Transformer 9mva

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 519amps.? or 4)Both of the above...? if not please explain me in details...... thanx
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Hello
In article

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.
John
--
John Rye
Hadleigh IPSWICH England
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snipped-for-privacy@yahoo.co.in wrote:

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.
Bill Shymanski
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