Voltage sags aren’t that swell

This audio was created using Microsoft Azure Speech Services


It’s good to get a warning when a dip is in the road ahead. And it’s good to take it seriously, as I learned as a teenager with my new driver’s license. I chose not to slow down to the recommended speed, and hit a dip that was much dippier than I had anticipated. By the way, if you happened to live in my neighborhood back then you’d wonder if the bigger dip was on the road or the guy behind the wheel. Anyway, I was even more surprised when after hitting the dip the car started steering funny and pulling to the left. When I got home my explanation began with – “funny story Dad…”. Let’s just say I had some summer jobs in my future.

Probably the most common type of power quality event that can impact your data center is the short-duration voltage sag. By definition, the duration of a voltage sag lasts anywhere from 0.5 cycles to one minute, but most typically are 2 – 20 cycles long. These short term disturbances happen all the time, and can originate from inside a power system, but the majority of these events come from the utility grid. Today’s power grids are strained, and short-term variations and transients tend to increase as the grid reaches capacity. Plus, a loaded grid means more switching takes place upstream; every switching event is a potential source of power quality disturbance.

Two factors determine the intensity of a voltage sag: duration of the event, and amplitude of the event. The lower the voltage dips (amplitude), and the longer it stays below its nominal value (duration), the more potential for trouble.

Know your system

Knowing how voltage sags affect your data center is a key part of its design, construction, and operation. Know your system. Power quality meters are a key element in your arsenal. Once you have a feel for the sag’s impact on your system, consider aligning the voltage sag detection set point thresholds in your power quality meters to make sure you capture events that might become a problem, and avoid capturing events that fill up alarm logs with excess noise.

Information is power

Power Quality impacts the entire system. Advanced PQ meters and power monitoring software have the potential to tell you more about the quality of your power than ever before. Make sure to consider all the parts of the system that are needed to keep your facility on line when considering immunity to voltage sags and other short term disturbances, including the cooling / mechanical side of the system.

In the next blog, I’ll cover exactly how a good power monitoring system that includes software-based tools and meters with built-in intelligence will keep you ahead of the curve.

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  • Vanya Ignatova

    9 years ago

    For sure, voltage sags are main issue, not only on the road.
    They are quite frequent, studies have estimated that a typical site will see from 59 to 85 voltage sags per year.
    And their impact – an unexpected downtime cause very important financial losses in Data Centers.

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