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This article was originally published in the November/December 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

 

 

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Home Energy Magazine Online November/December 1993


LIGHTING

 

 


Understanding Power Quality

 


by Anibal T. De Almeida


The home electronics revolution and the growing number of utility programs promoting compact fluorescent lamps are ushering the issue of power quality into the residential sector.


Power quality is a relatively obscure concept, limited mostly to conversations among utility engineers and physicists, but as electronic appliances take over the home, it may become a residential issue as well. By the year 2000, electronic loads may account for half of U.S. electrical demand, and much of that growth in electronic load will take place in the residential sector.

Power quality is a two-pronged issue, with electronic equipment playing both villain and victim. Most new electronic equipment, while more efficient than its mechanical predecessors, consumes electricity differently than traditional mechanical appliances. While older devices like incandescent bulbs use power as it is supplied by the utility, electronic devices draw currents in bursts, altering the electricity that flows through them, so that what comes out the other side and returns to the grid is distorted. This dirty power underutilizes utility equipment and increases line losses. Thus, some of the efficiency gained through improvements in appliances is lost in the transportation of the electricity that runs them. Additionally, utilities must invest in filters and capacitors to clean this dirty power. Poor-quality power also causes transformers, cables and other transmission equipment to burn out more quickly, thus increasing utility equipment costs.

For customers, power quality first emerged as an issue in industrial and manufacturing facilities. Tiny power disturbances wreak havoc with the increasingly complicated, computerized machinery found along assembly lines today. Computers crash and data scrambles. The stakes are high--work stoppages can cost a company up to $500,000 an hour, and power-related problems may cost U.S. companies $25 billion a year.

In general, the more sophisticated equipment is, the more sensitive it is to variations in power quality. Household appliances that were once simple mechanical devices--like furnaces, air conditioners and heat pumps--are going electronic. And the home electronics revolution has made video recorders, personal computers, microwave ovens and digital clocks--all sensitive to power distortions--commonplace in American homes. As computers get smaller and faster, they become increasingly sensitive to power quality problems. Efficiency improvements on the horizon for refrigerators include electronic motors, which will also make them prey to the same power disturbances that leave the VCR clock blinking at us dumbly when we walk in the front door.

The Future of Fluorescents

Utilities are beginning to pay attention to electronic loads in the residential sector, particularly as they pursue demand-side management goals by promoting new, more efficient technologies. Many utilities are focusing promotional efforts on electronically ballasted compact fluorescent lamps (CFLs), which are capturing a growing segment of the CFL market from their magnetically ballasted predecessors. Although the electronic CFLs are more expensive than magnetically ballasted CFLs, they offer improved energy efficiency, lighter weight, and higher quality light, without the flickering and strobing effects of magnetic ballasts (see So Many Sockets, So Little Time, HE Mar/Apr '93, p.7). However, CFLs can also create power quality problems, though these problems may be avoided with circuitry that corrects for poor power factor and harmonic distortion.

Green Seal is lighting the way to higher quality CFLs in this country with a new CFL standard, and is currently testing a variety of bulbs for the honor of wearing the Green Seal logo (see Consumers and CFLs, p. 11). Green Seal is a non-profit organization which develops standards and product certifications designed to reduce the environmental impacts associated with the manufacture, use and disposal of products. Their Class A CFL standard requires a power factor rating of at least .9, and THD of less than 33%. Just this year, similar utility rebate eligibility requirements motivated several companies--including Panasonic, Phillips, and GE--to introduce corrected integral electronic CFLs.

Although U.S. manufacturers have been slow to produce power-corrected CFLs, a Taiwanese manufacturer, Electrotech, in 1991 introduced a competitively priced modular CFL boasting a power factor of 0.9 and total harmonic distortion (THD) of around 30%. The modular lamps offer substantial life-cycle cost savings over the all-in-one integral lamps because the ballasts can be reused. When an integral lamp fails, the whole unit must be discarded, although the most expensive component of the lamp, the ballast, may still have enough life in it to outlast four more bulbs. (Ballasts typically have a lifespan of about 50,000 hours; lamps, up to 10,000.)

Utilities are investing massive amounts of money to get CFLs into customer homes. Many require high power factor and low THD for eligibility for rebates and other incentives. [Power quality is] an issue for the utility more than it is for the consumer, says Bob Gilleskie of San Diego Gas and Electric Company. You see scattered problems now, and I believe that unless end-use manufacturers take steps to improve their products this area, the problem could be significant in the future. But there are enough people out there who are asking for better products. Manufacturers are getting the message.

I see the manufacturers solving the power factor and THD problem in 1993 and 1994, says Bill Grimm, senior energy management analyst for Southern California Edison. We're trying to create productive utility partnerships. When you're trying to transform markets, you need as much help as you can get. Edison recently signed agreements with 11 CFL manufacturers for rebates on 150,000 fluorescent bulbs.

With new bulbs entering the market all the time, the field of products can be a bit overwhelming. The National Lighting Product Information Program's April 1993 Specifier Reports: Screwbase Compact Fluorescent Lamp Products is one tool to use in attempting to comprehend this rapidly changing market. The report details a survey of CFL product manufacturers, and the program's own comprehensive testing of a long list of CFL products for a variety of characteristics, including efficacy, light output, power quality, total harmonic distortion and life. To obtain a copy, send a check for $30 ($15 each for 10-99 issues; $10 for $100 or more) to: Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Tel.: (518) 276--8716; Fax: (518)276-2999. n

 


Victims

Advanced electronic equipment is more sensitive to power disturbances than traditional machinery, as this graph of sensitivity thresholds illustrates. For instance, a voltage sag resulting in 70% of normal voltage for six cycles

(point A) will trip an adjustable-speed drive (ASD), while neither computers

nor breakers will be affected. A voltage sag to 60% at 12 cycles (point B)

will affect computers as well as ASDs but still lies outside the sensitivty threshold for breakers.

 


...and Villains Too

Traditional, electromagnetic equipment will draw sinusoidal current from a sinusoidal voltage. But electronic equipment, which converts ac power to dc power does not draw current for the entire voltage waveform interval. The resulting current irregularities can cause disturbances, such as impulses and voltage loss, on the power distribution system.

 


(Top) Motors and incandescent lights
(Middle) Computers and electronic equipment
(Bottom) Dimmers and variable-speed drives


A Power Quality Primer

Power quality is affected from both sides of the grid, referring both to the quality and reliability of power as supplied by the utility, and to the type and effect of customer loads on the transmission system. While power supply is generally regular and reliable, it can be interrupted or altered by storms and overloading of the system. Conversely, an ideal load--with a power factor of 1 and no harmonic distortion--draws power that matches that supplied by the utility, an even sinusoidal voltage waveform (see Figure A), while loads that deviate from that form can strain and increase inefficiency in the transmission system. Deviations from the regular sinusoidal wave, both on the utility side and on the customer side, result in poor power quality.

Brownouts are long-term (hours-long) voltage sags caused by system overload. U.S. utilities use rolling blackouts to avoid brownouts, because brownouts tend to damage equipment, but such fluctuations are common in developing countries.

Harmonics are a distortion of the utility-supplied waveform (see Figure B) and are caused by non-linear, (distorted) loads, which include motor controls, computers, office equipment, compact fluorescent lamps, light dimmers, televisions and, in general, most electronic loads. High harmonics increase line losses and decrease equipment lifetime.

Harmonic distortion refers to the difference between the shape of the current wave drawn by a device, and the shape of the voltage wave supplied to that device. Total harmonic distortion (THD) measures the degree to which the input is distorted, and is the relative value of all the harmonics combined, as a percentage of the fundamental current.

Power factor is a measure of how the current is being used to transmit power. It is a number between zero and 1, with 1 indicating perfect power factor. For example, electric resistance heaters and incandescent bulbs have a perfect power factor of 1, while newer electronic equipment, like electronically ballasted compact fluorescents, have lower power factors. When a load draws current that is not in phase with the voltage waveform, or draws a current that differs from the sinusoidal waveform provided by the utility, the power factor is less than 1. Poor power factor causes inefficiency in the delivery of electricity to the end-user, requiring more energy to compensate for losses on the line. For example, a load with a power factor of .5 will require twice as much current as a load with a power factor of 1 for the same amount of usable power. A low power factor is a power drain that decreases system efficiency.

Reliability refers to the probability of maintaining a continuous supply of electricity without interruption. Utilities generally design systems to lose only one hour of service in a period of ten years.

Sags (undervoltages) when very large loads start up, or as a result of a serious overload on the system (see Figure C).

Spikes are brief spurts of voltage (in the millisecond to microsecond range), during which voltage can shoot up a hundred times higher than normal (see Figure D). Spikes are caused by lightning and by the switching of large loads or sections of the power system network; they can disrupt the operation of data processing equipment and damage electronic equipment. Spikes can be suppressed by connecting the sensitive load to a transient absorber (also called surge arrester, varistor or VDR), which acts as a security valve when voltage goes substantially above normal. Computers and office equipment should be connected to sockets fitted with spike suppression.

Voltage Level. Household appliances and equipment are designed to operate at peak performance and efficiency at 120 volts, the voltage normally supplied to the home. Both undervoltages and overvoltages (sags and spikes) can affect equipment lifetime and operating efficiency. Voltage level can be checked with an alternating current voltmeter. Deviation from the rated level should be maintained within 112 to 126 volts.

Voltage Waveshape. The supply normally provided by the utility, besides having a constant voltage, is a smooth sinusoidal waveform.

 


Figure A. Normal power supply.

 


Figure B. Harmonic distortion.

 


Figure C. Sags and swells.

 


Figure D. Surges, spikes and impulses.

 

 

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