Transformer Base Conversion

[timm33333]

Base change formulas are used to change the transformer-impedance from transformer-base to system-base. Can these formulas also be used to convert the impedance from one transformer base to another transformer base?

For example for a 10 MVA 34.5kV-12.47kV transformer: the impedance is given to be 8% at a base of 34.5kV-13.8kV. To change this impedance to a base of 34.5kV-12.47kV:
(Z at 12.47kV) = (Z at 13.8 kV) * (13.8^2/12.47^2) = 8% * 1.224 = 9.79 %

Is it correct? Thanks.

 

[xnuke]

While your math is correct for the change of base formula, I see no reason why a manufacturer would give the transformer percent impedance using different ratings for the base voltages than the rated voltages. That seems weird and wrong.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cranky108]

Yes I believe you can, and we do operate several transformers at different base voltage, then the rated voltage.

I also recall a 6.9kV generator operating on a 7.2kV system.

 

[7anoter4]

You don't change the base voltage, you change the rated secondary voltage. It is not the same transformer so Z it may be different. You have to know the new short-circuit voltage or the new Z.