ABB UC Series Maintenance Guide

1ZSE 5492-124 en, Rev. 6 | Maintenance guide UCG 23
3.17.5 Restoring the gas cushion
Check the oil level in the oil conservator one month after
oil filling. If the oil level is lower now than when the oil filling
was finished (correct for temperature differencies!) and no
leakages are observed, the gas cushion has been solved in
the oil and has to be restored.
The procedure below is used for on-load tap-changers
without oil filter unit for continuous oil filtration. In case the
on-load 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 the oil filter
unit manual.
In case the on-load tap-changer consists of more than one
unit, do the restoring in one unit at a time.
WARNING
The oil in the diverter switch housing may be hot. Be
cautious!
WARNING
Before any work is carried out on the on-load
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.
CAUTION
To operate the on-load tap-changer with a too small
or no gas cushion means a risk for a trip of the
pressure relay.
Procedure
1. Close the valve in the tube to the conservator.
2. Connect the oil pump to the oil valve. (For connection
dimensions, see the dimension drawing for the on-load
tap-changer), see Fig. 10.
3. Open the oil valve and the air release valve.
4. Start the oil pump and drain approximately 15 litres of oil
into a clean and dry container.
5. Close the air release valve.
6. Close the oil valve and disconnect the pump.
7. Connect the output side of the pump to the oil valve.
8. Open the oil valve.
9. Open the valve in the tube to the conservator!
10. Pump the earlier drained 15 litres of oil back into the
diverter switch housing.
11. Close the oil valve and disconnect the pump.
12. The level in the oil conservator and the gas cushion are
now restored.
If the on-load tap-changer consists of more than one unit,
proceed with the other one until the last one has been
restored.
CAUTION
Avoid to do the restoring in damp weather since
moisture will get into the diverter switch housing. If
the restoring has to be done in such weather, the
incoming air has to be dehydrated and the drained oil
protected from water.
Open
Air release valve
Oil valve
Oil valve
Open end wrench
key width 6 mm
Fig. 10. Air release valve (position may be on another vertical flange).

ABB UC Series Specifications

General

Brand	ABB
Model	UC Series
Category	Industrial Equipment
Language	English