Sf6 Purity Analyzers In Switchgears

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 the IEC Standard 60376 tables.

In operation and during electrical failures, pollutants are generated which, depending on their?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 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 gas analyzers have the ability to measure purity and humidity Dew/Frost Point, and other characteristics.?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 is 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.

SF6 Decomposition Products From Electrical Arc In Gas Switchgear:

During normal load or short-circuit current switching, SF6 molecules are ionized and fragmented by the arc.

The figure shows the main reaction processes and where each is likely to occur. SF4, as the main decomposition product from the electric discharge, first reacts with H2O on the inner wall surface resulting in SOF2. The metal fluorides remain as powder or dust on the surface. H2O is released in this reaction and therefore available for further reactions with SF4 or for the much slower conversion of SOF2 to SO2. In this process, H2O is in fact not consumed but plays the role of a catalyst. The factors involved are:

• ?The formation rate of decomposition products is a function of the arc or discharge energy.
• Transport speed (diffusion or convection) of decomposition products from the arc or discharge location to the walls of the compartment.
• Gas mixing of decomposition products created at surfaces (diffusion or convection)
• The reaction rate of decomposition products at wall surfaces
• The adsorption rate of decomposition products in adsorber materials depends on the properties and the location of the absorber material

Source: Switchgear Content website: