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Brushing Up On Voltage Sags

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

SinewaveVoltage sags don't get the press that power interruptions do, so you may not realize that they're the number one occurrence affecting power quality. Since facilities of all types rely on sensitive electronic equipment, voltage sags have become a costly problem. Understanding how they originate can help you find the right protection for your needs.

What causes voltage sags?

A sag is a reduction in voltage (typically at least 10 percent) that doesn't hang around for long. They usually last for less than a second, although they can go on for up to a minute.

Sags can originate on either side of your electric meter, and their exact source is often difficult to pin down. Frequently, they are caused by equipment within your facility; a bunch of motors starting at the same time for example. Outside, wind, lighting and trees falling on power lines can produce sags. The source doesn't have 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 monitors measure and record power as it enters your facility and display graphs of voltages at daily, weekly or monthly intervals. More sophisticated models use software to track voltage sags and other power quality disturbances.

Plotting the curves

What is the acceptable level for voltage sags? This is determined by power quality curves, which represent the intensity and duration of voltage disturbances. The Computer Business Equipment Manufacturers Association initially created the curves as a realistic maximum allowable voltage that equipment can withstand without damage. Power quality curves were also adopted by the Information Technology Industry Council (ITIC).

The ITIC curve plots the percentage deviation from nominal voltage for a given length of time the voltage variation is permitted before performance issues or damage occurs. For example, the limit for voltage occurrences greater than 1 second duration might be +/-10 percent of nominal.

ITIC Curve

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: 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

First, fix the problem causing the sag and then focus on upgrading your equipment so that it's more capable of riding through voltage issues. Implement a program to specify and purchase electrical equipment that is more tolerant of voltage variations.

If the initial two approaches are not sufficient, take steps to compensate for voltage sags when they do occur. Install uninterruptible power supply (UPS) devices. They condition 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.


Image source: iStock and 

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