A new approach to modeling three-phase transformer connections

The modeling of three-phase transformer connections for power-flow and short-circuit studies can take on many forms, depending upon the assumptions made. Too many times, simplifying assumptions lead to answers that are totally wrong. Exact models of the various connections must be used if correct results are to be achieved. The exact models must satisfy Kirchhoff´s voltage and current laws, and the ideal relationship between the voltages and currents on the two sides of the transformer windings and the models must represent any phase shift that is a result of the particular connection. New models for all common three-phase transformer connections have been developed that satisfy all of the requirements stated above. This paper limits the discussion to the ungrounded wye-delta connection and the model developed for use in power-flow and short-circuit studies