The most important insulating gases —— SF6
In electrical equipment, the heat transfer characteristics of medium have great influence on the operating temperature and efficiency of equipment. In some cases, the ability to transfer heat is the deciding factor in choosing an insulating medium. As a gaseous medium, its heat transfer characteristics mainly depend on its thermal conductivity, specific heat capacity and viscosity. Sulfur hexafluoride is the dominant insulating gas because of its good heat transfer properties.
Sulfur hexafluoride packing
The classical heat conduction is to consider the molecular thermal diffusion movement of gas, so that the molecules in the high-temperature region carry higher internal energy and migrate to the region with lower temperature, resulting in heat transfer in space. Molecular motion here refers to thermal motion, not macroscopic relative motion. As long as there is temperature difference in space, there is heat conduction. In high temperature arc, sulfur hexafluoride will decompose and ionize, and with energy consumption, it has great influence on heat conduction process. Therefore, the thermal conductivity at this time can be considered as composed of two parts, namely, the standard thermal conductivity and components related to decomposition and ionization reactions. However, for gaseous medium, its heat transfer ability is often not pure conduction, but largely depends on convection heat transfer. Convective heat transfer is due to the existence of local pressure difference, so that the gas medium produces macroscopic movement and diffuses heat out. The convective heat transfer ability of gas medium is related to the specific heat capacity and viscosity of gas.
Secondly, the diameter of sulfur hexafluoride molecules is larger than that of oxygen and nitrogen molecules in the air, so that the average free travel of free electrons in sulfur hexafluoride gas is greatly shortened, so that it is not easy to accumulate energy in the electric field, so that the collision free ability of free electrons is reduced. In sulfur hexafluoride, the insulation strength of sulfur hexafluoride gas is very high because the net collision free coefficient of electricity is very small, and negative ions with low mobility are easy to combine with positive ions to form neutral molecules. The insulation strength of the sulfur hexafluoride gas remains essentially constant in the range from power frequency to communication frequency of the applied ac voltage. Therefore, sulfur hexafluoride gas is also widely used in many off-frequency electrical and electronic equipment, such as coaxial cable, waveguide, radar equipment, etc.
Sulphur hexafluoride has few disadvantages as an insulating gas