Question

what precautions would be necessary if two transformers were to be connected in parallel to supply additive kVA and one transformer had an additive polarity and the second transformer had a subtractive polarity

Answer 1

Solution:

For the parallel operation of two single phase transformers it is necessary that they should have same polarity. Polarity of the two transformers must be same if the are supposed to be operated in parallel mode of operation. It is okay if they have unequal efficiencies, or unequal kVA ratings, as parallel connection is still possible.

if two transformers were to be connected in parallel to supply additive kVA and one transformer had an additive polarity and the second transformer had a subtractive polarity.

The secondary voltage will be different for which a circulating current will flow through the two secondaries if the transformers are connected in parallel. This circulating current will act as the extra load and a large circulating current flow through out the winding without any useful work being done and cause heating of transformers even on no load.

It will be heavy short circuit current flowing in both the transformers. And due to over heating of transformer , transformer will be damaged.