Analysis of the advantages and disadvantages of SF6 circuit breakers
2024-11-06 10:49:33 格雷诺（厦门）开关有限公司
SF6 circuit breaker is a circuit breaker that uses SF6 gas as the arc source and insulating medium. It belongs to the gas-blown circuit breaker like the air circuit breaker, but the difference is: ① the working pressure is lower; ② during the arc blowing process, the gas is not discharged to the atmosphere, but circulated in a closed system.
1 Advantages of SF6 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 has great use value for the deionization and arc extinguishing of the residual arc column. That is, SF6 has very good negative charge, and its molecules can quickly capture free electrons to form negative ions. The conductive effect of these negative ions is very slow, which accelerates the recovery rate of the arc gap dielectric strength, so it has good arc extinguishing performance. Under a 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 performance of SF6 gas makes SF6 circuit breakers have the following advantages: strong breaking capacity, high breaking voltage, more continuous breaking times, suitable for frequent operation, low noise, no fire hazard, low electromechanical wear, etc. It is an excellent "maintenance-free" circuit breaker. It is increasingly used in high voltage. Commonly used SF6 circuit breakers are LN1-35 and HB36.
2 Disadvantages of SF6 Pure SF6 gas is a good arc extinguishing medium. If it is used in low-voltage electrical appliances that are frequently operated, due to the arc action of frequent operation, metal vapor reacts with SF6 gas decomposition products and combines to form fine powders with good insulation (hydrofluoride, sulfur-based acid salts, etc.), which are deposited on the contact surface and severely corrode the contact material, thereby sharply increasing the contact resistance and making the sealed contacts filled with SF6 gas unable to work reliably. Therefore, it is not suitable to use SF6 as an arc extinguishing medium for low-voltage electrical appliances that are frequently operated.
Because SF6 gas will decompose corrosive gas at high temperature during discharge, it will have serious corrosive effect on aluminum alloy, and will also damage phenolic resin laminated materials and porcelain insulation. If SF6 and N2 are mixed, when the SF6 content exceeds 20% to 30%, its insulation strength is the same as that when fully filled with SF6, and the corrosiveness is greatly reduced. Therefore, SF6 is often mixed with N2. In SF6 circuit breakers, the water content of SF6 gas must be strictly regulated and cannot exceed the standard. Water will produce hydrofluoric acid (H2O+SF4→SOF2+2HF) with SF4 in the arc decomposition product and corrode the material. When the moisture content reaches saturation, it will also condense on the surface of the insulating parts, significantly reducing the insulation strength and even causing surface discharge. The operating experience and the above analysis show that: SF6 circuit breakers have a small insulation structure. If the water content of SF6 gas is high, the insulation level will be greatly reduced, the contact resistance will increase sharply, and damage or explosion accidents will easily occur during operation. Therefore, all manufacturers and operating departments require strict sealing processes and stipulate that the water content of SF6 gas must not exceed the standard. my country's standard is that the water content of SF6 gas should be less than 300ppm (volume ratio). SF6 circuit breakers use SF6 gas as the arc extinguishing medium. Under normal circumstances, SF6 is a non-flammable, odorless, non-toxic inert gas with a density of about 2 times that of air. However, under the action of the arc, a small part of SF6 gas will be decomposed to generate some toxic low-fluoride compounds, such as SOF2, SF4, SOF4 and SO2F2, which have an impact on human health and also corrode and deteriorate metal parts. Therefore, in SF6 circuit breakers, adsorption devices are generally installed, and the adsorbents are activated alumina, activated carbon and molecular sieves. The adsorption device can completely adsorb the toxic substances generated by the decomposition of SF4 gas under the high temperature of the arc.
3 Maintenance of SF6 circuit breakers
(1) Before maintenance of SF6 circuit breakers, the circuit breaker should be opened, the operating power should be cut off, the energy of the operating mechanism should be released, and the gas in the circuit breaker should be recovered by the SF6 gas recovery device. The residual gas must be pumped out by a vacuum pump to make the vacuum degree in the circuit breaker lower than 133.33Pa.
(2) The circuit breaker is filled with high-purity nitrogen (purity above 99.99%) at a suitable pressure, and then vented. Repeat this process twice to minimize the residual SF6 gas and its organisms inside.
(3) During dismantling and maintenance, the relative humidity of the environment should not be greater than 80%. The workplace should be dry and clean, and ventilation should be strengthened. Maintenance personnel should wear nylon work clothes and hats, wear gas masks and goggles, and use latex film gloves. Smoking is strictly prohibited in the workplace. Hands and faces should be washed during work breaks, and personal hygiene should be emphasized.
(4) When white powdery decomposition products are found in the container during the disassembly of the circuit breaker, they should be cleaned with vacuum cleaner or soft toilet paper, and collected in a sealed container and buried deep to prevent spread. Do not use compressed air to blow or other methods that cause the powder to fly to remove it.
(5) The metal parts of the circuit breaker can be cleaned with detergent or gasoline. Insulating parts should be cleaned with anhydrous alcohol or acetone. Seals cannot be cleaned with gasoline or chloroform. Generally, all should be replaced with new ones.
(6) Parts and seals in contact with SF6 gas can be coated with a thin layer of HL#8 or HL#10 polytetrafluoroethylene grease. The flange surface outside the seal should be coated with neutral vaseline or #2 antifreeze grease. For imported foreign products, appropriate grease should be selected according to the requirements of the instruction manual. Flange gaps and flange connection screws should be sealed with 703 sealant.
(7) The adsorbent in the circuit breaker container should be replaced during disassembly and maintenance, and the replaced adsorbent should be properly handled to prevent contamination from spreading. The newly replaced adsorbent should be dried in an oven at 200-300℃ for more than 12 hours, and then immediately loaded into the circuit breaker after natural cooling. The exposure time in the air should be minimized. The amount of adsorbent loaded is 1/10 of the mass of SF6 gas filled into the circuit breaker.
(8) If the circuit breaker is disassembled and not reassembled in time, the insulating parts should be placed in an oven or drying room to keep them dry.
