This paper reports on advanced transformer modeling facilities suitable for large-scale optimal power flow studies. The new transformer models are developed from first principles and incorporated into an existing Newton-based optimal power flow computer program for highly robust iterative solutions. A three-winding transformer model with tap ratios in all three windings is shown to be a general case for existing two-winding transformer models and the classic load tap-changing and phase-shifting transformer models. The newly developed transformer models add a great deal to software flexibility and are amenable to more realistic electric energy studies. This is partly due to the transformer models being fitted with complex tap changers in each winding and a nonlinear representation of the magnetizing branch. The three-winding transformer model interfaces easily with reactive power plant models, e.g., static VAr compensators.
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