Single-line diagram

In power engineering, a single-line diagram (SLD), also sometimes called one-line diagram, is a simplest symbolic representation of an electric power system. A single line in the diagram typically corresponds to more than one physical conductor: in a direct current system the line includes the supply and return paths, in a three-phase system the line represents all three phases (the conductors are both supply and return due to the nature of the alternating current circuits). The one-line diagram has its largest application in power flow studies. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols. Instead of representing each of three phases with a separate line or terminal, only one conductor is represented. It is a form of block diagram graphically depicting the paths for power flow between entities of the system. Elements on the diagram do not represent the physical size or location of the electrical equipment, but it is a common convention to organize the diagram with the same left-to-right, top-to-bottom sequence as the switchgear or other apparatus represented. A one-line diagram can also be used to show a high level view of conduit runs for a PLC control system. 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.