At trade shows and distributors, you undoubtedly hear about the latest and greatest product offerings from a large contingent of vendors in electrical contractors’ domain, including those that sell power quality (PQ) monitoring equipment. Here is a quick review of the fourth generation (4G) of monitors; this may help you decide if the new stuff is for you or if the old and reliable will do for another year.
4G PQ monitors continue to move up the data-information-answers-knowledge-wisdom hierarchical pyramid (though some combine answers and knowledge into a single level), providing responses to questions such as “What caused that oscillatory transient?” or “Did the sag originate upstream toward the electrical source or downstream toward the load?” The need for answers is more important, partially because of the need for increased productivity, attrition of knowledgeable power quality engineers in utility companies and other industries, and the reliance on people with less experience. Instrument hardware and software technology advancements made it possible to provide such answers in an affordable platform.
There also has been the revision of two comprehensive power quality instrumentation standards: No. 1159, from the Institute of Electrical and Electronics Engineers (IEEE) and 61000-4-30, from the International Electrotechnical Commission (IEC). Instruments that comply with these standards should give the same results when measuring and monitoring voltage and current harmonics and interharmonics; voltage fluctuations (flicker); short- and long-term rms voltage variations (sags, swells, interruptions); voltage imbalance (or unbalance); power frequency variations; direct current (DC) voltage in alternating current (AC) systems; impulsive and oscillatory transients; voltage notching; and noise. While most instruments have four voltage and four current channels, there are thousands of parameters that can be calculated, including power parameters, such as watts, volt-amperes, volt-amperes reactive, power factor, demand and energy.
A word of caution about specmanship with some products: Phrases that indicate a product is designed to meet a particular standard or is the world’s best don’t have much value to the user. Similarly, lack of adequate specifications with regard to a critical feature or parameter accuracy can lead to questionable measurements and decisions, such as amplitude and phase accuracy over the bandwidth and amplitude range for a current probe used in power measurements. Your applications will determine if you need more stringent accuracies and “Class A” compliance, such as finding who is to blame for the equipment failure (the equipment manufacturer, the facility manager or the utility company) or if you are just doing spot checks for changes in load and harmonic currents.
While the voltage measuring inputs may appear similar in most instruments, some differentiating features may be important. Safety ratings—such as 1,000 volts (V) CAT III, 600V CAT IV—are becoming more common, though most applications require measurements at 480V or below. Differential inputs (two input leads per voltage phase) versus single-ended with a common connection for all the other voltage input channels (usually for the neutral) can provide more flexibility when measuring voltages other than three-phase circuits. The sampling rate determines the highest frequency components that can be correctly measured. The odds of accurately capturing a 10--microsecond transient with a sampling rate of 128 samples per cycle aren’t good, neither can it measure some of the higher order harmonics above the 63rd that are showing up with the 24-pulse converters in some newer adjustable speed drives.
Besides the similar concerns, current inputs have even more variables with which to contend. A single current input realistically cannot be used to produce three-phase power results. Current levels are rarely balanced in most systems. The current input on the instrument may have a 0.1 percent accuracy rating, but the current probe or transformer (CT) that is placed around the conductor to actually acquire the current signals may have a 2 percent amplitude accuracy over the range of interest. Typical iron-core clamp-on CTs usually need at least 10 percent of the full-scale current rating to energize the core properly for accurate readings. Readings of 3 amps (A) on a 300A CT are usually inaccurate, especially in the harmonic content. Rogowski coils or “flex-CTs” don’t exhibit the same low-end problems, but they have their own limits due to their design. They work on an antenna-like principle, so they may pick up signals from other conductors in high harmonic or electrically noisy environments. And neither of those types of CTs can measure DC current levels along with the AC, though you may see what looks like DC current in the data. These types of measurements require either Hall-effect probes or shunt resistors.
So before you wander down the aisles at an industry event, such as the upcoming NECA Show, think through the requirements of your applications. AEMC, Amprobe, Eaton Corp., E-Mon, Fluke Corp., GE Energy, Hioki USA, Ideal Industries Inc., Megger, Siemens Industry Inc. and Summit Technologies Inc. are some of the instrument vendors that you are likely to find. The products range from clamp-on PQ meters to permanently installed, multipoint power quality monitoring systems, and prices range from hundreds to thousands of dollars. As with any job, it comes down to the right tool to get the best results for the tasks that you are likely to encounter.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.