Article from Questline
Voltage sags don't get the press that power interruptions do, so you may not realize that they're the No. 1 occurrence affecting power quality. Because 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%) 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're caused by equipment within your facility; a bunch of motors starting at the same time for example. Outside, wind, lightning 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 only indicate if a sag has occurred. More sophisticated models measure and record power as it enters your facility and display graphs of voltages at daily, weekly or monthly intervals.
Plotting the curves
What is the acceptable level for voltage sags? This is determined by the power quality curve developed by the Information Technology Industry Council (ITIC) as a realistic allowable voltage that equipment can withstand without damage or misoperation. Although the curve was initially created for computer equipment, it has been generally applied to general power quality evaluation.
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 10 seconds duration might be +/-10% of nominal.
Typically, electronic equipment can withstand 100% voltage sags, provided they last for less than 20 milliseconds (ms) in duration (one cycle), but voltage sags greater than 30% that last more than 20 ms will likely create problems.
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's more tolerant of voltage variations.
Next, take steps to compensate for voltage sags when they do occur. Install online uninterruptible power supply (UPS) devices. They condition power by correcting voltage sags.
Dynamic sag correctors, also known as constant voltage regulators, are another option. These devices use diode/rectifiers with capacitors to double voltage without using a transformer. When a supply line voltage sag is detected, the dynamic sag corrector static bypass switch is opened, and inverter switching devices are controlled to provide power to the load to compensate for the sag.
By taking the time understand how voltage sags can occur and the risks they present to your facility, you can take the right steps to protect your critical equipment and your bottom line.