Conditions for Parallel operation of 3 phase transformer: 1) Voltage ratings of both the transformers must be identical. 2) Percentage / p.u. impedances should be equal in magnitude. 3) X/R ratio of the transformer windings should be equal. 4) Transformer terminals with identical polarity must be connected to each other on secondary side. 5) Phase displacement between primary & secondary voltages must be same. 6) Phase sequence of both must be same.