Sunday, August 17, 2014

SF6 Circuit breaker; SF6 Circuit breaker Applications

SF6 Circuit Breaker
As clear from the name circuit breaker which operates in Sulphur Hexfluoride gas is known as SF6 circuit breaker. It is most widely used in HT Breakers.
SF6 has excellent insulating property.
SF6 Gas has high electro-negativity. Which means that they have ver high affinity to absorb electrons. Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.
 SF6 gas can absorb electron in two different ways
 SF6  + e ----->     SF6-
SF6  + e ----->     SF5-  + F
You can see from above that SF6 when comes in contact with free electrons then they absorb the electrons and these –ve ions are much heavier then free electrons which will leads to lower mobility of charged particle in SF6 gas as compared to other gases. As everyone knows that this charged particle mobility is majorly responsible for conducting electric current through a gas. Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength.

There are various other properties other then higher dielectric strength such has this gas has also unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property as this gas has lower gaseous viscosity due to lesser molecular mobility.
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 above 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.

Properties Of SF6 (Sulfur Hexafuoride) Gas

a) 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.

b) Toxicity Of Arc Products

Toxic decomposition products are formed when SF6 gas is subjected to an elec­tric arc. The decomposition products are metal fluorides and form a white or tan powder. Toxic gases are also formed which have the characteristic odor of rotten eggs. Do not breathe the vapors remaining in a circuit breaker where arcing or corona dis­charges have occurred in the gas.
Evacuate the faulted SF6 gas from the circuit breaker and flush with fresh air before working on the circuit breaker.

c) Physical Properties

SF6 is one of the heaviest known gases with a den­sity about five times the density of air under similar conditions. SF6 shows little change in vapor pressure over a wide temperature range and is a soft gas in that it is more compressible dynamically than air.
The heat trans­fer coefficient of SF6 is greater than air and its cooling characteristics by convection are about 1.6 times air.

d) 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.
The exact dielectric strength, as compared to air, varies with electrical configuration, electrode spacing, and electrode configuration.

e) Arc Quenching

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

f) 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.

Disadvantages of SF6 CB
There is main disadvantage associated with SF6 Gas is that SF6 gas is a Greenhouse gas.  There are so many safety regulation has been 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 OCB.
SF6 is the most potent greenhouse gas with a global warming potential that is 23,900 times greater than that of carbon dioxide (CO2); it is also very persistent in the atmosphere with a lifetime of 3,200 years
There are mainly three types of SF6 CB depending upon the Voltage level of application-
1.         Single interrupter SF6 CB applied for up to 245 KV(220 KV) system.
2.         Two interrupter SF6 CB applied for up to 420 KV(400 KV) system.
3.         Four interrupter SF6 CB applied for up to 800 KV(715 KV) system.
Working of SF6 Circuit Breaker
As all circuit breakers will work on the same principle there is only difference in arc quenching medium. In Air circuit breakers air is used as quenching medium in the same way in SF6 circuit breakers SF6 gas is used as quenching medium.
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 in breaker and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for re utilize.
The working of SF6 circuit breaker is little bit different in modern 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 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.

Always keep an eye on SF6 Levels in circuit breakers, don't operate them if SF6 level indicates in red zone. If you try to operate the breaker in that position then there are very chances that explosion may takes place.

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