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6. Oil filling and draining
6.1 Filling methods and restrictions
Oil filling can be carried out at atmospheric pressure or under
a vacuum. The wall between the diverter switch housing and
the transformer tank is designed to withstand a vacuum on
one side and atmospheric pressure on the other. It is not
permitted to have a vacuum on one side and the pressure of a
high oil column on the other.
CAUTION
The pressure difference between the inside and
outside of the tap-changer cylinder may not exceed
100 kPa at any time.
Oil filling may be carried out in different ways
depending on what the transformer manufacturer
finds convenient as long as the instructions above are
followed and the tap-changer is filled with oil to the
correct level.
The methods below are recommended, and if they are
followed in detail, no pressure limits will be exceeded
and the oil levels will be correct.
6.2 Before filling
Upon commissioning, open the cover and check that the
diverter switch housing is dry and clean and that the diverter
switch is in place. Check that the bottom valve is closed.
Follow step 11 in Section 3.2.3.
Tightening torque 40 Nm (see Fig. 18).
6.3 Filling at atmospheric pressure
1. Open the conservator valve, if any.
2. Remove the breathing device on the conservator for the
tap-changer.
3. Remove the cover.
4. Pump oil into the diverter switch housing via the oil
valve, see Fig. 47 (for connection dimensions, see the
dimensional drawing for the tap-changer).
The valve is located at ground level.
5. Refit the cover as per Section 4.3.
When filling more than one unit, fill all of them
according to points 3 to 5.
42 Installation and commissioning guide UCL/VUCL | 1ZSC000562-AAZ en
6. Continue to pump in oil until the conservator is filled to its
correct level. Also see Section 1.5.
7. Shut the oil valve and disconnect the pump.
8. Refit the breathing device. Make sure that the connection
to the breathing device is properly sealed.
WARNING
Do not energize the transformer earlier than three
hours after oil filling at atmospheric pressure. This
waiting period is needed to allow air bubbles to
dissipate.
6.4 Filling under vacuum
Filling under a vacuum is not necessary but may be carried
out with the conservator. After filling under a vacuum, no
standing time is needed. The methods below ensure that no
permitted pressure differences are exceeded.
For vacuum filling without the conservator connected,
contact the manufacturer for advice.
6.4.1 Oil conservator filled under vacuum
1. Establish a connection between the oil conservator for the
transformer and the oil conservator for the tap-changer.
2. Open the valve between all tap-changer units and the
conservator and close the oil valve.
3. Put the transformer under vacuum. (The tap-changer is
put under vacuum automatically.)
4. Let oil in through the oil valve of the tap-changer. (For
connection dimensions, see the dimensional drawing for
the tap-changer.)
5. When the needle of the oil level indicator in the oil
conservator starts to move, close the oil valves on all
units. Open one at a time and close each when the needle
in the oil level indicator starts to move. Fill the conservator
to the right level before closing the oil valve for the last
unit. (This is done to make sure that all units are properly
filled. Differences in flow rates might otherwise cause
some of the units to not completely fill.)
6. When atmospheric pressure is restored in the transformer,
remove the connection between the two conservators
and connect the breathing device to the oil conservator
for the tap-changer. Make sure that the connection to the
breathing device is properly sealed.
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