Photo from RH system company gas analyzer model 973
SF6 gas plays a key role in switchgears and the percentage of gas purity has a direct effect on the switching.
Commercially, SF6 is supplied in pressurized bottles or liquid tanks. The gas of these bottles has a minimum degree of purity of 99.9% and may possess the following impurities according to IEC 60376:
- carbon tetrafluoride (CF4) <0.03%
- oxygen + nitrogen (air) <0.03%
- trace water
- CO2 trace
For more information about the permitted levels of impurities please refer to IEC Standard 60376 tables.
In operation and during electrical failures, pollutants are generated which, depending on its concentration, reactivity and toxicity, modify the characteristics of gas for insulation and arc extinguishing. This may affect the safety of the installation and people. Gas handling operations can also cause the incorporation of pollutants.
For treatment and possible reuse, it is necessary to know the levels of contaminants in the gas.
As a precaution and safety equipment for measurement, the checks must always be performed in the following order: a measure of acidity; moisture measurement, and measurement of the purity Measuring the purity or volume rate of SF6 is one of the indicators of the quality of gas.
A typical gas purity meter is shown in Figure. SF6 purity measurement is displayed directly in % Volume SF6.
Today, most of gas analyzers have the ability to measure purity and humidity and Dew/Frost Point, and other characters. SF6 purity is typically measured using conductivity or speed of sound measurement techniques.
Impurities that reduce the percentage of SF6 by volume are mainly handling the gas, the filling and emptying of compartments that may inadvertently incorporate air and water vapor due to erroneous operation, incomplete evacuation of pipes and vessels, leaks in connections, and seal surfaces. Systems are often used in field density meters.
These meters are normally calibrated using air so the accuracy of the equipment may be impaired if the pollutant gas is air. In practice there are contaminants that are difficult to detect such as the presence of oil, it can be introduced into the equipment during filling operations or during handling of SF6 and usually deposited on the inner surfaces and can be carbonized to form conductive layers with the consequent danger to the team. Their presence can be avoided with teams equipped with pumps without oil lubrication.
In the following video(source: RH system company gas analyzer video from youtube website), we can briefly see the decomposition and composition of SF6 gas during the operation of circuit breaker contacts.
For more information about the decomposition and composition of SF6 gas please read this article