1. SF6 Gas Density Monitoring
Very low temperature and liquefaction situation with Trafag 87X6 SF6 gas density monitor
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2. The Reference Gas Principle (Theory)
SF6 filled switchgear
SF6 filled reference chamber
separating membrane DM
The temperatures of the gas in the switchgear and the gas in the reference chamber of 87x6 density monitor are „the same“ (depends on the mounting situation)
Conclusion: „fully temperature compensation over the whole temperature range“
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3. The Density Monitor with Reference Chamber (Picture shows the “older” 87x0 density monitor generation)
Tappet moves when
SF6 leaks from tank and
triggers alarm switch
...inside the monitor
Reference chamber filled with gas
SF6 gas inlet (connection to tank)
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4. Behavior of bimetalic-compensated Systems
Contact 1, falling
(bimetal comp, non-Trafag)
True isochor
Tolerance band of Trafag
DM for calibration
Contact 2, falling
(bimetal comp, non-Trafag)
True isochor
Tolerance band of Trafag
DM for calibration
The accuracy of the alarm levels
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5. Density Monitor at temperature below -55C
Due to the mechanical system the density monitor will still operate below -55°C
May be the specifications like accuracy and so on are out of the from Trafag specified values on the datasheets
The tolerances of the mechanical system may be are to high for a proper function
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6. Liquefaction?
The reference chamber of 87x6 density monitor is filled at previous alarm density level (1. stage)
In regular operation the density at switch breaker is higher (somewhere between filling pressure and previous alarm level) as previous alarm level in reference chamber of 87x6 density monitor
Therefore, the liquefaction in switch breaker starts at higher temperature (that means earlier) as in the reference chamber of 87x6 gas density monitor
Benefits:
Small local liquefaction in the switch breaker will not be detected by the 87x6 density monitor
Serious liquefaction which can create a critical situation for the switch breaker get detected by the 87x6 density monitor
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