Conditions for Paralleling Two or More Transformers

The voltage has to be the same, the frequency has to be the same and all phases have to be the same, for starters. Each tranny has to have its own protection arrangements for seconds, and a protocol for what happens if one of the two transformers goes off-line is the third requirement.

To add to what PWSlack said, it is best for the transformers to be the same kVA rating and very close % impedance, or they will not share the load equally. If they are different ratings, they can't necessarily be expected to share the load proportionally, so it can cause problems with overloading/overheating.

If they have on-load tap changers (OLTC), there needs to be additional control circuitry for the OLTCs to work in parallel to prevent runaway tap changing, circulating currents, and uneven load sharing.

The transformers to be connected in parallel to share the load proportional to their KVA ratings, the following conditions are required to fulfill.

1. Primary windings of the transformers should be suitable for the supply system voltage and frequency.

2. The transformers should be properly connected with regards to polarity.

3. The ratings of both primaries and secondaries should be identical. In other words, the transformers should have the same turn ratio i.e. transformation ratio.

4. The percentage impedances should be equal in magnitude and have the same X/R ration in order to avoid circulating currents in secondary loop formed by two transformers secondary windings. So they do not share load KVA in proportional to their KVA ratings. And causes more heating in the secondaries.

5. With transformers having different KVA ratings, the equivalent impedances should be inversely proportional to the individual KVA rating if circulating currents are to be avoided.

Generally paralleling of transformers is avoided to share the load. So the load is divided during the design stage such that each load can be fed by individual transformer. This solves a big problem that if one of the parallel transformers were fail it could not be found matching transformer to replace with new one especially having the same percentage impedance of the failed transformer

Added to all the above replies vector group should be same.

For detailed information on transformer parallelling see the link below

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