Basics of power management equipment

Electrical power enters the plant, drives the equipment, shines the lights, conditions the air, makes the coffee, and runs the printer that prepares your paycheck — and the check to pay for all this electricity. Are you getting your money's worth? Probably not.

By Jack Smith, Senior Editor, Plant Engineering Magazine April 8, 2004
Key Concepts
  • Power monitoring reveals your power system’s characteristics.

  • Power quality data are analyzed by comparisons among parameters, locations, or surveys.

  • Good power management decisions require accurate measurements and proper data analyses.

Permanently installed monitors
Portable power monitors
Electrical power management plan checklist
Power monitor features

Electrical power enters the plant, drives the equipment, shines the lights, conditions the air, makes the coffee, and runs the printer that prepares your paycheck — and the check to pay for all this electricity.

Are you getting your money’s worth? Probably not. How can you know if you are getting what you pay for unless you have a power management plan in effect (see “Electrical power management plan checklist”)?

A power management plan begins with knowing how much electrical energy you are using, how and where you are using it, its quality, and how much it is costing you. But just knowing is like driving your car by looking in the rear-view mirror — it gives you a clear picture of where you have been, and perhaps where you are; it does precious little to tell you where you’re headed.

Once you know how much power you use, where you use it, and its quality, you can begin to make decisions that affect the quality of your electrical power, the price you pay, and even the quality and cost of the products you manufacture. Responsible electrical power management begins with a plan and includes the equipment required to monitor, analyze, and maintain costs and power quality in your plant.


When you take a trip in your car, it helps to have a roadmap to direct you. When you take responsibility for the cost and quality of your electrical power, your roadmap is your management plan; your roadway is your power monitoring equipment.

Electrical power monitoring gives you the ability to measure all aspects of power quality and record them in appropriate detail — whether you monitor your power continually or during a temporary power study. It enables you to see the characteristics of your power system and keep tabs on its relative health.

For example, an emergency load-shedding system can reduce the total plant load automatically to keep key plant processes operating on the remaining capacity in the event of utility or generator loss. Power monitors provide real-time power data ranging from basic metering, such as voltage, current, and frequency, to power quality data including harmonic analysis, waveforms, and transient detection. Some monitors allow users to designate field loads and power sources, and then prioritize load shedding based on the steady state conditions and the instantaneous electrical system characteristics.

Electrical power system monitoring records events that can range from microseconds to hours. To judge the quality of your power, many measurements and evaluations should be performed simultaneously and in real time. Parameters and events that typical state-of-the-art power monitoring systems record include:

  • Root-mean-square (rms) voltage and current

  • Power and power consumption, which includes Watts (W), voltamperes (VA), reactive power (VAR), power factor (PF), displacement power factor (DPF), demand, and kilowatt-hours (kWh)

  • Voltage sags, swells, and outages

  • Voltage transients

  • Voltage phase imbalance

  • Flicker (periodic voltage fluctuations that cause annoying modulation in lighting)

  • Ground current

  • Harmonic distortion.

  • RMS measurements are the basis for determining power levels and capturing events. Voltage, current, and real power should be measured as rms values. Then VA, VAR, PF, and DPF can be calculated from them. Sags, swells, and outage captures are based on rms values as well. Some power monitors use analog rms measurement techniques and average the rms measurement over several cycles. In this case, cycle-to-cycle variations can get lost. A better approach is to look at every cycle.

    The accuracy of power consumption and harmonics measurements depends not only on sampling rates, but also on processing throughput. Some monitors may sample for a limited period and suspend monitoring while they perform the power and harmonics calculations. However, monitors employing digital signal processors can measure every cycle, without “blind spots.”

    Intermittent voltage events such as sags, swells, outages, and transients are difficult to capture. A good power monitor is able to analyze both the 5-min outage and the 200-

    The ability of any digital monitor to capture and display transients depends on sampling rates and the design of the sampling system. Monitors equipped with peak detection circuitry can capture amplitude and phase of transients, but cannot display the shape of the transient. Monitors with high-speed sampling systems can both capture transients and display their waveforms, similar to a digital oscilloscope.

    Power monitors can be permanently installed or portable. Permanently installed power monitors allow continuous monitoring and provide for scheduled reporting of critical power cost and quality parameters to key decision makers (Fig. 1). Portable power monitors enable troubleshooting or power surveys wherever they are needed.

    Permanently installed monitors

    Continuous monitoring provides the data to perform a post mortem on disruptive power disturbance events. These data help you determine how and where events occurred. Harmonics must be monitored continually to ensure that the incremental addition of loads over time does not cause excessive heating, which can lead to the premature failure of transformers, conductors, and circuit breakers.

    Continuous tracking of power consumption and rms quantities provide useful data for planning future plant expansion and for ensuring that existing and future utility feeder and substation capacities are adequate. These data also are useful for planning additional discrete loads or new backup power systems.

    A comprehensive view of the power system is helpful in plants that schedule maintenance shutdowns once or twice per year. Archiving power survey reports that contain all aspects of power in great detail provides the ability to compare existing and historical conditions to help predict when a failure will occur. Alarm functions of continuous power monitors can alert key personnel when a power problem is detected by sending messages to pagers or PC screens.

    Portable power monitors

    Portable power monitors have rugged sealed cases that allow them to be transported to various locations within a plant (Fig. 2). They are especially useful when temporary monitoring is necessary or where existing power equipment will not accommodate the installation of a permanently installed monitor.

    Many portable power monitors have nearly the same features as their permanently installed counterparts. However features unique to portables center around mobility and flexibility.

    Power meter portability enables you to perform power system surveys by analyzing costs vs. benefits of power system improvements as well as verifying savings generated by system improvements. Although permanently installed monitors can help you reduce energy costs, the flexibility of a portable unit lets you document high demand caused by equipment startup, then sequence load startups to reduce demand peaks. It also enables you to identify inefficient equipment or equipment in need of maintenance or upgrade. Portables can augment your preventive maintenance efforts by allowing you to create a baseline of machine or motor operating characteristics, and then compare future readings to baseline data to determine maintenance requirements.

    Both permanent and portable monitors capture electrical anomalies. However, portables allow you to troubleshoot power quality problems by taking the monitor to the source. Although both have many of these capabilities, the flexible features of typical portables allow you to:

    • Identify harmonic distortion levels and sources

    • Solve wiring and grounding problems

    • Document and troubleshoot sags and swells that can cause nuisance tripping and product defects

    • Capture and analyze potentially damaging voltage transients

    • Log waveforms and other data, then upload them to a PC for analysis.

    • Standard practice is to perform power surveys during one business cycle. For most plants this means monitoring for a week to see the effects of shift changes, maintenance procedures, and other weekly occurrences that affect power usage. Every day 5,184,000 cycles of voltage and the same number of current cycles occur on every phase. High-speed sampling combined with creative memory management provides microsecond detail, single-cycle visibility, and daily trends to provide ongoing power quality tracking.


      Equipment is used to capture critical data required for responsible electrical energy management. However, analysis of this captured data is accomplished primarily by software. A brief overview is presented here. Further power management software coverage will be published in the June issue.

      The power of modern PCs and software, along with the connectivity of networks and the internet, put powerful analytical tools within a click of the mouse. These tools allow you to convert power quality data into an index that can be tracked over time. This power quality index is determined based on the relationship of each recorded event to an appropriate power tolerance curve. Software allows you to delve further into the data from which the index was derived. You can then compare and trend any parameter against any other parameter; compare data among locations; or compare one survey with another.

      Power tolerance curves provide an indication of the likelihood that an event will cause an equipment failure. The vertical axis represents the magnitude of the event and the horizontal axis the duration of the event. The longer the duration of the event, and the more extreme it is, the more likely it is to cause a problem. Equipment type, location, and sensitivity, as well as power monitoring point determine which of the various standard power tolerance curves to use. The ANSI curve defines the maximum excursions of voltage with respect to time that can be expected at the service entrance from a utility. The CBEMA curve and the newer ITIC curve describe the sensitivity of electronic equipment. Some systems allow you to define your own curves based on the needs of your plant.

      Power tolerance curves focus your attention on worst-case events. When you click on an event on the power tolerance curve, the analysis software links the rms and ac waveform information to provide a plot of the steady-state event along with the transition events, which can be expanded for further analysis.


      Good electrical power management means making good decisions based on accurate measurements and proper analyses of those data. A well-executed power management plan along with useful data and power system best practices helps you minimize cost while maximizing power quality. Power management decisions include:

      • Load shedding or curtailment

      • Starting or stopping a production line based on energy cost vs. production needs

      • Whether to start cogeneration or peak-shaving generators (Fig. 3)

      • Whether to operate plant lighting at less than 100% output based on time of day, peak billing periods, and illumination requirements.

      • The level of power monitoring depends on the criticality of your plant’s operation. Mission-critical applications, such as data centers, air traffic control, or financial transaction facilities strive for 100% uptime. Your plant must include its risk tolerance level in any equation used to derive a power management plan. Monitoring the electrical power delivery and distribution infrastructure of your plant should be and integral part of any proactive power management program.

        PLANT ENGINEERING magazine extends its appreciation to Cooper Power Systems; Eaton Electrical | Cutler-Hammer; Power Measurement; Reliable Power Meters; Rockwell Automation; Siemens Energy & Automation; Schneider Electric/Square D; and Toromont CAT for the use of their materials in the preparation of this article.

        Electrical power quality quiz

        True or false? Answers
        Voltage sags usually affect all 3-phase voltages equally. False . Most sags involve a single phase on the utility system.
        Ground faults in a facility always cause protective overcurrent devices to operate. False . Ungrounded systems are intended to continue in operation during a ground fault.
        Voltages to ground on an ungrounded system can exceed the nominal phase-to-phase voltage. True . During ferroresonance conditions, voltages to ground on an ungrounded circuit can exceed the nominal phase-to-phase voltage.
        Ground rods installed at sensitive equipment can reduce nuisance tripping, but will introduce a safety hazard for employees and risk equipment damage. True . The National Electric Code (NEC) prohibits ground rods that serve as the sole equipment grounding means.
        Portable monitoring equipment provides the same level of monitoring as a permanent system. False . Portable equipment should never be used in place of a permanent monitoring system.
        Source: Schneider Electric/Square D

        Electrical power management plan checklist

        The following checklist is a good place to start when designing a power management plan. Remember to match your power management requirements to the features of equipment you select.

        Identify goals and objectives in terms of energy management, power quality, system reliability, and uptime

        Identify available resources
        —Money for production, system integration, and training costs
        — Time required for equipment installation, training, and associated learning curve
        — Manpower requirements for system evaluation and selection, installation, training, and usage

        Collect records and documentation, which include rate structures, utility names and contact information, and accurate facility one-line diagrams

        Involve the appropriate personnel according to goals and objectives
        —Plant engineering
        —Power quality engineering
        — Maintenance
        — IT
        — Accounting
        — Plant management

        Evaluate and select power management equipment based on:
        — Installation requirements
        — Support and maintenance requirements
        — Reputation of potential vendors
        — Training
        —Turnkey availability

        Obtain system training

        Plan for system maintenance

        Plan for long-term operability.

        Power monitor features

        State of the art power monitors provide energy and billing information necessary to measure and control energy costs; power quality information to help troubleshoot, diagnose, and minimize unplanned downtime; and included I/O for status and control applications. Typical features include:

        Calculation of kWh usage and peak demand for the period that matches the utility billing interval

        Metering in actual dollar usage units as well as engineering units

        Demand metering

        Cost allocation of mains and feeders

        Utility revenue meter accuracy

        Displays energy data viewable anytime, anywhere, using a standard web browser

        Detects and records subcycle disturbances such as utility capacitor bank switching

        Detects and records potentially damaging voltage transients caused by lightning strikes, static discharge, or equipment operation

        Captures waveforms when utility reclosers operate

        Alarms on and captures ground fault waveforms

        Provides line-to-line (L-L) and line-to-neutral (L-N) sag/swell alarms

        Provides multiple levels of information on each event

        Multiple waveform captures of adjustable resolution and duration, which display the details of an event and the conditions surrounding an event

        Data recorder to log key electrical values

        Minimum, maximum, and average functions, which indicate extremes and normal operating conditions

        Event log with time stamp, which allows you to determine sequence of events

        Digital status displays

        Displays analog input values in applicable engineering units

        Monitors solid-state transfer switch status

        Capable of performing simple, automated control functions, such as load shedding for peak demand shaving, without custom programming.