There are several reasons PQube monitors are widely recognized as the best instruments for large photovoltaic arrays: every PQube provides 5-times-per-second power flow measurements, they each come with a NIST-trace Accuracy Certificate, they capture 256-samples-per-cycle disturbances as well as detailed bi-directional power flow, you can set them up to monitor DC and AC simultaneously, and the PQube's ethernet-based remote communication with standard emails and web pages is often perfect for PV arrays.
So a couple of weeks ago, I had the pleasure of solving a PV array problem -- actually, an instrumentation problem -- in Hawaii with Paul Spracklen of Xtreme Power and David Rose of Sandia National Labs. The 1 megawatt PV array, located in a remote field on a small island, was coupled to one of Xtreme Power's impressive DC storage systems. This big PV array is fairly low to the ground, so to keep the plants from growing up and over it, a flock of surprisingly curious sheep roam underneath the panels.
The PQube monitor at the site was showing unusual readings, and Paul, David, and I were there to figure out what was going on.
The PV array provides a significant fraction of the island's power, so we quickly learned that we could not do our work during the day. Everything would have to be done at night. Outdoors. In the dark. Not how I would ordinarily prefer to work on 480 volts and 12.6 kilovolts...
After some investigation, we suspected that there was substantial noise present, probably in the unregulated 2kHz - 150 kHz region. I replaced the standard current transformer cables with shielded cables, and did some judicious re-routing of the twisted-pair current signal wires at the PQube, and that solved the problem.
Later calculations showed that, given the high noise level at this location, a differential capacitive coupling of less than half a picofarad would explain the odd readings we had seen.
This part of the frequency spectrum -- 2kHz - 150kHz -- is pretty much the Wild West. There are very few regulations. And as power inverters get bigger, we're seeing more and more problems around the world.
Claudia Imposimato's Working Group 6 of IEC Subcommittee 77A is writing a Standard for conducted immunity testing in the 2kHz - 150 kHz region, and my own Working Group 9 is writing a Standard method for measuring conducted power disturbances in this region. Please let me know if you have had problems yourself -- there don't seem to be any regulations, anywhere in the world, that limit either emissions or susceptibility in this part of the frequency spectrum. Time to fix that!
David is still working on solving a different problem at the site: why the mobile-phone-based ethernet modem stopped working a few months ago. But I have great confidence that he and Thomas Pua, PSL's expert on this, will be able to figure it out soon. Meanwhile, the PQube at the site is happily running along, storing all its measurements on its 4-gigabyte SD card for later analysis.
(By the way, during the night I managed to actually photograph a rare Ethernet bug. Here it is.)
In February, my old friend Dr. Professor Lothar Fickert of the University of Graz asked me a simple question: in English, what is the correct name of a "Kabelauff�hrung"? In other words, what do we call the place where a cable makes the transition from overhead-cable to underground-cable? (Click on the small picture above for a bigger image.)
I was uncertain myself about the correct English word, so I asked several of my favorite experts, and was surprised to get a wide range of answers: "cable tee-off pole" or "cable terminal pole" depending on whether it is in the middle of, or at the end of, an overhead line; "riser"; "transition from aerial to undergound"; "riser pole"; and, to my astonishment, "dip". Yikes.
One joker, who shall remain nameless here, suggested we adopt the word "Kabelauff�hrung" into English (just as we have adopted "sushi", and "intelligence", and "gesundheit") because the word is so perfectly descriptive.
It's also interesting that there isn't any agreement about the direction of the coupling -- some people see it as coming from the ground up ("riser"), and some see it as coming from the overhead down ("transition from aerial to underground").
What do you call this transition??! Please let me know.
English sure is a difficult language.
The good news: IEC Standard 61000-4-30 is quite precise about how to measure many power quality problems, and a "Class A" Certificate is a good indication that a PQ instrument is well-designed.
The bad news: several instrument manufacturers are claiming to be "Class A" when they aren't.
There are lots of different, technically-justifiable algorithms for measuring power quality disturbances. IEC 61000-4-30 was published 10 years ago because different PQ meters used different algorithms, perfectly legitimately, and therefore produce different results when connected to the same signal. (A simple example: two cycle-by-cycle true-RMS PQ monitors are connected to the same signal; one of them uses the positive-going zero-crossing as the beginning of the cycle, and the other uses the negative-going zero-crossing. What happens when the signal drops to zero volts for one cycle, then returns to nominal? The two instrument will produce completely different readings.)
IEC Standard 61000-4-30 fixes this problem. Instruments that correctly follow 61000-4-30 Power Quality Measurement Methods will produce the same reading, every time, for a long list of power quality parameters:
Class A in the Standard is designed to ensure that two compliant instruments, when connected to the same signal, produce the same results. Unfortunately, some instrument makers have misunderstood, and have self-certified their instruments thinking Class A is simply an RMS-accuracy requirement of 0.1% of nominal. It isn't.
If you need Class A, look for an instrument that has a Power Standards Lab Class A Certificate. (PSL is the only worldwide lab that does this certification work.) There are lots of legitimately-Certified instrument suppliers, and all of them have been tested rigorously at PSL. You can trust their results.
(And PSL's tiny, low-cost PQube instrument is Class A for frequency, voltage magnitude, dips, swells, interruptions, and unbalance.)
In the United States, the Federal Aviation Administration is now using PQube monitors to track the power at remote radar control centers. Here's my favorite Application Note about a difficult power problem at a remote mountain-top radar center.
At airports in Europe, PQubes are monitoring critical Control Tower functions. At large airports in China, PQubes are catching disturbances on more down-to-earth loads, like escalator (moving staircase) power. We've seen PQubes used in Latin America to solve power problems on sensitive baggage inspection systems. And everywhere in the world, we see PQubes keeping track of the ground power that is supplied to sophisticated aircraft when they're sitting at a gate.
(PQubes automatically adapt to 400 Hz power, too, so we see them flying occasionally on Airbus and Boeing aircraft, tracking any disturbances and variations on the on-board power buses. And if you've ever wondered how you get an earth connection on an airplane system while it's flying, let me know... it's interesting, especially on the new non-conductive carbon-fiber structures.)
We ship every PQube monitor pre-configured with its own, unique email account on the PQube.com server.
You can plug your PQube's ethernet cable almost anywhere you can plug in a laptop, and your PQube will be ready to send you emails. It really is that simple. (The only thing you have to do yourself is open up your PQube's setup.ini file and type in your own email address, so your PQube knows where to send emails: disturbance graphs, daily summaries of power flow, and more.)
I know how easy it is, because I can do it myself in hotel rooms when I'm half asleep.
(By the way, every PQube comes with an individual Accuracy Certificate, too, directly traceable to the United States National Institute of Standards and Technology (NIST). For example, here's the NIST-trace Accuracy Certificate for the PQube that's installed in my office: http://www.powerstandards.com/
Sales of PQubes in 2011 set a new record, and our monthly sales broke records in January, then February, then March, too. Thank you all for your wonderful support of PSL! We're proud of our products, and we're especially proud that you trust us with your critical applications. Thank you.
If you would like to get together, I will be in Germany, Slovenia, Croatia, and Italy (for an IEC standards meeting) later in April - details here. As always, it would be a pleasure to visit with you.
With my best wishes for you to enjoy a peaceful and happy Spring -
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Alex McEachern
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(I have sent this e-mail to you at '[recipient]', because you are on my personal world-wide list of 23,240 engineers, educators, and students interested in power quality. If you no longer wish to receive it, please let me know.)