Choosing the
most appropriate method of cooling for a particular application is a common
problem in transformer specification. No clear rules can be given, but the
following guidance for mineral oil-immersed transformers may help. The basic
questions to consider are as follows:
1. Is
capital cost a prime consideration?
2. Are
maintenance procedures satisfactory?
3. Will the
transformer be used on its own or in parallel with other units?
4. Is
physical size critical?
ONAN
This type of
cooling has no mechanical moving parts and therefore requires little, if any,
maintenance. Many developing countries prefer this type because of reliability,
but there is an increasing cost penalty as sizes increase.
ONAF
A
transformer supplied with fans fitted to the radiators will have a rating, with
fans in operation, of probably between 15% and 33% greater than with the fans
not in operation. The transformer therefore has an effective dual rating under
ONAN and ONAF conditions.
The
transformer might be specified as 20/25MVA ONAN/ONAF. The increased output
under ONAF conditions is reliably and cheaply obtained.
Applying an
ONAN/ONAF transformer in a situation where the ONAF rating is required most of
the time is undesirable since reliance is placed on fan operation. Where a
‘firm’ supply is derived from two transformers operating in parallel on a
load-sharing basis the normal load is well inside the ONAN rating and the fans
would only run in the rare event of one transformer being out of service.
Such an
application would exploit the cost saving of the ONAF design without placing
too much emphasis on the reliable operation of the fans. Note that fans create
noise and additional noise mitigating precautions may be needed in
environmentally sensitive areas.
OFAF
Forcing the
oil circulation and blowing air over the radiators will normally achieve a
smaller, cheaper transformer than either ONAF or ONAN. Generally speaking, the
larger the rating required the greater the benefits.
However, the
maintenance burden is increased owing to the oil pumps, motors and radiator fans
required. Application in attended sites, with good maintenance procedures, is
generally satisfactory. Generator transformers and power station interbus transformers
will often use OFAF cooling.
ODAF/ODWF
These are
specialized cooling categories where the oil is ‘directed’ by pumps into the
closest proximity possible to the winding conductors. The external cooling
medium can be air or water.
Because of
the design, operation of the oil pumps, cooling fans, or water pumps is crucial
to the rating obtainable and such transformers may have rather poor naturally
cooled (ONAN) ratings. Such directed and forced cooling results in a compact
and economical design suitable for use in well-maintained environments.
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