What is an SF6 Circuit Breaker?

An 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 gases have strong electronegativity and insulating properties, making them a better option than air and oil circuit breakers.

Why SF6 gas is used in circuit breakers

SF6 gases are used in circuit breakers because they have high dielectric strength and high cooling effects.

It also has a unique property of quick recombination after arc breaking and is usually a good transfer of heat energy.

That is why it is preferably used in circuit breakers to quench the arc more than air.

Types of sulfur hexafluoride circuit breaker

There are three types of SF6 CB, they include:

  • Non puffer type.
  • Single pressure puffer type
  • Double pressure puffer type

Non puffer type sf6 breaker

This is the first type of SF6 breaker produced without a puffer cylinder. The breaker comprises a gas chamber and an interrupter unit connected by a valve.

The valve is connected to the movement of the breakers’ contacts. Each time the contacts separate, the valve opens to release the blast of SF6 gas, which serves as an extinguisher.

Single pressure puffer type breaker

This type of SF6 breaker usually comes with two fixed contacts and a puffer cylinder.

The movable puffer cylinder filled with SF6 gas acts as a bridge between the two fixed contacts, and is also responsible for making and breaking the circuit when there is a power surge.

Double Pressure Puffer Type SF6 Circuit Breaker

This type of SF6 circuit breaker also uses compressed SF6 gas in a cylinder to quench the arc. But unlike the single pressure puffer type that has two fixed contacts, this one has a fixed and a moving contacts.

However, this type of breaker is no longer in use.

Working principles

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 and disadvantages of SF6 breakers

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.

Disadvantages

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

Interrupter unit

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.

Gas system

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.

Maintenance

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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