Three-phase Transformer Modeling.

We present a simple and transparent method to derive the admittance matrices of three-phase banks of single-phase transformers in an unbalanced setting. The basic idea is to explicitly separate a transformer model into an internal model that specifies the characteristics of the constituent single-phase transformers, and a conversion rule that maps its internal variables to its terminal variables. Its admittance matrix can then be derived by eliminating the internal variables from the set of equations describing the internal model and the conversion rule. The method is general and provides rigor and clarity, facilitating extensions. It also reveals a transformer model that is unified and modular.