An type circuit breaker that uses pressurized SF6 gas to extinguish the arc is called an SF6 circuit breaker. SF6 gas has exceptional dielectric, arc quenching, chemical, and other physical properties, proving its superiority over other arc quenching media, such as oil or air. The SF6 circuit breaker is mainly divided into three types.
- Non-puffer piston circuit breaker
- Single-puffer piston circuit breaker.
- Double-puffer piston circuit breaker.
Circuit breakers require an insulating medium to prevent electrical arcing. Air and oil were used for this purpose in the past, but their arc extinguishing properties were relatively slow. High-voltage circuit breakers, however, require quick arc extinction properties for swift voltage recovery. This is why SF6 circuit breakers are preferred over oil or air circuit breakers. They possess excellent properties for quick arc extinction, making them ideal for high voltages up to 760 kV.
Properties of Sulphur hexafluoride Circuit Breaker
Sulfur hexafluoride is highly desirable for electrical equipment and power systems due to its excellent insulating and arc-quenching properties.
- It is a colorless, odorless, non-toxic, and non-inflammable gas.
- SF6 gas is a stable and inert gas that is five times denser than air.
- It has a higher thermal conductivity than air, which helps cool current-carrying parts more efficiently.
- SF6 gas is highly electronegative, which facilitates the removal of free electrons by forming negative ions upon discharge.
- It has a unique property of quick recombination after removal of the energizing spark, making it 100 times more efficient than an arc-quenching medium.
- Its dielectric strength is 2.5 times that of air. The gas dielectric strength increases at high pressure, although it is 30% less than dielectric oil.
- Moisture can damage SF6 circuit breakers as it can create hydrogen fluoride that attacks the parts of the circuit breakers.
Construction of SF6 Circuit Breakers
SF6 circuit breakers can be divided into two main components: the interrupter unit and the gas system.
Interrupter Unit – The unit is connected to the SF6 (sulfur hexafluoride) gas reservoir, a non-flammable and non-toxic gas with excellent electrical insulation properties. The unit has slide vents in the moving contacts that enable high-pressure gas to enter the main tank and extinguish the electrical arc that forms during an interruption event. This gas also cools the contacts and prevents them from overheating. Overall, the Interrupter Unit is critical in ensuring high-voltage electrical networks’ safe and efficient operation.
Gas System – SF6 circuit breakers are designed with a highly effective closed-circuit gas system. This system has two separate chambers, a low-pressure and a high-pressure chamber. The use of SF6 gas in circuit breakers is quite expensive, which is why it is reclaimed after every operation. To ensure optimal performance, the system has been designed with a low-pressure alarm equipped with warning switches. This alarm is incredibly useful as it alerts the user when the gas pressure drops too low. If this occurs, the dielectric strength of gases decreases, and the arc-quenching ability of breakers is impaired. Therefore, the warning alarm is essential for maintaining the breakers’ effectiveness and longevity.
Working Principle of SF6 Circuit Breaker
Under normal operating conditions, the contacts of the breaker are securely closed. However, when a fault occurs in the system, the contacts are pulled apart, creating an electric arc between them. This sudden displacement of the moving contacts is perfectly synchronized with a valve, which allows high-pressure SF6 gas to enter the arc-interrupting chamber at a pressure of around 16kg/cm^2.
The SF6 gas is crucial in extinguishing the electric arc by absorbing free electrons and creating ions that do not act as charge carriers. As a result, the dielectric strength of the gas is significantly increased, and the arc is quickly put out. After serving its purpose, the gas pressure is reduced to approximately 3kg/cm^2 and is stored in a low-pressure reservoir for re-use.
During opening operations, a piston attached to the moving contacts uses puffer piston pressure to generate arc quenching pressure.
Advantage of SF6 circuit breaker
SF6 circuit breakers provide numerous advantages over traditional breakers.
- SF6 gas is an excellent choice for circuit breakers due to its insulating and arc-extinguishing properties.
- The gas is safe to use because it does not catch fire and is stable. The products resulting from its decomposition are also safe from causing explosions.
- The high dielectric strength of SF6 significantly reduces the distance between conductors.
- The performance of this device is not affected by any changes in atmospheric conditions.
- It operates quietly and prevents overvoltage by extinguishing the arc at a natural current zero.
- During arcing, no carbon particles are formed, and there is no reduction in dielectric strength.
- It requires very little maintenance and does not require an expensive compressed air system.”
- SF6 is commonly used for clearing short-line faults, switching, and opening transmission lines without issues
Disadvantages of SF6 circuit breakers
- SF6 gas can be dangerous in case of a leak from the breaker tank. This is because SF6 gas is heavier than air, and when it escapes, it settles in the surrounding area. This can cause suffocation for those who are present in the vicinity. It is essential to take necessary precautions and safety measures to prevent such incidents.
- Moisture is a serious threat to the SF6 breaker tank, and its ingress can cause a wide range of issues that may lead to multiple failures. When moisture enters the breaker tank, it can react with the SF6 gas, forming various corrosive byproducts that can damage the breaker’s components. Moreover, moisture can also create electrical arcs and discharge paths, which may cause a short circuit, leading to a catastrophic failure of the breaker.
- It is essential to regularly clean the internal components in a clean and dry environment.
- Gas transportation and maintenance require specialized facilities to ensure its quality.
Maintenance of SF6 Circuit Breaker
- Checks for gas leaks to ensure safety
- Check all the mechanical components of the circuit breaker
- To ensure smooth movement, it is recommended to apply lubrication to the parts that move
- Test functionality of the control circuits
- Replace the components that are worn out.
Applications
SF6 circuit breakers have numerous applications in the electrical industry.
- Power Transmission and Distribution: SF6 circuit breakers are widely used in high-voltage power systems to protect important equipment like transformers and circuit lines. These circuit breakers help to prevent damage and keep everything running smoothly.
- Industrial Plants: Industrial facilities use circuit breakers to protect against short circuits and overloads.
- Railways: SF6 circuit breakers are used in railway systems to ensure a reliable power supply and safe power distribution. These circuit breakers help prevent power outages and protect against electrical accidents by interrupting the flow of electricity when necessary. They are an essential part of railway electrification systems.
- Substations: SF6 circuit breakers are utilized in power substations to regulate and protect the electrical grid.
- Power Generation: SF6 circuit breakers are reliable for safeguarding generators in power generation facilities.
- Renewable Energy: Renewable energy systems, such as wind farms and solar power plants, use them.
Conclusion:
In modern electrical systems, SF6 circuit breakers are essential for reliable operation and safety. They possess exceptional insulating properties and high arc-quenching capability, making them indispensable in high-voltage applications.
