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{{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/ |archive-url=https://web.archive.org/web/20220325112732/https://standards.ieee.org/ieee/493/3402/ |url-status=dead |archive-date=March 25, 2022 |access-date=2018-01-09}} (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 motors, transformers, heaters, power supplies) or fault current (overload or short circuit) elsewhere on the system.{{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}}

Voltage sags are defined by their magnitude or depth, and duration. A voltage sag happens when the RMS voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.{{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}} Some references define the duration of a sag for a period of 0.5 cycle to a few seconds,{{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}} and a longer duration of low voltage would be called a sustained sag.{{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}} 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)."

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.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 However, since power systems are usually grids including hundreds of 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.{{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 }}

Related concepts

The term sag should not be confused with a brownout, which is the reduction of voltage for minutes or hours.{{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}}

The term transient, as used in power quality, is an umbrella term and can refer to sags, swells, dropouts, etc.{{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}}

{{anchor|Voltage swell}}Swell

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.{{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.}}

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.
A line-to-ground fault will cause a voltage sag until the protective switchgear (fuse or circuit breaker) operates.
Some accidents in power lines such as lightning or a falling object can cause a line-to-ground fault.
Sudden load changes or excessive loads
Depending on the transformer connections, transformers energizing
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 {{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}}
The fault impedance
Type of fault
The voltage before the sag occurs
System configuration, e.g. system impedance and transformer connections

References

Sag

Category:Power engineering