impedance calculation of three transformer bank

[budhiman]

Hello

I was wondering how will the total %Z of the three transformer bank calculated. This is for fault current calculations. Will it be averaged?

 

[magoo2]

If you put a short circuit on a single phase transformer and apply enough voltage to get rated current, the % of rated voltage will be equal to the %Z of the transformer. That is how it is measured in the first place. It should be obvious that a low impedance transformer will draw more current than a high impedance one.

The %Z for each transformer is calculated separately. If the impedances are different enough and you have a three phase fault, you will get different fault currents in each phase. The higher impedance transformers will draw less current than the low impedance ones.

If you have a phase to phase fault or a single phase to ground fault, again with different impedances, the resulting fault currents can be calculated with symmetrical components and they will be different depending again on how different the impedances are and what phase are involved. The results are not as obvious with the three phase fault current case and you would have to pick some specific examples and run through the calculations.

All these comments assume that the source impedance ahead of the transformer bank are not significant.

 

[prc]

%Z of three phase bank is same as %z of single phase unit.

 

[stevenal]

prc is correct if the three transformers are matched in kVA and impedance. See magoo2's answer if not.