One-line diagram
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In power engineering, a one-line diagram is a simplified notation for representing a three phase power system. The one-line diagram has its largest application in power flow studies.
The theory of three phase power systems tells us that as long as the loads on each of the three phases are balanced, we can consider each phase separately. In power engineering, this assumption is usually true (although an important exception is the asymmetric fault), and to consider all three phases requires more effort with very little potential gain.
A one-line diagram is usually used along with other notational simplifications, such as the per-unit system.
A secondary advantage to using a one-line diagram is that the simpler diagram leaves more room for non-electrical, such as economic, information to be included.