What is a sulfur hexafluoride circuit breaker used for? How does it work?

Circuit breakers that use SF6 gas with excellent arc extinguishing performance and insulation properties as the arc extinguishing medium are called SF6 circuit breakers and are widely used in power systems. It is suitable for frequent operations and occasions requiring high-speed breaking. In my country, it is recommended to use SF6 circuit breakers for 7.2-40.5, especially for almost all 126KV and above. But not suitable for high altitude areas.

The SF6 circuit breaker can not only cut off and connect high-voltage lines and various no-load and load currents during normal operation of the system, but also when the system fails, through the action of the relay protection device, it can automatically, quickly and reliably cut off various overload currents and short-circuit currents to prevent the occurrence and expansion of the scope of the accident.

A circuit breaker that uses sulfur hexafluoride (SF6) gas as arc extinguishing medium and insulating medium, referred to as SF6 circuit breaker. It belongs to the same type of air-blown circuit breaker as the air circuit breaker, but the difference is:

①The working air pressure is low;

②During the arc blowing process, the gas is not discharged to the atmosphere, but is recycled in a closed system. The use of sulfur hexafluoride as arc extinguishing medium in circuit breakers began in the early 1950s. Due to the excellent characteristics of this gas, the voltage and current parameters of this single-break circuit breaker are much higher than those of compressed air circuit breakers and oil-less circuit breakers, and do not require high air pressure and a considerable number of series breaks. In the 1960s and 1970s, SF6 circuit breakers have been widely used in ultra-high voltage and large-capacity power systems. In the early 1980s, 363 kV single-break, 550 kV double-break and SF6 circuit breakers with rated breaking currents of 80 and 100 kA were successfully developed.

The SF6 circuit breaker uses a sulfur hexafluoride density relay to monitor changes in gas pressure. When the pressure of sulfur hexafluoride gas drops to the first alarm value, the density relay acts and reports the gas supply pressure signal. When the pressure of sulfur hexafluoride gas drops to the second alarm value, the density relay operates and reports the locking pressure signal. At the same time, the tripping and closing circuit of the switch is disconnected to realize opening and closing locking.

Below 150°C, SF6 has good chemical inertness and does not interact chemically with metals, plastics and other materials commonly used in circuit breakers. When decomposed into various components under the high temperature caused by a high-power arc, they will be re-synthesized within a very short time after the arc is extinguished. There is no carbon element and no air in SF6, which can avoid contact oxidation. The dielectric strength of SF6 is very high and increases with increasing pressure. At 1 atmosphere, the dielectric strength of SF6 is approximately 2 to 3 times that of air. When the absolute pressure is 3 atmospheres, the dielectric strength of SF6 can reach or exceed that of commonly used insulating oils. SF6 has good arc extinguishing performance. In a simple switching arc extinguishing chamber, its arc extinguishing ability is 100 times greater than that of air. In SF6, when the arc current is close to zero, there is a very high temperature only on the center of the arc column with a small diameter, and it is surrounded by a non-conductive layer. In this way, the arc gap dielectric strength will recover quickly after the current crosses zero.

SF6 circuit breakers have two structural layout forms. One is the porcelain column structure. The circuit breaker consists of three independent single phases and a hydraulic and electrical control cabinet. Each phase is composed of four arc extinguishing chambers (fractures) with two pillar porcelain sleeves connected in series. The arc extinguishing chamber and the pillar porcelain sleeve are filled with SF6 gas at rated pressure. The second is the tank structure. The circuit breaker adopts a bidirectional longitudinal blowing arc extinguishing chamber. When opening, the cylinder and moving contact are driven to move through the arm box transmission mechanism. 

SF6 molecules and free electrons have very good mixing properties. When electrons and SF6 molecules come into contact, they are almost 100% mixed to form heavy negative ions. This performance is of great use value for the deionization and arc extinguishing of the remaining arc columns. That is, SF6 has very good electronegative properties, and its molecules can quickly capture free electrons to form negative ions. The conductive effect of these negative ions is very slow, thus accelerating the recovery rate of the arc gap dielectric strength, so it has good arc extinguishing performance. Under the air pressure of 1.01×105Pa, the arc extinguishing performance of SF6 is 100 times that of air, and it does not deteriorate after arc extinguishing and can be reused. The excellent insulation and arc extinguishing properties of SF6 gas give SF6 circuit breakers the following advantages: strong breaking capacity, high fracture voltage, high number of consecutive breaks, suitable for frequent operations, low noise, no fire hazard, low mechanical and electrical wear, etc. It is a "no maintenance" circuit breaker with excellent performance.

With the rapid development of the national economy, the demand for electricity in all walks of life has also increased sharply. The load on the power system has increased day by day, and the requirements for power supply reliability have gradually increased. The switches in substations have gradually been replaced by oil switches and new SF6 circuit breakers. SF6 circuit breakers have been widely used in power systems.

1. Introduction to the working of sulfur hexafluoride circuit breaker:

Sulfur hexafluoride circuit breaker is a new type of circuit breaker that uses sulfur hexafluoride gas as insulating medium and arc extinguishing medium. Sulfur hexafluoride gas is a colorless, odorless, non-toxic, non-flammable inert gas. It has high electrical strength and good arc extinguishing performance, and its dielectric strength far exceeds that of traditional insulating gases. Therefore, its use in electrical equipment can reduce equipment size, eliminate fire hazards, and improve the reliability and safety of power systems.

2. Structure of sulfur hexafluoride circuit breaker:

Sulfur hexafluoride circuit breaker: It consists of three parts: body structure (adopting a three-box structure), operating mechanism, and arc extinguishing device. It has the advantages of simple structure, small size, light weight, large current breaking capacity, rapid arc extinguishing, high number of interruptions allowed, and long maintenance cycle. It is the direction for the promotion and application of power systems in the future.

3. Working Principle of Sulfur Hexafluoride Circuit Breakers:

Sulfur hexafluoride circuit breakers are typically filled with sulfur hexafluoride gas at 3-5 atmospheres as internal insulation. During the circuit breaker's opening process, the moving contact drives a piston to compress the gas, creating an airflow to extinguish the electric arc.

The basic structure of the arc-extinguishing chamber of a sulfur hexafluoride circuit breaker consists of a moving contact, an insulating nozzle, and a compressed gas piston connected together, driven by an operating mechanism via an insulating connecting rod. The stationary contact is tubular, while the moving contact is a socket type. Both the moving and stationary contacts are inlaid with copper-tungsten alloy at their ends. The insulating nozzle is made of high-temperature and corrosion-resistant polytetrafluoroethylene (PTFE).

When the circuit breaker opens, the moving contact and piston move together to the right. After the moving and stationary contacts separate, an electric arc is generated. As the piston moves rapidly to the right, the gas on the right side is compressed, generating an airflow that passes through the nozzle and longitudinally blows the arc, extinguishing it. Afterward, the gas in the arc-extinguishing chamber is discharged into the switch body through the inner hole of the stationary contact and the cooler.

When the circuit breaker closes, the operating mechanism moves the moving contact, nozzle, and piston to the left, causing the stationary contact to insert into the moving contact seat, ensuring good electrical contact between the moving and stationary contacts, thus achieving the closing purpose.

The sulfur hexafluoride (SF6) switch uses a SF6 density relay to monitor changes in gas pressure. When the SF6 gas pressure drops to the first alarm value, the density relay activates, reporting a signal indicating the replenishment pressure. When the SF6 gas pressure drops to the second alarm value, the density relay activates, reporting a signal indicating the lockout pressure, and simultaneously disconnects the switch's trip and close circuits, achieving tripping and closing lockout.