Balanced systems
The theory of three-phase power systems tells us that as long as the loads on each of the three phases are balanced, the system is fully represented by (and thus calculations can be performed for) any single phase (so called per phase analysis).[7][8] In power engineering, this assumption is often useful, and to consider all three phases requires more effort with very little potential advantage.[9] An important and frequent exception is an asymmetric fault on only one or two phases of the system.
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 space for non-electrical, such as economic, information to be included.
Buses
The lines in the single-line diagram connect nodes – points in the system that are
"electrically distinct" (i.e., there is nonzero electrical impedance between them). For sufficiently large systems, these points represent physical busbars, so the diagram nodes are frequently called buses. A bus corresponds to a location where the power is either injected into the system (e.g., a generator) or consumed (an electrical load).[3] A steady-state of each bus can be characterized by its voltage phasor; the system state is defined by a vector[4] of voltage phasors for all the buses.[5] In a physical system the state is calculated through power system state estimation, since the end of the 20th century this process involves direct simultaneous measurements (synchrophasor) using the phasor measurement units.[6]