Variable Impedance Transformer Models for use in Real-Time Security Analysis Functions

Traditional constant leakage reactance transformer models used in state estimation and load flow oriented real-time functions are not always appropriate for two-winding and three-winding transformers. In some cases the leakage reactance varies as much as fifty percent (50%) over the full transformer tap range. For three-winding transformers, the reactance for one of the legs of the model may change sign over the full transformer tap range. This paper presents the transformer models used for these variable leakage reactance transformers. Field state estimation results are included.