For a single phase, 285MVA, generator step up transformer (which is part of a 3-phase bank), 23kV - 425kV, how do I calculate the transformer impedance at the minimum and maximum taps? The transformer has an off load tap changer on the high voltage winding.
You look at the test report and see what the impedances are at those taps. It may be linear, but it may not. There was a thread here not too long ago that you might look up.