Editing Three-phase electric power (section)

== Transformer connections ==
A "delta" (Δ) connected transformer winding is connected between phases of a three-phase system. A "wye" (Y) transformer connects each winding from a phase wire to a common neutral point.

A single three-phase transformer can be used, or three single-phase transformers.

In an "open delta" or "V" system, only two transformers are used. A closed delta made of three single-phase transformers can operate as an open delta if one of the transformers has failed or needs to be removed.<ref name="Fowler" /> In open delta, each transformer must carry current for its respective phases as well as current for the third phase, therefore capacity is reduced to 87%. With one of three transformers missing and the remaining two at 87% efficiency, the capacity is 58% ({{frac|2|3}} of 87%).<ref name=power>{{cite journal |first=J. B. |last=Gibbs |title=Three-Phase Power from Single-Phase Transformer Connections |journal=Power |publisher=McGraw-Hill |date=April 27, 1920 |volume=51 |issue=17 |page=673 |url=https://books.google.com/books?id=u91QAAAAYAAJ&pg=PA68-IA21 |access-date=21 December 2012}}</ref><ref>H. W. Beaty, D. G. Fink (ed.), ''Standard Handbook for Electrical Engineers''. 15th ed., McGraw-Hill, 2007, {{ISBN|0-07-144146-8}}, pp.&nbsp;10–11.</ref>

Where a delta-fed system must be grounded for detection of stray current to ground or protection from surge voltages, a grounding transformer (usually a [[zigzag transformer]]) may be connected to allow ground fault currents to return from any phase to ground. Another variation is a "corner grounded" delta system, which is a closed delta that is grounded at one of the junctions of transformers.<ref>{{Cite web|url=http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/2700DB0202.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/2700DB0202.pdf |archive-date=2022-10-09 |url-status=live|title=Schneider}}</ref>