Have I Got A Deal For You!

Reading through the websites and marketing Material of the major power quality instrument manufacturers in the hopes of figuring out which power quality monitor to buy can leave one feeling confused, frustrated and wondering which way to turn. Contacting the sales department will undoubtedly have someone tell you why their product is the only product to meet your needs, even before finding out what your needs are. With all of the mergers and acquisitions of such companies, it is always humorous to hear the salesperson at a trade show who was vehemently describing the shortcomings of a competitor’s product last year to be praising its superior performance after the competitor bought his company.

It is clearly very difficult to determine what works for your application from a brochure or other marketing material. A list of specs can be daunting and misleading. Phrases like “designed to meet standard X” are a far cry from “certified performance per standard X.” Why should you care if the instrument can timestamp data down to 4 microseconds if you are primarily interested in doing energy-efficiency studies on the equipment in your facility to see which item makes financial sense to replace? Doubling the price for looks or features that you will never use could be money better spent buying two instruments so you can do simultaneous monitoring at the point of common coupling (utility meter) and at the equipment with problems.

So, rather than write another article listing which products are out there and all of the features that you may never use, let’s begin with how to start the procurement process. As I alluded to above, start with a needs assessment. What do you primarily plan to use the instrument for? Is it troubleshooting three-phase circuits? Is it doing benchmark power quality audits when commissioning a new facility? Is it a long-term, ongoing monitoring system? As an electrical contractor, you could be involved with any of these, though troubleshooting equipment failures usually comes to the top of survey lists. An even more specific task that tops the list is using the instrument as a three-phase power meter. So if that’s your need, using a single-phase meter with the “multiply by 3” isn’t going to fit the bill, nor are voltage-only monitors. Likewise, if you need to measure/monitor a variety of polyphase circuits, including delta, wye with neutral-to-ground, open delta, 2½ element, and so on, you can rule out common reference input configurations and opt for one with full differential inputs.

Like the microprocessor industry, increasing speed with each generation has resulted in sampling rates of 512 samples per cycle or higher. To the average user, this means higher costs and larger amounts of memory consumed with each waveform recorded, but with a possibility of new insight into problems being investigated. Higher sampling rates translate into a wider bandwidth of frequencies that can be accurately measured. To get a good root-mean-square (rms) reading for looking into sources and effects of sags and swells, tests have shown that even 128 samples per cycle is adequate in many cases. But when dealing with the newer adjustable-speed drives and other equipment that uses 24-pole (or pulse) converters or higher, the first harmonic frequency that one is likely to find is above the “Nyquist limit” for a 128-sample-per-cycle instrument (4,320 versus 3,840 hertz for a 60-hertz fundamental frequency).

Flexibility in selecting from a wide range of trigger conditions and parameters to be monitored is a tradeoff against simplicity in setup for the occasional users. How easy and intuitive is it to get the instrument configured to meet your monitoring need? If you are doing a benchmark audit, you would like the instrument to be capable of seeing all that is out there by setting it to capture the full range of parameters as defined by IEEE 1159 and IEC 61000-4-30. If the equipment is apparently tripping offline at the same time that the lights blink, capturing an adequate amount of voltage and current waveforms and rms values before and after a trigger on a sag will go a long way to a fast resolution. What’s adequate? Again, it depends on the application, for a large horsepower monitor may need several seconds’ worth.

How easy it is to navigate the visual interface—whether in the instrument itself or using a PC/tablet/phone—can only be evaluated effectively with a hands-on demo of the product. And this isn’t the sales rep pressing the buttons for you. Take it for a test drive. Do what you think you do most often and even things you only occasionally need to do. Something as simple as a phasor diagram to determine if the voltage and current connections are properly made for the wiring configuration is very helpful; it may save you having to gather a week’s worth of worthless power data from incorrect pairing of voltage and current channels. Most products have PC software for doing detailed analysis and getting the relevant data and information into a report. Be wary of automated report writing claims, as those are often just boilerplate templates that have little value in describing what really happened.

To get the best deal, you need to do your homework first so that you are getting what you need for your application, not just the latest whiz-bang toy out there.

About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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