First, I'm always getting asked by the linemen if they can bank an "X" impedance transformer with a "Y" impedance transformer. The old REA specs we have never even mention it except in the delta/delta configuration. So we (I) have always assumed that it doesn't matter in the wye/delta and wye/wye banks that we use. Is this true?
Secondly, when the REA specs DID refer to impedance balancing, they would say that the tansformers had to be within plus or minus "X" percent of each other. Were they referring to the "percent" as stamped on the nameplate, or an actual arithmetical percentage between the two (or three) values?
My application notes state that a D-Y bank would share equally a balanced 3ph load even though the impedance of the individual transformers may not be equal. That identical transformers should be used in D-D and open D banks. Also, Y-Y should be avoided.