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