View source for Voltage sag

{{short description|Short-duration reduction in the voltage of an electric power distribution system}}
A '''voltage sag''' (U.S. English) or '''voltage dip'''<ref name=IEEE493>{{cite web |title=IEEE 493-2007 - IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems |website=The IEEE Standards Association |date=2007-02-12 |url=https://standards.ieee.org/ieee/493/3402/ |access-date=2018-01-09}}</ref> (British English) is a short-duration reduction in the [[voltage]] of an [[electric power distribution]] system. It can be caused by high current demand such as [[inrush current]] (starting of [[electric motor]]s, [[transformer]]s, heaters, power supplies) or fault current ([[electrical overload|overload]] or [[short circuit]]) elsewhere on the system.<ref name=Bollen>{{cite book |last=Bollen |first=Math H.J. |title=Solving power quality problems: voltage sags and interruptions |year=1999 |publisher=IEEE Press |location=New York |isbn=978-0-7803-4713-7 |page=139}}</ref>

Voltage sags are defined by their magnitude or depth, and duration.<ref name=Karady/> A voltage sag happens when the RMS voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.<ref name=Bollen/><ref name=UTS>{{cite web |title=Industrial Voltage Regulator Power Conditioner |url=http://www.ustpower.com/Support/SupportKnowledgeBase/KnowledgeBasePowerQualityProblems/Sag_Dip.aspx |publisher=Utility Systems Technologies |access-date=25 September 2013}}</ref> Some references define the duration of a sag for a period of 0.5 cycle to a few seconds,<ref name=Newnes/><ref name=Teodorescu>{{cite book |title=Grid Converters for Photovoltaic and Wind Power Systems |year=2011 |publisher=Wiley-IEEE Press |isbn=978-1-119-95720-1 |author1=Remus Teodorescu |author2=Marco Liserre |author3=Pedro Rodríguez}}</ref> and a longer duration of low voltage would be called a "sustained sag".<ref name=Newnes>{{cite book |title=Practical grounding, bonding, shielding and surge protection. |year=2004 |publisher=Newnes |location=Oxford |isbn=978-0-08-048018-3 |page=134 |author=Vijayaraghavan, G, Mark Brown, and Malcolm Barnes}}</ref> The definition of voltage sag can be found in IEEE 1159, 3.1.73 as "A variation of the RMS value of the voltage from nominal voltage for a time greater than 0.5 cycles of the power frequency but less than or equal to 1 minute. Usually further described using a modifier indicating the magnitude of a voltage variation (e.g. sag, swell, or interruption) and possibly a modifier indicating the duration of the variation (e.g., instantaneous, momentary, or temporary)."<ref name=Karady/>

== Voltage sag in large power system==
The main goal of the [[power system]] is to provide reliable and high-quality electricity for its customers. One of the main measures of [[power quality]] is the voltage magnitude. Therefore,  Monitoring the power system to ensure its performance is one of the highest priorities.<ref>Association IS (2014) IEEE trial-use recommended practice for voltage sag and short interruption ride-through testing for end-use electrical equipment rated less than 1000V. Standard P1668-2014</ref> However, since power systems are usually grids including hundreds of [[busbar|buses]], installing measuring instruments at every single [[busbar]] of the system is not cost-efficient. In this regard, various approaches have been suggested to estimate the voltage of different buses merely based on the measured voltage on a few buses.<ref name="Jalalat Liasi 2022 p. ">{{cite journal | last1=Jalalat | first1=Hamed | last2=Liasi | first2=Sahand | title=Optimal location of voltage sags monitoring buses, based on the correlation rate between network buses with considering the probability distribution of variables | journal=Electrical Engineering | publisher=Springer Science and Business Media LLC | date=2022-11-16 | volume=105 | pages=509–518 | issn=0948-7921 | doi=10.1007/s00202-022-01674-6 | s2cid=253619994 }}</ref>

==Related concepts==
The term "sag" should not be confused with a  [[brownout (electricity)|brownout]], which is the reduction of voltage for minutes or hours.<ref>{{cite book |last=Standler |first=Ronald B. |title=Protection of electronic circuits from overvoltages |year=1989 |publisher=Wiley |location=New York |isbn=9780471611219 |page=40}}</ref>

The term "transient", as used in [[power quality]], is an [[umbrella term]] and can refer to sags, swells, dropouts, etc.<ref name="VedamSarma2008">{{cite book |author1=R. Sastry Vedam |author2=Mulukutla S. Sarma |title=Power Quality: VAR Compensation in Power Systems |year=2008 |publisher=CRC Press |isbn=978-1-4200-6482-7 |pages=4–5 and 23}}</ref>

=={{anchor|Voltage swell}}Swell==
{{See also|Voltage surge}}
'''Voltage swell''' is the opposite of voltage sag. Voltage swell, which is a momentary increase in voltage, happens when a heavy load turns off in a power system.<ref name=Kazibwe>{{cite book |last=Kazibwe |first=Wilson E. |title=Electric power quality control techniques |year=1993 |publisher=Van Nostrand Reinhold |location=New York |isbn=978-0-442-01093-5 |page=11 |author2=Sendaula, Musoke H.}}</ref>

==Causes==
Several factors can cause a voltage sag:
* Some electric motors draw much more current when they are starting than when they are running at their rated speed.<ref name=Bollen/><ref name=Kazibwe/>
* A line-to-ground fault will cause a voltage sag until the protective [[switchgear]] ([[fuse (electrical)|fuse]] or [[circuit breaker]]) operates.<ref name=Bollen/><ref name=Kazibwe/>
* Some accidents in [[power line]]s such as [[lightning]] or a falling object can cause a line-to-ground fault.<ref name=Kazibwe/>
* Sudden load changes or excessive loads<ref name=Kazibwe/>
* Depending on the [[transformer]] connections, transformers energizing<ref name=Teodorescu/>
* Voltage sags can arrive from the power utility, but most are caused by local in-building equipment. In residential homes, voltage sags are sometimes seen when refrigerators, air-conditioners, or furnace fans start up.

'''Factors that affect the magnitude of sag caused by faults:'''
* The distance between the victim and the fault source <ref name=Karady>{{cite web |last=Karady |first=George |title=Effect of voltage sags on loads in a distribution system |url=https://pserc.wisc.edu/wp-content/uploads/sites/755/2018/08/T-16_Final-Report_Oct-2005.pdf}}</ref>
* The fault impedance<ref name=Karady/>
* Type of fault<ref name=Karady/>
* The voltage before the sag occurs<ref name=Karady/>
* System configuration, e.g. system impedance and transformer connections<ref name=Karady/>

==See also==
* {{annotated link|Low voltage ride through}} (LVRT)
* {{annotated link|Supply voltage sag}}
* {{annotated link|Voltage spike}}

==References==
{{Reflist}}

[[Category:Voltage stability|Sag]]
[[Category:Power engineering]]