2transformers in parallel with different kvA rating

Hi guys,
Just wanna know, is it possible to run 2transformers with different kvA rating in parallel if yes how? We had two transformers each 5MVA
running in parallel, since our customer's demand increased we decided to upgrade the one transformer to a 20MVA, now will this be possible, and how?
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On 5 Feb, 09:29, snipped-for-privacy@eskom.co.za wrote:

The following conditions must exist for transformers to operate satisfactorily in parallel:
1. Connection diagrams must be identical. Paralleling transformers with different connection diagrams is similar to short circuiting their secondary windings.
2. Voltage ratios must be the same. If voltage ratios are not the same, circulating currents will flow in the secondaries with no or little load, and the division of load will be improper.
3. Percent impedance, including primary and secondary leads to each transformer, should be nearly equal. If the impedance's are equal and the turns ratios are identical, the paralleled transformers will divide the load currents (properly) in proportion to their kVA ratings. If the percent impedance's are different, the transformer with the lower percent impedance will take more than its proper share of the load.
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On 2/5/07 1:29 AM, in article snipped-for-privacy@m58g2000cwm.googlegroups.com,

I don't know your or your employer's background, but you are asking for professional engineering advice that, legally, can be given only by licensed engineers. Moreover, if your company is in the business of advising clients, they need to have true expertise on the subject.
Bill -- Fermez le Bush--about two years to go.
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wrote:

Theoretically yes you can operate in parallel if the transformer impedances are equal when expressed on a common MVA base and the turns ratio are identical, also the angle of displacement between pri and sec need to be identical.
In practice however, would it be wise to parallel transformers of such disparity? I would seriously consider changing both transformers to 12.5MVA with an emergency rating of 25MVA should one be offline.
You can get reasonable prices for secondhand transformers nowadays so the existing ones could be sold to offset the investment.
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Salmon Egg wrote:

That may be true in some countries but not all. In many countries there is no such thing as a "licenced engineer". In many countries, electrical installations are set up (I won't say designed) and run by those who have had no formal engineering training or education at all.
That may be worth thinking about when choosing a room overseas - or boarding a ferry or other passenger vessel.
--
Sue






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