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

The Road to Perdition…..

….is paved with good intentions. And the road to poor power quality is paved with power conditioning devices. Four examples:

1) Reviewing a power quality study, wherein the complaint is a chiller that is shutting down due to excessive voltage imbalance. The voltage imbalance chart showed unusual step changes in voltage balance, often exceeding 2%. The problem: the site had a tap-switch voltage regulator installed, and as individual phases were regulated, voltage balance jumped from relatively balanced to relatively imbalanced. The solution: Bypass the voltage regulator.

2) A second study, with imaging artifacts affecting an MRI system after several years of satisfactory service. The system is protected by a UPS, and shows a higher than typical number of load related events. The problem: the UPS requires the battery string to supply the maximum imaging system load. As the batteries have aged, they were unable to support the peak load. The solution: service the UPS / battery string; in all likelihood the battery string needs to be replaced. A different UPS, that did not rely on the battery supply peak power, would not have had this problem.

3) Finally, a site with repetitive (~ 10 minutes) and severe low frequency transients. A facility based capacitor bank, intended to correct voltage and power factor, is malfunctioning – the capacitors pull in, and then shut down 45 seconds later (for so far unknown reasons) – probably internal fault or error indications. Ten minutes later, the caps try again. A CT scanner has been damaged repeatedly by this problem.

Been a While

Life gets busy, eh? Apologies for not blogging here of late. A few updates….

The good folks at Dranetz-BMI stopped by; I went to a seminar in Springfield, and liked what I saw, and they brought down the PX-5 power analyzer. FINALLY something from them that I think we can work with. I’ve always been a BMI kind of power quality person, never liked Dranetz (from the 626 backwards text printing, to the penchant for the 656 and 4300 to fill up memory) but I think in the PX-5, they have finally melded the BMI and Dranetz technologies (giving you options with each philosophy), the memory technology has become cost effective, and the GUI / operating system seems flexible and powerful (as opposed to powerful and cumbersome). I’m strongly recommending these to some clients and look forward to developing reports and analysis tools for them.

Magnetic fields are back on my mind – an upcoming site visit to a clinic on Long Island, and some inquiries for a magnetic field training program. I recently picked up a DC gaussmeter, and I am thinking of putting together a small book or manual on B-field hunting…..

Finally, you can catch me live at the upcoming Power Quality Conference in Long Beach, CA – I’ll be presenting my paper “Optimizing Mains Impedance: Real World Examples” on Thursday morning (October 26th), then chairing a workshop on Wiring and Grounding. And I’ll be working with TEAL Electronics at the upcoming RSNA meeting in Chicago – look for us in the South Building – Hall A, Booth 1815. Nothing like late November in Chicago to get you in the holiday spirit, put the chill of winter into your bones, and get your corns and bunions aching!

Looking for a Job?

For whatever reason, I’ve been getting a bunch of phone calls from people looking for a job. I think my internet and industry footprint is large enough that people find me pretty quickly via search engines – and are surprised that its for the most part, just me. In any case….here’s a couple of resources for those on the hunt

Lineal Recruiting Services (Trumbull, CT) – www.lineal.com – I’ve never actually met Lisa Lineal but I’ve seen her ads in Power Electronics, Power Quality, EC&M, etc. ever since I’ve been working (1983). Scary huh?

Power Technology Associates (Sharon, MA) – www.powercareers.com – I think I’ve spoken to these folks on occasion, and have probably met some of them at various shows and conferences.

Hope these help – good luck on your job search!

Harmonic Panic

Just looked over some power quality studies and an associated string of emails. The expert commented that the high harmonics on the neutral (49% and 51%, two panels) were a major problem, and required “doubling the neutral”.

Of course, the fact that the actual neutral currents averaged 10-15 amps, and never exceeded 250 Amps maximum (on a service entrance panel rated for, oh, 800 Amps) apparently got by this expert. And the voltage harmonics, 2% maximum on one panel, 3% on the other, were a little elevated, but not in any way problematic.

This, my friends is why I treat every diagnosis of high harmonics as suspect unless I see the data and run the numbers. Because too many people know how to hook up meters, push the “automatic report” button, and jump to conclusions. And not enough people have a clue what it all means.

How Good Is Your Ground?

An article in EE-Evaluation Engineering, as passed along by a client. The article is by Vladimir Kraz, Credence Technologies, and J. E. Patrick Gagnon, Texas Instruments. Good piece.

My comment: the article is written from the perppective of a controlled environment (i.e. – semiconductor test, wafer fab, etc) where there is a lot of thought and investment in a good, controlled, ground plane / grid / system – and the goal is to get connected to this ground plane as solidly as possible. For a lot of the systems that I work with (i.e. – medical imaging) there is no deliberately conceived ground system – so things are a lot murkier. If one is trying to break up problematic ground loops / paths, the last thing one might consider is improving the system coupling to the outside world….

Oh, Just Use the Ground

I’m back from a site visit; an MRI system has been having issues. On site, I discovered that a large UPS, which apparently requires 5 wires (3 phase, neutral, and ground) has been connected with a short jumper or bond between the neutral and ground – since a neutral was not run to the device. This was with the sanction (in fact, at the direction of, via the installation manual) of the manufacturer (not a small or fly-by-night vendor, I might add).

The result: 8 Amps of current (presumably from the input rectifier filters) on the protective ground conductor. At another site, we measured 16 Amps of current on the neutral-ground bond (not all of which flowed to the source on the ground conductor, mechanical mounting and conduit connections presumably taking some of the current)

OK, folks, it’s Power Quality 101. YOU DON’T USE THE GROUND TO CARRY CURRENT.

I guess it shocks me (heh-heh, thats a pun) that in 2006 people are still doing the same sort of things that got the whole power quality mess started in the old days.

I got to drag out my leakage current and ground current meters and put into practice some of the things I wrote about in my last Power Quality paper: Leakage and Ground Currents: Measurement Techniques

Keeping It Clean

I direct your attention to this article, in the May 2006 Issue of Evaluation Engineering.

Robert Close, who works for my client TEAL Electronics Corporation, put together a nice article. But the three waveform graphics embedded therein are whats really important. Those are screenshots from the TEALwatch Viewer – an Excel based data vieweer that I put together for the new TEALwatch power monitor.

Nice to see your stuff in the media….