Correct Answer - Option 3 : Polarity
For parallel connection of single phase transformers, primary windings of the transformers are connected to source bus-bars and secondary windings are connected to the load bus-bars.
Various conditions that must be fulfilled for the successful parallel operation of single phase transformers:
1. Same voltage ratio & turns ratio (both primary and secondary voltage rating is same).
2. Same percentage impedance and X/R ratio.
3. Identical position of tap changer.
4. Same KVA ratings.
5. Same phase angle shift (vector group are same).
6. Same frequency rating.
7. Same polarity.
8. Same phase sequence.
Some of these conditions are convenient and some are mandatory.
The convenient are: Same voltage ratio & turns ratio, same percentage impedance, same KVA rating, same position of tap changer.
The mandatory conditions are: same phase angle shift, same polarity, same phase sequence and same frequency.
When the convenient conditions are not met paralleled operation is possible but not optimal.
For the parallel operation of three phase transformers same conditions are applicable with the additional condition that the transformers should be belong to same vector group.