Transformer terminal equivalents obtained via admittance measurements are suitable for simulating high-frequency transient interaction between the transformer and the network. This paper augments the terminal equivalent approach with a measurement-based voltage transfer function model which permits calculation of voltages at internal points in the regulating winding. The approach is demonstrated for a single-phase three-winding transformer with its tap setting in the middle position. Three internal points in the regulating winding are included that represent the mid-point and the two extreme ends. The terminal equivalent modeling makes use of additional common-mode measurements to avoid error magnifications to result from the ungrounded tertiary winding. The final model is used in a time domain simulation where ground-fault initiation results in a resonant voltage build-up in the winding. It is shown that that the peak value of the resonant overvoltage can be higher than during the lightning impulse test, with unfavorable network conditions. Additional measurements show that the selected tap position affects the terminal behavior of the transformer, changing the frequency and peak value of the lower resonance point in the voltage transfer between windings