I was provided the data to model a 230/34.35 kV 3 winding transformer for a wind farm with the +/- 12.5 % regulation and +/- 8 taps. The primary-secondary impedance value for the transformer is 11.80% at nominal and 12.4% (8R tap position, 258.75 kV) and 11.81% (8L tap position, 201.25 kV) all using 124 MVA base - Voltage control is on the primary (230 kV) and secondary (34.5 kV) is held constant. Recently, however, realizing that the voltage regulation should be on the secondary, the transformer mfr provided the impedance values of 15.72% (for the 8R, 30.667 kV), 11.87% (for Nominal, 34.5 kV), and 9.03% (for 8L, 39.481 kV). They suggested this as the same impedance value. I have 2 questions, 1. Is what they stated valid and if so, would the short circuit calculation at the lowest tap position still valid? 2. If you were provided the 2 sets of impedance data as above, which one would you use for the short circuit evaluation since the 9.03% impedance would represent a much larger than the 11.81% ? Thanks for any advice.
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