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 Y. 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 Y, 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.