drawn between the fixed
and moving contacts. The air blast immediately transfers the arc to the arcing
electrode and is consequently quenched by the axial flow of air .
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Axial Blast ACB with side moving contact
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Cross Blast Air Circuit Breaker
The
working principle of Cross Blast Air Circuit Breaker is quite simple. In this
system of air blast circuit breaker the blast pipe is fixed in perpendicular to
the movement of moving contact in the arcing chamber and on the opposite side
of the arcing chamber one exhaust chamber is also fitted at the same alignment
of blast pipe, so that the air comes from blast pipe can straightly enter into
exhaust chamber through the contact gap of the breaker. The exhaust chamber is
spit with arc splitters. When moving contact is withdrawn from fixed contact,
an arc is established in between the contact, and at the same time high
pressure air coming from blast pipe will pass through the contact gap and will
forcefully take the arc into exhaust chamber where the arc is split with the
help of arc splitters and ultimately arc is quenched.
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Cross Blast Air Circuit Breaker
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