Comprehensive understanding of sulfur hexafluoride circuit breakers (SF6 circuit breakers)!

First, let's talk about SF6 circuit breakers. We know that the biggest difference between SF6 circuit breakers and vacuum circuit breakers is the difference in arc extinguishing media, so it is necessary to explain the properties of SF6 gas first. SF6 is a colorless, odorless, non-toxic, non-flammable inert gas with a molecular weight of 146.07, which is about 5 times that of air. SF6 has good chemical properties at room temperature. It only decomposes into S atoms and F atoms at high temperatures of several thousand degrees when the arc burns. After cooling, most of them are recombined into the original molecules; at high temperatures, a few SF6 atoms react with trace oxygen in the vapor of the contact metal material to form toxic low fluorides such as SOF2, SOF4, SF4, and SO2F2. In addition, SF6 has good insulation properties. At the same air pressure, its insulation strength is 2.5~3 times that of air. Increasing the gas pressure of SF6 can obtain higher insulation strength, but this increase is not linear. It shows a saturation trend at higher gas pressure, and the more uneven the electric field is, the lower the gas pressure when it is saturated. SF6 also has a strong electron affinity. When burning an arc in SF6, it captures a large number of free electrons, quickly reduces the conductivity of the arc, increases the arc column resistance, and promotes the extinction of the arc. In addition, the thermal conductivity of SF6 is 2~5 times higher than that of air, and the arc heat dissipation is large, which helps to extinguish the arc and quickly improve the dielectric recovery strength after the arc current passes zero, so it is also a very good arc extinguishing medium.

Compared with high-voltage, ultra-high-voltage and ultra-high-voltage sulfur hexafluoride circuit breakers, the structure of 12~40.5kV sulfur hexafluoride circuit breakers is relatively simple, and the design uses gas of the same pressure as insulation and arc extinguishing, that is, single-pressure type; the arc extinguishing principles of the current circuit breaker arc extinguishing chamber are mainly the following: rotary arc type; compressed gas type; thermal expansion type. Among them, the arc-spinning type and the thermal expansion type use the magnetic field force generated by the current in the arc-extinguishing chamber to drive the arc to move and make the break free quickly, or use the high temperature generated by the arc burning to make the SF6 expand rapidly and blow off the arc, so they are also called self-energy arc-extinguishing chambers. Obviously, the arc-extinguishing ability of these two structures is related to the size of the interrupted current; the compressed gas type drives a piston to blow the compressed SF6 gas to the arc to achieve the purpose of forced cooling when the moving contact separates. The arc-extinguishing ability of this structure has nothing to do with the size of the interrupted current.

As a significant technical feature of sulfur hexafluoride circuit breakers, it has weak arc-extinguishing ability when interrupting low-value and low-current arcs, so the current will not be suddenly cut off before the natural zero crossing, resulting in the so-called "cut-off" phenomenon, so there is no danger of resulting cut-off overvoltage. It is precisely because of this "soft" breaking characteristic that the 40.5kV sulfur hexafluoride circuit breaker is favored by operators. It is mainly selected to control transformers in order to safely cut off no-load transformer operations. This is why sulfur hexafluoride circuit breakers are still used to a certain extent in 35kV systems today.