Adding transformer banking requirements

Adding transformer from existing open delta or two single phase
transformer to make it three single phase transformer
are the same requirements to parallel transformer operation
list below are the requirements.

1. The voltage ratings of both primaries and secondaries must be indentical.
2. The transformer must be properly connected with regards to polarity.
3. The ratio of the equivalent resistance to reactance of all transformers
should be the same.
4. The transformer percent impedance should be the same.