Three-phase transformer modelling for unbalanced conditions Part II: general formulation

Part I of this two-part paper proposed a model for two-winding three-phase transformers under steady-state unbalanced conditions. Although the model is very simple (it only uses two parameters), it is valid for different core designs (three-legged, five-legged and triplex). The nodal equations for four examples are developed step by step in Part I. In part II, phase and sequence nodal equations are systematically obtained for all possible phase shifts and all winding connections, including the isolated wyes and the impedance grounded wyes on one or both transformer sides. The proposed model encompasses the usual model in the literature. The formulation has been tested with the IEEE 4 node test feeder. Three numerical application examples illustrate that the model provides accurate results (compared with the results of a non-linear model) if the transformer is not highly saturated.