Frequency-Dependent Modeling of Power Transformers With Ungrounded Windings

A linear wideband transformer model for the purpose of simulation of electromagnetic (EM) transients has previously been developed by the author. The model was obtained via frequency-domain measurements and rational approximation of the terminal admittance matrix . In the case of transformers with ungrounded windings, the model may produce erroneous results due to a near singular condition at low frequencies. This paper shows that the problem can be overcome by explicit measurement and modeling of the zero sequence system. Blocks of , which correspond to ungrounded windings, are modified by removal of their zero sequence components. A new zero sequence system is obtained by dedicated measurements and the two functions are approximated with rational functions which are combined to produce the final model. Application to a wye-delta-connected transformer demonstrates the validity of the approach.