Commissioning
Small power transformers can be
transported to site complete with oil, bushings, tap changers and
cooling equipment. It is then a relatively simple matter to lift them
onto a pole or plinth and connect them into the system.
Large transformers are subject to
weight restrictions and size limitations. When they are moved by road
or rail and it is necessary to remove the oil, bushings, cooling
equipment and other accessories to meet these limitations. Very large
transformers are usually carried on custom-built transporters.
Once a transformer of this size arrives
on site, it must be lifted or jacked onto its plinth for re-erection.
In some cases with restricted space it may be necessary to use
special techniques, such as water skates to maneuver the transformer
into position.
When the transformer has been erected
and the oil filled and reprocessed, it is necessary to carry out
commissioning tests to check that all electrical connections have
been correctly made and that no deterioration has occurred in the
insulation system.
These commissioning tests are selected
from the routine tests and usually include winding resistance and
ratio, magnetizing current at 440 V, and analysis of oil samples to
establish breakdown strength, water content and total gas content. If
oil samples indicate high water content then it may be necessary to
dry the oil using methods addressed in the following section.
Maintenance
Transformers require little maintenance
in service, apart from regular inspection and servicing of the OLTC
mechanism. The diverter contacts experience significant wear due to
arcing, and they must be replaced at regular intervals which are
determined by the operating regime.
For furnace transformers it may be
advisable to filter the oil regularly in a diverter compartment in
order to remove carbon particles and maintain the electrical
strength.
The usual method of protecting the oil
breather system in small transformers is to use silicone gel
breathers to dry incoming air; in larger transformers refrigerated
breathers continuously dry the air in a conservator. Regular
maintenance (at least once a month) is necessary to maintain a silica
gel breather in efficient working order.
If oil samples indicate high water
content then it may be necessary to dry the oil using a heating
vacuum process. This also indicates high water content in the paper
insulation and it may be necessary to redry the windings by applying
a heating and vacuum cycle on site, or to return the transformer to
the manufacturer for reprocessing or refurbishment.
An alternative procedure is to pass the
oil continuously through a molecular sieve filter. Molecular sieves
absorb up to 40 per cent of their weight of water.
Diagnostics and repair
In the event of a failure, the user
must first decide whether to repair or replace the transformer. Where
small transformers are involved, it is usually more economic to
replace the unit. In order to reach a decision, it is usually
necessary to carry out diagnostic tests to identify the number of
faults and their location.
Diagnostic tests may include the
surveillance tests, and it may also be decided to use acoustic
location devices to identify a sparking site, low-voltage impulse
tests to identify a winding fault and frequency response analysis of
a winding to an applied square wave to detect winding conductor
displacement.
If the fault is in a winding, it
usually requires either replacement of the winding in a repair
workshop or rewinding by the manufacturer, but many faults external
to the windings, such as connection or core faults can be corrected
on site.
Where a repair can be undertaken on
site it is essential to maintain dry conditions in the transformer by
continual purging using dry air. Any material taken into the tank
must be fully processed and a careful log should be maintained of all
materials taken into and brought out of the tank.
When a repair is completed, the
transformer must be re-dried and re-impregnated, and the necessary
tests carried out to verify that the transformer can be returned to
service in good condition.
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