Center tapped wye-delta transformer bank test case

On many distribution feeders one or more customers require single-phase and three-phase service. The single-phase service consist of two 120 volts circuits and one-240 volt circuit. This provides service to the "lighting" load. A typical three-phase load is an induction motor. The most common transformer bank connection for providing this combination service is the ungrounded wye-delta with a center tap on the "lighting" transformer. The other two transformers are referred to as the "power" transformers. The resulting secondary is four-wire with the center tap on the lighting transformer grounded. The unbalanced loading and unequal impedances of the transformers lead to an unbalanced three-phase voltage supply to the induction motor. It is important that the distribution engineer be able to accurately calculate the unbalanced voltages resulting from a particular combination of loading and transformer ratings. This paper presents a new IEEE test feeder with the required data for the analysis of the center tapped wye-delta transformer bank. The results of the analysis of the test feeder are included.