The power divides between parallel-connected transformers in a relationship which is inversely proportional to their impedances; a low-impedance transformer operated in parallel with a higher-impedance unit will pass the greater part of the power and may be overloaded. A mismatch in loading of up to 10 per cent is normally acceptable.
For clarification please note to example below:
Transformer 1 is thus overloaded by 20 % and transformer 3 by 6 %. Since the individual transformers should not be subjected to overload, the transformers may only assume a partial load such that the impedance voltage of each is uk = 4 %, as in the case with transformer 1. Therefore,
If this output is not sufficient, another 160 kVA transformer with ukr = 4 % shall have to be installed.