Why transformers connected in parallel should have same voltage ratio?

Necessary conditions for parallel operation of transformers:

1. Voltage ratings mentioned on the nameplate of the transformers to be connected in parallel must be the same means the primaries of all the transformed must have the same magnitude of line voltage.
2. The transformers must be connected in parallel with the correct polarity.
3. The phase sequence of the secondaries of all transformers to be connected in parallel must be the same.
4. As a part of the phase sequence matching the phase displacement between primary and secondary line voltages must be the same for all transformers. This condition can be fulfilled if all transformers belong to the same phasor group.

Desirable conditions for parallel operation of transformers:

• The voltage ratios of the transformer to be connected in parallel should be the same to avoid circulating currents.
• All the transformers should share the common load proportional to their kVA ratings.
• The (X /R) ratio of transformers should be equal to avoid the operation of transformers at different power factors.