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4. Drying
The tap-changer is dried together with the transformer using
one of the following processes: alternating hot air and vacuum,
or vapor phase at a temperature of no more than 135 °C
(275 °F).
4.1 Precautions before processing
1. If not done earlier, lift the diverter switch out from the
housing; see Section 3.2.1, steps 5 and 7. The pressure
relay with its test vent and oil filter, if any, should be
removed and protected from dust, e.g. placed in a plastic
bag or in their original packing.
2. Do not expose the diverter switch housing to any pressure
difference between the inside and outside during the vapor
phase process. During the hot air and vacuum process,
the maximum permitted pressure differential is 100 kPa at
a temperature of 135 °C (275 °F).
During the vapor phase process, the bottom valve of
the diverter switch housing should be open. To open the
bottom valve, proceed as follows; see Fig. 14:
a. Remove the plug on top of the oil drainage tube.
b. Use the bottom valve key through the oil drainage tube.
c. Turn the valve counter-clockwise to its stop,
approximately 6 turns.
d. Refit the plug.
e. Remove the O-ring in the lower flange (for mounting on
active part only) before the process.
4.2 Precautions after processing
CAUTION
To avoid seizing, do not operate the tap-changer,
neither during the drying process nor afterwards, until
it is filled with oil.
1. Make sure that all liquid has been drained from the diverter
switch housing after the vapor phase process has been
carried out. When cover-mounted, close the bottom valve;
see Section 3.2.3, step 10. When yoke-mounted, the valve
is closed during refitting of the top section.
CAUTION
Make sure that the bottom valve key is removed after
the valve has been closed.
2. The cleats on the upper section of the tap selector that
hold the conductors between the diverter switch housing
and the tap selector should be retightened (tightening
torque 15 Nm) and locked by a method specified by the
transformer manufacturer for similar screw joints.
28 Installation and commissioning guide UCL/VUCL | 1ZSC000562-AAZ en
3. If a tie-in resistor from ABB is supplied, its screw joints
are to be retightened (tightening torque 15 Nm) and
locked using the method specified by the transformer
manufacturer for similar screw joints.
4.3 Installation of diverter switch
When yoke-mounted, carry out the steps in Section 3.2.3
before installing the diverter switch.
WARNING
For VUCL: The vacuum interrupters are sensitive and
the diverter switch must thus be handled with care.
CAUTION
Check the serial numbers to make sure that the
diverter switch is mounted in the correct housing; see
Fig. 3.
CAUTION
Make sure that the diverter switch housing is clean
and dry, and that no foreign objects (tools, etc.) are
left in the housing.
CAUTION
Carefully lower the diverter switch into its housing
so that neither the diverter switch nor the housing is
damaged.
The diverter switch is provided with guide slots that fit against
the guide bar and the oil drainage tube in the diverter switch
housing; see Fig. 23. Rotate the diverter switch so the
crescent-shaped guide slot is aligned with the oil draining
tube.
When the diverter switch is lowered, visually check that its
plug-in contacts are aligned with the contacts in the cylinder
wall.
In order to ensure that the diverter switch pin has engaged
the coupling disk, carry out at least three tap change
operations in one direction. A distinct sound is heard when
the diverter switch falls into place.
If no sound is heard, the reason can be that the pin fits
directly into the slot or that the diverter switch might need to
be pushed down while operating the motor-drive.
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