Base MVA for Transformer impedance on ETAP 1

[mastergate]

I have a tansformer as follows:
Voltage: 60kV / 10kV
Rated power: 45/60/75 MVA (ONAN/ONAF/ONAF)
Impedance voltage at rated power and rated voltage at 75°C: 10% +/- 7.5% tol.(at 45 MVA)

ETAP does not allow to change base MVA (it is always the biggest rating). Please how to specify on ETAP the correct impedance?

Thank you.

 

[cuky2000]

How relevant is to change he MVA base if the end result is the same for the given MVA choose by the program?

 

[dpc]

I find it extremely curious that ETAP would force the use of highest forced cooled rating as a base. You might want to double-check that. For an ANSI-rated transformer, the impedance is always based on the loweest self-cooled rating. IEC generally uses the highest forced cooled rating.

In any event, you can easily adjust the %Z by changing bases. (%Z at 45 MVA) = (%Z at 75 MVA) x 45/75

Cheers,

Dave

 

[Kiribanda]

mastergate,
Why donot you contact ETAP HELP desk for this type of program related specific questions?

 

[mastergate]

@dpc

Z Base: This value is used as the base MVA for the transformer impedance and depends on the standard selection.
ANSI: Base MVA = Class1 MVA.
IEC: Base MVA = the largest available class MVA.

 

[argotier]

dpc is right but take into account that if you have the Z at 45 MVA and need it at 75 MVA the conversion is the other way around, like:

(%Z at 75 MVA) = (%Z at 45 MVA) x 75/45