Editing Single-line diagram

{{Short description|Simplest symbolic representation of an electric power system}}
[[Image:One-line diagram.svg|thumb|right|300px|A typical single-line diagram with annotated power flows. Red boxes represent [[circuit breaker]]s, grey lines represent three-phase bus and interconnecting conductors, the orange circle represents an [[electric generator]], the green spiral is an [[inductor]], and the three overlapping blue circles represent a double-wound [[transformer]] with a tertiary winding.]]
In [[power engineering]], a '''single-line diagram''' ('''SLD'''), also sometimes called '''one-line diagram''', is a simplest symbolic representation of an electric power system.{{sfn|Oliver|1991|p=38}}<ref name=csd>
{{citation
| first1 = Thomas | last1 = McAvinew
| first2 = Raymond | last2 = Mulley
| title = Control System Documentation
| page = 165
| publisher = ISA
| isbn = 1-55617-896-4
| year = 2004}}
</ref> A single line in the diagram typically corresponds to more than one physical [[Electrical conductor|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).{{sfn|Oliver|1991|p=38}}   

The single-line diagram has its largest application in [[Power flow study|power flow studies]]. Electrical elements such as circuit breakers, transformers, capacitors, [[Busbar|bus bars]], and conductors are shown by standardized schematic symbols.<ref name=csd/> 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 single-line diagram can also be used to show a high level view of conduit runs for a [[Power-line_communication|PLC]] control system.

== 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 [[busbar]]s, 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]]).{{sfn|Meier|2006|p=197}} A steady-state of each bus can be characterized by its voltage [[phasor]]; the ''system state'' is defined by a vector<ref name="Ahmad2013">{{cite book | author = Mukhtar Ahmad | date = 2013 | title = Power System State Estimation | publisher = Artech House | page = 166 | isbn = 978-1-60807-511-9 | oclc = 1259189630 | url = https://books.google.com/books?id=T5Pq6Z8h8vsC&pg=PA166}}</ref> of voltage phasors for all the buses.<ref name="PadiyarKulkarni2019">{{cite book | author1 = K. R. Padiyar | author2 = Anil M. Kulkarni | date = 4 February 2019 | title = Dynamics and Control of Electric Transmission and Microgrids | publisher = John Wiley & Sons | page = 12 | isbn = 978-1-119-17338-0 | oclc = 1043202630 | url = https://books.google.com/books?id=9i97DwAAQBAJ&pg=PA12}}</ref> 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 unit]]s.<ref name="Dagle2018">{{cite book | title = Power Electronics and Power Systems | last1 = Dagle | first1 = Jeff | chapter = Importance of Synchrophasor Technology in Managing the Grid | date = 30 May 2018 | pages = 1–11 | publisher = Springer International Publishing | issn = 2196-3185 | eissn = 2196-3193 | doi = 10.1007/978-3-319-89378-5_1 | isbn = 978-3-319-89377-8 | s2cid = 115678159 | url = }}</ref>

==Balanced systems==
The theory of three-phase power systems tells us that as long as the [[Electrical load|load]]s 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'').<ref name=guile_paterson>
{{citation
| first1 = A.E. | last1 = Guile
| first2 = W.| last2 = Paterson
| title = Electrical Power Systems
| edition = 2nd
| page = 4
| publisher = Pergamon
| isbn = 0-08-021729-X
| year = 1977}}</ref><ref name="RAMARKURUSEELAN2013">{{cite book | author1 = S. Ramar | author2 = S. Kuruseelan | date = 25 March 2013 | title = Power System Analysis | publisher = PHI Learning Pvt. Ltd. | pages = 8 | isbn = 9788120347335 | oclc = 1026831292 | url = https://books.google.com/books?id=ahntULiMFgwC&pg=PA8}}</ref> In power engineering, this assumption is often useful, and to consider all three phases requires more effort with very little potential advantage.<ref>
{{Citation
| first = Nasser | last = Tleis
| title = Power System Modelling and Fault Analysis
| publisher = Elsevier
| year = 2008
| page = 28
| isbn = 978-0-7506-8074-5}}</ref> An important and frequent exception is an [[asymmetric fault]] on only one or two phases of the system. 

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 [[Energy economics|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 sequence]]s are identified on the diagrams.  For example, in general a [[Electric generator|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==
* [[Electrical drawing]]

== References ==
{{Reflist}}

== Sources ==
* {{cite book | first1 = Kenneth G. | last1 = Oliver | date = 1991 | title = Basic Industrial Electricity: A Training and Maintenance Manual | publisher = Industrial Press Inc. | pages = 38–41 | isbn = 978-0-8311-3006-0 | oclc = 1000410657 | chapter = Single Wire Diagrams | chapter-url = https://books.google.com/books?id=grl_sNGEOiIC&pg=PA38}}
* {{cite book | first1 = Alexandra von | last1 =  Meier | date = 30 June 2006 | title = Electric Power Systems: A Conceptual Introduction | publisher = John Wiley & Sons | page = 197 | isbn = 978-0-470-03640-2 | oclc = 1039149555 | url = https://books.google.com/books?id=bWAi22IB3lkC&pg=PA197}}

[[Category:Electric power distribution]]
[[Category:Diagrams]]