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A Closer Look at Voltage Sags

Key Points
  • Voltage sags are the most common occurrence affecting power quality and they can be very costly. 
  • The source of a sag can be difficult to locate, since they can occur inside or outside facilities. 
  • UPS equipment and constant voltage regulators can protect your equipment from voltage sags.

Source: http://www.sxc.hu/
Power lines

While they are not as well-known as power surges, voltage sags are the most common occurrence affecting power quality and they can be very costly. Computer equipment and machinery have become increasingly susceptible to voltage sags. Understanding how voltage sags originate can help you protect your facility.

What causes voltage sags?

A voltage sag is a reduction in voltage for a short period of time. A sag can last between eight milliseconds and one minute, although they often occur for no more than one second.

They can originate on either side of the electric meter and their exact source is often difficult to locate. Frequently, they are caused by equipment within a facility. For example, a large number of motors starting up at the same time or a short circuit can result in nuisance shutdowns from sags. Outside your facility, switching operations, as well as wind, lightning and trees falling on power lines can produce sags. The source does not need to be close by. A voltage sag on a power grid can impact facilities within a 100-mile radius.

A power quality monitor is the most commonly used tool for detecting voltage sags. Simple power quality monitors measure and record power as it enters a facility and display graphs of RMS voltages on daily, weekly or monthly intervals. More sophisticated monitors use software to track voltage sags and other power quality disturbances compared with standard power quality curves.

Source: http://www.energy.gov/
Voltage sags
Plotting the curves

What is the acceptable level for voltage sags? This is generally determined by power quality curves; a plot of voltage magnitude versus time. Power quality curves represent the intensity and duration of voltage disturbances. The Computer Business Equipment Manufacturers Association (CBEMA) initially created the curves as a realistic maximum allowable voltage that equipment can withstand without damage. Other power quality curves that are in use today were developed by the American National Standards Institute (ANSI) and the Information Technology Industry Council (ITIC).

The ANSI curves plot the deviation from nominal voltage as a percentage of nominal voltage compared to the duration or the maximum length of time the voltage is permitted to reach. For example, the limit for voltage occurrences greater than 1 second duration might be ± 10 percent. The ITIC and CBEMA curves also plot voltage with respect to duration, but as a percentage of absolute voltage. Typically, electronic equipment can withstand high voltages provided they last for less than one millisecond in duration, but voltages greater than +10 percent or -20 percent for between 0.5 and 10 seconds duration will likely create problems.

Note that these curves are merely guidelines, and some electronic equipment may require higher power quality conditions than those represented in these standards.

Reducing the impact of voltage sags

Sophisticated uninterruptible power supply (UPS) equipment conditions power by correcting voltage sags. Constant voltage regulators (CVR) are another option. These devices incorporate capacitors, which help to regulate voltage as the incoming primary voltage changes. CVRs function similar to UPS units; however, they do not include batteries to make them uninterruptible. Consider specifying and purchasing electrical equipment that is more tolerant of voltage variation.

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