Trfr %Z varaition with size

[veritas]

Hi

I have a 23/11kV, 45/58MVA, delta-star trfr. Z% = 9.8% @45MVA. How do I determine what Z% is at 58MVA? Is there a general formula one can apply to calculate impedances at the different ratings for the same transformer?

Thanks in advance.

Veritas

 

[magoo2]

You really shouldn't have to recalculate the %Z for the 2 MVA ratings, but if you must

the % Z at 58 MVA will be 45/58 * 9.8% or about 7.6%.

 

[alehman]

You should set the base MVA of your calculations to match the 45MVA rating if possible, so you don't have to do the conversion.

 

[veritas]

magoo2

Thanks for that. However, I get that the new impedance is about 12.63% and not 7.6% (58/45 * 9.8%). Zbase = 11.756ohms @45MVA and it is 9.12ohms @58MVA. Thus for a given ohmic impedance the pu impedance must be greater if I express it at 58MVA than 45MVA.

Thanks again.

 

[magoo2]

Sorry I goofed. Your result is correct.

You should get the same ohms at both bases.

The 12.63% at 58 MVA is correct.

 

[veritas]

No worries mate, your input is much appreciated.

Kind regards.