Negative inductance and numerical instability of the Saturable Transformer Component in EMTP

Numerical instability of the EMTP built-in model, the Saturable Transformer Component for the three-winding case, has been the topic of interest of the EMTP community for the past twenty years. Through theoretical analysis and computer simulation, the source of the numerical instability of the component is identified as the use of the negative inductance in the equivalent circuit of the transformer. A simple modification of the structure of the component is suggested, and the numerical instability is eliminated. The modification is supported by theoretical analysis and verified by the benchmark test of the GPU line energization test of 1973. The stability of the three-winding transformer equivalent circuit with a negative inductance is discussed in detail. A topology-based and duality-derived single-phase three-winding transformer model is described.