Correct Oil Level; Restoring The Gas Cushion - ABB UBB Series Installation And Commissioning Manual

6.4 Correct oil level

For installations where the 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 tap-changer should be the same.
The oil level of the oil conservator of the tap-changer should
in no case be higher than the oil level of the transformer oil
conservator.
At +20 °C (68 °F), oil is filled 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 10 °C (18 °F) increase of temperature, adjust the
oil level upwards a tenth of the scale range of the oil level
indicator.
-
For every 10 °C (18 °F) decrease of temperature, adjust
the oil level downwards a tenth of the scale range of the oil
level indicator.

6.5 Restoring the gas cushion

WARNING
Before any work is carried out on the tap-changer:
Make sure that the transformer is disconnected and
that earthing is properly carried out. Obtain a signed
certificate from the engineer in charge.
After oil filling under vaccum a gas cushion has to be formed.
Proceed as follows:
1. Shut the conservator valve.
2. Connect a pump to the oil valve and start to suck out oil.
Open the air vent to the tap-changer and let air in. Stop
the pumping after about 2 liters of oil has been sucked out
from the tap-changer.
WARNING
The oil in the selector switch housing may be hot. Be
cautious!
3. Shut the air vent and the oil valve and disconnect the
pump.
4. Open the conservator valve and an appropriate gas
cushion will be formed.
If the tap-changer has been filled with degassed oil and has
a very low frequency of operation, the gas cushion may have
been absorbed by the oil. To avoid fault trip by the pressure
relay, the gas cushion should be restored as above.
CAUTION
To operate the tap-changer with a too small or no gas
cushion means a risk for a trip of the pressure relay.
The procedure above is used for tap-changers without oil filter
unit for continuous oil filtration. In case the tap-changer is
equipped with an oil filter unit for continuous oil filtration from
ABB, and it is installed according to our recommendations,
follow the instructions in "Oil filter unit for tap-changers,
manual" for restoring the gas cushion.
CAUTION
Avoid to do the restoring in damp weather since
moisture will get into the selector switch housing.
If the restoring has to be done in such weather, the
incoming air has to be dehydrated.
1ZSE 5492-118 en, Rev. 5 | Installation and commissioning guide UBB 35