00267516

7/28/2019 00267516

1/5

Power Quality SolutionsEPRI accelerates an already wide-ranging programof research to study and solve power quality problemsJ ohnDouglas, Electric PowerResearch Institute

he proliferation of microelectronic processors in awide range of equipment, from home VCRs and digi-tal clocks to automated industrial assembly lines andT ospital diagnostic systems, has increased the vul-nerability of such equipment to power quality problems.These problems include a variety of electrical disturbances,which may originate in several ways and have very differenteffects on various kinds of sensitive loads.What were once considered minor variations in power,usually unnoticed in the operation of conventional equip-ment such as lights and constant-speed motors, may nowbring whole factories to a standstill. A power interruption or30percent voltage sag lasting mere hundredths of a second,for example, can reset programmable controllers for anassembly line, while adjustable-speed drives for motorizedequipment on the assembly line may themselves be sensi-tive to voltage harmonics and transients. The cost of suchinterference can be substantial. One glass plant, for instance,estimatesthat a five-cycle interruption, an outage of less thana tenth of a second, can cost about $200,000, and a majorcomputer center reports that a 2-second outage can costsome $600,000.As a result of this vulnerability, increasing numbers ofindustrial and commercial facilities are trying to protectthemselves by investing in equipment to improve powerquality, such as uninterruptible power supply (UPS) systems,surge suppressors, and isolation transformers. Corporatedata processing centers, for example, can justify an increaseof as much as 45 percent in their capital investment foron-site power distribution facilities in order to ensure ade-quate electricity quality. Products and services related topower quality now represent a multibillion-dollar market inthe United States alone.Which approach to power quality is best, however, de-pends largely on specific circumstances. Some problems,such as harmonic interference between different electricalloads on the same line inside a building, are often solvedmost easily on a customer's premises. Others, such as mo-mentary power disruptions caused by lightning strikes orsubstation switching operations, may be less expensive tocorrect on the utility's side of the meter. Determining themost cost-effective approach in particular situations requiresThis ar t ic le is repr in ted f rom the December 7993 i ssue o f the EPRlJourna l . Background in for mat io n for th is ar t ic le was provided b yWade Ma lco lm , Ben Bane r j ee , and Marek Samo ty j , Cus tomerSystems Div is ion; Vasu Tah i l ian i and Harshad Meh ta , E lectr ica lSystem s Div is ion; and V i to Longo, In tegra ted Energy SystemsDiv is ion.

First, more information about the causes of power qual-Second, the development of new analytical tools for

There are also major opportunities, including severalbased on recent developments in power electronics, to im-prove power-conditioning equipment for both sides of themeter. More-efficient inverters for online UPS systems, forexample, will reduce losses from the power that continu-ously flows through them. Thyristor-based controllers fordistribution systems will provide new options for reducingpower interruptions that originate on utility lines. Equipmentmanufacturers are also incorporating power quality protec-tion into their products. Examples include filters to mitigatethe effect of harmonics, special circuits to provide powerfactor correction, and capacitors that provide enough storedenergy to enable digital clocks to ride through brief interrup-tions. EPRl is working with manufacturers in some of thesedevelopment efforts.Responding to these needs and opportunities, EPRl isaccelerating an already wide-ranging program of research tostudy and solve power quality problems. Surveys are underway to better understand the causes and effects of powerdisturbances, as seen both on utility systems and at individ-ual customer sites. The power quality characteristics of im-portant categories of sensitive equipment, such as personalcomputers, are being measured, and more-advanced powerconditioners are being developed for use by customers. EPRlis also developing advanced power electronic controllers foruse on utility distribution systems.

ity problems in generalutilities to use in advising their customers.

AssessingPower QualityAs the need for more information about power qualitybecame apparent overthe past few years, EPRl launched twomajor studies to determine the extent and nature of prob-lems and to identify potential solutions. One effort wasaimed at customers and has produced a series of targetedhow-to documents for dealing with power quality problemsin various end-use situations. A standardized power qualityassessment procedure was also produced as part of thiseffort. The second study, now at its midpoint, involves theuse of 300 power quality monitoring nodes on the distribu-tion systems of 24 utilities throughout the United States andis being extended to customer sites. Preliminary results haverecently become available and are providing valuable in-sights into the causes and impacts of various utility systemevents that affect power quality. Both studies were under-taken by Electrotek Concepts, Inc., of Knoxville, Tennessee.IEEE Power Engineering Review, March 1994 J

7/28/2019 00267516

2/5

Several important conclusions emerged from the end-user study. The researchers found that, despite growingconcern over power quality, many large ndustrial customersneed to be better educated about how specific problemsaffect their equipment and about the potential solutions thatare available. The three kinds of equipment found to be mostsusceptible to common disturbances are programmablelogic controllers, automated data processors, and adjust-able-speed drives (ASDs). The four kinds of disturbance thatpresent the most problems with these types of equipmentare voltage sags, momentary interruptions, transients, andharmonics. The cost of losses associated with these prob-lems may exceed the original purchase price of the affectedequipment many times over.The initial phase of the study was followed by 38 detailedcase studies at 17 participating utilities. The 32 cases thathave been completed have revealed a variety of specificpower quality problems facing important industrial and com-mercial customers.Automotive plants, for example, are major power con-sumers; their peak loads are 20 MW or more, typically sup-plied by two separate distribution lines. While the presenceof multiple, redundant lines means that a transmission sys-tem fault or the lossof a substation transformer will typicallycause only a brief voltage sag, even such momentary eventsmay have a significant cost. Following a voltage sag, forexample, the restarting of assembly lines may require clear-ing the lines of damaged work, restarting boilers, and repro-gramming automatic controls, for a typical cost of about

$50,000 per incident. One automaker estimated that the totallosses from momentary power glitches at all its plants run toabout $10 million a year. The researchers concluded thatmost automotive manufacturing equipment could be engi-neered to withstand small voltage dips and that constant-voltage transformers could help sensitive equipment ride outlarger voltage sags.EPRls national study of power quality on utility distribu-tion systems began in 1992 and will continue through 1995.Monitoring sites were chosen to produce statistically validconclusions about how distribution systems in general re-spond to various power quality problems. Results so farindicate that the most mportant cause of momentaryvoltagesags is lightning strikes. In the majority of sags, the voltagedrops to about 80 percent of nominal value. In terms ofduration, sags tend to cluster around three values: 4 cycles(the typical clearing time for faults), 30 cycles (the instanta-neous reclosing time for breakers), and 120 cycles (the de-layed reclosing time for breakers). The effect of a voltage dipon equipment depends on both its magnitude and its dura-tion; about 42 percent of the sags observed to date weresevere enough to exceed the tolerance standard adopted bycomputer manufacturers.Another common event that can cause power qualityproblems is capacitor switching on the distribution system.Although such switching is a long-established practice thatusually doesnt affect customer equipment, problems canarise at some large industrial sites that have their own trans-formers and capacitor banks for Dower factor correction. In

1994AlbertaExpositiandConferenceonPoCall or Papers

Abstractdeadline: April 15, 1994Conferencedate:October 12-15,1994

The organizing committee i s callingAlberta Exposition and Conference oconferences have historically focusedonend user.Telecommunications,are especially encinclude:

e0e EM1andRFI concerns

Power quality effects on equipment reliPower quality effects ondata reliability

tions

420- 6276.Authorsof acceptedabstracts will bedetailed instructions for authorswillbe issue

such cases, a resonant oscillation maybe set up in the customers line thatmagnifies the effect of the utilitysswitching operation. A momentary over-voltage some three to four times thenominal rating may thus be created, trip-ping or even damaging protective de-vices and equipment. Electronicallybased ASDs for industrial motors areparticularly susceptible to these tran-sients. A planned extension of the EPRldistribution power quality project willinclude simultaneous monitoring ofpower disturbances on both sides of themeter at affected sites.Remedies on theCustomer Sideof the MeterA variety of technical options areavailable to correct power quality prob-lems. The choice of which to use, andwhether to focus on the customers orthe utilitys side of the meter, dependslargely on specific circumstances, butresearch to date has provided some im-portant guidelines.The majority of minor power distur-bances in commercial buildings can beremedied by making improvements infacility wiring and grounding. Sensitiveloads and electrically d i r ty loads, for ex-ample, can be isolated on dedicatedbranch circuits to minimize nterference.Care must be taken to prevent poweranomalies from passing between suchdedicated circuits by way of shared neu-tral wires or ground connections. TheNational Electrical Code stipulates thatthe steel frame of a commercial buildingbe bonded o ground at the utilityserviceentrance. Problems can arise, however,

IEEE Power Engineering Review, March 1994

7/28/2019 00267516

3/5

if bonds are made inside a building be-tween neutral wires and the groundedframe, creating inadvertent connectionsbetween loads and allowing stray currentsto cause disruptions.In many industrial plants, adjustablespeed drives require special consideration,since they are a major source of somepower quality problems, especially har-monics, and are particularly susceptible toothers, such as voltage surges and sags.ASDs work by first rectifying constant-fre-quency, constant-voltage ac power to dcand then inverting this dc power to createa variable-frequency, variable-voltage acoutput. This process feeds harmonic volt-age distortions back onto the line, wherethey can interfere with sensitive electronicloads. In turn, the dc section of an ASD isparticularly susceptible to momentaryovervoltages coming from the outside.In general, harmonics caused by ASDsare not a problem unless these devicesmake up a major part of the load suppliedbya customer's transformer. But, ifthe sitealso has capacitors for power factor correc-tion, the resulting resonant circuit maymagnify the effect of a particular harmonicfrequency. EPRl research has verified thatone cost-effective solution to this problemis the installation of line filters next to largeASDs and motors to reduce the magnitudeof harmonic distortion. Conversely, to pre-vent nuisance tripping of small ASDs inresponse to transient voltages from theoutside, which are also magnified by on-site power-factor-correcting capacitors, asmall inductor (choke) can be placed inseries on a line to reduce current surges.Sometimes, however, more sophisti-cated technology is needed to solve powerquality problems on a customer's prem-ises, and EPRl studies have indicated a par-ticular need for new power-conditioningdevices capable of providing integratedprotection against multiple kinds of powerdisturbances. Responding to this need,EPRl has worked with Public Service Elec-tric &Gas Company (PSE&G) and Westing-house Electric Corporation to develop anactive power line conditioner (APLC), thefirst device to combine active harmonic fil-tering, line voltage regulation, and tran-sient voltage surge protection in a single, compact unit. A5-kVA single-phase version of the APLC was commercializedin late 1992, and 50-kVA and 150- kVA three-phase versionsare now also available. Other products in the APLC familywill be offered in the near future.Instead of using conventional passive filter circuits, whichremove only specific harmonics from a line, the APLC'spower electronic circuits automatically adapt to changes inthe harmonic spectrum and actively inject signals onto a linethat cancel the disturbances. This approach eliminates theneed to calculate what harmonic currents are likely to bepresent or to retune filters as load conditions change. Inaddition, the APLC regulates incoming voltage to compen-sate for sags and surges on the utility line. Since the 5-kVAdevice is about the size of a personal computer and weighsonly 125 pounds, itcan easily be installed close to sensitiveloads.

One of the first demonstrations of anAPLC took place at a pharmaceutical plantin the PSE&G service territory, where har-monic distortion from ASDs was interferingwith the computer-controlled operation ofthe plant's quality control laboratory. Prob-lems of this kind, which had been interrupt-ing plant operations several times a month,were completely eliminated through the in-stallation of a prototype APLC. During theyear-long test, the device also maintainedpower quality for the lab during a particu-larly severe thunderstorm."The APLC provides a low-cost solutionfor several kinds of power quality prob-lems,'' says research manager Ben Baner-jee of EPRl's Customer Systems Division."We estimate that APLC technology couldsave utilitycustomers many millions of dol-lars over the next five years. Further devel-opment efforts are also under way toincorporate some energy storage capacityinto the APLC to enable it to ride throughshort power outages without affecting theperformance of sensitive loads."The Customer Systems Division is alsoworking on a variety of other power qualitydevices for end-use applications. One ex-ample is a hybrid filter that adds a seriesactive element to an existing passive filterand controls harmonics by actively adjust-ing series impedance. New England PowerService Company is demonstrating a proto-type hybrid filter in an 800-horsepower ASDapplication at a sewage treatment plant.Another example is an advanced solid-stateload center for commercial and industrialfacilities, which will integrate a variety ofcontrol functions related to power quality,including load monitoring, fast load switch-ing, and protection against fault currentsand overvoltages. Also under investigationare advanced UPS concepts that featurenovel means of storing energy, such asflywheels and supercapacitors. And a newgeneration of ASDs is being developed thatwill reduce harmonics and be more capableof riding through power disturbances."This growing array of power qualitytechnologies will give utilities and their cus-tomers more flexibility in addressing powerdisturbances," says Wade Malcolm, man-ager of the Power Electronics & ControlsProgram in the Customer Systems Division. "In addition, weare working with equipment manufacturers to incorporatepower-conditioning capabilities into their products."

Remedieson the Utility Sideof he MeterMeanwhile, EPRl is also developing advanced technologyfor use by utilities to improve overall distribution systemreliability and to keep power quality problems that originateon distribution systems from reaching customers. This tech-nology, the basis for a concept EPRl calls custompower,involves a combination of power electronic controllers, dis-tribution automation equipment, and an integrated commu-nications protocol, which together will enable utilities tomeet the power quality needs of industrial and commercialcustomers with sensitive loads.Custompowerservicewouldprobably be offered initially to large individual customers orto clusters of smaller customers in industrial parks.5EEE Power Engineering Review, March1994

7/28/2019 00267516

4/5

Integrated approach tosdvi ng power quali ty problems

Voltage sag

Overvoltage

Disturbance 1 PossibleCauseLightningstrikeTree or animalcontactFault on anotherphaseLoad rejection

Interruption

TransientHarmonicdistortion

I

Blown fuseBreaker operationLightningstrikeUtility switchingNonlinear oadsFerroresonance

Electricalnoise customer wiringor grounding

Utility-SideSolutionDynamic voltagerestorerStatic conditionerDynamic voltagerestorerFault CurrentLimiterHigh energysurge arresterSolid state circuitbreaker 'static condensorHigh energys u r g e arresterFilterStatic condensorDynamic Voltagerestorer

"Power quality problems can generally be categorizedinto two groups," says Vasu Tahiliani, manager of the Cus-tom Power Distribution Program in EPRl's E lectrical SystemsDivision. "One group includes waveform distortions thatusually arise on the customer's premises and can best besolved there. The other group includes power interruptionsand voltage sags that originate on the distribution system.Custompowerwil l give utilities new options for overcomingthis second group of problems and providing customers withhigh-quality power. I believe this can sometimes be thelowest-cost approach, although specific problems have to behandled on a case-by-case basis."The basic custom power configuration would use powerelectronic controllers at a substation with two independentdistribution feeders to provide essentially uninterruptible,robust power to customers. In the event of a power flowinterruption on one feeder, a solid-state breaker (SSB)wouldswitch over to the other feeder, reducing the outage time fora fault to one cycle or less. Meanwhile, a static condenser(STATCON) would hold voltage constant and supply mo-mentary power so that the customer load would remainessentially unaffected by the feeder switching.Both of the basic custom power controllers just describedare now being developed for EPRl by Westinghouse Science& Technology Center. SSB and STATCON prototypes arebeing designed to use currently available gate-turnoff thyris-tors for 15-kV distribution systems. Laboratory-scale ver-sions of both devices have been constructed and tested toprove concept feasibility. Utility field testing of full-scaleprototypes is scheduled to begin in late 1994 or early 1995.Commercial versions of the controllers are expected to beready for market in 1995 or 1996.SSBs have several advantages overthe mechanical circuitbreakers now commonly used in distribution systems. Notonlycan anSSB react much morequicklythan its mechanicalcounterpart, but itcan also be used repeatedly without de-grading performance, whereas a conventional circuit breakerhas to be refurbished after repeated use. Each SSB hasthyristors that conduct normal load current and interruptexcessive fault currents. A surge arrester in parallel with thethyristor switches provides protectionagainst transient over-

Customer-SideSolutionLine conditionerUPSLine conditionerVoltage regulatorUPS

UPSMotor gen setLine conditionersurge suppressorLine conditionerFilter

GroundingShieldingLine conditionerFilter

voltages that develop during faults. Inaddition to their role in providing pre-mium-quality power to customers withsensitive loads, SSBs will allow utilitiesto operate their distribution systems atincreased short-circuit capacity and toexecute automated load managementmore efficiently.A STATCON is connected as a shuntbetween a distribution line and ground; itsupports voltage in the line by exchang-ing power with the line during differentparts of a cycle. A thyristor inverter in theSTATCON rectifies a portion of the acpowerfromthe ineto dcpowertochargea large capacitor and then inverts this dcpower to ac for reinjection to the line asneeded. The capacitor both acts as asource of constant voltage and providesenough energy to supply real power tothe line for a few cycles during voltagesags or interruptions.Development work has also begun ona variety of other electronic controllersenvisioned as part of custom power serv-ice. A dynamic voltage restorer would beconnected in series with a distributionline to override momentary voltage sagsand swells by inserting a compensatingvoltage into the line. Thyristor-switched capacitors wouldbring distribution system capacitors online in smaller incre-ments in order to follow load changes more closely andreduce the transients now associated with large capacitorswitching operations."Our aim in developing custom power technology is totake a cost-competitive approach to improving power qual-ity," says EPRl research manager Harshad Mehta. "Ad-vanced electronic controllers will provide new flexibility toutilities as they implement distribution automation plans andwork with customers to find the most expedient and eco-nomical way to provide uninterruptible quality power."

Services forMembersTo be in a better position to advise their customers aboutthe best solutions to power quality problems, utilities need

a better understanding of the relationship between distribu-tion systems, customer systems, and a variety of end-usetechnologies. To address this need,EPRl's Power ElectronicsApplications Center (PEAC) in Knoxville, Tennessee, haslaunched a research project on system compatibility. Specifi-cally, this project is aimed at determining how well existingelectronic equipment can tolerate power disturbances, whatdisturbances the equipment generates, and how well itper-forms its intended function. Equipment is chosen for evalu-ation by EPRl member utilities participating in the project.The first two technologies tested under the project werethe electronic ballasts used in fluorescent lamps and thetransient voltage surge suppressors widely sold to protectsensitive equipment, such as computers. Tests were con-ducted at the new Power Quality Test Facility (PQTF)at PEAC.The tests showed that, although electronic ballasts produceabout18percent more illumination perwatt of electric powerconsumed than conventional magnetic ballasts, they createmore than twice as much current distortion and are morevulnerable to transients on the power system. Tests on threebrands of transient voltage surge suppressors revealed widevariations among the brands in their response to surges andsteady-state overvoltages, and uncovered some deficienciesthat were reported to the manufacturers.

IEEE Power Engineering Review, March 1994

7/28/2019 00267516

5/5

Another PEAC service is the Power Quality Hotline, avail-able to EPRl member utilities. With this service, a utilityengineer can obtain immediate, high-level help over thephone to deal with specific power quality problems. DukePower, for example, recently used the hotline to ask forassistance n finding out why some customers' digital clockswere gaining as much as 2 hoursduring the night.After Dukefaxed a graph of waveforms captured by monitors on theaffected circuit, engineers used a simulator at PQTF to repro-duce the effect and directed Duke maintenance personneltoward possible causes. F ollowing this lead, the Duke teamfound that voltage pulses were being sent onto the circuit bysubstation equipment that was about to fail. Replacing theequipment not only solved the mystery of the speedingclocks but also probably prevented a costly blackout on thecircuit.Two kinds of training courses are provided by PEAC forutility personnel concerned with power quality. The first typeis an intensive, hands-on, 3-day course in which participantsuse PQTF equipment to simulate a variety of power qualityproblems and have an opportunity to use some of the com-mercially available products designed to correct those prob-lems. The second type of course involves customizedinstruction by PQTF staff on a utility's premises and may beattended by utility customers as well as utility personnel.To further technology transfer in the power quality area,EPRl has established the North American Power QualityTesting Network and is also setting up regional power qualityservice centers. The national network includes not onlymember utilities but also university researchers, public agen-cies (such as the National Institute of Standards and Tech-nology), and the Canadian Electrical Association. The firstregional power quality service center opened recently in theNorthwest with half a dozen utility members. The center iscurrently sponsoring several activities of regional interest,including power quality workshops, the creation of a data-base of power quality consultants, and the publication ofmarket-specific power quality bro-chures.

used most effectively to combat various power quality prob-lems. The center is also working with manufacturers toimprove product performance.The optimal placement of power-conditioning equipmenteventually comes down to economics. Some cases are clear:a home computer is most easily protected against transientovervoltages by a $20 surge suppressor; a new industrialpark might be able to obtain uninterruptible power morecost-effectively through dual-feeder custom power serviceoffered by the local utility. Between these extremes, how-ever, the decision about how best to provide premium-qual-ity power depends ona variety of site-specific factors and oncontinuing technological developments.Looking to the future, EPRl's Integrated Energy SystemsDivision (IESD), jointly with the Electrical Systems and Cus-tomer Systems divisions, is conducting a market study ofpower quality that will measure utility and customer atti-tudes toward power disturbances and their potential solu-tions. "We want to make sure that EPRl's response to powerquality issues is as effective as possible in meeting thedemands of the industry," says Vito Longo, a research man-ager in IESD."EPRI is committed to giving member utilities the infor-mation and technologies they need to work with customersconcerned about power quality," says Marek Samotyj, man-ager for power electronics end-use systems in EPRl's Cus-tomer Systems Division. "Our case studies and theevaluations performed at PEAC are helping to establishmuch-needed standards in what has been a chaotic field ofendeavor."Vasu Tahiliani of the Electrical Systems Division empha-sizes the need for "coordination atall levels: research, devel-opment, and application. Particularly as more custom poweroptions become available, we expect to work even moreclosely with the suppliers of this equipment and with mem-ber utilities in determining which technologies are mostappropriate for a specific application."

IntegratedApproachBecause of the complex natureof power quality problems, an in-tegrated approach is needed in ad-dressing them: in choosingpower-conditioning equipment, indeciding whether to use the equip-ment on the customer's or the util-ity's side of the meter, and inconducting research to head offfuture problems, as the use of sen-sitive electronic devices becomeseven more widespread. Substan-tial progress is being made in eachof these areas.A wide variety of power-condi-tioning equipment is now avail-able for on-site use by utilitycustomers, ranging from smallplug-in surge suppressors to mul-timegawatt uninterruptible powersupplies. No single product cansolve all the kinds of power qualityproblems that customers are likelyto encounter. PEAC has conductedtests on many of these devices andhas published technical commen-taries on how each type can be

IEEE Power E ngineering Review,March 1994 7