Five-legged wound-core transformer model: Derivation, parameters, implementation, and evaluation

The ability to predict or confirm ferroresonance and to evaluate its severity depends primarily on the correctness of the transformer model used in the computer simulation. An equivalent circuit is developed here for the widely used three-phase grounded-wye to grounded-wye five-legged wound-core distribution transformer. Recently developed equivalent circuits are generally correct, but cannot faithfully reproduce all of the ferroresonant modes possible in this transformer. This new equivalent circuit is derived using duality transformations. Core saturation surd losses and coil capacitances are included in the model. Model parameters are obtained via measurement, obviating the need for proprietary manufacturer design information. The model is implemented in EMTP. Simulation results are presented and compared to measurements. A technique for predicting ferroresonance on a system level, based on the use of bifurcation diagrams and Poincare sections, is developed and implemented using EMTP. The model performs quite well, even though coil capacitances are not entirely included in the model. One of the key advancements is the development of a bifurcation simulation method to predict ferroresonant behaviors for a large range of capacitance values.