  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 in tap
position Nom+ with inclusion of three internal points in the regulating winding
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.
