The Costs of Power Quality

    At a time when the quantity of power and its cost have become more of a concern than ever for energy users, concerns about quality have made power decisions increasingly difficult. Power quality problems are costly. When poor power quality damages equipment, downtime and repair or replacement costs reduce productivity, increase overhead, and can lead to loss of customer confidence, which could reduce sales.

    Power quality problems are not limited to sensitive electronic components, either. Harmonic distortion, for instance, may also affect motors and transformers, and may make the entire system vulnerable to damage. 

    In addition, the proliferation of ever-more-sensitive electronic equipment in applications from off ice buildings to the factory floor has brought the issue of power quality to the forefront. As more and more sophisticated testing and measuring equipment and just general electronics have been introduced, businesses have become more vulnerable to power anomalies. These days, power events that a few years back had little or no effect on day-to-day business-a surge, a lightning strike, or a problem with a piece of equipment-may have serious consequences. 

    Also, solid-state, switching-type "active" circuitry has replaced "passive" electronics-interlocked components that caused little or no distortion to the standard 60-hertz (Hz) sine wave-in the past decade. The active circuitry places more stress on the grid than the older passive electronics. 

Examples 
    Lighting systems consume 50% of the energy consumed in commercial and industrial buildings. Newer solid-state electronic ballasts that have largely replaced magnetic products incorporate an active switching device that introduces harmonic distortion to the line. 

    The harmonics manifest themselves by distorting the nice, smooth sine wave, which causes all kinds of problems in the grid. The harmonics cause stress in transformers in neutrals which normally see no current. Harmonic distortion on the line causes current on the neutral and creates stress on the system that causes transformers to overheat. 

    Variable-speed drives, which are becoming very common on industrial equipment, also include an active component. These devices reduce overall energy consumption, but they add distortion on the line.   Even ubiquitous personal computers add to the harmonic distortion problem, since they have switching power supplies. These are active components that can reflect back on the line. 

    Manufacturers of these products have started to produce devices that contro l the amount of harmonic distortion the devices put on the line. In fact, it is becoming an active specification in many instances. 

    Some utilities monitor and respond to the level of harmonic distortion caused by a device. Many lighting rebate programs mandate ballasts rated for less than a given level of total harmonic distortion (THD), often 12%. Utilities are also upgrading their grids to better manage currents in the neutral.

Power Anomalies 

   For convenience, power anomalies are often broken down into six categories. (For more information on power quality causes and effects, see the following page: Power Quality Disturbances, Causes, and Impacts

• Harmonic distortion is distortion of the sinusoidal voltage or current wave shape

• Electromagnetic interference, or EMI, consists of high-frequency, low-voltage signals coupled onto the power lines

• Transients are subcycle disturbances with a very fast voltage change; they are also called spikes, impulses, and surges

• Momentary interruptions refer to a loss of voltage ranging from less than a cycle to several cycles 

• Voltage sags and swells are variations in the voltage from about one half cycle to several seconds. Sags refer to a reduction in the voltage, while swells deal with a voltage increase

• Undervoltage or overvoltage is a sag or swell that lasts longer than several seconds

Finding and Fixing Power Quality Problems

The increased sensitivity of building systems makes it more important that energy managers better monitor the quality of the power to their facilities. Better monitoring is especially important to companies that produce a lot of distortion as well as companies that are very susceptible to distortion. 

Active systems are being put in place to help correct problems such as harmonic distortion by filtering out undesirable harmonics. These capacitive types of equipment-some new high-speed equipment can capture an event that occurs within milliseconds of the occurrence-are being sold to actively try to compensate for power events. 

A power quality audit can help identify power quality problems. The second step, dealing with power quality problems, usually means installing subsystems-equipment that will compensate for those problems. 

The objective of the power quality audit is to determine the source of power quality problems and to devise a solution to that problem. During the audit, monitoring devices are placed throughout the facility in order to determine precisely where in the system power quality problems are being found, as well as to monitor the quality of the power being supplied by the electric utility. 

Typically, after two to three weeks of monitoring, the power quality professional has enough data to recommend ways to solve any power quality problems. The system then often undergoes constant monitoring, either by the energy user or by an outside consultant, to ensure smooth operation of the facility.

Summary

Increasing demand continues to add stress to the power grid, and relief doesn't seem likely in the near future. New technology will produce less electronic disturbance on the line, so in 10 or 20 years, many of these problems may be resolved. 

Today, however, this combination of stress on the grid and internal sources of power quality degradation means the best action for many energy managers to take is to get the problem under control through a power quality audit and remediation program.

Stedi-Power, Inc
5044 B U Bowman Drive #102
Buford, Georgia 30518
PHONE: (678) 546-6780

Last Updated: 07 Jun 2004
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