HVDC TRANSFORMER BUSHINGS
PHI 530.1425.2500
PHI 265.850.1500
Kepco project
IS 2664 GB
Page
27
of
70
•
Two hours after the completion of filling operation, measure the gas pressure and the dew point at
atmospheric pressure and record the data.
•
Repeat the same measurements after 24h and record the data.
•
If the pressure loss is negligible and the dew point increase between the two measurements is less than
5°C proceed as follow:
o
Remove all Nitrogen by putting the bushing under vacuum until the residual pressure measured
at the bushing level will be <10 Pa abs (0.1 mbar abs).
o
Keep the vacuum for at least 12 hours.
o
Proceed with the gas filling operation (see
Chapter 8
).
•
If there is evidence of pressure drop it is necessary to investigate the problem with the support of a GE
Energy Connections Specialist.
o
If the pressure drop is negligible but the dew point variation is > 5°C, it is necessary to perform
another cycle of vacuum and Nitrogen flushing.
9.2
Procedure for bushings not equipped with the gas filling station
The procedure is the same as in Paragraph 9.1, except the fact that the equipment must be directly connected
to the DILO valves located at the bushing flange.
10
Gas density monitor
The bushing is equipped with two gas density monitors manufactured by Electronsystem
The two instruments are different as one is a normal gas density monitor, while the other is equipped with an
electronic pressure transducer 6-20 mA for remote indication (hybrid type).
These devices ensure a great accuracy in the measurement, as they are based on the continuous and direct
comparison between the gas density inside the bushing and the gas density of a built-in reference chamber filled
with the same gas. This system is different and more accurate respect to the normal instruments that estimate
the gas density through a pressure measurement corrected by a bimetal element that takes into account the
ambient temperature.
The next
Figure 15
shows the instruments and its graduated scale.
Gas density monitor (example)
Instrument scale
Figure 15 – Electronsystem gas density monitors
The indication on the scale is expressed in MPa abs, referred to a gas temperature of 20°C. This means that
the instrument converts the gas density into a value of pressure referred to 20°C, independently from the
effective temperature of the gas.
With this system, the instrument reading practically does not change with the gas temperature, that is a function
of the bushing loading condition and ambient temperature.
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