Introduction to Sulfur Hexafluoride Circuit Breaker (GCB)

  The sulfur hexafluoride circuit breaker, abbreviated as GCB, is a type of circuit breaker that uses sulfur hexafluoride gas as both the arc extinguishing medium and the insulating medium. The use of sulfur hexafluoride as an arc extinguishing medium in circuit breakers began in the early 1950s. Due to the excellent properties of this gas, the single-break of this type of circuit breaker significantly surpasses compressed air and low-oil circuit breakers in terms of voltage and current parameters, and does not require high gas pressure or a large number of series breaks. By the 1960s and 1970s, sulfur hexafluoride circuit breakers were widely used in ultra-high voltage and high-capacity power systems. By the early 1980s, 363 kV single-break, 550 kV double-break, and rated breaking currents of 80 and 100 kA sulfur hexafluoride circuit breakers had been successfully developed. Today, they are extensively used in the power sector and have become an indispensable piece of equipment.

  Compared to traditional circuit breakers, sulfur hexafluoride circuit breakers offer the following significant advantages.

  1. They can fully utilize the arc-blowing effect of the gas flow, with a small arc extinguishing chamber volume, simple structure, large breaking current, short arcing time, no reignition or restrike when breaking capacitive or inductive currents, and low overvoltage.

  2. Long electrical life. They can continuously break 19 times at full capacity of 50kA, with a cumulative breaking current of up to 4200kA, and are suitable for frequent operations.

  3. High insulation level. At a gas pressure of 0.3MPa, sulfur hexafluoride gas passes various insulation tests with a large margin. After a cumulative breaking current of 3000kA, each break can still withstand a power frequency voltage of 250kV for 1 minute at 0.3MPa. Even when the sulfur hexafluoride gas pressure is reduced to zero, it can withstand a power frequency voltage of 166.4kV for 5 minutes.

  4. Excellent sealing performance. The sulfur hexafluoride gas has low moisture content. The arc extinguishing chamber, resistor, and support are divided into independent gas compartments, which do not need to be opened during on-site installation. After installation, they are connected with automatic joints, making installation and maintenance convenient and preventing dirt and moisture from entering the circuit breaker.

  5. Comprehensive self-protection and monitoring systems. The signal cylinder in the circuit breaker enables self-protection. The density relay monitors sulfur hexafluoride gas leakage, and the pressure switch and safety valve monitor the hydraulic system pressure, ensuring the safety of the hydraulic system. Additionally, the hydraulic mechanism uses a valve system that prevents "slow separation under pressure loss," and the mechanism can be locked on the body to ensure operational safety.

  Although sulfur hexafluoride circuit breakers have many advantages, they also have several drawbacks due to the inherent limitations of sulfur hexafluoride. Under high temperatures during discharge, sulfur hexafluoride gas decomposes into corrosive gases, which severely corrode aluminum alloys and damage phenolic resin laminated materials and porcelain insulation. When sulfur hexafluoride is mixed with nitrogen, if the sulfur hexafluoride content exceeds 20% to 30%, its insulation strength is the same as that of pure sulfur hexafluoride, but the corrosiveness is significantly reduced. Therefore, sulfur hexafluoride is often mixed with nitrogen. In sulfur hexafluoride circuit breakers, the moisture content of the sulfur hexafluoride gas must be strictly controlled to not exceed the standard. Water reacts with sulfur tetrafluoride, a decomposition product of the arc, to form hydrofluoric acid, which corrodes materials. When the moisture content reaches saturation, it can condense on the surface of insulating components, significantly reducing insulation strength and even causing surface discharge. Operational experience and the above analysis indicate that due to the small insulation structure volume of sulfur hexafluoride circuit breakers, if the moisture content of the sulfur hexafluoride gas is high, the insulation level will greatly decrease, and the contact resistance will sharply increase, leading to damage or explosion during operation. Therefore, manufacturers and operational departments require strict sealing processes and stipulate that the moisture content of sulfur hexafluoride gas must not exceed the standard. The Chinese standard specifies that the moisture content of sulfur hexafluoride gas should be less than 300ppm (by volume).

  In addition, a small portion of sulfur hexafluoride gas decomposes under the action of an arc, generating some toxic low-fluoride compounds that affect human health and corrode and degrade metal components. Therefore, sulfur hexafluoride circuit breakers are generally equipped with adsorption devices, with adsorbents such as activated alumina, activated carbon, and molecular sieves. The adsorption device can completely adsorb the toxic substances generated by the decomposition of sulfur tetrafluoride gas under the high temperature of the arc.

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