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1ZSE 5492-115 en, Rev. 5
6 Oil lling
6.3 Filling under vacuum
Oil lling under vacuum is not necessary. If it is to be carried out, the on-load tap-changer
and the transformer tank should be put under vacuum simultaneously.
It is assumed that a vacuum-proof conservator is in use.
1. Open the conservator valve, if any.
2. Dismantle the pipe to the breather, or, if simpler, remove the air relief valve or the dehy-
drating breather.
3. Connect the transformer to the conservator for the on-load tap-changer.
4. Apply vacuum.
5. Connect the oil lling equipment to the oil valve on the on-load tap-changer tank. Open
the valve and let oil into the tank until the correct level is shown on the oil level indicator.
For correct oil level see section 6.4.
6. Shut the oil valve and disconnect the lling equipment.
7. When lling of the transformer is completed disconnect the vacuum pump and let air
into the conservator or expansion volume.
8. Reassemble the pipe or the breather. The connections must be airtight; therefore use
sealing tape on the threads and O-rings in the anges.
6.4 Correct oil level
For installations where the on-load tap-changer and the transformer tank have a common oil
conservator, the instructions for the transformer should be used.
If possible, the oil levels of the oil conservators of the transformer tank and the on-load tap-
changer should be the same. The oil level of the oil conservator of the on-load tap-changer
should in no case be higher than the oil level of the transformer oil conservator.
At +20 °C (68 °F), oil is lled to the level where the pointer of the oil level indicator points
half-way between MIN and MAX.
For other temperatures than +20 °C (68 °F), proceed as follows:
■ For every 13 °C (23 °F) increase of temperature, adjust the oil level upwards a tenth of
the scale range of the oil level indicator.
■ For every 13 °C (23 °F) decrease of temperature, adjust the oil level downwards a tenth
of the scale range of the oil level indicator.
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