Considerations for Power-Quality Monitoring

Sept. 1, 2007
Identify the root cause and quantify the solution required to fix power-system problems

Power-quality monitoring has become increasingly common as the capabilities of commercially available monitoring systems have increased and costs have fallen. Power-quality engineers have used various power-monitoring tools for many years. In recent years, several trends have converged, resulting in the wide application of power-monitoring systems beyond power quality or power-management specialists.

Today, power monitors can track virtually any electrical parameter of interest. With all of the monitoring options available, a number of questions need to be asked: "What should we monitor?" "Which type of power-monitoring system should be selected: permanent or portable?" "What does my power-monitoring system need to measure, how fast does it need to sample, and how accurate does it need to be?"

When the lights flicker in a building, the first question a facility manager typically asks is: "What happened?" Unfortunately, in most cases, he/she does not know. The facility manager knows that the power went out momentarily - or did it? Characterizing the event by the voltage magnitude and duration, and evaluating the current coincidentally, helps identify the root cause and quantify the solution required to fix the problem.

If the company is adversely affected by such power-system events, corrective action may be warranted. But, without knowing exactly what happened (not only during this event, but over a period of time), the company probably doesn't have enough information to make a well-informed decision about which corrective action to pursue. For example, the company might invest in an uninterruptible power supply (UPS), which can ride through outages and interruptions for minutes, but the problem might actually have to do with transients (short-term, high over-voltage conditions).

Basic considerations when selecting a monitoring system and analyzing power-quality issues include:

  • Monitoring at the incoming service (from the utility) and downstream (near critical loads) to help distinguish where problems may have originated.
  • Monitoring for a complete business cycle at a time when the problem is likely to occur (typically for a 1-week minimum).
  • High sampling speeds, preferably 10 times the frequency of interest, to characterize fast transients caused by utility switching events or lightning. Often, transients occur, but the monitoring equipment may not sample fast enough to capture the event.

While portable monitors are more suitable for troubleshooting measurements, permanent monitors offer several important benefits: They're always monitoring, they're proactive and can help notify users of problems in advance, and they capture power quantities useful for energy-saving opportunities. Many companies track energy usage, peak demand, power factor, etc. in order to minimize their power bills. They may also compare their measurements with their power bills to make sure there isn't a discrepancy.

Remote communications with power-monitoring systems are essential to log data from such systems and communicate with users (paging the facility manager following an event, for example). Remote access via the Web is the preferred choice so that facility managers can access data remotely at any time and, secondly, so that experts can support the analysis from virtually anywhere in the world.

Dan Carnovale is power quality solutions manager at Cleveland-based Eaton Corp. (

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