Inclusion of Neutral Points in Measurement-Based Frequency-Dependent Transformer Model

Measurement-based wide-band black-box transformer models are well suited for simulation studies of high-frequency transients in power systems. The inclusion of neutral points in such models can be difficult when the model is required to simulate voltage transfer to windings with high-impedance loads. The difficulty arises because it is then necessary to include the small winding capacitive currents in the measurement and model extraction procedure, while at the same time the capacitances can be asymmetrical with respect to the winding ends. This paper introduces a new measurement procedure which can accurately capture the effect of such capacitive asymmetries. The procedure is demonstrated for a wye-wye connected transformer. The measurement and subsequent passive rational modeling results in two alternative models. The first is a four-terminal model with bonded high-voltage and low-voltage terminals, suitable for simulation of lightning overvoltages in distribution systems. The second is a full eight-terminal model, suitable for general purpose transient studies.