SF6 gas has excellent arc quenching capacity, it attracts free electron from the air, it is electronegative, colourless, odourless, it is non-inflammable. It is suitable for arc quenching in high voltage breaker.
Well that’s what we know about SF6 gas, or that’s what we have written in our exams. But, it’s not enough. Yes, there are more things that you don’t know about SF6 gas. Let’s get introduce with SF6 gas practically. To get the details of SF6 gas read the below article.
> Get Introduce With SF6 Gas Practically.
Most of the high voltage equipment are insulated with SF6 gas. High voltage equipment including circuit breaker, Current transformer, HV bushings, Gas Insulated Switchgear (GIS), Transformers, Cables, etc, all has SF6 as an insulating medium. Sf6 gas is generally preferred from 72.5kV to 1200kV voltage classes. From this you can imagine the excellent insulating properties SF6 gas has.
SF6 gas remains stable up to 500° C, which makes it suitable for breaking high current up to 63,00 A or 63kA and above. It has no colour, no odour, and if you would try to burn it, you’ll actually end up with nothing, as it is incombustible.
Sf6 gas possess excellent insulating properties when pressurized between 2 to 6 bar. Of course, bar is the unit of pressure. It also offers 2 to 3 times more dielectric strength than the air at the same pressure between 2 to 6 bar.
Fig- SF6, air, oil dielectric breakdown strength as a function of pressure.
Pressure of the gas varies, manufacturer to manufacturer. Some high voltage equipment manufacturer offer 6 bar pressure in their equipment, some offers 6.5 bars, some may have 7 bar, depends on the dielectric strength, a manufacturer wants. The dielectric strength of the gas varies with the pressure.
The gas is 5 times heavier than the air with a molecular weight of 146 and specific gravity of 6.14 g/l (gram per litre). Well, I have no idea what’s that mean. But it’s a scientific fact about the gas. Below -40°C the gas can liquefy i.e. it may get converted into liquid, and hence special care must be taken in such situation to maintain the temperature. Generally, the equipment is provided with heaters who maintains the temperature.
Industrial SF6 gas used in high voltage equipment like circuit breakers is specified with 99.9% purity. The amount of impurities allowed in the gas is specified in the IEC 60376: 2005. Table shown below from IEC 60376: 2005, provides the total amount of impurities allowed in industrial SF6 gas per kilo gram.
The above table from IEC also provides methods to determine the impurity level in the gas. IEC60376: 2005 is applicable to SF6 gas used in electrical equipment.
The gas is good insulator at normal temperature, but during the arcing period in circuit breaker it becomes a good conductor and, carries current until next current zero occurs. The gas cools very rapidly within few micro seconds and the sulphur and fluorine ions recombines very quickly to form the stable insulating SF6.
Medium voltage equipment like vacuum circuit breaker are generally ‘sealed for life’ but, the SF6 circuit breaker can be opened for inspection after several thousand operations. Leakage rates of Sf6 gas guaranteed by manufactures for different equipment like circuit breaker, GIS etc., is less than 0.5% per annum or max up to 1% per annum.
Now, if you wish to buy SF6 gas believe me you will be amazed. Yes, because you will receive the gas in liquid state. Yes, you read that right. SF6 gas is transported in liquid state. And that’s amazing. This done by reducing the temperature of the gas bellow -40°C.
Fig- SF6 gas cylinders
By the way, SF6 stands for sulfur hexafluoride. In this gas six florine atoms are attached to central sulfur atom, and hence the name is SF6.
Fig – SF6 Gas
After all of this information we can conclude, SF6 gas is very good in electrical properties but, one of the biggest disadvantage of SF6 gas is, it is one of the dangerous greenhouse gas. According to the Intergovernmental Panel on Climate Change, SF6 is the most powerful greenhouse gas. And hence special cares must be taken to avoid any release of gas in the atmosphere.
More than 10,000 tons of SF6 are produced per year, most of which (over 8,000 tons) is used as a dielectric medium in electrical industry. From this you can imagine the importance of the gas in electrical industry and that’s why it is important to have full information about the gas. So, I hope this article will help.
If you think the article is helpful, do share It with your friends. You can also watch my video on SF6 gas click on the link given below – http://www.theelectricalguy.in/the-things-you-dont-know-about-sf6-gas-2/