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!

Om Street 2017: Better, Faster, Stronger

We survived the 2017 edition of Om Street: Yoga on LaSalle road, my yoga studio’s annual “yoga in the streets” event that I’ve been audio engineering since the event’s inception in 2011.

Some history via the blogosphere:

Just finished cleaning up the audio equipment from the event this morning; three bins and one large duffel bag loaded with equipment that needed to get sorted, re-wrapped as necessary, and restowed for the next gig.

It was a pretty quick and pain-free recovery this year, owing to a few factors:

  • I ended up wrapping the bulk of the long cables myself – 4 x 100′ XLR (audio) cables, 4 x 100′ extension (AC power) cords, and all of the 1/4″ and speakon speaker cable (several hundred feet worth). There were just 2 x 100′ AC power cords that someone else wrapped and needed to be re-wrapped. In past years the cables were a bit of a hot mess.
  • I had a great assistant (yay, Steve!) who was dedicated to the band, so when I unpacked the storage bin from the band, cables were nicely  wrapped, mic stands nicely folded, mics and headphones all bagged up neatly. He also did a yoeman’s job of setting the band up (mics, direct boxes, and monitor headphones) so I could focus on other things.

Honestly, the only bin that was a bit of a hot mess was the main audio station bin (that I packed, last) because it was kind of the catch-all for everything lying around.

It’s a huge outdoor yoga event:

Om Street 2017 Wide Shot

Om Street 2017 (Breck Macnab Photography / West Hartford Yoga)

And it’s pretty much me handling the audio all by myself. Some things that helped out this year:

  • I added a pair of low cost wireless stick mics for emcee / stage mics this year. In previous years I used the studio wired mics but that’s a couple of fairly long XLR cables I do not need to worry about setting up or striking. I picked them up for Q&A for some larger workshops we have, but they proved useful for Om Street as well.
  • I did some more work on the band monitor setup, adding a small mixer, buying some dedicated cables / adapters, buying a gamer headset for myself (with a small headset mic on a boom), and setting it all up ahead of time to get levels. So the band had great monitors in both ears and we had a talk-back channel, with one band mic and my headset mic going only to the monitor channel, so we could communicate during the practice.
  • I took the time to kit out the audio equipment. Typically I show up with audio stuff in bins sorted mostly by who owns the equipment (yoga studio, or me) and general function (audio cables, power, speaker cabloes, mic stands, etc.). I would end up running around a lot during set-up, getting things to the right place This year I put everything for the band in one bin (power strip, mics, stands, direct boxes, cables, monitor headphones and amplifier) and everything for the main audio station in a second bin. I also got some of those big family zip-lock bags and kitted together all the cables and adapters for the satellite PA systems (200′ and 400′ down the road) so I could unload those with the speakers, stands, and amplifier, and not have to walk back and froth so much.

This year I took the time to sketch out the audio schematic:

RFFlow - Om Street 2017

I’ll add some links to “sure to be posted” video of the event, but here are a few news articles that have already made it to press:

Tale of Two Power Systems – UPS Edition

This one nearly fooled us; we recalled the “two power systems” nature of a recent site and so when a second data set came in with somewhat similar characteristics, we thought it might be more data from the same facility. But this is a completely different site, and a completely different problem!

Looking over a power monitoring data set recently; we came across a site with a dual personality. The site in question had a marginally high total harmonic distortion (THD ~ 5.5%) from 5/15/17 up until 5/24/17 (specifically, at 3:50 pm). After that, the THD trended much higher, rising as high as 12% (with a large amount of visible high frequency noise).

A “before / after” look at the voltage and current waveforms provides more evidence, with visible notching on the voltage waveform “before”; and very high levels of high frequency noise (broad spectrum “after”.

Before 5/24/17 After 5/24/17

Individual harmonics similarly supported the findings of the THD log, with harmonics under 3% before 5/24/17, and very high harmonics across the spectrum after 5/24/17.

Before 5/24/17 After 5/24/17


Finally, the “before / after” affect was also seen in the Neutral-Ground voltage – with severely high NG voltages after 5/24/17; consisting mainly of high frequency components.

Before 5/24/17 After 5/24/17

The clue to understand this puzzle is that the RMS voltage of the device under test was very stable and well regulated (probably a UPS or power conditioner output) before the 5/24/17 date, and higher / less well regulated after the 5/24/17 date.

And here is the “moment of truth” when the voltage changes from moderately distorted / notched to severely distorted with high frequency noise.

Moment of Truth

So what’s the scoop? We’re not on site, but here’s our bet – that there is a UPS supplying 480Y/277 VAC power to the load, but is itself being fed 480 VAC Delta (ungrounded and/or no neutral). During Inverter operation, the UPS works fairly well (although we bet the notching and higher THD are not normal for this device). But when switched to Bypass, the load loses the neutral reference, and is picking up noise from the UPS rectifier and/or inverter circuitry.

The trip report notes “No power problem suspected, multiple tube failures, want to eliminate power as an issue.” They probably assume UPS installed = no power issues (and they are probably not experiencing sags / swells / etc. on other facility loads). But if they are operating a system requiring 480Y/277 VAC from 480 VAC Delta, and relying on the UPS to provide a neutral connection point, they are probably having some serious grounding, reference, and noise issues!

Tale of Two Power Systems

Looking over a power monitoring dataset recently; we came across a site with a dual personality. The site in question had low total harmonic distortion (THD ~ 1.5%) from 4/1/17 up until 4/10/17 (specifically, at 2:00 pm). After that, the THD fluctuated much higher, rising as high as 5.4% (outside of manufacturer requirements for medical imaging equipment).

Tale2Power THD

A closer “before / after” look at the voltage and current waveforms provides more evidence, with visible notching on the voltage waveform “after”; monitored current showed some noise but was similarly low.

Tale2Power Waveforms Before Tale2Power Waveforms After
Before 4/10/17 After 4/10/17

Individual harmonics similarly supported the findings of the THD log, with all harmonics higher, and 5th harmonics exceeding 3%.

Tale2Power Harmonics Before Tale2Power Harmonics After
Before 4/10/17 After 4/10/17

Tale2Power NG RMS
Finally, the “before after” affect was also seen in the Neutral-Ground voltage – with noise voltages evident although the lower frequency voltages were not much higher.

Tale2Power NG Before Tale2Power NG After
Before 4/10/17 After 4/10/17

The funny thing is that the RMS voltage and the current of the device under test were not significantly different before / after the 4/10/17 date, in terms of RMS level or in terms of stability: sags, swells or fluctuations.
Tale2Power RMS

So what’s the scoop? We’re not on site, but odds are good that some facility load (we’re betting air conditioning, but could be other facility loads) got switched on at this time. Alternately, perhaps the facility transitioned to an alternative power source. But whatever the reason, this is clearly a tale of two power systems, and we’re curious about it!

UPS Overload and Bypass: CT Scanner Load

A quick consulting project came over the transom this week. A 150 KVA UPS, protecting a CT scanner, was occasionally overloading and transferring to bypass.

UPS Bypass 02

Here, the transition to Bypass is evident by the step change in voltage from a rock solid 480 VAC (UPS Inverter) to a very high 515 VAC (Bypass)

UPS Bypass 01

Drilling in a bit more, we see the CT Scanner switch on (point “A”) with a maximum current of 245 Amps and a resultant collapse of the UPS output, a short period where the CT current drops and the UPS output stabilizes, then a transition to Bypass (point “B”). Note the increase in voltage while operating on Bypass.

At the end of the CT scan (point “C”) the voltage rises due to impedance. And the UPS stays in Bypass for an extended period (point “D”) needing to be manually reset.

UPS Bypass 03

A close-up of the “start of scan” waveform shows the nature of the inrush current (higher for just once cycle) – although the UPS voltage drops more than usual, it does not really fold or collapse.

Nothing really unusual here – some finger pointing at impedance (not really an issue, the voltage drop on the unregulated bypass was just 2.7% at full load) and voltage distortion (under 3% voltage distortion on the UPS input) – neither of which is a problem. The UPS got sized in based on power monitoring, which apparently did not capture peak load condition.

I suggested that the higher voltage on Bypass (515 VAC = 7% higher than nominal) would mean lower observed current, although that did not factor into the calculations (they were monitoring further upstream on a 208 VAC source). The UPS vendor is going to see if they can tweak the protection circuitry a bit to be able to survive and supply this short overload without a bypass transition.

Remembering Joe Briere

I found out this morning that Joe Briere of Computer Power Northeast passed away back in March – obituary.

Joe and I go back a long ways. When I worked at Philips Medical in Shelton (pre-1995) he stopped in a few times to see if there was any business there; he was plying his trade as a power quality consultant in the medical imaging field, doing a lot of work with Siemens Medical, so it was natural that our paths would cross. He was one of those “I’ll help you with your power quality issues and maybe sell a transformer, voltage regulator, power conditioner, Uninterruptible Power Supply, or Surge Protective Device along the way” kind of consultants that inevitably made a lot more money than me (who chose not to sell or rep products, just provide technical services). He was always a straight-shooter, never found him to oversell or over-promise, and was a hands-on guy who knew his way around grounding, the electrical code, isolation transformers, etc. We did not agree on everything, but I always respected his opinions and experience.

For a few years there (2001 – 2003) Joe and I would often find ourselves meeting at Siemens trouble sites across the country as the service organization tried to get a handle on power and grounding issues, so I got to know him pretty well, poking around hospital electrical systems and sharing a beer and a meal afterwards.

Have not been in contact for many years (my last contact with Joe was 2003, and with Computer Power Northeast was 2009) but I’d pop over to the website now and then to see if they were still in business. Joe was probably retirement age when I started working closely with him, and reportedly kept busy well into his 80’s. His business partner called for a little consulting project this morning and shared the news.

Was a long time ago in a galaxy far, far away. Power Quality Consulting was kind of the wild west back then and the folks who knew what they were doing, were not afraid to open an electrical panel and make some measurements, and could sort out technical issues that left others scratching their heads were a rare breed. RIP, old friend.

Support Your Independent Power Quality Consultant (or Pay a Lot More)

Came across this one on a Linked In power quality board.

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Skiers blowing up UV light bulbs

I am looking for a small three phase voltage regulator/ power conditioner to fix utility voltage fluctuation problem that is blowing expensive UV lamps in the water purification system in a small town. You see, the water filtration facility ( or shack ) is connected to the grid at the end of a long utility line; and it’s closest neighbor is a ski hill. When the sky lifts; or the pumps for the snow making machines start/ stop, the 600V gets high voltage spikes of up to 640V. This is blowing up the UV bulbs.

I need to replace the existing auto transformer with a 600V – 3 wire input, and a 277V – 3 wire output power conditioner/ voltage regulator. Probably will need something in the 10-15kVA range.

Open Delta Autotransformer

Any ideas for good quality US or Canadian manufacturer?

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Spoiler Alert: odds are pretty good that this site does not need a “power conditioner/ voltage regulator”. The culprit here is almost certainly the transformer used to convert 600 VAC (ph-ph) to 277 VAC (ph-n) for the lighting circuits. They are using autotransformers hooked up in an Open Delta configuration – no doubt something like this:

Open Delta Wiring

Problems:

  1. The voltages from X1 / X2 / X3 to neutral are not going to be balanced or particularly stable – they will definitely change with load. X3-N will in fact be 347 VAC. The other phases will be all over the place, and will fluctuate with load.
  2. The voltages from X1-X3 and X2-X3 will be fairly stable (direct magnetic coupling) but X1-X2 (the open delta phase) will fluctuate.

I’d probably recommend measuring the X1 / X2 / X3 voltages (Ph-Ph, Ph-N, and Ph-G) and when they come up unbalanced, I’d know where the problem was.

Now a dozen or so power quality experts (with a product to sell and a commission to accrue) had lots of suggestions for power conditioning or voltage regulation solutions. These might fix the problem, but at a much higher cost / complexity.

The best solution is mostly likely a simple Delta-Wye isolation transformer, 600 VAC primary, 480Y/277 VAC secondary.

Delta Wye Wiring

The problem is – nobody gets much of a commission on a 15 KVA isolation transformer (maybe a $1500 box plus installation, breakers, etc.) so who wants to recommend THAT?

Your friendly neighborhood independent power quality consultant, that’s who . . .

Ground Resistance Testing

I just sent a client a document from a seminar that I created and led in 1996. (The seminar client is long out of business).

It’s nice to be (a) the old dog who was around back in the day, and (b) a bit of a digital pack rat. Also interesting that the technical issues of 2017 are no so different from the technical issues of 1996.

Here’s a snapshot of that document (pretty slick for 1996, no?) – and no guarantees that the IEC / UL references or requirements are still valid. But the concept – that measuring ground resistance with a low current ohmmeter is going to give you sketchy results – remains valid.

Ground Resistance Testing