Switchgear

Electrical Switchgear
Definition of Switchgear
A switchgear or electrical switchgear is a generic term which includes all the
switching devices associated with mainly power system protection. It also
includes all devices associated with control, metering and regulating of electrical
power system. Assembly of such devices in a logical manner forms a
switchgear. This is very basic definition of switchgear.

Switchgear and Protection
We all familiar with low voltage switches and re-wirable fuses in our home. The
switch is used to manually open and close the electrical circuit in our home and
electrical fuse is used to protect our household electrical circuit from over current
and short circuit faults. In same way every electrical circuit including high voltage
electrical power system needs switching and protective devices. But in high
voltage and extra high voltage system, these switching and protective scheme
becomes complicated one for high fault current interruption in safe and secure
way. In addition to that from commercial point of view every electrical power
system needs measuring, control and regulating arrangement. Collectively the
whole system is called Switchgear and Protection of power system. The
electrical switchgear have been developing in various forms.

Switchgear protection plays a vital role in modern power system network, right
from generation through transmission to distribution end. The current interruption
device or switching device is called circuit breaker in Switchgear protection
system. The circuit breaker can be operated manually as when required and it is
also operated during over current and short circuit or any other faults in the
system by sensing the abnormality of system. The circuit breaker senses the
faulty condition of system through protection relay and this relay is again actuated
by faulty signal normally comes from current transformer or voltage transformer.

A switchgear has to perform the function of carrying, making and breaking the
normal load current like a switch and it has to perform the function of clearing the
fault in addition to that it also has provision of metering and regulating the various
parameters of electrical power system. Thus the circuit breaker includes circuit
breaker, current transformer, voltage transformer, protection relay, measuring
instrument, electrical switch,electrical fuse, miniature circuit breaker, lightening
arrestor or surge arrestor, isolator and other associated equipment.

Electric switchgear is necessary at every switching point in the electrical power
system. There are various voltage levels and hence various fault levels between

the generating stations and load centers. Therefore various types of switchgear
assembly are required depending upon different voltage levels of the system.

Besides the power system network, electrical switchgear is also required in
industrial works, industrial projects, domestic and commercial buildings.

Electrical Circuit Breaker
What is Circuit Breaker ?
Definition of Circuit Breaker : - Electrical Circuit Breaker is a switching device
which can be operated manually as well as automatically for controlling and
protection of electrical power system respectively. As the modern power system
deals with huge currents, the spacial attention should be given during designing
of circuit breaker to safe interruption of arc produced during the operation of
circuit breaker. This was the basic definition of circuit breaker.

Introduction to Circuit Breaker
The modern power system deals with huge power network and huge numbers of
associated electrical equipment. During short circuit fault or any other types of
electrical fault these equipment as well as the power network suffer a high stress
of fault current in them which may damage the equipment and networks
permanently. For saving these equipments and the power networks the fault
current should be cleared from the system as quickly as possible. Again after the
fault is cleared, the system must come to its normal working condition as soon as
possible for supplying reliable quality power to the receiving ends. In addition to
that for proper controlling of power system, different switching operations are
required to be performed. So for timely disconnecting and reconnecting different
parts of power system network for protection and control, there must be some
special type of switching devices which can be operated safely under huge
current carrying condition. During interruption of huge current, there would be
large arcing in between switching contacts, so care should be taken to quench
these arcs in safe manner. The circuit breaker is the special device which does
all the required switching operations during current carrying condition. This was
the basic introduction to circuit breaker

Working Principle of Circuit Breaker
The circuit breaker mainly consists of fixed contacts and moving contacts. In
normal "on" condition of circuit breaker, these two contacts are physically
connected to each other due to applied mechanical pressure on the moving
contacts. There is an arrangement stored potential energy in the operating
mechanism of circuit breaker which is realized if switching signal given to the
breaker. The potential energy can be stored in the circuit breaker by different
ways like by deforming metal spring, by compressed air, or by hydrolic pressure.
But whatever the source of potential energy, it must be released during operation.
Relaese of potential energy makes sliding of the moving contact at extremely fast
manner. All circuit breaker have operating coils (tripping coils and close coil),
whenever these coils are energized by switching pulse, the plunger inside them

displaced. This operating coil plunger is typically attached to the operating
mechanism of circuit breaker, as a result the mechanically stored potential
energy in the breaker mechanism is released in forms of kinetic energy, which
makes the moving contact to move as these moving contacts mechanically
attached through a gear lever arrangement with the operating mechanism. After a
cycle of operation of circuit breaker the total stored energy is released and
hence the potential energy again stored in the operating mechanism of circuit
breaker by means of spring charging motor or air compressor or by any other
means. Till now we have discussed about mechanical working principle of
circuit breaker. But there are electrical characteristics of a circuit breaker which
also should be consider in this discussion of operation of circuit breaker.

Let's have a discussion on electrical principle of circuit breaker

The circuit breaker has to carry large rated or fault power. Due to this large power
there is always dangerously high arcing between moving contacts and fixed
contact during operation of circuit breaker. Again as we discussed earlier the arc
in circuit breaker can be quenching safely if the dielectric strength between the
current carrying contacts of circuit breaker increases rapidly during every current
zero crossing of the alternating current. The dielectric strength of the media in
between contacts can be increased in numbers of ways, like by compressing the
ionized arcing media since compressing accelerates the deionization process of
the media, by cooling the arcing media since cooling increase the resistance of
arcing path or by replacing the ionized arcing media by fresh gasses. Hence a
numbers of arc quenching processes should be involved in operation of circuit
breaker.

Types of Circuit Breaker
According different criteria there are different types of circuit breaker

According to their arc quenching media the circuit breaker can be divided as

1) Oil Circuit Breaker
2) Air Circuit Breaker
3) SF6 Circuit Breaker
4) Vacuum Circuit Breaker

According to their services the circuit breaker can be divided as

1) Outdoor Circuit Breaker
2) Indoor Breaker

According to the operating mechanism of circuit breaker they can be divided as

1) Spring operated Circuit Breaker
2) Pneumatic Circuit Breaker
3) Hydrolic Circuit Breaker

According to the voltage level of installation types of circuit breaker are referred
as

1) High Voltage Circuit Breaker
2) Medium Voltage Circuit Breaker
3) Low Voltage Circuit Breaker.

Arc in Circuit Breaker
Before going through details arc quenching or arc extinction technologies
employed in circuit breaker we should know first what is arc actually.

What is arc ?
During opening of current carrying contacts in a circuit breaker the medium in
between opening contacts become highly ionized through which the interrupting
current gets low resistive path and continues to flow through this path even the
contacts are physically separated. During the flowing of current from one contact
to other the path becomes so heated that it glows. This is called arc.

Whenever, on load current contacts of circuit breaker open there is an arc in
circuit breaker, established between the separating contacts. As long as this arc
is sustained in between the contacts the current through the circuit breaker will
not be interrupted finally as because arc is itself a conductive path of electricity.
For total interruption of current the circuit breaker it is essential to quench the arc
as quick as possible. The main designing criteria of a circuit breaker is to provide
appropriate technology of arc quenching in circuit breaker to fulfill quick and safe
current interruption. So before going through different arc quenching techniques
employed in circuit breaker, we should try to understand "e;what is arc"e; and
basic theory of arc in circuit breaker, let’s discuss.

Thermal Ionization of gas
There are numbers of free electrons and ions present in a gas at room
temperature due to ultraviolet rays, cosmic rays and radioactivity of the earth.
These free electrons and ions are so few in number that they are insufficient to
sustain conduction of electricity. The gas molecules move randomly at room

temperature. It is found an air molecule at a temperature of 300 oK (Room
temperature) moves randomly with an approximate average velocity of 500
meters/second and collides other molecules at a rate of 10 10 times/second. These
randomly moving molecules collide each other in very frequent manner but the
kinetic energy of the molecules is not sufficient to extract an electron from atoms
of the molecules. If the temperature is increased the air will be heated up and
consequently the velocity on the molecules increased. Higher velocity means
higher impact during intermolecular collision. During this situation some of the
molecules are disassociated in to atoms. If temperature of the air is further
increased many atoms are deprived of valence electrons and make the gas
ionized. Then this ionized gas can conduct electricity because of sufficient free
electrons. This condition of any gas or air is called plasma. This phenomenon is
called thermal ionization of gas.

Ionization due to electron collision
As we discussed that there are always some free electrons and ions presents in
the air or gas but they are insufficient to conduct electricity. Whenever these free
electrons come across a strong electric field, these are directed towards higher
potential points in the field and acquire sufficiently high velocity. In other words,
the electrons are accelerated along the direction of the electric field due to high
potential gradient. During their travel these electrons collide with other atoms and
molecules of the air or gas and extract valance electrons from their orbits. After
extracted from parent atoms, the electrons will also run along the direction of the
same electric field due to potential gradient. These electrons will similarly collide
with other atoms and create more free electrons which will also be directed along
the electric field. Due to this conjugative action the numbers of free electrons in
the gas will become so high that the gas stars conducting electricity. This
phenomenon is known as ionization of gas due to electron collision

Deionization of gas
If all the cause of ionization of gas are removed from an ionized gas it rapidly
come back to its neutral state by recombination of the positive and negative
charges. The process of recombination of positive and negative charges is known
as deionization process. In deionization by diffusion, the negative ions or
electrons and positive ions move to the walls under the influence of concentration
gradients and thus completing the process of recombination.

Role of arc in Circuit Breaker
When two current contacts just open, an arc bridges the contact gap through
which the current gets a low resistive path to flow so there will not be any sudden
interruption of current. As there is no sudden and abrupt change in current during
opening of the contacts, there will not be any abnormal switching over voltage in

the system. If i is the current flows through the contacts just before they open, L is
the system inductance, switching over voltage during opening of contacts, may be
expressed as V = L.di/dt where di/dt rate of change of current with respect to time
during opening of the contacts. In the case of alternating current arc is monetarily
extinguished at every current zero. After crossing every current zero the media
between separated contacts gets ionized again during next cycle of current and
the arc in circuit breaker is reestablished. To make the interruption complete and
successful, this re-ionization in between separated contacts to be prevented after
a current zero.

If arc in circuit breaker is absence during opening of current carrying contacts,
there would be sudden and abrupt interruption of current which will cause a huge
switching over voltage sufficient to severely stress the insulation of the system.
On the other hand, the arc provides a gradual but quick, transition from the
current carrying to the current breaking states of the contacts.

Arc Interruption or Arc Quenching or Arc Extinction
Theory
Arc Column Characteristics

At high temperature the charged particles in a gas are rapidly and randomly
move, but in absence of electric field, no net motion is occurred. Whenever an
electric field is applied in the gas, the charged particles gain drift velocity
superimposed on their random thermal motion. The drift velocity is proportional to
the voltage gradient of the field and particle mobility. The particle mobility
depends upon the mass of the particle, heavier particles, lower the mobility. The
mobility also depends upon mean free paths available in the gas for random
movement of the particles. Since every time a particle collides, it losses its
directed velocity and has to be reaccelerated in the direction of electric field
again. Hence net mobility of the particles is reduced. If the gas is in highly
pressure, it becomes denser and hence, the gas molecules come closer to each
other; therefore collision occurs more frequently which lowers the mobility
particles. The total current by charged particles is directly proportional to their
mobility. Therefore the mobility of charged particles depends upon the
temperature, pressure of the gas and as well as nature of the gas. Again the
mobility of gas particles determines the degree ionization of gas.

So from above explanation we can say that ionization process of gas depends
upon nature of gas (heavier or lighter gas particles), pressure of gas and
temperature of gas. As we said earlier the intensity of arc column depend up on
the presence of ionized media between separated electrical contacts, hence,
special attention should be given in reducing ionization or increasing deionization
of media between contacts. That is why the main designing feature of circuit

breaker is to provide different pressure control methods, cooling methods for
different arc media in between circuit breaker contacts.

Arc Interruption or Arc Quenching or Arc Extinction
in Circuit Breaker
Heat loss from Arc

Heat loss from arc in circuit breaker is taken place through conduction,
convection as well as radiation. In circuit breaker with plain break arc in oil, arc in
chutes or narrow slots nearly all the heat loss due to conduction. In air blast
circuit breaker or in breaker where a gas flow is present between the electrical
contacts, the heat loss of arc plasma occurs due to convection process. At normal
pressure the radiation is not a significant factor but at higher pressure the
radiation may become a very important factor of heat dissipation from arc plasma.
During opening of electrical contacts, the arc in circuit breaker is produced and it
is extinguished at every zero crossing of the current and then it is again
reestablished during next cycle. The final arc extinction or arc quenching in circuit
breaker is achieved by rapid increase of the dielectric strength in the medium
between the contacts so that reestablishment of arc after zero crossing cannot be
possible. This rapid increase of dielectric strength in between circuit breaker
contacts is achieved either by deionization of gas in the arc media or by replacing
ionized gas by cool and fresh gas.

There are various deionization processes applied for arc extinction in circuit
breaker, let us discussed in brief

Deionization of gas due to increasing pressure

If pressure of the arc path increases, the density of the ionized gas is increased
which means, the particles in the gas come closer to each other and as a result
the mean free path of the particles is reduced. This increases the collision rate
and as we discussed earlier at every collision the charged particles loss their
directed velocity along electric field and again they are reaccelerated towards
field. It can be said that over all mobility of the charged particles is reduced so the
voltage required to maintain the arc is increased. Another effect of the increased
density of particles is a higher rate of deionization of gas due to the recombination
of oppositely charged particles.

Deionization of gas due to decreasing temperature

The rate of ionization of gas depends upon the intensity of impact during collision
of gas particles. The intensity of impact during collision of particles again depends
upon velocity of random motions of the particles. This random motion of a particle
and its velocity increases with increase of temperature of the gas. Hence it can be

concluded like that if temperature of a gas is increased; its ionization process is
increased and opposite statement is also true that is if the temperature is
decreased the rate of ionization of gas is decreased means deionization of gas is
increased. Therefore more voltage required to maintain arc plasma with a
decreased temperature. Finally it can be said that the cooling effectively
increases the resistance of the arc. Different types of circuit breakers employ
different cooling techniques which we will discuss later in the course of circuit
breakers.

Vacuum Arc or Arc in Vacuum
As there is no such media the arc in vacuum circuit breaker differs from general
arc in circuit breaker. In vacuum arc the electrons, ions and atoms are all derived
from the electrodes itself. The absolute vacuum is not practically possible to
create so there are some gases in practical vacuum chamber but the gas
pressure here is so low that it does not have any significant role in conduction
process during arc. In this sense the vacuum arc is therefore really a metal
vapour discharge. The vacuum arc can be divided into two main regions, the
cathode region and the plasma region.

Cathode Region of Arc Plasma
The vapour necessary to sustain vacuum arc comes mainly from the cathode
spots. Each spot carries a mean current dependent on the cathode material,
which is about 100 A for copper. The current density at the spots is estimated to
be 1010 - 1011 A/m2, depending on the cathode material. The cathode spots move
on the cathode surface. At higher current the numbers of catheads spots is
increased due to repulsion the motion of parallel spots and their movements
become more random.

Whenever the current carrying contacts open in a circuit breaker, cathode spots
are formed depending upon the current flowing through the contacts. At high
current multiple numbers of cathode spots formed which constitute the main
source of vapour for the arc in vacuum circuit breaker. The cathode surface in
normally not perfectly smooth and may have many micro projections on the
surface. When current carrying contacts are being separated in a vacuum circuit
breaker the current flowing in the circuit will be concentrated at those projections
as they form the last point of contacts. Due to their small area of contact the
projections are sufficiently heated up and they suffer explosive evaporation and
supply the vapour for formation of arc in vacuum circuit breaker.

The vapour which has high density at the cathode spot, expands into the vacuum
and perhaps at a distance of 10 mm from the cathode. The an electron traversing

the inter electrode gap experiences condition of high pressure near the cathode
where the mean free path is quite less than that of low pressure in the plasma
where it is the order of 10 mm.

At low currents, the voltage drop in the plasma region of low current is negligible.
The voltage gradient is less than 0.01 V/mm. At high current, the gradient may be
increased up to a few V/mm.

Stability of Vacuum Arc

The power frequency current passes through the contacts in circuit breaker,
crosses current zero point 100 times in a second. It is always desirable to
interrupt the current during it passes the zero value otherwise there will be current
chopping effect which may causes switching over voltage in the system.
Therefore, it is necessary to interrupt the arc as long as it is stable for a half cycle
duration particularly it should continue to exist when the current approaches to
zero. The stability of arc in vacuum circuit breaker depends upon the contact
materials, pressure of metal vaour and circuit parameters such as voltage,
current, inductance and capacitance. It is observe that higher vapour pressure in
low temperature is better stability of arc. Some metals like Zn, Bi also show better
stability of vacuum arc. Like vapour pressure thermal conductivity of contact
material is also a major factor regarding stability of vacuum arc in circuit breaker.
If the contact metal is good conductor of heat, the contact surface temperature
will fall in faster rate thus metal vapour will be condensed fast hence due to the
lack of vapor the vacuum arc will be interrupted. But if the metal used for circuit
breaker contacts is bad conductor of heat, the metal vapour will not condense fast
and the arc continues thus vacuum arc in circuit breaker becomes stable. For
successful and safe current interruption in vacuum circuit breaker, both arc
extinction at proper point of time and the stable arc are required. It is observed
that the metal having high boiling and melting point gives low vapour in high
temperature but in the same time it becomes poor conductor. Again the metal
having low melting and boiling points gives more vapour at high temperature and
in the same time it becomes good conductor. Therefore, to combine these
contradictory properties in one single material, alloys of two or more metals or a
metal and nonmetal have to be made. Some example of alloys used as the
materials to make vacuum circuit breaker contacts are copper – bismuth, silver –
lead, copper – lead etc.

Extinction of vacuum arc

Successful current interruption by a vacuum arc depends upon how fast the metal
vapour is condensed into the anode and shield near at current zero. At current
zero crossing the numbers of cathode spots are decreased to very few as the
current falls and ultimately becomes zero at exact current zero. The metal vapour
density becomes also very less because during this current zero maximum metal
vapour is condensed into anode and shield. The density of metal vapour

becomes so low throughout the gap during zero crossing that the gap is
substantially becomes an insulator which prevents re-ionization of vacuum arc in
circuit breaker after current zero.

Oil Circuit Breaker
Mineral oil has better insulating property than air. In oil circuit breaker the fixed
contact and moving contact are immerged inside the insulating oil. Whenever
there is a separation of current carrying contacts in the oil, the arc is initialized at
the moment of separation of contacts, and due to this arc the oil is vaporized and
decomposed in mostly hydrogen gas and ultimately creates a hydrogen bubble
around the arc. This highly compressed gas bubble around the arc prevents re-
striking of the arc after current reaches zero crossing of the cycle. The Oil Circuit
Breaker is the one of the oldest type of circuit breakers.

Electrical Switchgear
Electrical Circuit Breaker
Vacuum Arc
Oil Circuit Breaker
Air Circuit Breaker
SF6 Circuit Breaker
Vacuum Circuit Breaker

Operation of Oil Circuit Breaker
The operation of oil circuit breaker is quite simple let’s have a discussion.
When the current carrying contacts in the oil are separated an arc is established
in between the separated contacts. Actually, when separation of contacts has just
started, distance between the current contacts is small as a result the voltage
gradient between contacts becomes high. This high voltage gradient between the
contacts ionized the oil and consequently initiates arcing between the contacts.
This arc will produce a large amount of heat in surrounding oil and vaporizes the
oil and decomposes the oil in mostly hydrogen and a small amount of methane,
ethylene and acetylene. The hydrogen gas can not remain in molecular form and
its is broken into its atomic form releasing lot of heat. The arc temperature may
reach up to 5000oK. Due to this high temperature the gas is liberated surround
the arc very rapidly and forms an excessively fast growing gas bubble around the
arc. It is found that the mixture of gases occupies a volume about one thousand
times that of the oil decomposed. From this figure we can assume how fast the
gas bubble around the arc will grow in size. If this growing gas bubble around the
arc is compressed by any means then rate of de – ionization process of ionized

gaseous media in between the contacts will accelerate which rapidly increase the
dielectric strength between the contacts and consequently the arc will be
quenched at zero crossing of the current cycle. This is the basic operation of oil
circuit breaker. In addition to that cooling effect of hydrogen gas surround the
arc path also helps, the quick arc quenching in oil circuit breaker.

Types of oil circuit breakers
There are mainly two types of oil circuit breakers available

Bulk Oil Circuit Breaker or BOCB

Bulk Oil Circuit Breaker or BOCB is such types of circuit breakers where oil is
used as arc quenching media as well as insulating media between current
carrying contacts and earthed parts of the breaker. The oil used here is same as
transformer insulating oil.

Minimum Oil Circuit Breaker or MOCB

These types of circuit breakers utilize oil as the interrupting media. However,
unlike bulk oil circuit breaker, a minimum oil circuit breaker places the
interrupting unit in insulating chamber at live potential. The insulating oil is
available only in interrupting chamber. The features of designing MOCB is to
reduce requirement of oil, and hence these breaker are called minimum oil
circuit breaker.

Bulk Oil Circuit Breaker
Construction of Bulk Oil Circuit Breaker

The basic construction of Bulk Oil Circuit Breaker is quite simple. Here all moving
contacts and fixed contacts are immerged in oil inside closed iron vessel or iron
tank. Whenever the current carrying contacts are being open within the oil the arc
is produced in between the separated contacts. The large energy will be
dissipated from the arc in oil which vaporizes the oil as well as decomposes it.
Because of that a large gaseous pressure is developed inside the oil which tries
to displace the liquid oil from surrounding of the contacts. The inner wall of the oil
tank has to withstand this large pressure of the displaced oil. Thus the oil tank of
bulk oil circuit breaker has to be sufficiently strong in construction. An air cushion
is necessary between the oil surface and tank roof to accommodate the displaced
oil when gas forms around the arc. That is why the oil tank is not totally filled up
with oil it is filled up to certain level above which the air is tight in the tank. The
breaker tank top cover should be securely bolted on the tank body and total
breaker must be properly locked with foundation otherwise it may jump out during
interruption of high fault current. In these type of equipment where expansible oil

is enclosed in an air tight vessel (oil tank) there must be a gas vent fitted on the
tank cover. Naturally some form of gas vent always is provided on the cover of
bulk oil circuit breaker tank. This is very basic features for construction of Bulk Oil
Circuit Breaker.

Arc quenching in bulk oil circuit breaker

When the current carrying contacts in the oil are separated an arc is established
in between the separated contacts. This arc will produce rapidly growing gas
bubble around the arc. As the moving contact move away from fixed contact the
length of arc is increased as a result the resistance of the arc increases. The
increased resistance causes lowering the temperature and hence reducing the
formation of gasses surround the arc. The arc quenching in bulk oil circuit breaker
takes place when current passes through zero crossing. If we go through the arc
quenching phenomenon more thoroughly we will find many other factors effects
the arc quenching in bulk oil circuit breaker. As the gas bubble is enclosed by the
oil inside the totally air tight vessel, the oil surround it will apply high pressure on
the bubble, which results highly compressed gas around the arc. As the pressure
is increased the de – ionization of gas increases which helps the arc quenching.
The cooling effect of hydrogen gas also helps in arc quenching in oil circuit
breaker.

Single Break Bulk Oil Circuit Breaker

In single break bulk oil circuit breaker there is one pair of current carrying
contacts for each phase of power circuit. The each pair of current carrying
contacts in this bulk oil circuit breaker consists of one fixed contact and one
moving contact. Fixed contact is stationary contact and moving contact moves
away from fixed contact during opening of the circuit breaker. As the moving
contact is being moved away from fixed contact the arc is produced in between
the contacts and it is extinguished during zero crossing of the fault current, due to

the reasons as explain in previous chapter. As the days go on further research
works have been done to improve better arc control in single break bulk oil circuit
breaker. The main aim of development of bulk oil circuit breaker is to increase the
pressure developed by the vaporization and dissociation of oil. Since in large gas
pressure, the mean free paths of electrons and ions are reduced which results in
effective deionization. So if the pressure can be increased, the rate of
deionization is increased which helps to quick arc extinction. It has been found
that if the opening of fixed and moving contacts is done inside a semi closed
insulated chamber then the gas bubble created around the arc will get less space
of expansion, hence it becomes highly compressed. These semi closed insulated
arcing chamber in bulk oil circuit breaker is known as side vented explosion pot or
cross jet pot. The principle of operation of cross jet pot is quite simple let’s have a
discussion. The pressure developed by the vaporization and dissociation of the oil
is retained in the side vented explosive pot by withdrawing the moving contact
through a stack of insulating plates having a minimum radial clearance around the
contact. Thus there is practically no release of pressure until the moving contact
uncovers one of the side vents. The compressed hydrogen gas can then escape
across the arc path, thus exerting a powerful cooling action on the ionized

column. When the current zero
is reached, the post arc resistance increased rapidly due this cooling action. At
higher breaking currents larger will be the pressure generated and a bulk oil
circuit breaker gives its best performance at the highest current within its rating.
These single break bulk oil circuit breaker may have problem during clearing low
currents such as load current of the breaker.

Various improvement in the design of pressure chamber or side vented explosive
chamber have been suggested to overcome the problem of low current
interruption. One solution of this is providing a supplementary oil chamber below
the side vents. This supplementary oil chamber is known as compensating
chamber which provides fresh source of oil to be vaporized in order to feed more
clean gas back across the arc path during clearing low current.

Double Break Bulk Oil Circuit Breaker

Various improvements in the design of bulk oil circuit breaker have been
suggested to satisfactory and safe arc interruption especially at currents below
the rated maximum. One solution to this problem is to use an intermediate
contact between tow current carrying contacts. The arc is here split into two parts
in series. The aim here is to extinguish the second arc quickly by using the gas
pressure and oil momentum due to the first arc. In Double Break Bulk Oil Circuit
Breaker, there are two fixed contact and are bridged by one moving contact. The
moving contact is fitted with driving mechanism of the oil circuit breaker by means
of an insulated rod. As the moving contact bridge moves downwards the contact
gaps are created with fixed contacts at both end of the intermediate moving
contact bridge. Hence arcs are produced at both contacts gap.

Minimum Oil Circuit Breaker

As the volume of the oil in bulk oil circuit breaker is huge, the chances of fire
hazard in bulk oil system are more. For avoiding unwanted fire hazard in the
system, one important development in the design of oil circuit breaker has been
introduced where use of oil in the circuit breaker is much less than that of bulk oil
circuit breaker. It has been decided that the oil in the circuit breaker should be
used only as arc quenching media not as an insulating media. Then the concept
of minimum oil circuit breaker comes. In this type of circuit breaker the arc
interrupting device is enclosed in a tank of insulating material which as a whole is
at live potential of system. This chamber is called arcing chamber or interrupting
pot. The gas pressure developed in the arcing chamber depends upon the current
to be interrupted. Higher the current to be interrupted causes larger the gas
pressure developed in side the chamber, hence better the arc quenching. But this
put a limit on the design of the arc chamber for mechanical stresses. With use of

better insulating materials for the arcing chambers such as glass fiber, reinforced
synthetic resin etc, the minimum oil circuit breaker are able to meet easily the
increased fault levels of the system.

Working Principle or arc quenching in minimum oil circuit breaker

Working Principle of minimum oil circuit breaker or arc quenching in
minimum oil circuit breaker is described below. In a minimum oil circuit
breaker, the arc drawn across the current carrying contacts is contained inside
the arcing chamber. Hence the hydrogen bubble formed by the vaporized oil is
trapped inside the chamber. As the contacts continue to move, after its certain
travel an exit vent becomes available for exhausting the trapped hydrogen gas.
There are two different types of arcing chamber is available in terms of venting
are provided in the arcing chambers. One is axial venting and other is radial
venting. In axial venting, gases (mostly Hydrogen), produced due to vaporization
of oil and decomposition of oil during arc, will sweep the arc in axial or
longitudinal direction.

Let's have a look on working principle Minimum Oil Circuit Breaker with axial
venting arc chamber

The moving contact has just
been separated and arc is
initiated in MOCB.

The ionized gas around the arc
sweep away through upper vent
and cold oil enters into the
arcing chamber through the
lower vent in axial direction as
soon as the moving contact tip
crosses the lower vent opening
and final arc quenching in
minimum oil circuit breaker
occurs

The cold oil occupies the gap
between fixed contact and
moving contact and the
minimum oil circuit breaker
finally comes into open position.

Whereas in case of radial venting or cross blast, the gases (mostly Hydrogen)
sweep the arc in radial or transverse direction.

The axial venting generates high
gas pressure and hence has high dielectric strength, so it is mainly used for
interrupting low current at high voltage. On the other hand radial venting produces
relatively low gas pressure and hence low dielectric strength so it can be used for
low voltage and high current interruption. Many times the combination of both is
used in minimum oil circuit breaker so that the chamber is equally efficient to
interrupt low current as well as high current. These types of circuit breaker are
available up to 8000 MVA at 245 KV.

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.

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.

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.

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.

SF6 Circuit Breaker
A circuit breaker in which the current carrying contacts operate in Sulphur
Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker.

SF6 has excellent insulating property. SF6 has high electro-negativity. That
means it has high affinity of absorbing free electron. Whenever a free electron
collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms
a negative ion.

The attachment of electron with SF6 gas molecules may occur in tow different
ways,
-
1) SF6 + e = SF6
2) SF6 + e = SF5 - + F

These negative ions obviously much heavier than a free electron and therefore
over all mobility of the charged particle in the SF6 gas is much less as compared
other common gases. We know that mobility of charged particle is majorly
responsible for conducting current through a gas.

SF6 Circuit Breaker

Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very
high dielectric strength. Not only the gas has a good dielectric strength but also it
has the unique property of fast recombination after the source energizing the
spark is removed. The gas has also very good heat transfer property. Due to its
low gaseous viscosity (because of less molecular mobility) SF6 gas can efficiently
transfer heat by convection. So due to its high dielectric strength and high cooling
effect SF6 gas is approximately 100 times more effective arc quenching media
than air. Due to these unique properties of this gas SF6 Circuit Breaker is used
in complete range of medium voltage and high voltage electrical power system.
These circuit breakers are available for the voltage ranges from 33KV to 800KV
and even more.

Disadvantages of SF6 CB

The SF6 gas is identified as a greenhouse gas, safety regulation are being
introduced in many countries in order to prevent its release into atmosphere.

Puffer type design of SF6 CB needs a high mechanical energy which is almost
five times greater than that of oil circuit breaker.

Types of SF6 Circuit Breaker
There are mainly three types of SF6 CB depending upon the voltage level of
application

1) Single Interrupter SF6 CB applied for up to 245KV(220KV) system
2) Two Interrupter SF6 CB applied for up to 420KV(400KV) system
3) Four Interrupter SF6 CB applied for up to 800KV(715KV) system

Working of SF6 Circuit Breaker
The working of SF6 CB of first generation was quite simple it is some extent
similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a
high pressure reservoir. During operation of SF6 circuit breaker this highly
compressed gas is released through the arc and collected to relatively low
pressure reservoir and then it pumped back to the high pressure reservoir for
reutilize.

The working of SF6 circuit breaker is little bit different in moder time. Innovation
of puffer type design makes operation of SF6 CB much easier. In buffer type
design, the arc energy is utilized to develop pressure in the arcing chamber for
arc quenching.

Here the
breaker is filled with SF6 gas at rated pressure. There are two fixed contact fitted

with a specific contact gap. A sliding cylinder bridges these to fixed contacts. The
cylinder can axially slide upward and downward along the contacts. There is one
stationary piston inside the cylinder which is fixed with other stationary parts of
the SF6 circuit breaker, in such a way that it can not change its position during
the movement of the cylinder. As the piston is fixed and cylinder is movable or
sliding, the internal volume of the cylinder changes when the cylinder slides.

During opening of the breaker the cylinder moves downwards against position of
the fixed piston hence the volume inside the cylinder is reduced which produces
compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents
which were blocked by upper fixed contact body during closed position. As the
cylinder move further downwards, these vent openings cross the upper fixed
contact, and become unblocked and then compressed SF6 gas inside the
cylinder will come out through this vents in high speed towards the arc and
passes through the axial hole of the both fixed contacts. The arc is quenched
during this flow of SF6 gas.

During closing of the SF6 circuit breaker, the sliding cylinder moves upwards and
as the position of piston remains at fixed height, the volume of the cylinder
increases which introduces low pressure inside the cylinder compared to the
surrounding. Due to this pressure difference SF6 gas from surrounding will try to
enter in the cylinder. The higher pressure gas will come through the axial hole of
both fixed contact and enters into cylinder via vent and during this flow; the gas
will quench the arc

Vacuum Circuit Breaker
A vacuum circuit breaker is such kind of circuit breaker where the arc
quenching takes place in vacuum. The technology is suitable for mainly medium
voltage application. For higher voltage Vacuum technology has been developed
but not commercially viable. The operation of opening and closing of current
carrying contacts and associated arc interruption take place in a vacuum chamber
in the breaker which is called vacuum interrupter. The vacuum interrupter
consists of a steel arc chamber in the centre symmetrically arranged ceramic
insulators. The vacuum pressure inside a vacuum interrupter is normally
maintained at 10 - 6 bar.

The material used for current carrying contacts plays an important role in the
performance of the vacuum circuit breaker. CuCr is the most ideal material to make
VCB contacts. Vacuum interrupter technology was first introduced in the year of 1960.
But still it is a developing technology. As time goes on, the size of the vacuum
interrupter is being reducing from its early 1960’s size due to different technical
developments in this field of engineering. The contact geometry is also improving with

time, from butt contact of early days it gradually changes to spiral shape, cup shape and
axial magnetic field contact. The vacuum circuit breaker is today recognized as most
reliable current interruption technology for medium voltage system. It requires minimum
maintenance compared to other circuit breaker technologies.

Advantages of vacuum circuit breaker or VCB
Service life of Vacuum Circuit Breaker is much longer than other types of circuit
breakers. There is no chance of fire hazard as oil circuit breaker. It is much
environment friendly than SF6 Circuit breaker. Beside of that contraction of VCB
is much user friendly. Replacement of Vacuum Interrupter (VI) is much
convenient.

Operation of Vacuum Circuit Breaker
The main aim of any circuit breaker is to quench arc during current zero crossing,
by establishing high dielectric strength in between the contacts so that
reestablishment of arc after current zero becomes impossible. The dielectric
strength of vacuum is eight times greater than that of air and four times greater
than that of SF6 gas. This high dielectric strength makes it possible to quench a
vacuum arc within very small contact gap. For short contact gap, low contact
mass and no compression of medium the drive energy required in vacuum circuit
breaker is minimum. When two face to face contact areas are just being
separated to each other, they do not be separated instantly, contact area on the
contact face is being reduced and ultimately comes to a point and then they are
finally de-touched. Although this happens in a fraction of micro second but it is the

fact. At this
instant of de-touching of contacts in a vacuum, the current through the contacts
concentrated on that last contact point on the contact surface and makes a hot
spot. As it is vacuum, the metal on the contact surface is easily vaporized due to
that hot spot and create a conducting media for arc path. Then the arc will be
initiated and continued until the next current zero. At current zero this vacuum arc
is extinguished and the conducting metal vapour is re-condensed on the contact
surface. At this point, the contacts are already separated hence there is no
question of re-vaporization of contact surface, for next cycle of current. That
means, the arc cannot be reestablished again. In this way vacuum circuit breaker
prevents the reestablishment of arc by producing high dielectric strength in the
contact gap after current zero.

There are two types of arc shapes. For interrupting current up to 10kA, the arc
remains diffused and the form of vapour discharge and cover the entire contact
surface. Above 10kA the diffused arc is constricted considerably by its own
magnetic field and it contracts. The phenomenon gives rise over heating of
contact at its center. In order to prevent this, the design of the contacts should be
such that the arc does not remain stationary but keeps travelling by its own
magnetic field. Specially designed contact shape of vacuum circuit breaker make
the constricted stationary arc travel along the surface of the contacts, thereby
causing minimum and uniform contact erosion.

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