The transformers are connected in parallel when load on one of the transformers is more than its capacity. The reliability is increased with parallel operation than to have single larger unit. The cost associated with maintaining the spares is less when two transformers are connected in parallel.
The following conditions are to be satisfied while connecting three phase transformers in parallel.
i) The transformers connected in parallel must have same polarity so that the resultant voltage around the local loop is zero. With improper polarities there are changes of dead short circuit.
ii) The relative phase displacements on the secondary sides of the three phase transformers to be connected in parallel must be zero. The transformers with same phase group can be connected in parallel.
As the phase shift between the secondary voltages of a star/ delta and delta/ star transformers is 30o, they can not be connected in parallel. But transformers with + 30oand - 30ophase shift can be connected in parallel by reversing phase sequence of one of them.
iii) The voltage ratio of the two transformers must be same. This prevents no load circulating current when the transformers are in parallel on primary and secondary sides. As the leakage impedance is less, with a small voltage difference no load circulating current is high resulting in larger I2 R losses.
iv) The ratio of equivalent leakage reactance to equivalent resistance should be same for all transformers. If there is difference in this ratio, the phase angle of the two currents show divergence. One transformer will operate at higher p.f. while other transformer will operate at lower p.f. Due to this given load is not proportionately shared by them.