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Maintenance guide UCG
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1ZSE 5492-124 en, Rev 6
1.2 Maintenance schedule
Maintenance of the on-load tap-changer consists of three
major steps:
– Inspection
– Overhaul
– Contact replacement
1.2.1 Inspection
It is recommended to perform an inspection once a year. This
principally concerns the motor-drive mechanism and refers
to a visual inspection inside the motor-drive cabinet to check
that nothing is loose and the heater is functioning.
In the motor-drive mechanism a counter registers every tap-
changer operation. During inspection the counter is read. If
possible, motor and counter are tested by operating one step
and then back.
If the on-load tap-changer has its own oil conservator, the
breather and the oil level indicator are checked according to
the instructions from the transformer manufacturer.
The inspection is carried out while the transformer is in
service.
If the on-load tap-changer is equipped with an oil filter unit
from ABB, it should be inspected once a year according to
the oil filter unit manual.
1.2.2 Overhaul
The contact life and the frequency of operations or the time in
service determine the time interval between overhauls.
The number of operations run by the on-load tap-changer is
recorded by a counter, housed in the motor-drive mechanism
cabinet. The registered number of operations should be noted
at every inspection and overhaul.
The on-load tap-changer should normally be overhauled
regularly at intervals of 1/5 of the estimated contact life. The
relevant information is stated on the rating plate. Hereby, the
contact wear can be followed and necessary preparations can
be made for replacing the contacts.
If the tap-change operations occur infrequently and a very
long time elapses until the number of operations amounts to
1/5 of the contact life, the interval between overhauls should
be limited to the time stated on the rating plate (normally 7
years).
Fig. 2 shows the general arrangement of an on-load tap-
changer type UC. The main components are the spring-
operated diverter switch and the tap selector with sliding
contacts. For maintenance the diverter switch is lifted.
The contacts are then immediately accessible and can be
inspected for wear. The drive-shafts should not be dismantled
when lifting the diverter switch.
Maintenance is normally not required on the parts operating
in the oil of thetransformer tank. However, when the on-load
tap-changer has made one million operations a check of the
tap selector is recommended.
The diverter switch has its own housing separate from
the transformer oil. This is to prevent contamination of the
transformer oil since the diverter switch oil deteriorates due
to the switching operations. The oil needs to be checked and
filtered at regular intervals to maintain adequate dielectric
strength as well as to prevent mechanical wear.
It is necessary to inspect the contacts and clean the insulation
parts of the diverter switch as well as to clean the housing
inside at regular intervals.
The main components of the diverter switch are:
– Fixed main contacts
– Moving main contacts
– Fixed transition contacts
– Moving transition contacts
– Transition resistors
– Spring-driven polygon link system
Besides the maintenance of the diverter switch and cleaning
of the oil, the motor-drive mechanism should be checked and
lubricated.
The pressure relay, the device that protects the transformer
from damages due to excessive pressure in the diverter
switch housing, should also be checked.
One on-load tap-changer of UCG type may consist
of one, two or three units driven by a common
motor-drive mechanism. The instructions in this
guide deals with one unit. If there are two or three
units, all work decribed should be carried on all units
unless otherwise is stated. If more than one diverter
switch is lifted out at the same time, make sure the
right diverter switch is lowered into the right housing
(compare with the serial numbers, see Fig. 2).
