Facility Power Monitoring: Or, Where Did All the Electrons Go?
Power Measurements
If most of the available power and quality measurements are recorded, the PS250 can store about 4,600 records. If such data were sampled every 3 minutes, the memory could store data for nearly 10 days of continuous operation.
Figure 5 shows an example of power consumption logged over 2 days. Power consumption was measured on one phase of the three-phase 480-volt ac lighting circuit at MTDC. Timers turn on much of the center's lighting at about 5 a.m. Power usage fluctuates throughout the workday, then drops off at 5 p.m. It peaks again at 6:30 p.m. when the cleaning crew comes in, then drops for the night at around 8 p.m. when the cleaning crew departs.
Figure 6 shows power consumption on all three phases of another lighting circuit. Note a recurring load on the A-phase circuit late at night. This indicates the power consumed by exterior building lights that are controlled by a timer.
Although figures 5 and 6 show instantaneous power consumption at prescribed time intervals, it is possible to calculate and display power consumption in kilowatt-hours.
Figure 5—Power logged for one phase
of electrical supply over 2 days.
Figure 6—Power logged for three phases
of electrical supply over 6 days.
Load Analysis
If a facility is experiencing electrical problems, particularly reliability issues with computers or communications equipment, some form of power quality analysis may be needed. Many electrical devices depend on a stable voltage relatively free of noise and transients. Analysis of load currents and harmonics may identify problems that can be resolved by redistributing loads on a building's circuits or by adding filtering devices such as K-rated transformers.
The waveform in figure 7 is typical of a reactive load: the voltage and current are out of phase. Although both the voltage and current waveforms are sinusoidal—the crest factors of 1.4 indicate as much—the true power is about half of what you would calculate if the voltage and current were measured without regard to phase. This type of waveform is typical of an electrical motor when it is being started.
Figure 7—Voltage and current for a reactive load
such as an electrical
supply motor that
is being started (P.F. stands for Power Factor).
Nonlinear load currents and harmonics are typical of electronic power supplies, especially those on older computers and test equipment. The waveforms shown in figure 8 were recorded on a radio service monitor manufactured in the 1980s.
Note that the load current flows briefly when the voltage is near its peak. The crest factor of 2.4 is an indication of the nonlinear current flow. The power factor of 0.73 also indicates that current is not flowing in conjunction with the line voltage. The crest factor in particular indicates a substantial amount of harmonic energy in the powerline. While the relatively modest power consumed by this device doesn't warrant the installation of a K-rated transformer, a much larger load with similar crest factor and power factor could.
Figure 8—The voltage and current for an electronic power
supply show
harmonic energy (P.F. stands for power factor).
Conclusions
Portable power-quality monitors are useful tools for gathering information on power consumption that is more detailed and circuit specific than is available on the facility's power meter or from the utility company's bills. Fluke, AEMC, Hioki, GridSense, and Summit Technologies all manufacture products suitable for monitoring the power conditions of electrical circuits with up to 480 volts and with one to three phases.
The most common application anticipated for Forest Service facilities is documenting where and when power is consumed. Such information can be used to review policies or select new technologies that can reduce energy consumption and costs. The monitors also may be helpful when analyzing source and load power if the electrical system requires troubleshooting. In either case, power-quality meters can help Forest Service facility managers optimize electrical power consumption and distribution.
About the Author
Ted Etter joined MTDC in 2002 to work on electronics projects. He has spent more than 25 years working in the areas of electronic instrumentation and display technology. He received a bachelor's degree in mathematics from the University of Oregon in 1992 and a master's degree in teacher's education from Eastern Oregon State University in 1993. Before coming to MTDC, he taught courses in programming, digital circuits, data communications, radio frequency communications, robotics, microprocessors, and operating systems at the University of Montana College of Technology.
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