SULFUR HEXAFLUORIDE CIRCUIT

BREAKER

Group No. 3

Group Members

Sr No. Name Roll No.

1) Aryan Mishra 214120037
2) Atharva Khamkar 214120031
3) Neel Patil 214120043
4) Chandan Bhalekar 214120007
5) Omkar Rane 204120041

PROF.P.Y.DESHBHRATAR
PROF.S.D.KAMBLE
AIM:
Construction, Types & Working of Sulfur Hexafluoride (SF6) Curcuit Breaker.

THEORY:

What is SF6?

Types, Working, and Application of Sulfur Hexafluoride Circuit Breaker.

What is SF6 circuit Breaker:

Sulfur Hexafluoride protect electrical power stations and distribution systems by

interrupting electric currents, when tripped by a protective relay. Instead of oil, air, or a
vacuum, a sulfur hexafluoride circuit breaker uses sulfur Hexafluoride (SF6) gas to cool and
quench the arc on opening a circuit.

Advantages over other media include lower operating noise and no emission of hot gases, and
relatively low maintenance.

Developed in the 1950s and onward, SF6 circuit breakers are widely used in electrical grids at
transmission voltages up to 800 kV, as generator circuit breakers, and in distribution systems
at voltages up to 35 kV.

Sulfur hexafluoride circuit breakers may be used as self-contained apparatus in outdoor air-
insulated substations or may be incorporated into gas-insulated switchgear which allows
compact installations at high voltages.

Here we are going to discuss about types, construction and applications of SF6 circuit breaker.

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Construction of SF6 Circuit Breakers:
SF6 circuit breakers mainly consist of two parts, namely the interrupter unit and the gas
system.

1. Interrupter Unit –
This unit consists of moving and fixed contacts comprising a set of current-carrying parts
and an arcing probe. It is connected to the SF6 gas reservoir. This unit consists slide vents
in the moving contacts which permit the high-pressure gas into the main tank.

2. Gas System –
The closed circuit gas system is employed in SF6 circuit breakers. The SF6 gas is costly, so
it is reclaimed after each operation. This unit consists low and high-pressure chambers with a
low- pressure alarm along with warning switches. When the pressure of the gas is very low
due to which the dielectric strength of gases decrease and an arc quenching ability of the
breakers is endangered, then this system gives the warning alarm.

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Working Principle:

SF6 circuit breaker uses sulphur hexafluoride gas as an arc quenching medium. SF6 gas
is used because it has high dielectric strength which is acts as a good arc quenching
medium. SF6 is an electro-negative gas and has a high capacity to absorb free electrons.

SF6 circuit breakers contact are opened in the high-pressure flow of SF6 gas and arc is stuck
between breakers contact. The free electrons in the arc are suddenly absorbed by gas to
form negative immobile ions and the arc gets quench rapidly

SF6 circuit breaker is manufacture for voltage rating from 6.6Kv to 760Kv also is an inert,
non-toxic, non-flammable gas. SF6 has dielectric strength 2.35 times that of air at
atmospheric pressure.

The pressure on the moving contacts is possible thanks to the potential energy stored in the
mentioned pressure mechanism. When an overload occurs in the electrical circuit, the
operating coil is charged with energy and a plunger connected to the mechanism of the
moving contacts, allows the energy stored in this mechanism to be released, allowing the
moving contacts to separate as well.

As the moving contacts separate, the circuit inside the CB (circuit breaker) opens,
interrupting the flow of current and protecting the system from further damage.

However, it is important that you also understand the concept of “arc”.

When electrical current passes through an air gap from an energized component to a
neutral component, a plasma discharge known as arc occurs. As an example, lightning is
a very large arc, crossing atmospheric space from a cloud to the ground or to another
cloud.

Arcing can also occur in household electrical wiring, but also within circuit breakersduring
operation, which can damage them and cause fires if the arc is not controlled.

Therefore, the mechanism of circuit breakers also seeks to prevent or control, as much as
possible, the generation of these electric arcs.

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Properties:

There are 3 Properties of Sulfur Hexafluoride (SF6) circuit breaker.

Physical Property –

 It is non-flammable gas.
 It is colorless and odorless gas.
 It has excellent thermal conductivity
 It has high density and heavier than air.
 It liquefies at low temperature which is pressure-dependent

Chemical Property -

 SF6 gas is stable and inert.

 It is non-toxic in its pure form but its products are.
 It has high electronegativity meaning it has a strong affinity for free electrons.
 It recombines very easily after arc quenching for re-utilization.
 They are non-corrosive.

Electrical Property –

 It has superior dielectric strength which is directly proportional to the pressure.

 Its arc quenching capabilities is almost 100 times better than air.
 The frequency of the voltage does not affect its dielectric strength

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Types of Sf6 circuit Breaker:

I. Double pressure type SF6 circuit breaker

Under normal conditions, the contacts are connected blocking the SF6 gas inside. When a fault
occurs, the moving contact separates from the fixed contact and an arc is produced. Due to the
movement of contacts, the highly pressurized SF6 gas is released.The pressure inside the
cylinder is higher than the pressure outside, therefore, the gas flows out. The nozzle helps
increase the speed of the gas which quenches the arc as discussed earlier.The released SF6 gas
is filtered, recombined, compressed and then stored in the cylinder for further reutilization. The
system required to maintain the gas is quite complicated and expensive. Therefore, these
circuit breakers are obsolete and not used anymore. Furthermore, due to changes in pressure,
the gas temperature falls down very quickly which may liquefy the gas. Therefore, a heater
system is also used to prevent the gas from liquification.ype SF6 circuit breaker-

Such SF6 circuit breaker has a fixed contact and moving contact. The moving contact is hollow
from inside having a cylinder that stores compressed SF6 gas. The tip of the moving contact is
designed in such a way to form a nozzle that increases the speed of the gas when it passes
through it. The fixed contact is designed in such a way when it is in the closed position, it
blocks the flow of SF6 gas. When the contacts separate, the path for gas flow is opened which
releases a blast of SF6 gas. It has the same working operation as an air blast circuit

breaker except the gas is recombined, compressed and stored in the gas cylinder again. Which
makes it very complex and quite expensive gas system is required for operation.

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II. Single pressure type SF6 circuit breaker-

Under normal conditions, the puffer cylinder is in a closed position where the connecting bridge
connects between the two fixed contacts. The cylinder is filled with SF6 gas. Since the vents are
closed by the fixed contact, the SF6 gas remains inside the cylinder. When a fault condition
occurs, the cylinder moves downward to break the connection between the fixed contacts. The
medium ionizes and an arc is generated between the contacts. Due to downward movement and
the stationary piston, the volume inside the cylinder decreases which compresses the SF6 gas
inside. There are vents inside the cylinder that are previously blocked by the upper fixed
contact. The vents open up when the cylinder moves further down. The compressed SF6 gas
moves out at high speed through the vents. The flow of the SF6 gas quenches the arc.

There are two fixed contacts having a small gap between them. A puffer cylinder is placed on top of it.
The cylinder is filled with SF6 gas. There is a stationary piston inside the cylinder. When the cylinder
moves its volume varies due to the fixed piston. The piston also pushes on the gas increasing its
pressure while the pressure is also increased by utilizing the arc energy. There are vents inside the
cylinder and fixed contacts for in and out of the gas. The vents inside the cylinder are blocked by the
fixed contact which opens when the cylinder moves downward. The hole inside the fixed contact is used
for the inlet of gas when the cylinder goes into the closed position. The cylinder can move upward and
downward by using the moving arm.

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III. Non-Puffer Type SF6 Circuit Breaker-
Under normal conditions, the contacts remain closed and the compressed SF6 remains in the gas
chamber. When a fault occurs, the moving contact begins to move and separate from the fixed
contact. The medium surrounding the contacts gets ionized and an arc is produced between the
contacts. At the same time, the gas chamber’s valve will open and release the pressurized SF6
into the arc chamber. The SF6 quenches the arc as explained before. The blast of SF6 also cools
off the arc. The SF6 flows out through the outlet where the gas is recombined. The gas is
recompressed and stored in the gas chamber for reuse. The gas system is fully enclosed and it is
constantly monitored for any leakage.

It consists of a gas chamber and interrupter unit. The SF6 gas is compressed and stored inside a
gas chamber. The gas chamber is connected to the interrupter unit through a valve. The valve is
linked with the movement of the contacts. as soon as the contacts separate, the valve is opened
to release a blast of SF6.The arc quenching occurs inside the interrupter unit or arc chamber. It
has two types of contacts i.e. moving contact and fixed contact. The contacts are hollow
cylinders. The fixed contacts include arc horns that are used for protection against arcs. The tip
of the arc horns is covered with copper-tungsten to have extra resistance. The moving contact
includes a vent. The vent is used as an outlet for the SF6 gas from the arc chamber. The moving
contact moves back and forth to make or break the circuit. It is attached with an actuator
through an insulating rod that actuates the whole moving contact assembly by moving back and
forth. The actuator is synchronized with the gas chamber’s valve which releases the gas at the
same movement the contact separates.

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Advantages of Sf6 CB:

• SF6 gas is non-inflammable.

• By products produced after decomposition are not explosive.
• No current chopping. Capacitive currents are interrupted without restriking.
• Arcing time is very short. So no contact erosion.
• Contact gap may be kept small because superior arc quenching property.
• No risk of fire as the gas is non-inflammable
• As there are no carbon deposits, so insulation problems are neglected.
• Very short arcing time
• Quick operation
• Noiseless operation
• They can interrupt large currents
• As these breakers are totally sealed and enclosed, so they are particularly
suitablewhere fire explosion exists. E.g. Coal mines
• No moisture and dust problems
• Minimum maintenance required.

Disadvantages of SF6 CB:

• The by products produced in SF, during arcing is dangerous for the operating personnel.
• SF6breakers are costly due to the high cost of SF6
• Since SF gas has to be reconditioned after every operation of the breaker.
• Hence additional equipment is required for this purpose.
• These breakers are costly due to high cost of SF6 gas.
• Additional equipment is required to recombine and recondition the gas after
eachoperation .Imperfect joint may lead to leakage of gas.
• Arched SF6 gas is poisonous.

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Maintenance and Precaution of SF6 circuit Breaker:

Toxicity-

SF6 is odorless, colorless, tasteless, and nontoxic in its pure state. It can, however, exclude
oxygen and cause suffocation. If the normal oxygen content of air is reduced from 21 percent to
less than 13 percent, suffocation can occur without warning. Therefore, circuit breaker tanks
should be purged out after opening

Dielectric Strength-
SF6 has a dielectric strength about three times that of air at one atmosphere pressure for a given
electrode spacing. The dielectric strength increases with increasing pressure; and at three
atmospheres, the dielectric strength is roughly equivalent to transformer oil. The heaters for SF6 in
circuit breakers are required to keep the gas from liquefying because, as the gas liquifies, the pressure
drops, lowering the dielectric strength.

Arc Quenching-
SF6 is approximately 100 times more effective than air in quenching spurious arcing. SF6 also has ahigh
thermal heat capacity that can absorb the energy of the arc without much of a temperature rise.

Electrical Arc Breakdown-

Because of the arc-quenching ability of SF6, corona and arcing in SF6 does not occur until way past the
voltage level of onset of corona and arcing in air. SF6 will slowly decompose when exposed to continuous
corona.

All SF6 breakdown or arc products are toxic. Normal circuit breaker operation produces small
quantities of arc products during current interruption which normally recombine to SF6.

Arc products which do not recombine, or which combine with any oxygen or moisture present, are
normally removed by the molecular sieve filter material within the circuit breaker.

Toxicity-
Faulted SF6 Gas - Faulted SF6 gas smells like rotten eggs and can cause nausea and minor
irritation of the eyes and upper respiratory tract. Normally, faulted SF6 gas is so foul
smelling no one can stand exposure long enough at a concentration high enough to cause
permanent damage.
Solid arc products - Solid arc products are toxic and are a white or off-white, ashlike
powder. Contact with the skin may cause an irritation or possible painful fluoride burn. If
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 solid arc products come in contact with the skin, wash immediately with a large amount of
water. If water is not available, vacuum off arc products with a vacuum cleaner.

Clothing and safety equipment requirements:

When handling and re moving solid arc products from faulted SF6, the following
clothing and safety equipment should be worn:
Coveralls - Coveralls must be worn when removing solid arc products. Coveralls are
not required after all solid arc products are cleaned up. Disposable coveralls are
recommended foruse when removing solid arc products; however, regular coveralls can
be worn if disposable ones are not available, provided they are washed at the end of
each day.
Hoods - Hoods must be worn when removing solid arc products from inside a faulted dead-
tank circuit breaker.

Gloves -Gloves must be worn when solid arc products are hah-died. Inexpensive, disposable
gloves are recommended. Non-disposable gloves must be washed in water andallowed to drip-
dry after use.

Boots - Slip-on boots, non-disposable or plastic disposable, must be worn by employees who
enter eternally faulted dead-tank circuit breakers. Slip-on boots are not required afterthe
removalof solid arc products and vacuuming. Nondisposable boots must be washed inwater
and dried after use.

Respirator - A cartridge, dust-type respirator is required when entering an internally faulted

dead-tank circuit breaker. The respirator will remove solid arc products from air breathed, but
it does not supply oxygen so it must only be used when there is sufficient oxygen to support
life. The filter and cartridge should be changed when an odor is sensedthrough the respirator.
The use of respirators is optional for work on circuit breakers whose interrupter units are not
large enough for a man to enter and the units are well ventilated. Air-line-type respirators
should be used when the cartridge type is ineffective due to providing too short a work time
before the cartridge becomes contaminated and anodor is sensed. When an air-line respirator is
used, a minimum of two working respiratorsmust be available on the job before any employee
is allowed to enter the circuit breaker tank.

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Disposal of waste - All materials used in the cleanup operation for large quantities of SF6 arc
products shall be placed in a 55 gal drum and disposed of as hazardous waste.

The following Item should be disposed of :

 All solid arc products.

 All disposal protective clothing

 All cleaning rags.

 Filters from respirator

 Molecular Sieve from breaker and gas cart

 Vaccum filter element.

Specifications of SF6 circuit breaker:

• SF6 circuit breakers are mainly used for the protection of very high voltage
circuits up to800 kV from fault current.
• It can safely break and depower a high-voltage circuit for any kind of
inspection ormaintenance.
• Each interrupter unit is capable of handling 60 kA in the range of 80 kV.
• SF6 circuit breakers have high carrying current rating (4 kA to 40 kA).

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CONCLUSION :

Sulphur Hexafluoride or SF6 circuit breaker is a type of circuit breaker that uses pressurized
SF6 gas to extinguish the arc. It is a dielectric gas having superior insulating and arc quenching
properties far better than air or oil. It is used for arc quenching in high voltage circuit breakers
up to 800 kV in power stations, electrical grids etc.

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