The Wye-Delta Center Tapped Transformer Bank

A common way of providing service to a combination three-phase motor and single-phase lighting load is to use a wyedelta transformer bank. The question is whether or not the neutral should be grounded or ungrounded. The purpose of this paper is to present the pros and cons of grounding or not grounding the wye connection. Initially the grounded connection will be studied in a general sense for when all three-phases are connected and then the effects of open phases. Following that the ungrounded case will be studied. As basic transformer theory is applied it will be shown that in both connections there will be "backfeed voltages" on the transformer side of the open phase.