transformer bank

[tonyflair]

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

 

[peebee]

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.

 

[rbulsara]

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

 

[Zzero]

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%?

 

[dpc]

Yes

 

[Zzero]

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.

 

[davidbeach]

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