High-Frequency Modeling and Simulation of a Single-Phase Three-Winding Transformer Including Taps in Regulating Winding

TL;DR

This paper develops a measurement-based high-frequency model for a three-winding transformer with taps, enabling detailed internal voltage analysis during transient events, and demonstrates its impact on resonant overvoltages in simulations.

Contribution

It introduces a novel measurement-based voltage transfer function model for internal points in a transformer winding, enhancing high-frequency transient simulation accuracy.

Findings

Resonant overvoltages can exceed lightning impulse test levels.

Tap position significantly influences internal voltage behavior.

The model accurately predicts internal voltages during ground faults.

Abstract

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.