- SF6 Circuit Breaker Definition: An SF6 circuit breaker is defined as a type of circuit breaker that uses sulfur hexafluoride gas to quench electrical arcs.
- Dielectric Strength: SF6 gas has high dielectric strength, making it highly effective in insulating and quenching arcs.
- Types of SF6 Circuit Breakers: These breakers are available in single interrupter (up to 245 kV), two interrupter (up to 420 kV), and four interrupter (up to 800 kV) types.
- Environmental Impact: SF6 gas is a greenhouse gas, and safety regulations are necessary to limit its release into the atmosphere.
- Modern Operation: Modern SF6 circuit breakers use a puffer mechanism where the arc energy generates pressure in the arcing chamber to quench the arc efficiently.
What is an SF6 Circuit Breaker
SF6 gas has excellent insulating properties and high electronegativity, meaning it strongly attracts free electrons. It absorbs electrons to form negative ions, which are heavier and less mobile, leading to high dielectric strength.
Ions are obviousely much heavier than a free electron and therefore over all mobility of charged particles in SF6 gas is much less as compared to the other common gases.
We know that the mobility of charged particles 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.
SF6 gas not only has good dielectric strength but also quickly recombines after the arc is removed. Additionally, it has excellent heat transfer properties.
Due to its low gaseous viscosity, SF6 gas can efficiently transfer heat by convection. So due to 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 a complete range of medium voltage and high voltage electrical power system. These circuit breakers are available for the voltage ranges from 33KV to 800 KV and even more.
Disadvantages of SF6 Circuit Breaker
- The SF6 gas is identified as a greenhouse gas, safety regulation is being introduced in many countries in order to prevent its release into the atmosphere.
- Puffer type design of SF6 CB needs a high mechanical energy which is almost five times greater than that of oil circuit breaker.
Types of SF6 Circuit Breaker
There are mainly three types of SF6 circuit breakers depending upon the voltage level of the application:
- Single interrupter SF6 CB applied for up to 245 kV (220 kV) system.
- Two interrupter SF6 CB applied for up to 420 kV (400 kV) system.
- Four interrupter SF6 CB applied for up to 800 kV (715 kV) system.
Working of SF6 Circuit Breaker
The working of SF6 CB of first generation was quite simple and 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 in the breaker and collected to a relatively low-pressure reservoir and then it is pumped back to the high-pressure reservoir for reutilizing.
The working of SF6 circuit breaker is a little bit different in modern time. The innovation of puffer type design makes the 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 contacts 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 cannot change its position during the movement of the cylinder. As the piston is fixed and the cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.
When the breaker opens, the cylinder moves down against the fixed piston, reducing the volume inside and compressing the SF6 gas. The cylinder has side vents that are blocked by the upper fixed contact body when closed.
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 a fixed height, the volume of the cylinder increases which introduces low pressure inside the cylinder compared to the surrounding.
Because of this pressure difference, SF6 gas from the surroundings enters the cylinder. The higher pressure gas flows through the axial holes of the fixed contacts and into the cylinder, quenching the arc in the process.
