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transformer bank

transformer bank

transformer bank

(OP)
If I'm considering a 3 phase transformer bank (made from 3 single phase transformers -> 25kV - 120/208 V, 50kVA, Z=2%, wye-wye), how would I model such a thing as one unit ?
(i.e. what would the impedance be, if I were modeling the above as a 3 phase transformer for computer simulations ?).

Thanks in advance

RE: transformer bank

Seems to me you'd still use 2% as a transformer impedance.  No sqrt(3) required.  But transformer rating should be entered as 3x single-phase tx kVA rating.

Think of it this way:
120=I*Zt1-ph
208=I*Zt3-ph
Either way, the transformer current I is the same; that means Zt3 = Zt1 * sqrt(3).

But with per unit,
1pu=1pu*Ztpu (regardless of voltage).

Note that the conversion from %Z to Zpu will change, though, as the kVA base and transformer kVA change from 1-phase to 3-phase calcs.

RE: transformer bank

It can be modelled as a 150 kVA, 3 phase, 208V unit with 2%Z as peebee also mentioned.

RE: transformer bank

I have a similar question.  I have three each 10MVA, 12.8% single phase transformers.  However they are connected delta-delta.  Would the equilvalent three phase, positive sequence impedance also be 12.8%?

RE: transformer bank

Yes

RE: transformer bank

Why?
I can see with a wye-wye connection that the line to neutral voltage divided by rated current is impedance, but with delta-delta, there isn't a neutral.

RE: transformer bank

So?  You can still divide line-to-line by sqrt(3) can't you?

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