When I was learning carpentry from a master carpenter, I tried to do something with a tool close at hand instead of going to get the correct tool for the job. Of course, I butchered the piece of wood and eventually had to get the right tool, which got the job done in a fraction of the time that I wasted with the convenient-to-reach tool. The same lesson applies to power quality tools, which range from a simple screwdriver to a $24,000, 32-gigahertz (GHz) spectrum analyzer. Not only can you waste time and not get the answer you are looking for, you might even be led to the wrong answer using the wrong tool for the task.
Before going into the variety of tools available, here’s a quick safety reminder. Always assess the hazards and skills required for the task, and ensure you or whoever is doing it is a “qualified person” per the National Electrical Code definition. Ensure all personnel within the possible hazard area wear the proper personal protective equipment per NFPA 70E and other local requirements. And wherever possible, make connections on de-energized circuits only. Even something as simple as taking a panel cover off to tighten a screw can be disastrous. Accidents don’t always happen to someone else.
On the low end of the price range is an item in everyone’s tool kit: a digital multimeter (DMM). A DMM can measure a number of steady-state power quality phenomena, such as voltage imbalance. It can also be used to find voltage drops across contacts and other devices that should have very low drops. Excessive neutral-to-ground voltage is often a steady-state condition.
Clamp-on power meters are slightly more expensive ($300–$3,000) but used similarly. Though only single-phase, they can be useful for current imbalance and power factor, and many have limited harmonic measuring capabilities. Be wary of the 3 assumptions some meters make, which contend that all three phases are identical and, therefore, give you three-phase answers with a simple multiplication. Also, most clamp-on meters use current transformers that cannot measure (or tolerate) direct current (DC).
Power loggers generally have capabilities similar to power meters but can take unattended readings for extended periods. They are useful for finding time-correlated problems, such as the voltage drops at a certain time each day. Several manufacturers offer both single- and three-phase loggers ($500–$3,500) that come with software for downloading the data onto a computer for analysis. Some plug right into an outlet to let you piggyback the equipment being monitored for simple and safe connections.
Most electrical contractors doing power quality work have several monitors, which can monitor a wide range of power quality phenomena. Read the specifications and the user’s guide before taking one out to troubleshoot for a suspected power quality problem. This is especially important when looking for transients and higher order harmonics. If the sampling rate of the instrument is 64 times per cycle, it is not possible to determine harmonics above the 32nd, and even that is suspect in the real world of measuring. If the current probe is a Rogowski coil (flex-probes) and you are measuring in a room with a half dozen 500-horsepower motors running off adjustable speed drives, much of the current data is going to be skewed by the antenna-like pickup characteristic of those probes. If using a current transformer that isn’t rated for DC and there is an inrush current condition on a saturated transformer with a DC offset, it won’t produce reliable data.
However, using the instrument within its limitations provides a wealth of data that virtually no other instrument can simultaneously do for you. Right in the sweet spot of power quality monitors are capturing the waveforms of disturbances, such as the arcing transients that occur before the voltage sag is cleared by the distribution circuit protection device or the slight frequency and phase shift that occurs when switching from utility power to a backup power source that resulted in a particularly susceptible piece of equipment dropping off line. Whether doing a benchmark survey to compare the site data to the commissioning data or troubleshooting a process interruption that only occurs once per month but with large financial consequences, a power quality monitor in the $3,000–8,000 price range can do exactly that.
Though they don’t have the same triggering, capture and characterization functionality as a power quality monitor, a high-speed (200 megahertz–1 GHz) digital oscilloscope ($3,000–5,000) can be a valuable tool to have at your disposal when looking at noise or transients that are above the bandwidth of power quality monitors. Likewise, a spectrum analyzer ($10,000–15,000) can provide a more complete and wider picture of the steady-state signals that are present in a system and fall below the fundamental frequency or above the harmonic range of most power quality analyzers. For random or burst signals, a noise logger ($4,000–8,000) is an invaluable tool, and for a hands-free, no-contact look for hot spots that can result from high impedance contacts or harmonic losses in motors and transformers, the thermal or infrared camera ($4,000–20,000) is the tool of choice.
Of course, the most used tool for power quality tasks is likely the screwdriver (priceless).
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.