It is a common practice to serve combination three-phase and single-phase loads from an unsymmetrical three-phase transformer bank and a four-wire secondary. Depending upon the loads and company standards, nine different transformer connections can be considered along with either an open four wire or a quadraplex secondary, The selection of the proper connection and transformer ratings can be achieved by the correct modeling and analysis of a system consisting of an equivalent source, transformer bank, secondary, and loads. This paper develops the models for the nine transformer connections, the secondary, and the loads, A computer program has been written that allows the analysis of the system for loading and short-circuit studies. The paper includes several examples of different normal and abnormal operating conditions on some of the transformer connections.
