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.
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.
The
SF6 insulated Switchgear contains the same components as a conventional outdoor
substation. Fig (1) illustrates the construction of typical bay.
1-phase Bus
enclosure.
2- Isolator.
3-Earthing
Switch.
4-C.B
puffer type.
5- CT's
6-Line
Isolator.
7- VT.
8-High
Speed Earthing Switch.
9-Cable
sealing End.
10-Operating
mechanism (cabinet).
11-Conductor
tube.
12-Epoxy
partition
 |
| Fig (1) illustrates the construction of typical bay |
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.
Advantages of GIs and Application Aspects:
1-Compactness.
The
space occupied by SF6 installation is only about 8 to 10 % of that a conventional outdoor
substation.
High cost is partly compensated by saving in cost of space. A typical 420/525 kV
SF6 GIs
requires
only 920 m2 site area against 30.000 m2 for a conventional air insulated
substation.
2-Choice of
Mounting Site.
Modular
SF6 GIS can be tailor made to Suit the particular site requirements.
This results is saving of otherwise Expensive
civil-foundation work. SF6 GIS can be suitably mounted indoor
on
any floor or basement and SF6 Insulated Cables (GIC) can be taken through walls
and terminated
through
SF6 bushing or power cables.
3- Reduced Installation Time.
The
principle of building block construction (modular construction) reduces the
installation time to a few weeks. Each conventional substation requires several
months for installation.
In
SF6 substations, the time-consuming high cost galvanized steel structures are
eliminated. Heavy foundations for galvanized steel structures,
Equipment
support structures etc are eliminated. This results in economy and reduced
project execution time. Modules are factory assembled, tested and dispatched
with nominal SF6 gas. Site erection time is reduced to final assembly of
modules.
4-Protection
from pollution.
The
external moisture. Atmospheric Pollution, snow dust etc. have little influence
on SF6 insulated substation. However, to facilitate installation and
maintenance, the substations are generally housed inside a small building.
5- Increased Safety.
As
the enclosures are at earth potential there is no possibility of accidental
contact
by
service personnel to live parts.
6-Explosion-proof
and Fire-proof installation.
Oil
Circuit Breakers and oil filled equipment are prone to explosion. SF6 breakers
and SF6 filled equipment are explosion proof and fire-proof.
Disadvantages of GIS:
1- High cost compared to conventional outdoor substation.
2-Excessive
damage in case of internal fault. Long outage periods as
Repair
of damaged part at site may be difficult.
3-Requirement
of cleanliness is very stringent. Dust or moisture can cause
internal
flashovers.
4-Such
substations are generally in door. They need a separate building.
This
is generally not required for conventional outdoor substations.
5-Procurement
of gas and supply of gas to site is problematic.
Adequate
stock of gas must be maintained.
6-Project
needs almost total imports including SF6 Gas. Spares conventional
substation
is totally indigenous up to 400 kV.
