Sneaky Low Frequency Transients

Low frequency transients, sometimes called Utility Switching Transients or Power Factor Correction Capacitor Switching Transients, can be pretty hard to identify. Traditional power monitoring equipment has never done a particularly good job at spotting these – folks of a certain age will recall that the BMI-4800 power monitor would throw a frequency error (either 61.9 Hz or 64.0 Hz) if the transient caused an extra zero-crossing – sometimes that was the only way to detect the transient, and savvy engineers would use these frequency faults as a diagnostic tool.

Looking through a lot of Fluke 1750 data sets over the years (we’re looking at Site #4472 this week), we’ve gotten pretty good at pulling these transients out of the 100s or 1000s of transient events captured. Some detection tools:

  • Some transients do indeed trigger a voltage transient event, but need to be carefully reviewed because the reported magnitude is often that of the higher frequency leading edge
  • Many transients are accompanied by a rise in RMS voltage, so carefully adjusting the voltage swell threshold can often help to spot these.
  • In Wye systems, many transients cause a Neutral-Ground swell event, which can often be spotted.

In a recent data set; none of these indicators worked out. We were very fortunate that the first current event captured (with a current swell threshold set to 10 Amps, a typical threshold for our reports) was a transient event – so we happened to notice it.

Low Frequency Transient

Current Triggered Event #1

Then, identifying the duration of the current swell event (~ 17 msec, much shorter than the normal equipment loading) and the amplitude (between 15-20 Arms, normal equipment current swells were much higher) we were able to sort through 100s of current triggered events to find nine (9) low frequency transients in the data.

Low Frequency Table

Normally, we would not be so concerned about these transients, which are comparatively minor, simply looking at the voltage waveforms, with no significant overvoltage nor multiple voltage zero-crossings, However, the associated current swell (70-100 A peak) indicates something in the equipment under test is sensitive to or reacting to these transients, and drawing a slug of current. So they are worth looking into….

Low Frequency Transient 2

Current Triggered Event #646

Om Street 2017: Time Lapse Video

“On July 22, 2017, over 2500 people gathered on LaSalle Road in West Hartford, CT to “get their asana in the street!” The weather was perfect and the energy was incredible, making the 7th annual OM Street a huge success. The 75 minute all-levels yoga class was led by Barbara Ruzansky of WHY (http://www.westhartfordyoga.com) and included assistants from 40 area studios and businesses. Every studio that gathered their tribe, and every individual that put a mat or a chair on LaSalle represented a community, a coming together of like-minded spirits into something like a neighborhood, like a family.”

Remarkably, I made it into the video frame this year (directly behind, to the left of the tree at the far right lower corner of the shot, blue shirt) standing at the audio table, keeping an eye on the sound. And at the end (around 2:50) I take the front speakers down…

It’s kind of amazing to be part of this each year, to be the adult in charge of the sound system, and that, seven years running, we’ve never had a significant technical issue!