One very general distribution-transfomer configuration reference is
www.cooperpower.com/Library/pdf/R201902.pdf It sounds like you may be describing an
open-wye-primary, open-delta-secondary bank similar to figure 21.
If the two 1ø transfomers are not of equal kVA ratings, it is because the utility intends for the lighting/general 1ø 3-wire loads directly connected to the larger transfomer via lo-side “bø” and “cø.” Additional 3ø 3-wire loads like motors are fed from both 1ø transformers. In figure 21, the transformer with X2 unconnected is typically the smaller transfomer. {The non-symmetrical bank is anticipated for intentionally unbalanced secondary-phase currents.} In figure 21, lo-side “aø” is connected only to 3ø loads [that are also served from lo-side “bø” and “cø.”] Lo-side “aø” in the diagram is also called the “stinger”/”wild”/”power” leg. [It is more common for US utilities to designate lo-side bø (or cø) as the high leg.]
It is normal for an open-delta-secondary bank to experience some ø-ø voltage unbalance on the loaded transfomer bank, but is typically limited to no more than 5%. In the usual 3-wire delta and 3- or 4-wire wye systems where all transfomeres are sized equally, phase currents are best equally balanced.
www.cooperpower.com/library/pdf/02022.pdf and,
www.cooperpower.com/library/pdf/R240306.pdf on basic distribution-transformer fuse data.
[This message has been edited by Bjarney (edited 05-21-2004).]
