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Guideline for Dew Point in SF6 Gas limits in Power system

SF6 (Sulfur Hexafluoride) is used in GIS/switchgears due to its high arc quenching 
capabilities and very high die-electric strength (2.5 times of air). Purity & Dew point 
of SF6 Gas are very important as an insulating media. 

General guidelines are necessary to establish an acceptable range of moisture and SO2 levels in SF6 Gas.

Dew point: The temperature at which the water vapor in the Gas begins to 
condense. It is absolute measure of moisture content available in gas. It is 
expressed in ºC/ºF, PPMv(Volume) and PPMw(mass). All three units refers to 
same physical interpretation. If value in one unit available other two can be 
derived using appropriate formulas. Mostly kit internally converts values to 
provide values in all three units.

There are certain factors that needs to be considered while taking dew point 
measurements and interpretation of results. i.e. instrument used, Date/time 
of measurement, pressure of the gas in enclosed equipment, sensor 
condition etc.

Factors affecting Dew Point measurement:

Dew point should be measured during date/time of year having max.
temperature, as it will give closest possible indication of water (moisture)
content presence inside the equipment. i.e. Dew point measuring
instruments gives the value of water (moisture) content presence in SF6 gas.
However, water vapor is also trapped inside equipment structures. At higher
temperature moisture from the structure slowly diffuses in SF6 gas. Hence,
at higher temperature it gives actual value of Dew point (moisture).

Measurements should be taken after filling gas in equipment at rated
pressure specified by manufacturer. It is also to be noted that after filling
fresh/recycled gas in equipment, the trapped moisture content inside the
structures diffuses into SF6 gas until hydro-equilibrium established. Hence,
after filling fresh/recycled gas measurement should be taken after 24 hours
to get the accurate value and should be repeated weakly and measure at
least 3 readings.

Most of the kits will provide corrected result at atmospheric pressure and
temperature. Thus, the results can be directly compare/match with standards
limit.

If instrument gives/prompts sensory error, action should be taken
accordingly to obtain reliable measurements. For example, if dew point
measuring instrument used chilled mirror method for measurement and it
gives/prompts error of ‘clean the mirror’, mirror should be cleaned by user
after referring instruction manual of that instrument.

For measurement of SO2, a sensor required and presence of sensor in kit
should be confirmed. It comes with validity and should be replaced after
expiry.

Analysis of results.

If results of Dew point & SO2 found not in order after comparing with
standard values, it should be reported to higher authority.

If moisture content is high manufacturer shall be contacted for further
directives. moisture content can be removed with the use of DILO machine
(which is available with filters).

If purity is lower than 97% gas should be replaced or filtration shall be done
with DILO machine (which is available with filters)

The above guideline for moisture measurement (Dew Point) in SF6 gas is to be
follow in AIS or GIS substation, where ever SF6 gas is used.
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