Hi,

when dealing with single phase transformers to be connected to form a bank of three phase power transformer, the impedance per transformer should be used. this is independent when it is connected as an auto transformer bank. But, since the auto transformer is like a three-winding transformer, the nameplate of the transformer will have illustrated three values of impedances measured with respect to each other (i.e. H-X, H-Y, X-Y). In this case, you have to solve the value of impedance per side of transformer (H, X, Y) because these data are needed in the power flow software. the formula for that is usually written in several excellent texts in power system. In the case of PSS/E, you will have to just input the nameplate parameters and the software engine will solve the required impedances.