A circuit breaker that uses sulfur hexafluoride (SF6) gas with excellent arc-extinguishing and insulating properties as the arc-extinction medium is called an SF6 circuit breaker. These circuit breakers are widely used in power systems and are suitable for frequent operations and cases that require rapid opening and closing. In China, SF6 circuit breakers are recommended for voltages between 7.2 kV and 40.5 kV, and especially for voltages above 126 kV, almost exclusively SF6 circuit breakers are used. However, they are not suitable for high-altitude areas.
The SF6 circuit breaker can not only turn off and on high-voltage lines and various no-load and load currents during normal system operation, but also, in the event of a system fault, quickly and reliably disconnect various overload currents and short-circuit currents through the action of relay protection devices, preventing the occurrence and expansion of accidents.
An SF6 circuit breaker utilizes SF6 gas as both the arc-extinction and insulation medium. It belongs to the gas-blown circuit breaker category, differing from air circuit breakers in the following ways:
It operates at a lower gas pressure.
During the arc-extinction process, the gas does not vent to the atmosphere but is recycled within a closed system. The use of SF6 as an arc-extinguishing medium in circuit breakers began in the early 1950s. Due to its excellent properties, the parameters of this circuit breaker in terms of voltage and current are significantly higher than those of compressed air and less oil circuit breakers, and it does not require high gas pressure or many series circuit openings. In the 1960s and 1970s, SF6 circuit breakers were widely used in ultra-high voltage large-capacity power systems. In the early 1980s, 363 kV single-break, 550 kV double-break SF6 circuit breakers capable of interrupting currents of 80 and 100 kA were successfully developed.
SF6 circuit breakers utilize SF6 density relays to monitor gas pressure changes. When the SF6 gas pressure drops to the first alarm value, the density relay operates and signals for the supplementary gas pressure. When the pressure drops to the second alarm value, the density relay operates again, signaling a lockout pressure and simultaneously disconnecting the switching's trip and close circuits, achieving a lockout condition.
At temperatures below 150°C, SF6 exhibits good chemical inertness and does not react chemically with metals, plastics, or other materials commonly used in circuit breakers. Even when decomposed into various components due to the high temperatures from powerful arcs, it can quickly re-synthesize after the arc is extinguished. Since SF6 contains no carbon and is free of air, it avoids contact oxidation. SF6 has a very high dielectric strength, which increases with pressure. At 1 atmosphere of pressure, the dielectric strength of SF6 is approximately 2 to 3 times that of air. At an absolute pressure of 3 atmospheres, its dielectric strength can reach or exceed that of commonly used insulating oil. SF6 has excellent arc-extinguishing capabilities; in a simple arc-extinguishing chamber, its ability to extinguish arcs is 100 times greater than that of air. In SF6, when the arc current approaches zero, only a very small diameter of the arc column maintains a high temperature, while a non-conductive layer surrounds it. Therefore, once the current reaches zero, the dielectric strength of the arc gap recovers quickly.
There are two structural arrangements for SF6 circuit breakers: one is the porcelain column structure, which consists of three independent single-phase units and a hydraulic/electrical control cabinet. Each phase consists of two porcelain insulators with four arc-extinguishing chambers (breaks) connected in series. Both the arc-extinguishing chambers and the insulator columns are filled with SF6 gas at rated pressure. The second is the tank structure, which adopts a bidirectional longitudinal blowout arc chamber. During opening, a drive mechanism in the elbow box moves the cylinder and moving contacts.
SF6 molecules and free electrons have excellent mixing properties. When electrons contact SF6 molecules, they almost completely mix, forming heavy negative ions. This characteristic is highly valuable for the deionization and extinguishing of residual arcs. Specifically, SF6 possesses good electronegativity; its molecules can quickly capture free electrons to form negative ions. The conductive effect of these negative ions is very slow, thereby accelerating the recovery rate of the dielectric strength in the arc gap. Consequently, SF6 exhibits excellent arc-extinguishing performance. At a pressure of 1.01 × 105 Pa, SF6's arc-extinguishing capability is 100 times that of air, and it does not degrade after extinguishing, making it reusable. The excellent insulating and arc-extinguishing properties of SF6 result in several advantages for SF6 circuit breakers: strong interruption capability, higher break voltage ratings, more permitted interruption cycles, suitability for frequent operations, low noise, no fire hazard, and minimal electromechanical wear. Thus, it is an outstanding "maintenance-free" circuit breaker.
With the rapid development of the national economy, the demand for electricity in various industries has sharply increased, leading to a growing load on power systems and increasing reliability requirements for power supply. As a result, substations are gradually replacing oil circuit breakers with new SF6 circuit breakers, which have already been widely used in power systems.
1. Working Introduction of SF6 Circuit Breakers:
SF6 circuit breakers are new circuit breakers that use sulfur hexafluoride gas as both the insulation and arc-extinction medium. SF6 gas is a colorless, odorless, non-toxic, and non-flammable inert gas with high dielectric strength and good arc-extinguishing properties, far exceeding those of traditional insulating gases. Therefore, its application in electrical equipment can reduce device size, eliminate fire hazards, and improve the reliability and safety of power systems.
2. Structure of SF6 Circuit Breakers:
SF6 circuit breaker consists of three parts: the main body structure (adopting a three-box co-box structure), operating mechanism, and arc-extinguishing device. Its advantages include simple structure, compact size, light weight, large breaking capacity, rapid arc extinction, high allowable number of interruptions, and long maintenance cycles, positioning it as the direction for future application in power systems.
3. Working Principle of SF6 Circuit Breakers:
The SF6 circuit breaker is usually filled with SF6 gas at a pressure of 3 to 5 atmospheres as internal insulation. During the disconnection process, the moving contact drives the piston to pressurize the gas, creating airflow to blow out the electric arc.
The basic structure of the SF6 circuit breaker's arc-extinguishing chamber consists of the moving contact, insulating nozzle, and gas-pressurizing piston connected together and driven by an operating mechanism via an insulated connecting rod. The fixed contact is shaped like a tube, while the moving contact is socket-shaped, with copper-tungsten alloy embedded at both ends of the moving and fixed contacts. The insulating nozzle is made from high-temperature and corrosion-resistant polytetrafluoroethylene.
When the circuit breaker opens, the moving contact and piston move to the right. After the moving and fixed contacts are separated, an electric arc occurs, and as the piston moves rapidly to the right, the gas on the right side gets compressed, generating airflow through the nozzle to blow out the arc. Afterwards, the gas in the arc-extinguishing chamber is discharged into the switch body through the holes in the fixed contact and a cooler.
When closing the circuit breaker, the operating mechanism drives the moving contact, nozzle, and piston to move to the left, allowing the fixed contact to fit into the moving contact socket, ensuring good electrical contact for closing.
The SF6 switch utilizes SF6 density relays to monitor gas pressure changes. When the SF6 gas pressure drops to the first alarm value, the density relay operates and signals for the supplementary gas pressure. If the pressure drops to the second alarm value, the density relay operates again, sending a lockout signal and simultaneously disconnecting the trip and close circuits, achieving a locking action for opening and closing.
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