Things you don’t know about SF6 gas!

• June 7, 2020
• Gaurav J

SF6 gas has excellent arc quenching capacity. The gas attracts free electron from the air. It is electronegative, colourless, odourless, it is non-inflammable. SF6 gas is suitable for arc quenching in high voltage circuit breakers. 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. In this tutorial let’s get introduce with SF6 gas practically. So, let’s start.

Most of the high voltage equipment like, circuit breaker, Current transformer, HV bushings, Gas Insulated Switchgear (GIS), Transformers, Cables, etc are insulated with SF6 gas. SF6 gas is generally preferred from including & above 72.5kV voltage class. 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 63,000 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. 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.

Pressure of the SF6 gas in an equipment depends upon the design of a manufacturer. Some high voltage equipment manufacturer, offer 6 bar as a normal 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 which helps in maintaining the required 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. SF6 gas is transported in liquid state. And that’s amazing. This done by reducing the temperature of the gas bellow -40°C.

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.

After all of this information we can conclude, SF6 gas is very good in electrical properties. 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 care must be taken to avoid any release of gas in to the atmosphere.

More than 10,000 tons of SF6 is 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.