There are other issues to look at when paralleling trasnformers. Circulating currents caued by mismatched voltage ratios and impedances can reduce the MVA capability of teh transformers. Ideally, only identical trasnformers should be paralelled.
For proper parallel operation, the voltage ratios, %impedances and X/R ratios must be the same.
If the voltage ratios are different this will give circulating current in most cases.
If the %impedances are the same but the X/R ratios are different the transformers may share the load but the sum of the individual transformer currents may exceed the total current. The effect of this is to reduce the combined capacity of the transformer pair.
If the X/R ratios are different the load sharing may shift if the load power factor changes.
For a rigorous solution, resolve the transformer impedances into reactance and resistance. Resolve the load current into reactive current and in-phase current and work from there.
respectfully
In paralleling two transformers, equal loading and safe operations, you have to ensure the following:
1. Same primary and secondary rated voltages.
2. Same kVA and Impedance Ratings.
3. Same Vector Group.
4. Same System Grounding Methods.
You may connect transformers with unequal kVA and Impedance ratings but you have to determine the maximum safe loading so you won't overload either of them. Let's try one example:
If the percent impedances, X/R ratios, voltages, and connections are the same, then the load will divide in proportion to the MVA ratings and rcwilson's equation MVA total = MVA 1 + MVA 2 is correct.
I agree with jghrist. It is of best engineering to parallel identically rated transformers. If both transformers are connected in parallel and are of the same rating then rcwilson's equation is correct = MVA1 + MVA2 is correct.
Going back to the equation KVA1*(Z1) = KVA2*(Z2)
If both transformers are identical say 1.5MVA, Z=6%
Then if transformer T1 is loaded = 1.5MVA then
1.5*(0.06) = KVA2*(0.06)
KVA2 = 1.5MVA therefore maximum safe loading for two connected transformers is KVA1 + KVA2 which is exactly rcwilson's equation.
However, be reminded that MVA1+MVA2 is correct as long as there are two transformers in parallel. if one is out (tripped out), then your safe loading is reduced by 50% otherwise one transformer will be overloaded by 150%.
Therefore it also of good engineering to size each transformer to have the capability to cater the total bus load or to have each transformers designed to have a FA rating or 150% of OA rating.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
The current under short circuit conditions is determined by the transformer impedance.
The directed voltage drop under normal loading, which determines load sharing, is predominantly influenced by the transformer regulation.
Transformers may have equal impedances but different X/R ratios and different voltage regulation.
Not to contradict the previous posters but to point out that they have included the disclaimer, "If both transformers are identical".
Equal %Z does not imply equal or similar transformers.
respectfully
