Unequal kVA transformers can share load acceptably if their voltage ratios and per-unit impedances are compatible, but the current does not divide simply by nameplate kVA. Convert impedances to a common voltage and power base, include connecting impedance, then solve current sharing. The smaller or lower-impedance unit may reach its winding-current limit before the total parallel-bank rating is reached.
Equal own-base impedance can give proportional sharing
Assume a 1 MVA and a 2 MVA transformer have identical voltage ratios and both have 6% impedance at the same impedance angle on their own ratings. On a 1 MVA common base, their impedances become 6% and 3%. With negligible connection impedance, current divides in the ratio 1:2, so both reach rated loading together at a 3 MVA bank load. The proportional result follows from the converted impedances; it is not a rule based solely on the two nameplate ratings.
A higher-impedance larger unit can leave capacity unused
Change the 2 MVA unit to 8% impedance on its own base, retaining the other simplifying assumptions. Its common-base impedance is now 4%, and current divides 40% to the 1 MVA unit and 60% to the 2 MVA unit. The smaller transformer reaches 1 MVA when the bank carries 2.5 MVA; the larger then carries only 1.5 MVA. The arithmetic sum of ratings remains 3 MVA, but it is not the unrestricted continuous bank capability in this example.
Apply limits to individual windings and connections
A final study uses complex impedances, actual taps and each connection path rather than the scalar values used in these examples. Check winding current, cooling state, bushings and cable capacity separately. Where voltage mismatch creates circulating current, the magnitude in a transformer can be greater than its useful share of the load. Report bank capability together with the limiting unit and operating condition, so that the owner does not apply one aggregate MVA limit to every configuration.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] Schneider Electric — Substation including generators and parallel operation of transformers.