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Facility Power Monitoring: Or, Where Did All the Electrons Go?

Ted Etter, Project Leader

Facilities engineers and supervisors may need special tools to analyze electrical power usage as they respond to rising energy costs and mandates to reduce energy consumption. While the monthly bill from the local electric utility indicates total power consumption, the bill does not show the breakdown for lighting, computers, machinery, and heating and cooling. Portable power-quality monitors can record power consumption on specific circuits within a facility, helping engineers and supervisors assess how electrical loads are distributed and where power usage can be altered.

Permanent and Temporary Metering

A policy requiring permanent power-quality meters in many Forest Service buildings is being drafted in response to the Energy Policy Act of 2005. Such meters record power consumption periodically and can download the information to a computer or deliver it over a network link. The threshold of building size or power consumption that will require installation of a permanent power-quality meter has not been decided. While logging power meters are available for about $1,500, the cost of installing and connecting them can be substantially more than their purchase cost.

Portable power-quality monitors (figure 1) provide the option of monitoring power conditions at a facility long enough to assess power usage on various circuits. Furthermore, the power-quality meters can be used to inspect circuits for load transients and high harmonic content. Portable power-quality meters cost $1,500 to $6,000. More expensive models include graphic liquid crystal displays that show powerline voltage, current waveforms, and logged data. Less expensive units primarily serve as data loggers, relying on attached computers for waveform displays and usage graphs.

Figure 1—The Summit Technology Inc.
PS250 portable power-quality monitor.

Portable power-quality monitors can be connected to the mains of a circuit breaker panel in a few minutes and programmed to measure a variety of electrical parameters on the circuits for periods of minutes to days. Typical measurement capabilities include monitoring voltage on 480-volt ac three-phase mains with current up to 1,000 amps.

A portable power-quality monitor may be useful for monitoring power conditions and usage throughout a forest or region. It can be used to document before and after power consumption during a building upgrade or to determine how much power typically is consumed by the occupant of a leased space within a building (even if that space isn't metered separately). If electrical equipment is failing frequently, power-quality meters can be used to monitor voltage transients, the power factor, and harmonics on powerlines that may be responsible for the failures.

Transients, Harmonics, and Phases

Ideally, the voltage at a facility has a pure sinusoidal waveform with a frequency of 60 hertz that doesn't fluctuate even if the load changes. Motors, lamps, and assorted electronic power supplies connected to a circuit change the load as they are turned off or on. For example, when a pump motor turns on, the increased load on the circuit may cause the voltage to sag briefly. Sudden changes in line voltage are referred to as transients. Large transients may harm or destroy electronic devices connected to the circuit.

Some loads connected to a circuit do not draw current in proportion to the line voltage. Such loads tend to alter the waveform of voltage on the line and add harmonics to it. For example, some loads tend to add odd harmonics so that the original 60 hertz sine wave has additional voltage signals with frequencies of 180 hertz (third harmonic), 300 hertz (fifth harmonic), and so on. Electronic power supplies are notorious for generating harmonic signals on powerlines. If the harmonic voltages are relatively powerful compared to the original line voltage, the signal has a large harmonic content and may harm or destroy electronic devices.

Residential buildings typically have voltage delivered in two phases: the equivalent of two 120-volt ac lines that have opposing phases. The voltage in one line is 180 degrees out of phase with the voltage in the other line. Most large buildings receive line power in three phases. In three-phase systems, two of the three lines carrying power are 120 degrees and 240 degrees out of phase with the other.

High-powered loads, such as large motors, may be driven by all three lines carrying power, but many circuits use only one of the three. For example, a circuit with 207 volts ac in three phases will show 207 volts on a voltmeter connected between any two of the three lines carrying power; the voltage between any one of the three lines carrying power and the neutral line will measure 120 volts. Wall outlets and 120-volt lighting circuits can be fed from any of the three lines carrying power.

Taking the Compact Model Out for a Spin

The Missoula Technology and Development Center (MTDC) was asked to evaluate a portable power-quality monitor and develop guidelines for its usage. After a brief market survey, the Power Sight PS250 by Summit Technology, Inc. was selected. Several options were weighed before our selection, such as the maximum rating of the current probes and whether harmonic content measurements were necessary. In the end, 1,000-amp probes were selected and the harmonic analysis option was not. An option for a fourth current probe to monitor neutral line current was not selected. The PS250 with three 1,000-amp probes (figure 2) cost about $2,000.

The PS250 has a two-line alphanumeric display that reports connection status and current measurements. When the device is first turned on and the probes are connected to a circuit, the PS250 checks for phase relationships between line voltage and current on each phase and can recommend reversing the current probe's orientation to obtain the proper measurements.

A personal computer (PC) is required for detailed reports and graphic displays of line voltage, current, and power consumption. The PS250 comes with a software package called Power Sight Manager (figure 3) that allows the PC to control measurement and logging parameters of the PS250 in addition to displaying waveforms and data logged by the meter. The PS250 connects to the PC through the PC's serial data port.

Figure 2—Components needed to monitor
power with the PS250 power-quality monitor.

Figure 3 —Components needed to monitor
power with the PS250 power-quality monitor.

The PS250 monitors voltage and current simultaneously to calculate and record power and waveform statistics such as the power factor and crest factor. The current in each phase of a circuit is measured with a split-core current probe. Voltage is measured by a clip lead attached to an exposed lead for each phase plus an additional lead for the neutral leg.

The voltage probes' insulated alligator clips permit connections to be made between the PS250 and the circuit while it is energized. An exposed circuit represents an extreme electrical shock hazard. While the voltage and current probes are well insulated, a user can accidentally come into contact with exposed high-voltage circuits. When working near the panel (figure 4), it's best to keep one hand in your pocket to prevent touching two exposed lines and completing the circuit with your body. Turning off power upstream may be advisable if there is any difficulty in establishing reliable test lead connections.

Figure 4—These voltage and current probes are
logging data on this three-phase electrical panel.

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