Nonlinear, lumped parameter transformer model reduction technique

Utility engineers often need nonlinear transformer models in order to investigate power system transient events. Methods exist to create accurate wideband reduced order linear transformer models, however, to date, a method of creating a reduced order wideband nonlinear transformer model has not been presented. This paper describes a technique that starts with a detailed nonlinear transformer model used for insulation design studies and reduces its order so that it can be used conveniently in EMTP. The method is based on linearization of the core's saturable characteristic during each solution time interval. The technique uses Kron's reduction approach in each solution time interval. It can be applied to any nonlinear lumped parameter network which uses electric parameter analogies (i.e., FEM networks). This paper outlines the nonlinear reduction technique. An illustrative example is given using the transient voltage response during saturation for a 785/345/34.5kV, YYD 500 MVA single phase autotransformer