Parallel Operation of Transformers

When the load current increases above the capacity of one transformer, we can connect one more in parallel instead of replacing the existing transformer with a bigger one.Following are some of the reasons for connecting transformers in parallel.
1)Higher Reliability
2)Better maintenance
3)Its is easy to make addition in existing setup
If a single large transformer is used then its failure will shutoff the installation. The parallel transformers solve this problem and improve reliability. It is possible to repair or maintain the transformers in parallel connection one by one without shutting off the plant. With increase in load current we can easily add new transformer to existing parallel unit.

Conditions to be satisfied

1)The transformers should be connected taking their polarity into consideration such that the net voltage around the loop is zero. If the polarities go wrong then there will be a short circuit.

2)The order in which the phase (RYB) reaches their maximum positive voltage must be same for both transformers, if not then they will be each pair phase will  be short-circuited

3)The voltage ratio of transformers should be the same. This will avoid the no load circulating current

4)The per unit impedance of the paralleled transformers should be approximately equal. The currents delivered by the two transformers  are proportional to their ratings of their per unit impedance are equal. The ratio of equivalent leakage reactance to equivalent resistance should be the same for all the transformers. If they are unequal, then the transformers will operate with different power factors.