If there is no same voltage ratio while conducting parallel operation of three-phase transformers, then it produces _________. 

Conditions for parallel operation of three-phase transformers

Necessary Conditions

1. The parallel connected transformers should have the same polarities and voltage ratio.  
2. If the voltage ratios are not the same, then the secondaries will not show the same voltage even if the primaries are connected to the same voltage supply. As a result of this, a current circulates in the secondaries and hence there will also be circulating currents on the primary side. Due to this, a considerable amount of current is drawn by the transformers even without the load.
3. Another necessary condition is the same operating frequency and same phase sequence of both transformers.

Desirable Conditions

1. The winding reactances to the resistances (X/R) ratios in the parallel connected transformers should be the same.
2. Same percentage impedance of both transformers.
3. Identical primary and secondary voltage ratings.
4. Impedances must be inversely proportional to the kVA ratings.