Application of vector fitting to state equation representation of transformers for simulation of electromagnetic transients

This paper presents an efficient methodology for transient modeling of power transformers based on measured or calculated frequency responses. The responses are approximated with rational functions using the method of vector fitting. The resulting model is dynamically stable since only negative poles are used, and input-output stability is assured by a novel adjusting procedure. The methodology is used to formulate two different transformer models: (1) a full phase domain model realized column-by-column, and (2) a model utilizing modal diagonalization. The application of the models is demonstrated for a 410 MVA generator step up transformer.