Transformers connection

I do not consider this a good idea. The primaries don't matter, but connecting the secondaries too should only be done with the right clocking. How big of a difference in voltages and currents you have, and what is the capacity of the transformers?

You don't. Transfortmers should have the same KVA rating and the %Z should be the same or very close.

Additional information http://seniordesign.engr.uidaho.edu/2008_2009/oltc/7400DB0701.pdf

GA. You gave a good link on the topic. All important points are covered. Both transformers should be of same vector group same voltage ratio ( the voltage tapping to be checked for same secondary voltage)

Mmmm:

The necessary for parallel operation

Transformers of unequal kVA ratings will share a load practically ( but not exactly) in proportion to their ratings, providing that the voltage ratios are identical and the percentage impedances (at their own kVA rating) are identical, or very nearly so in these cases a total of than 90% of the sum of the two ratings is normally available.

It is recommended that transformers, the kVA ratings of which differ by more than 2:1, should not be operated permanently in parallel.


Other condition necessary for parallel operation

1- All paralled units must be supplied from the same network.
2- Secondary cabling from the transformers to the point of paralling have approximately equal length and characteristics.
3- Voltage difference between corresponding phase must not exceed 0.4%

If the transformers have On-Load Tap Changers (OLTC), then paralleling is easily accomplished using Beckwith Electric Company's tap changer controls and paralleling equipment. See the following link for a number of good application notes from Beckwith that explains the most common method of paralleling control (circulating current method) and other options:

http://www.beckwithelectric.com/main/doccenter/doccenter-index.php#tapchanger

Beckwith's technical personnel are excellent to work with, and give great support of their products.

Hoping that other standard conditions of Parallel Operation - like Same Secondary Voltage Ratings, Same Vector Group, Percentage Impedance Difference within tolerable limits, Same Frequency, Same Phase Displacement, Same Phase Rotation, etc. are taken care - I would wish that the lowest capacity tarnsformer in the group is rated for not less than one third of the highset capacity transformer rating.