Voltage sag

A voltage sag (U.S. English) or voltage dip[1] (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload, or starting of electric motors.[2] The magnitude/depth and duration of voltage sags are frequently used to define them.[3] A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.[2][4] Some references define the duration of a sag for a period of 0.5 cycle to a few seconds,[5][6] and a longer duration of low voltage would be called a "sustained sag."[5] The definition of the voltage sag can be found in IEEE Std. 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)".[3]

The term "sag" should not be confused with a brownout, which is the reduction of voltage for minutes or hours.[7]

The term "transient," as used in power quality, is an umbrella term and can refer to sags, but also to swells, dropouts, etc.[8]

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.[9]

See also: Voltage surge

Causes

There are several factors which cause a voltage sag to happen:

  • Since electric motors draw more current when they are starting than when they are running at their rated speed, starting an electric motor can be a reason of a voltage sag.[2][9]
  • When a line-to-ground fault occurs, there will be a voltage sag until the protective switch gear operates.[2][9]
  • Some accidents in power lines such as lightning or a falling object can be a cause of line-to-ground fault and a voltage sag as a result.[9]
  • Sudden load changes or excessive loads can cause a voltage sag.[9]
  • Depending on the transformer connections, transformers energizing could be another reason for voltage sags happening.[6]
  • Voltage sags can arrive from the utility but most are caused by 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 [3]
  • The fault impedance[3]
  • Type of fault[3]
  • Level of pre-sag voltage[3]
  • System configuration e.g system impedance and transformer connections[3]

See also

References
  1. "IEEE 493-2007 - IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems". The IEEE Standards Association. 2007-02-12. Retrieved 2018-01-09.
  2. Bollen, Math H.J. (1999). Solving power quality problems : voltage sags and interruptions. New York: IEEE Press. p. 139. ISBN 978-0-7803-4713-7.
  3. Karady, George. "Effect of voltage sags on loads in a distribution system" (PDF).
  4. "Industrial Voltage Regulator Power Conditioner". Utility Systems Technologies. Retrieved 25 September 2013.
  5. Vijayaraghavan, G, Mark Brown and Malcolm Barnes (2004). Practical grounding, bonding, shielding and surge protection. Oxford: Newnes. p. 134. ISBN 978-0-08-048018-3.CS1 maint: multiple names: authors list (link)
  6. Remus Teodorescu; Marco Liserre; Pedro Rodríguez (2011). Grid Converters for Photovoltaic and Wind Power Systems. Wiley-IEEE Press. ISBN 978-1-119-95720-1.
  7. Standler, Ronald B. (1989). Protection of electronic circuits from overvoltages. New York: Wiley. p. 40. ISBN 9780471611219.
  8. R. Sastry Vedam; Mulukutla S. Sarma (2008). Power QVAR Compensation in Power Systems. CRC Press. pp. 4–5 and 23. ISBN 978-1-4200-6482-7.
  9. Kazibwe, Wilson E.; Sendaula, Musoke H. (1993). Electric power quality control techniques. New York: Van Nostrand Reinhold. p. 11. ISBN 978-0-442-01093-5.
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