The SF6 circuit breaker is an extinguisher that uses sulfur hexafluoride gas to cool and quench the arc.
It is the type of circuit breaker used to protect electrical grids, power stations, and distribution systems.
Sulfur hexafluoride gas has strong electronegativity and insulating properties, making them a better option than air and oil circuit breakers.
Types of sulfur hexafluoride circuit breaker
There are three types of SF6 CB, they include:
- Single interrupter SF6 CB, they use up to 245 kV (220 KV).
- Double interrupter SF6 CB, used on up to 420 kV (400 KV).
- Four interrupters SF6 CB, used on up to 800 kV (715 KV).
In a high-voltage circuit breaker like SF6, current interruption is achieved by separating the contacts in a gas medium.
The sf6 comprises both fixed and movable contacts enclosed in a gas system at a pressure of around 2.8 kg/cm2.
Whenever a fault arises, the movable contacts separate, and an arc strikes in between them.
Separation of the movable contacts is via an opening valve that allows a high-pressure flow of SF6 gas at 14 kg/cm2 to strike the arc and extinguish them.
Since SF6 gas is electronegative and has a powerful love for free electrons, it absorbs the conducting free electrons in the arc to form immobile negative ions.
Advantages of SF6 switchgear
The advantages include
- They are easier to maintain
- Produces non-hazardous gas
- Produce less noise because of their closed nature.
- Better extinguishers because of their dielectric property.
- Zero risks of a fire outbreak since the gas is non-flammable.
- Causes greenhouse effect: A greenhouse effect is when gases in the atmosphere such as CO2 trap the energies that supposed to radiate back to space, making the earth warmer.
SF6 gas has a similar effect if released into the environment.
According to the Intergovernmental Panel on Climate Change, SF6 is the most powerful greenhouse gas they have assessed, with a global warming potential of 22,200 times that of co2 when compared over a hundred-year period.
- It displaces oxygen: Since SF6 is heavier than air, it may displace oxygen. We should always be careful when entering a confined space to avoid the risk of oxygen displacement.
- It is expensive because of the cost of SF6 gas
- Since the breaker reconditions the gas after every operation, it requires additional equipment for the purpose.
Properties of SF6 CB
- It is colourless, odourless, and non-toxic in its pure state.
- It has good dielectric strength, about three times that of the air at one atmospheric pressure for electrode spacing. The dielectric strength also increases with an increase in pressure.
- Sf6 is 100 times more effective than air in quenching accidental arcing.
- Has high thermal conductivity
- It is an inert and stable gas, and its density is higher than air.
- Sf6 is an electronegative gas, which makes it an excellent insulator.
- It is a better extinguisher than an air medium.
- SF6 is non-flammable, meaning there is no risk of a fire outbreak.
Construction of sulfur hexafluoride gas breaker
The sulfur hexafluoride gas breaker comprises two key parts
- An interrupter unit
- The gas system
The interrupter of the Sulfur hexafluoride circuit breaker comprises a fixed and movable contact shield in a chamber called the arc interruption chamber.
The chamber is then connected to the gas reservoir of the circuit breaker. When the breaker senses fault, it releases the high-pressure gas from the reservoir to cool and extinguish the arc.
The gas system comprises a low and high-pressure chamber. It also has a low-pressure alarm and a switch that signals when the gas pressure becomes low.
A low-pressure gas can decrease the dielectric and extinguishing power of the breaker.
And since the SF6 gas is expensive, it is usually reformed after each operation.
A common problem with SF6 switchgear is gas leakage. When leakage occurs, the gas pressure decreases. This reduces the quenching ability of the circuit breaker.
To avoid this, always check your breakers often. You can check that via the pointer of the gas gauge.
If the gauge shows a decrease in gas pressure, subject it to a gas leakage test.
Gas leakage tests require soap water or an SF6 detector. If there is a leakage, consult an engineer for rectification.
You can also fill up the circuit breaker with the new SF6 gas to balance the pressure loss.
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