In order not to have circulation currents between transformers and the load be distributed among the
transformers working in parallel to their nominal power, when their are coupled in parallel the following conditions have to be met:
1. primary and secondary nominal voltages of the transformers have to be equal (U1nI=U1nII=U1;
U2nI=U2nII=U2). This condition is practically met by qualling the transformation reports, meaning kI=kII;
2. tri-phased transformers must have the same groups of connections.
3. The short-circuit nominal voltages of the transformers have to be equal, meaning:
ukIn% = ukIIn% their active and inductive components respectively be adequately equal;
4. identical marked terminals of the transformers windings be connected to the same network
conductor, both on the primary part and on the secondary part;
If the parallel coupled transformers comply with all the conditions, their vectorial diagrams at the load working built in relative units they come one over another. In this case, all the transformers are loaded with proportional loads with their nominal powers, and the transformers load currents are arithmetically summed.
Practically, only the second condition has to be thoroughly met, for the first and third conditions
tolerances being accepted.
Besides, the sum of their loads does not have to be higher than the sum of their nominal powers.