Modeling and analysis of unsymmetrical transformer banks serving unbalanced loads

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 quadruplex secondary. The selection of the proper connection and transformer ratings can be achieved by the correct modeling and analysis of a power system consisting of an equivalent source, transformer bank, secondary and loads. This paper develops these 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