Wyes and wye nots of three-phase distribution transformer connections

Abstract

The authors summarize the pros and cons of various three-phase transformer connections. Recommendations are provided regarding the standard types of connections a utility should offer for various service requirements. Consideration is given to warehousing costs, ferroresonance, neutral grounding, core construction, overvoltages, unbalanced loading, zero sequence sources, and through faults. Emphasis is placed on the exchange of information between customer and utility. An application for three-phase service form that can be used to determine service requirements, including voltage, grounded, ungrounded, three-phase load, single-phase load, motor characteristics, and fault current interrupting rating of secondary breakers and switchgear, is provided. It is recommended that a rural electric utility standardize on a limited number of transformer connections to serve three-phase loads. Recommended standard connections include: (1) four-wire service, grounded-wye-grounded-wye; and (2) three-wire service, wye-delta. Suggestions on utility-supplied information such as maximum available three-phase and line-to-ground fault currents are provided, and the operation of ungrounded systems is discussed. >