Avoiding the use of negative inductances and resistances in modeling three-winding transformers for computer simulations

Negative values of equivalent leakage inductance and winding resistance are often encountered when modeling three-winding transformers in power system simulations. These negative valued parameters can sometimes cause problems in arriving at a steady-state solution, and can result in unstable time-domain simulations. A modified equivalent circuit based on an autotransformer is proposed for three-winding transformer modeling. This model requires only positive values of inductances and resistances and thus leads to stable numerical solutions. Such a model can be derived for most practical situations. The superiority of the proposed model is demonstrated by means of an EMTP/ATP simulation of a test case involving a single-phase three-winding transformer.