An Explanation of Sulfur Hexafluoride

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  • Topic: Greenhouse gas, Switchgear, Dielectric strength
  • Pages : 6 (2072 words )
  • Download(s) : 101
  • Published : December 3, 2012
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First of all, what is sulfur hexafluoride (SF6)? SF6 is a non-toxic, nonflammable and noncorrosive gas. SF6 has almost the same chemical properties of a noble gas and it does not react with other materials. Under standard conditions, SF6 is colorless and odorless gas in high electrical components. Furthermore, SF6 also inhibits surface erosion and oxidation. It has high partial vapor pressure at both normal and low temperatures. SF6 also has an excellent heat transfer. Its high molecular weight and low viscosity enable SF6 to transfer heat more efficiently by convection. SF6 gas performed as an excellent insulating and arc-quenching. Therefore, it used as an interrupting medium in switchgear and circuit breakers instead of oil and air because it can improve the power system reliability.

SF6 can be used in different types of electrical switchgear such as Gas Insulated Switchgear (GIS). SF6 gas not only as arc-quenching medium but as insulation as well. With respect to the SF6 insulating gas pressure, there are two principal GIS designs: high pressure GIS functioning at about 405kPa, and low pressure GIS which is operating at 120kPa. While at higher voltage levels, the advantages of high pressure GIS become dominant. But the more suitable design is below the low pressure GIS at a rate voltage of 72.5kV. A higher SF6 gas pressure will increase the dielectric strength. As a result, for a given voltage level the required conductor spacing is reduced, resulting at a more compact design. Due to the economical and functional reasons, high voltage switchgear with SF6 gas is commonly used. The replacement of SF6 with air and oil in high voltage switchgear is because SF6 gas has the outstanding characteristics of the gas with the gas-tight metal enclosure. Some features like low operating energy for the switching devices, no risk of fire and not toxic hazard are available. Besides that, the space occupied by the switchgear is absolutely decreased and easier to install in difficult site conditions like unstable ground and in seismically active areas. High voltage switchgear with SF6 also possesses a high degree of operational reliability and safety to humans. The level of partial discharges will not increase and remain on the level during operation over the total service life time. Therefore deterioration of insulation by radiation or chemical reactions due to partial discharges will not happen. Moreover, it is totally unaffected by atmospheric conditions such as polluted or saline air in industrial and coastal areas or desert climates.

In addition, there are also some operational benefits of GIS of SF6 can be achieved like the failure of operation is extremely low and the protection of the insulating system from any degrading influence is good due to the outstanding performance level. These also will ensure the cost of maintenance to be reduced and has a long service life. Further, it has a low operating energy and makes easy to implement remote control and automation schemes. This can make sure a fast restoration of the service after a fault in the system. The first installation and operation also relatively low cost and it performs a high continuity of service.

SF6 gas has been chosen to be the insulation medium because it has high dielectric strength that prevent breakdown. Compressed SF6 mostly applied in AC power transmission apparatuses in the range of 66V to 500V. However, the SF6 gas insulated DC power transmission apparatuses can withstand for 100MV compact HVDC has been discovered recently. In the reality, there are some factors that we need to consider because they have an effect on the dielectric strength of SF6. Initially, the electrode material must take under consideration. When the breakdown strength exceeds about 200kV/cm under very high pressure, the material of electrodes in an SF6 gap will start to influence over breakdown. Steel electrodes have the larger uniform field breakdown strength compared to...
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