- 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 occur either inside or outside facilities.
- UPS equipment and constant voltage regulators can protect your equipment from voltage sags.
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. What are voltage sags, what causes them, and how can you protect your facility against them?
Voltage Sag Basics
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 typically occur for no more than one second.
Voltage sags can originate on either side of the electric meter and their exact source is often difficult to locate. They are frequently 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 shut downs 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 a daily, weekly, or monthly basis. More sophisticated monitors use software to track voltage sags and other power quality disturbances compared with standard power quality curves.
Plotting the Curves
What is the acceptable level of 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 in common 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%. The ITIC and CBEMA curves also plot voltage with respect to duration, but as a percentage of absolute voltage. Electronic equipment can typically withstand high voltages provided they last for less than 1 millisecond in duration, but voltages greater than +10% or -20% for between 0.5 seconds 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 a UPS unit; however, they do not include batteries to make them uninterruptible. Also, consider specifying and purchasing electrical equipment that is more tolerant of voltage variation.