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Maintenance guide UCG
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1ZSE 5492-124 en, Rev 6
3.17.6 Waiting period
CAUTION
Do not energize the transformer earlier than three
hours after oil filling in atmospheric pressure. This
waiting period is needed to allow airbubbles to
disappear.
3.18 Check of contact timing
Checking the contact timing is a good check of the condition
of the on-load tap-changer, but it is not necessary to check
contact timing at every overhaul. It is recommended to be
carried out after every 500 000 operations.
The test requires:
– Two indicator lamps (glow discharge lamps for the
minimum possible magnetisation of the transformer)
– Necessary leads
– Two stiff insulated leads or bars
WARNING
Never force DC current through the transformer
windings.
The insulated leads (or bars) are used for connection to
the moving contact arms of the tap selector via the plug-
in contacts of the diverter switch (They can be made of an
insulating tube with a lead inside.)
Connect the lamps as shown in Fig. 11.
The diverter switch contacts are designated as shown
in Fig. 12. Determine if x or v contacts are closed. In the
contact-timing diagram for the on-load tap-changer you can
find out the corresponding position. See Figs. 13 and 14.
The diverter switch housing shall be filled with transformer oil
up to the fixed main contacts to secure the correct function
of the dash pots during operation of the diverter switch, see
section 3.17.
At repeated operations in the same direction the selector
arms V and H operate every second time. When the direction
of operation is reversed, the contact arms will be at rest
during the first operation. The operation is then performed by
means of the diverter switch only.
When testing, the operation must be carried out in the
same direction as the previous operation.
The test is to be made on all three-phases and is to be
carried out as follows:
Find the exact position of the on-load tap-changer by
adjusting the disc brake as shown in the maintenance guide
for motor-drive mechanisms.
1. Note the number of whole turns and parts of turns on the
hand crank, during a slow manual operation.
2. Note when each tap-selector arm breaks and makes (the
corresponding lamp goes out or lights).
3. Note when the diverter switch flicks over (a distinct sound
is heard).
After this, compare the operations with the contact-timing
diagram applicable to the on-load tap-changer in Figs. 13
and 14.
One operation corresponds to 25 turns of the hand crank on
the motor-drive mechanism, type BUE and 15 turns for BUL.
Remove the equipment for contact-time measuring.
Mount the cover of the diverter switch housing according to
section 3.13.
Complete the oil filling according to section 3.17.
CAUTION
If the result is beyond the limits given here, please
contact ABB.
For BUE, the diverter switch must have switched over
before the 20th turn is finished with the hand crank.
For BUL, the diverter switch must have switched over
before the 12th turn is finished with the hand crank.
3.19 Putting into operation
Operate the on-load tap-changer to the position noted in
section 3.3.1. Put the LOCAL/REMOTE switch to REMOTE.
Reset the drag hands. Make sure that no tools or foreign
objects are left in the motor-drive mechanism cabinet. Close
the door. Make sure that nothing is left on the transformer
cover.
Sign the revision protocol and give it to the engineer in
charge and inform that the on-load tap-changer is ready for
energizing.
