I am from ferro alloys production side, i have a doubt that our
33/11kv transformer,9mva would to be overloaded......which can i do
1)i heard that upto 10% can be overloaded. can i..?
2)Can overload 10% of 9mva..ie upto 9.9mva..?
3)rated Highest primary current is 519amps.can i increase upto 10% of
4)Both of the above...?
if not please explain me in details......
Often one can overload a transformer of this size, BUT what is sensible will
be a function of :-
(a) The specification used for the design of the transformer.
(b) The type of cooling system used on the transformer.
(c) The ambient temperature.
(d) The load cycle required. That is it going to be overloaded continuously,
or only for 1 hour a day.
(e) The rating of any associated tap changer.
(f) The ratings of the cable and switchgear connecting it.
Sorry there is no easy answer.
Transformer overloading is a delicate art.
Like every non-trivial engineering question, the answer is always "it
depends" - you can't get a one-line answer from a news group, the answer
will come in the form of a long detailed report - you may wish to hire
an engineering consultant to evaluate the risks and benefits of
overloading the transformer.
You should first get all the nameplate data off the transformer, and the
transformer instruction manual. Contact the factory that built it and
see if you can get the original heat run test report for the unit - you
may even have that. How old is the unit? How hard has it been pushed in
the past? If it's more than a couple of decades old, it could have
deteriorated paper insulation.
Get the gas-in-oil analysis results for the last several years, see if
there's any indication the transformer is already in distress and
overheating in any part.
Check that all the cooling equipment is working at top efficiency - for
example, water-cooled heat exchangers are clean and supplied with clean
water at the rated (or lower) temperature - or if it is an outdoor
air-cooled unit, that all the fans and radiators are in good condition.
Most of the heating in a transformer will be because of copper losses
proportional to the square of current - you are unlikely to also
increase voltage as much as 10%. 10 % more current means 20% more heat
and more temperature rise in the windings - if a 20% increase in winding
temperature over cooling water exceeds the transformer insulation
rating, you are gambling on a short life for the unit.
What is the duty cycle of the proposed overload? There are IEEE guides
to cyclical loading of transformers that may be relevant.
My experience is limited to some exposure to arc furnace transformers,
where an increase in primary voltage increases the arc voltage and is
actually useful - I don't know if a ferroalloy furnace works the same
way. Arc furnace transformers are routinely overloaded for at least part
of each heat, but then arc furnace transformers in steelmaking shops
get changed out very frequently (4 times in 17 years, for example).
This is a classic engineering question and needs a lot of judgement and
experience to answer correctly. No professional engineer is going to
give you specific advice in a newsgroup - even if he or she is not paid
for an engineering opinion, he or she is still exposed to liability.
Judging from your voltages I expect you are in a 50 Hz country , and the
transformer may be built to IEC, not ANSI standards. You would be well
advised to hire a local professional engineer to study this problem, the
cost of his report will be very small compared to the cost of lost
production and a rewind if you damage the existing transformer.
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