Vacuum circuit breakers and SF6 (sulfur hexafluoride) circuit breakers are the two most widely used circuit breaker technologies in modern power systems. They are not mutually exclusive, but rather optimal solutions for different voltage levels and application scenarios.
◆Vacuum Circuit Breaker (VCB): Specializes in medium voltage applications (typically 10kV to 35kV, up to 72.5kV), using vacuum as the arc-extinguishing medium.
◆SF6 Circuit Breaker: Specializes in high voltage, ultra-high voltage, and ultra-high voltage applications (typically 72.5kV and above, up to 1100kV), using SF6 gas as the arc-extinguishing medium.
1. Core Working Principle
Vacuum Circuit Breaker (VCB)
Arc-extinguishing medium: High vacuum (typically above 10^-4 Pa). Vacuum has extremely high dielectric strength.
Arc-extinguishing principle: When the moving and stationary contacts separate in a vacuum, the current vaporizes the contact material, forming an arc composed of metal vapor. When the current passes through zero, the metal vapor rapidly diffuses and condenses on the shielding cover. The dielectric strength within the vacuum chamber quickly recovers, and the arc is extinguished.
SF6 Circuit Breaker
Arc extinguishing medium: SF6 (sulfur hexafluoride) gas. It is a colorless, odorless, non-toxic, and non-flammable inert gas with excellent electrical insulation properties and extremely strong arc extinguishing capability.
Arc extinguishing principle: It primarily utilizes the "gas blowing" principle. When interrupting the current, a compression cylinder compresses the SF6 gas, generating a high-pressure gas flow. When the current crosses zero, the high-speed SF6 gas blows toward the arc, removing heat while rapidly filling the arc gap with fresh, high-insulation SF6 gas, thereby extinguishing the arc.
2. Comparison of main features
| Characteristics | Vacuum Circuit Breaker (VCB) | SF6 Circuit Breaker |
|---|---|---|
| Arc extinguishing medium | Vacuum | SF6 gas |
| Operating voltage level | Mainly medium voltage (1kV - 72.5kV) | Mainly high voltage and ultra-high voltage (72.5kV - 1100kV) |
| Dielectric strength | High (vacuum is an excellent insulator) | Extremely high (SF6 has a dielectric strength 2-3 times that of air) |
| Arc extinguishing capacity |
Extremely strong, well-suited for interrupting small to medium currents and capacitive currents. |
Extremely strong, particularly adept at interrupting high currents and short-circuit faults. |
| Arc energy | Very low | Lower |
| Interrupting times |
Very high, with long electrical life and extended maintenance intervals. |
High, but generally lower than comparable VCBs. |
| Dimensions and size |
Small, compact design allows for easy integration into switchgear. |
Large, especially high-voltage products, require extensive gas pressure systems. |
| Environmental performance |
Good: The vacuum itself is pollution-free. |
Bad: SF6 is a potent greenhouse gas (GWP 23,900 times that of CO2), presents a risk of leakage, and is subject to strict regulation. |
| Maintenance requirements |
Low, long maintenance-free intervals |
High, requiring regular inspection of gas pressure, purity, and leaks |
| Operational overvoltage | Relatively high, prone to current interception, and may generate operational overvoltage. | Lower, with a smooth breaking process |
| Cost |
Lower cost in medium- and low-pressure applications |
High cost-effectiveness in high-pressure applications, though gas treatment and recovery equipment incur additional expenses |
| Main applications | 10kV, 35kV distribution network, industrial and mining enterprises, building power distribution, generator output protection | 110kV and above transmission networks, substations, power plant outlets, GIS switchgear assemblies |
3. Summary of advantages and disadvantages
Vacuum Circuit Breaker (VCB)
Advantages:
Environmentally friendly and pollution-free.
Interrupter is maintenance-free and has a long service life.
Compact, lightweight, and easy to install and integrate.
Fast operating speed and short arcing time.
Excellent fire and explosion resistance.
Disadvantages:
It is easy to generate cut-off overvoltage and needs to be used with a lightning arrester or RC absorption circuit.
Vacuum arc extinguishing chambers are non-serviceable; once leakage occurs, the entire unit must be replaced.
The limited capability to interrupt exceptionally high currents restricts its application at the highest voltage levels.
SF6 Circuit Breakers
Advantages:
Extremely high interrupting capacity, ideal for high-voltage and high-current applications.
Excellent breaking performance, low operating overvoltage.
Low noise, no fire risk.
Disadvantages:
Serious environmental concerns: SF6 is a greenhouse gas whose emissions are explicitly restricted under the Kyoto Protocol.
Complex structure, requiring a sophisticated gas monitoring and pressure maintenance system.
High production process requirements and high costs.
SF6 gas liquefies in low-temperature environments, affecting performance and requiring special design.
Conclusion
In the medium-voltage power distribution sector (35 kV and below), vacuum circuit breakers have become the undisputed king thanks to their environmental friendliness, compact size, and maintenance-free design.
In the high-voltage and ultra-high-voltage power transmission sector (72.5 kV and above), SF6 circuit breakers currently dominate due to their unparalleled breaking and insulation performance. However, they are facing significant environmental pressure and will inevitably be gradually replaced by environmentally friendly gas circuit breakers or series vacuum technology.
The choice of circuit breaker depends primarily on voltage level, breaking current, environmental requirements, and cost budget.
If you want to know more about circuit breakers, please contact us at:pannie@hdswitchgear.com.or add to Whatsapp/Wechat:+8618789455087.
