SF6 is essential for the transmission and distribution equipment and switchgear, because of its excellent dielectric, arc quenching, heat transfer, and chemical recombination properties.
This has been successfully proven since the 1960s for high-voltage equipment and since the beginning of the 1980s for medium-voltage equipment. The size of equipment and switchgear has been significantly reduced since the beginning of the SF6 technology. Nowadays, SF6 technology is even more important to bring bulk power at a high-voltage level closer to the consumers as for megacities. SF6 circuit breaker uses sulfur Hexafluoride (SF6) gas is used to extinguish the arc. There are some benefits and drawbacks of SF6 circuit breakers. Let’s list them one by one.
Advantages of SF6 Circuit Breakers
Some benefits of SF6 circuit breakers are listed below:
1. Enclosed construction
Despite all the technical advantages of the SF6 technology, SF6 is a potent greenhouse gas that is covered by the Kyoto Protocol. Therefore, SF6 must be managed within a closed cycle, avoiding any deliberate release into the atmosphere.
During the last 20 years, as a consequence, significant effort has been undertaken to reduce SF6 emissions. The focus for manufacturers and asset owners was on finding ways of increasing the tightness of equipment and reducing handling losses.
SF6 breakers are enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazard exists. There is no risk of fire as SF6 is non-inflammable. The closed gas enclosure keeps the interior dry so that there is no moisture problem.
2. Unique physical properties
Sulfur hexafluoride (SF6) is a synthetic compound showing a unique combination of physical properties: High dielectric strength, high arc quenching capabilities, and high heat transfer performance, which is about twice of air. SF6 circuit breaker uses Sulphur hexafluoride gas in its construction. The contacts of the breaker are opened in a high-pressure flow of SF6 gas and the arc is struck between them. This gas has a great extinguishing property. Many manufacturers prefer sulfur hexafluoride gas over oil and air. Sulfur hexafluoride has a high electronegativity, perfect for insulation.
SF6 is an odorless, tasteless, colorless, non-toxic, non-flammable, very stable, and inert gas at room temperature. Due to spontaneous recombination, the molecule is extremely stable even at very high temperatures in the arc plasma during the switching operation. The volumetric specific heat of SF6 is 3.7 times that of air. This has important consequences for reducing the effects of heating within electrical equipment.
SF6 technology is versatile. SF6 equipment is air-insulated substations, gas-insulated substations, and mixed technology solutions, all for application in indoor and outdoor environments in different layouts. It is used in both medium and high-voltage electrical systems. Several types of SF6 circuit breakers are designed by various manufacturers in the world in recent years which are rated for voltages from 3.6 to 760 kV.
5. Compact size
SF6 breaker has high compactness. Reduced dimensions save space in the switchgear room. Vacuum and Air Insulated Switchgear technologies are already available on the market as an alternative to SF6 in medium voltage, up to 52 kV. Given the fact that the SF6-free equipment needs much more space than SF6 equipment, users can select the technology which better fits their specific needs and evaluation criteria. The large difference in size, weight, and high compactness of equipment assures that SF6 technology will continue to play a fundamental role even in medium voltage, up to 52 kV.
6. High personnel safety
SF6 breakers are enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazard exists. Besides, there is no risk of fire as SF6 is non-inflammable. A closed gas enclosure keeps the interior dry so that there is no moisture problem.
7. High reliability and long lifetime
The sealed design of the SF6 circuit breaker avoids dust, rain, salt, ice penetration, and humidity condensation. The pressure of the SF6 gas is generally maintained above atmospheric; so good sealing of the gas chambers is vitally important.
8. Short installation time
Pre-assembled equipment for the SF6 breaker results in a shorter installation time. The application for mobile solutions is extremely easy. The pre-assembled equipment can be easily transported and installed on-site, reducing the time and effort required for assembly. Additionally, mobile solutions make it easy to transport and set up the equipment in various locations, making it a versatile option for different applications.
9. Less maintenance
No periodical need for SF6 replacement. Low maintenance cost, light foundation requirements, and minimum auxiliary equipment. Maintenance-free operation of the pole unit for 15-20 years under normal conditions.
10. Short arcing time
Due to the superior arc quenching property of SF6, such breakers have a very short arcing time. The conducting electrons in front of the arc are captured by the gas to form relatively immobile negative ions. The loss of these conducting electrons develops enough strength of insulation which will extinguish the arc.
The dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents. This is because SF6 gas has a higher ability to withstand electric fields, allowing it to interrupt currents at a higher voltage level without breaking down. This means that SF6 gas-insulated circuit breakers can handle higher currents and provide better protection for electrical systems.
12. Noiseless operation
SF6 breaker gives noiseless operation due to its closed gas circuit. SF6 is a highly stable, non-toxic, and non-flammable gas that is used as an insulating and arc-quenching medium in high-voltage circuit breakers. The closed gas circuit eliminates the need for external cooling and ventilation, which in turn eliminates the need for fans or other mechanical equipment that could create noise. Additionally, SF6 has excellent insulation properties which help to reduce electrical noise and arcing
13. Developing technology
In the MV field, when compact switchgear is required, SF6 is the single proposed solution. For loose components, however, SF6 technology is sharing the market with air for load break switches. But air market share is rapidly decreasing to the benefit of SF6, and with vacuum for CBs. Vacuum and SF6 CBs are modern solutions and will continue to expand due to the decreasing influence of minimum oil technology.
Disadvantages of SF6 Circuit Breakers
Some drawbacks of SF6 circuit breakers are listed below:
1. High cost
SF6 breakers are costly due to the high cost of SF6. SF6 is a highly specialized gas that is used as an insulating and arc-quenching medium in high-voltage electrical equipment. Because of its specialized nature and the fact that it is not readily available, the cost of SF6 is relatively high. This, in turn, makes SF6 breakers more expensive than other types of breakers.
2. Has to be reconditioned
SF6 gas has to be reconditioned after every operation of the breaker. This is because SF6 gas is a highly stable and non-reactive compound, but it can become contaminated over time due to exposure to environmental factors, such as moisture and dust. As a result, reconditioning is necessary to ensure that the gas remains pure and effective in providing insulation and arc-quenching properties. Additional equipment, such as a gas handling and recovery system, may be required to recondition the SF6 gas.
3. Needs special protection equipment
SF6 is non-toxic. However, it does not support life as it is not oxygen. As the gas is much heavier than air, under conditions of insufficient mixing with air the gas tends to accumulate at low levels. Areas below ground level, poorly ventilated, or unventilated areas may contain a high level of SF6 concentration. Personnel must be aware of the danger of asphyxiation in such places. In case of incorrect gas handling or uncertainty, gas compartments may contain a residual amount of SF6 and should not be entered without adequate ventilation and personal protection equipment.
4. Negative environmental impacts
During the last 50 years, SF6 technology has had a huge development because of the excellent insulation and arc quenching properties of the gas with no comparable equivalent at the moment. This has enabled the design and manufacturing of extremely compact equipment with optimized usage of material, high operational reliability, and safety, minimized fire load, and high availability. Despite all these advantages, SF6 is a potent greenhouse gas; which is covered by the Kyoto Protocol. As a consequence of that, the Electrical Industry has made a huge effort in developing and implementing a responsible application of SF6 in transmission and distribution networks.