3-winding transformer impedance at extreme taps.

[osizer]

I have the following data for a 3-winding transformer

S = 350/350/100 MVA (HV/LV/TV)
U = 410/230/36 kV (HV/LV/TV)
OLTC range: +/-12x1.5% located at HV side
Z(HV-LV) nominal tap = 13% @350
Z(HV-LV) max. tap = 14% @350
Z(HV-LV) min. tap = 13% @350

Can any transformer expert determine the ratio value corresponding to an impedance of 14%? Will it correspond to 483.8/230 or perhaps 336.2/230?

Logic tells me that the 14% will correspond to 483.8/230, due to the greater number of turns, but why then is the impedance for the minimum tap equal to the rated impedance?

 

[prc]

Maximum tap means the highest voltage tap (usually No1 tap), i.e., 483.8/230 kV

 

[FPelec]

Hello osizer,
Given the voltage ratio, I'm guessing that the transformer is actually an autotransformer (400/230 kV) with a tertiary winding (YNad), and a regulating winding connected to the HV line side. Am I correct?
If so, the variation of the short-circuit inductance strictly depends on the winding arrangement (see figure below).
The figure below shows the percent variation of the short-circuit voltage as a function of the tap voltage (expressed as a percentage of the nominal voltage), for different winding arrangements (with the leftmost winding being closest to the core leg, in the following schemes).