Single-line diagram

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). A steady-state of each bus can be characterized by its voltage phasor; the system state is defined by a vector of voltage phasors for all the buses. 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. ==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). {{citation {{Citation A single-line diagram is usually used along with other notational simplifications, such as the per-unit system. A secondary advantage to using a single-line diagram is that the simpler diagram leaves more space for non-electrical, such as economic, information to be included. ==Unbalanced systems==

When using the method of symmetrical components, separate single-line diagrams are made for each of the positive, negative and zero-sequence systems. This simplifies the analysis of unbalanced conditions of a polyphase system. Items that have different impedances for the different phase sequences are identified on the diagrams. For example, in general a generator will have different positive and negative sequence impedance, and certain transformer winding connections block zero-sequence currents. The unbalanced system can be resolved into three single line diagrams for each sequence, and interconnected to show how the unbalanced components add in each part of the system. ==See also==