Negative inductance and numerical instability of the Saturable TransformerComponent 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 th e EMTP community for the past twenty years. Through theoretical analysis an d computer simulation the source of the numerical instability of the compon ent is identified as the use of the negative inductance in the equivalent c ircuit of the transformer. A simple modification of the structure of the co mponent is suggested, and the numerical instability is eliminated. The modi fication 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 discu ssed in detail. A topology-based and duality-derived single-phase three-win ding transformer model is described.