Curious Voltage Regulation

Came across an unusual case of voltage regulation for a CT Scanner site recently.

Site voltage appears to be well (and artificially) regulated, with no daily voltage fluctuations typical of utility power but with small load related voltage drops. Site had no significant outages, sags, swells, or transients over two weeks of monitoring.
Typical voltage harmonics supports some sort of static, inverter based power source, with broad band harmonics. Elevated 5th, 7th, 11th and 13th harmonics are curious.
No apparent voltage regulation related to step change in load current – which is unusual for a static UPS or power conditioning device feeding a medical imaging load.
Moderate level of increased voltage distortion related to nonlinear load current – again, not typical for a medical imaging device powered directly from a properly sized UPS or power conditioner.

So, what’s going on?

My first guess was a slow, electro-mechanical voltage regulator (such as a motor driven Variac or Powerstat) and which shows up occasionally for medical imaging or other sensitive loads. However, that sort of regulator would not eliminate short utility sags or drop-outs, or affect the voltage harmonics or THD.

My next theory is that this site has a large UPS or static power conditioning device feeding an entire medical imaging department. The voltage drop and increased voltage distortion under load would be the result of voltage drops in distribution (impedance calculated to be roughly 60 mohms @ 480 VAC) – so perhaps the power conditioning device is located in the basement with normal distribution voltage drops (primarily wiring, perhaps a transformer) from there to the equipment.

No significant power issues found, just like to understand what I am seeing in the data!

Won’t Get Fooled Again . . .

No critique or judgement implied in the title of this post, just the lyrics of The Who’s song “…meet the new boss…same as the old boss…” percolated to the surface as I came across this press release last recently:

MTE Relaunches Prestigious TEAL® Brand for Precision Power (March 30, 2022)

Teal Electronics Corporation was my first and best client for many years. I worked closely with them developing and managing the PDU (Power Distribution Unit) for medical imaging systems with my then-employer Philips Medical Systems. I’ve consulted with them for ages – working on training, applications notes, customer support (remote and in the field).

I’m particularly fond of an Excel based viewer for their TEALwatch product – which you can see a bit of the GUI from here – Dedicated Line, Back to the Service Entrance

Times change, principals and technical contacts move on, we dropped the retainer in 2011, and my last invoice to them was 2015. They were purchased by MTE Corp in 2016, and despite a few fleeting contacts, my relationship with the company effectively ended. My guess is that the TEAL business unit itself got a bit lost in the shuffle.

They do seem to have tweaked the logo a bit. So that’s something . . .

I’m only slightly amused / disappointed that the sum total of the MTE / TEAL application library consists of 3 applications notes, the last survivors of a series I wrote for TEAL back in 1995….

Sneaky Voltage Swell Events

Long time, no post! A pandemic will do that to you . . . suffice it to say I am healthy, relatively happy, and staying busy with both my engineering work (mostly stable through the troubles) and my yoga studio work (very different these days but busier than ever with online classes and the need to provide appropriate technology).

Today’s engineering bon-bon involves a trio of voltage swell events. Seemingly caused by a short circuit (another facility load fault, perhaps utility lighting arrestors) that causes a 1/4 cycle drop-out on one phase and resultant voltage swell following. The swell has a serious overvoltage, likely to cause problems for many types of equipment / power supplies.

This event was captured as a minor RMS voltage swell (5.1% above nominal) – but the peak voltage is very high (540 V vs 395 V normal). Note the ~30 Amp peak current swell.
This event was barely detectable as an RMS voltage swell (4.8% above nominal) – once I recognized the nature of these events, I went looking for more and came up with just this one. Almost no resultant current swell.
This is the big smoking gun event. Even though it did not register as a sag / swell event at all (the voltage drop-out being balanced by the following swell, just 3.25% over nominal) it was captured as a current swell (the highest current event over 6 weeks of data capture) – 650 Amps waveform peak. Something within the client load saw this event as a problem and drew a slug of current.

I’m just the hired gun reviewing the data, pulling out events and issues, writing a report. So I won’t be following this to a resolution or further troubleshooting on site. But this is a good example why automated report writing is not always sufficient – the Fluke 1750 analysis tools would see these events as minor voltage swells (if that). It takes a human being with some experience (I’ve reviewed over 5000 power quality data sets since 2003) to see something unusual, scratch one’s head, and dig in a bit deeper.

The Shoemaker’s Children Go Barefoot (no more)

For someone who makes the bulk of her income working with power quality, my own computer systems have been fairly under-protected for many years.

I picked up a stand-by UPS (APC Model ES550) many years ago (maybe 10? hard to say, might have been my second device); it has served me reasonably well. And even though I’m well aware of the nature of stand-by UPS (time delay before inverter switches on, step wave inverter output) it’s done a pretty solid job of keeping my computer up and running.

A few days ago, my home office lost power for a bit – clocks were reset, the computer switched off – and I realized it was time to upgrade the office UPS. I picked up another APC – a line interactive, sine wave output model RS 1000MS – rated for 1000VA / 600W.

It’s got plenty of juice for my needs – sitting at about 20% of load / 37 minutes of battery time with my desktop, monitor, cable modem, and a small backup server and peripheral hard drive. I’m much enamored with the front panel LED screen and the PowerChute software. And while I have not set my computer up to hibernate at the command of the UPS, that’s a possibility.

I go back a long ways – when a buck a watt was perhaps a reasonable price to pay for a small UPS. So to get all this for about $150 – well, I’m not complaining.

And I took the time to run my house cable through the internal TVSS and the Ethernet from the cable modem back to the computer through the UPS – so I’ve got a better chance of surviving nearby lightning strikes / transients – related to both transient voltages and ground potential issues. I’m not at the point of driving a ground rod and connecting an external ground though. I’m down in a basement and close to the residence service panel, so not super worried about ground issues.

And I’ve also spent some time separating critical loads (computer, monitor, cable modem, exterior drives / servers) from less critical loads (printers, speakers), plugging these latter into the TVSS only outlets. And while I was down there with the system off, I spent some time untangling the cable spaghetti, wrapping and tying off cables, neatening things up.

Simplex Clock Correction Issues

I’m feeling a bit nostalgic this morning. Readers of a certain age will remember the classic Simplex clocks from school days. The clocks throughout the building had a special feature – a receiver and small control system that would permit the clocks to be synchronized or changed throughout the building, using a master control device. Useful to keep all the clocks at the same time, easily adjust for seasonal daylight savings time changes, and to reset the clocks in the event of power loss.

There’s a good overview of these systems here:

The Synchronous Wire system is the most popular system in the United States. Clocks are run using a power circuit that acts as its time base. The clocks receive an hourly correction which synchronizes both the minute and the second hand. Every 12 hours the clock receives a daily correction to keep the clock perfectly synchronized.

When I first started working in the medical imaging field (circa 1989), we’d run into issues with these clocks, a lot. Hospitals and health-care facilities were big users of these (there’s a clock in every patient room, hallway, and procedure room), and one particular piece of equipment (a Phillips “Classic” generator) was particularly susceptible. The generator used a motor-driven, linear variable autotransformer (think Variac or Powerstat) to adjust for line voltage changes – and the signal injected onto the mains by the clock controller (typically around 3500 Hz) would mess with the voltage regulation circuit, and the motor would “hunt” for the duration of the pulse (usually 5 or 10 seconds), The generator would be disabled or locked out while the motor was moving, and this would drive the docs and techs crazy (since it happened hourly).

Simplex Clocks

A waveform sample from a PowerLines trip report circa 2007, using Rx Monitoring Services power analyzer to capture the clock correction pulse.


I’ve also come across a few old power quality threads discussing these clock pulses causing standby / hybrid UPS systems (notoriously sensitive to anything that might indicate the start of an actual outage) to switch to inverter improperly.

Back in the day, the old BMI-4800 power analyzer had a “high frequency noise” detector which looked at broad spectrum harmonics or noise, and output a distinctive “picket fence” 24 hour log when these clocks were present (I still miss this diagnostic / reporting feature on modern power analyzers). I’m sure I have an example of this graph kicking around somewhere but can’t put my paws on it at the moment. I suspect any graphs I recall pre-date digital images (when I would create reports with blank boxes, and manually paste in photo-copied disturance graphs) so I’m not finding anything in trip reports or old PowerPoint presentations)

Resolving these issues? Sometimes we’d consult with the facility engineer – oftentimes these were turned up to “10” (maximum) and we could get the amplitude turned down to the point where it worked but did not cause problems. Sometimes we’d get the clocks reprogrammed to only correct 2x a day (noon and midnight) when it would be unlikely to affect the equipment. Some resourceful field techs developed a filter circuit to protect the regulation circuity; although that was sometimes not permitted (FDA requirements for x-ray equipment forbids modification or retro-fitting).

I don’t hear too much about these lately. Clocks are now often digital, controlled wirelessly or via ethernet. Switched-mode power converters have replaced old analog systems.


Cause and Effect: Arcing Transients and Equipment Faults

We recently reviewed some power monitor data for a client. Problem statement:

Breaker Q1 in the WCS electronic box trips on a sporadic basis. The breaker is the M4 and M5 fan motor overcurrent protection. We have replaced the breaker multiple times.

First pass, we noticed three very high current swells in the ground current data:

Ground RMS

We also saw 100s of very serious arcing voltage transients, not related to load current changes or other voltage events. The transients showed up on Phase-Neutral and Neutral-Ground, but the NG transients were much lower amplitude (secondary, not the primary issue).

Transient PhN Transient NG

Finally, we captured three current swell events that clearly show equipment faults to ground (notice the elevated ground current) immediately following voltage transients – cause and effect.

Fault1 Fault2 Fault3

Figuring out the cause of the transients is an exercise for the local service engineers or an onsite power quality engineer. But we’ve got a pretty clear linkage here between transients and equipment faults. Most of the time, power quality problems are a lot less concrete and clear.

Power Factor Correction Capacitors on a Timer

Came across an old friend this afternoon, low frequency transients related to utility or facility power factor capacitor switching, controlled via a timer (rather than sensing voltage, current, or power factor)

Minor PFC - Voltage Waveform

Here’s the voltage waveform – this seems to be a very minor transient, hardly worth noticing.

Minor PFC - Voltage and Current Waveform

Adding the current, we see a small ringing current related to the transient event. Oftentimes with more severe transients we see a large current swell.

Minor PFC - RMS Logs

The RMS voltage logs show a small but clear step increase in voltage at the time of the transient, clear sign of power factor correction capacitor switching.

Minor PFC - Table of Events

Finally, here is a table of all such transients captured. Notice how each transient occurs at 7:03 am, on different mornings. This sort of “same time every day” incident is a clear indication that the capacitor bank is on a timer control.

Not a super serious issue, this time, but interesting to come across a timer based system. They seem to be increasingly rare as more sophisticated controllers are brought online each year.

Power Standards Lab: Announcing: The PQube 3 Family

Came over the email transom this week:

We are pleased to let you know that we announced the commercial availability of two new PQube® 3 models.The range of PQube 3 analyzers now includes:

  • PQube 3 – ultra-precisee multi-function and multiple-circuit power quality and energy meter. Ideal for immediate diagnosis of power issues, power consumption analysis, as well as environmental sensing, and external process monitoring
  • PQube 3e – multi-load powerr consumption monitoring for 14 single-phase loads, or 4 three-phase loads measured simultaneously; drastically lowering the per-circuit cost of monitoring
  • PQube 3v – voltage quallity analyzer; ideal for price-sensitive applications where compliance is required, while load monitoring is not

Features of the PQube 3 analyzers include:

  • Ultra-compact form factor; fits virtually anywhere – ideal for embedding (DIN-rail)
  • Detection and recording of high-frequency impulses at 4 MHz
  • Measurement of 2 kHz – 150 kHz emissions (first instrument on the market to cover the entire frequency range)
  • Four analog and multiple current channels (up to 14 on PQube 3e)

Benefits of using the PQube 3 analyzers include:

  • No learning curve – very intuitive, with no software needed
  • PQube 3 generates information you can immediately use – disturbance and trend graphs sent directly to your iinbox
  • Additional channels and embedded sensors provide a precise set of measurements covering the power side, plus process monitoring, and environmental parameters
  • Measurements can be used for verification requirements; PQube 3 is certified Class A IEC 61000-4-30 Ed3, and is ANSI Class 0.2/0.2S revenue-grade accurate
  • Optional enterprise software enables fleet maintenance and data aggregation

Read the full press release here.

I’ve been a fan of PSL Founder Alex McEachern from back in the BMI days; have not had a lot of opportunity to work with the PSL devices / technology. To be honest, without an on-staff or on-call power guru (like me) to help sort out signal from noise, the technology seems a little hard to use / parse for the layperson. But cool as hell, from a power quality perspective….

Engineering Templates

Popped up on Facebook this week; a friend posted some guitar innards and a commenter referenced engineering templates – and it was off to the races.

Picket 16101

Picket 1610I – A personal favorite by dint of the transformers and the Delta-Wye transformer windings

Which is more terrifying:

  • I’m of an age where I actually used these for their intended purpose?
  • I still have these?
  • I knew exactly where they were? They’ve sat waiting patiently in the hanging file folder I put them in when I started consulting in ’95.

Apparently one can still purchase these – although I’m not sure how many are sold. I’ve not seen a drafting table in use (except perhaps ironically) for many years.

Truth be told my love affair with these tools goes back much further – Dad worked in IT back when the Univac brand was on top of the industry, and weekend trips to his office meant a morning of messing around with programming templates, making punch cards, shooting big rubberbands (used to bundle punch cards or print-outs). Every year at xmas we’d get a dot-matrix, ascii art peanuts calendar – I found a pretty representative sample at Hackaday.

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