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SF6 Gas Density Monitoring
Very low temperature and
liquefaction situation with
Trafag 87X6 SF6 gas density
monitor
05.02.07/ nip
The Reference Gas Principle
(Theory)
SF6 filled
switchgear
SF6 filled reference
chamber
DM
separating
membrane
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“
05.02.07/ nip
The Density Monitor with Reference
Chamber
(Picture shows the “older” 87x0 density monitor generation)
...inside the monitor
Tappet moves when
SF6 leaks from tank and
triggers alarm switch
Reference chamber filled with gas
SF6 gas inlet (connection
to tank)
05.02.07/ nip
Behavior of bimetaliccompensated Systems
Isochoren von xyz No. 06397
8.000bar
Contact 1, falling
(bimetal comp, non-Trafag)
7.500bar
True isochor
7.000bar
Tolerance band of Trafag
DM for calibration
6.500bar
Schalter 1 sinkend
Contact
falling
Schalter2,
2 sinkend
(bimetal
non-Trafag)
Isochorecomp,
5 bar
6.000bar
Isochore 6,4 bar
5.500bar
-8 kPa
True
isochor
+8 kPa
-8 kPa
5.000bar
Tolerance
band of Trafag
+8 kPa
DM for calibration
4.500bar
The accuracy of the
alarm levels
4.000bar
3.500bar
-40.0°C
05.02.07/ nip
-30.0°C
-20.0°C
3.000bar
-10.0°C
0.0°C
10.0°C
20.0°C
30.0°C
40.0°C
50.0°C
60.0°C
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
05.02.07/ nip
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
05.02.07/ nip
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