Voltage sag
A voltage sag (U.S. English) or voltage dip (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors.[1] A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.[1][2] Some references defines the duration of a sag for a period of 0.5 cycle to a few seconds,[3][4] and longer duration of low voltage would be called a "sustained sag".[3]
Contents
Related notions
The term "sag" should not be confused with brownout which is the reduction of voltage for minutes or hours.[5]
The term "transient" as used in power quality is an umbrella term and can refer to sags, but also to swells, dropouts, etc.[6]
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.[7]
Causes
There are several factors which cause a voltage sag to happen:
- Since the 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.[1][7]
- When a line-to-ground fault occurs, there will be a voltage sag until the protective switch gear operates.[1][7]
- Some accidents in power lines such as lightning or falling an object can be a cause of line-to-ground fault and a voltage sag as a result.[7]
- Sudden load changes or excessive loads can cause a voltage sag.[7]
- Depending on the transformer connections, transformers energizing could be another reason for happening voltage sags.[4]
- Voltage sags can arrive from the utility but most are caused by in-building equipment. In residential homes, we usually see voltage sags when the refrigerator, air-conditioner or furnace fan start up.
References
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