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北极星电力网技术频道 作者:电力论文3 2007/12/24 18:44:37
DidyouheartheoneaboutthecarthatwasdesignedtorunonaPC?Guaranteedtocrashatleastonceaday.
Justkidding.Butsinceyouuseyourcomputerregularly,you''reprobablyusedtotheoccasionalcrashes,hang-ups,etc.thatcomewiththeterritory.Ithappens,youre-boot,mutterafewwordsinthegeneraldirectionofRedmond,Washington,andoffyougoagainuntilthenextglitchcomesalong.
It''susuallyasoftwareproblem,maybeaccompaniedbyanominousmessagelike"YOURSYSTEMHASPERFORMEDANILLEGALOPERATION"or"INSUFFICIENTMEMORYTORUNTHISAPPLICATION".Butsometimesit"snotthesoftware"sfault.Itcouldbeaquestionofinadequatepowerquality.Powerquality?Sincewhendoespowerhave"quality"?It"seitheronorit"soff,right?Well,yesandno.BackinthedarkagesBC(beforecomputers),itusuallywassufficientthatthepowerturnedonwheneveryouflippedtheswitch.Anditusuallydid,exceptduringsummertimebrownoutswhentoomanypeopleturnedontheirairconditionersatthesametimeoraftertheoccasionalblackoutswhenlightning(orsomemotorist)struckapole.Otherthanthat,electricpowerqualitywasunremarkablyadequate.
That''snolongerthecase,becausethewayweuseelectricpowerhaschangedsignificantly.Justafewyearsago,mostoftheelectricalusesinourhomes(andinourfactoriesandoffices,forthatmatter)involvedlighting,electricmotorsandafewtransformers.Westillneedthosethingstoday,butwealsoincreasinglyrelyonwhatwemightcall"electronic"applicationsofelectricpower.Electronicapplicationsincludenotjustcomputersandtheirperipherals,butalloftheappliances,tools,instruments,thermostats,burglaralarms,audio/videoequipment,copiers,faxmachinesandothergadgetsthatcontainsomeformofmicrochipor"logiccircuits".It''sthoseelectronicgadgetsthataresofinickyaboutpowerquality.TheInstituteofElectricalandElectronicEngineers(IEEE)evenhasanameforthem;theycallthem"SensitiveElectronicEquipment,"andhavepublishedspecialrecommendedpracticestodealwiththemcorrectly.Thesethingssimplycan''ttoleratethesortoflapsesinpowerqualitythatlightbulbsandelectricmotorsroutinelyshrugoff.
Sincemoreandmoresensitiveelectronicequipmentisfindingitswayintoourhomes-especiallyashomeofficesbecomemorepopular-we''vepreparedthisarticletoexplaintheconceptofpowerqualityandhowitcanaffectyou.We''llalsoexplainhowtheproperuseofcopperwiringcanensurethatyoursensitiveelectronicequipmentfunctionsproperly.And,ifyou''reconsideringbuildinganewhomeorthinkingaboutre-wiringyourcurrentone,we''llmentionafewpointersthatwill"buildin"goodpowerqualityfromthestart.
Becauseofthevariouswaysweuseelectricity,powerqualitymeansdifferentthingstodifferentpeople.Still,therearethreebasicattributesthatwecanuseto"rate"powerquality,atleastqualitatively,fromthepointofviewofthehomeownerorhomeofficeuser.Theseattributesare:
Continuity,orhowcontinuousthesupplyis,meansmorethanthatthepowershouldsimplyturnonandstayonwheneveryoufliptheswitch.Itmeansthattherecanbenogaps,evenmomentaryones,whenpowerisflowing.That''simportantbecausesensitiveelectronicequipmentrequiresalmosttotalcontinuity,literallydowntothemillisecond.Forexample,you''veprobablyexperiencedsomethinglikethis:
Thefirstcasewasn''tyourfault,thesecondonewas.Eitherway,you''veexperiencedoneformofpoorpowerquality-aninterruption-andyoursensitiveelectronicequipmentcouldn''tcope.Whatcanyoudoaboutcontinuityproblems?Thequick-and-dirtysolutionistoinstallanuninterruptiblepowersupply,popularlyknownasaUPS,betweenyourequipmentandthewalloutlet.AUPSisessentiallyabatterythatiscontinuouslychargedbylinepower,itsacvoltagesuitablysteppeddownandrectifiedtodc.Thebattery''sdcpoweroutputisre-convertedintheUPSbacktoacpowerbyadevicecalledaninverter,whose"synthetic"acpoweristhenoutputtoyourequipment.Whenlinepowerisinterrupted,thebatteryactsasareservoir,providingafewminutestoafewhoursofpowersoyourequipmentcanrideouttheoutage.TherearemanytypesandsizesofUPSunits.Thebetteroneshavebuilt-insurgeprotectiondevices.They''recheapinsurance:UPSunitsthatareadequateforhomeofficesretailforabout$100-$200.Ifyourpowergridhasfrequentoutages,consideraconservativelyrated(longruntime)UPS.
Ontheotherhand,ifyou''veoverloadedthecircuitthatfeedsyourcomputer,youshouldthinkaboutinstallingaUPSanddistributingyourelectricalloadsbyaddingadditionalcircuits.Thisisthecorrectlong-termsolutiontofrequentcircuitbreakertrips.Thereareseveralbenefitstoaddingmorecircuits,butmaintainingcontinuouspowerisreasonenoughtogivetheideasomethought.Thecostofthefewadditionalyardsofcopperwireandafewhoursofanelectrician''stimecanbeinconsequentialcomparedwiththecostoflostdata.
Voltagevariationsareanothercommonsourceofproblemstohomecomputersandothersensitiveelectronicequipment.Voltagevariationscanbepositive(higherthannormal)ornegative(lower).Variationscanbehuge,reachingthousandsofvolts,butevensmallonescancauseproblems.Mostcomputerscanwithstandvariationsof±10(±12voltsona120-Vcircuit),althoughtheycandetect,andmaybeaffectedby,smallervariations.Thehumaneyecandistinguishalampflickercausedbya±1.5vchangeinvoltage,soifyoucanseetheeffectofavoltagechange,chancesaregoodthatyourcomputercanfeelit.Animportantpointtorememberisthatthecomputermanufacturersbasetheirvoltagerangeassumingatruesinewavevoltagewaveform.Ifthevoltagewaveformisflattoppedordistorted,yourcomputermaynotoperatewithinthestatedvoltagerange.We''llexplainsinewavedistortioninamoment.Let''sgetbacktothatoverloadedbranchcircuitwedescribedearlier.Thistime,wewon''tquitetripthecircuitbreaker.
Or,
Herewehavetointroducealittlealgebratoexplainourpoint.Thepower(P)thesepowersuppliesdrawisequaltothelinevoltage(E)atwhichtheyoperate(nominally120v)timesthecurrent(I)theydraw,orP=EI.Thepowerdrawnisconstant,fixedbytheneedsoftheequipment;soanydecreaseinvoltagemustbecompensatedbyaproportionalincreaseincurrent.Now,theamountofheatdissipatedbythepowersupply(specifically,byresistive(R)elementssuchasresistors,wires,etc.)isproportionaltothesquareofthecurrentflow,P=I2R.Therefore,theendresultofadropinvoltageatthewalloutletisariseinthetemperatureintheequipment.Atbest,thisexcessheatisrepresentswastedenergy;atworst,itcanshortentheequipment''slife.
Powersuppliesincomputersareusuallymorepowerfulthantheyneedtobebecausemanufacturersunderstandthatownersoftenaddequipment(additionalharddrives,tapebackups,etc.)afterthecomputerhasbeenplacedinservice.Asanincidentalbenefit,suchoversizedpowersuppliescanaccommodatetheextracurrentdrawnwhenlinevoltagedrops.Powersuppliesinmonitors,ontheotherhand,aresizedjustlargeenoughfortheexistingload,therebeingnoneedtoexpectanylateradditions.Highcurrentscausedbylowlinevoltagecandamagesuchpowersupplies.
Whenwespeakofvoltageintheconventionalsense,werefertothevoltagebetweenphaseandneutralconductors.However,avoltagecanalsoexistbetweentheneutralandgroundconductorsatdistanceswellremovedfromyourhouse''sserviceentrancepanel(ithastobezeroatthepanelbecausethat''swheretheneutralandgroundconductorsarejoined).Thisneutral-to-groundvoltageisnormallyzeroorclosetoit,butwhenthephase-to-neutralvoltagedrops,theneutral-groundvoltagerises.Infact,itrisesbyexactlyone-halfasmuchasthedropinphase-to-neutralvoltage,e.g.,reducingthephasevoltagefrom120vto110vincursa5-vincreasebetweentheneutralandgroundconductors.
Mostdesktopcomputerscantolerateasmuchas±10variationinphase-to-neutralvoltage,buttheyarequitesensitivetosmallchangesinneutral-to-groundvoltage.Thisisparticularlytruefornewercomputerswhoselogiccircuitrymayoperateatlessthanonevolt.(Laptopcomputersaren"tassensitivetothissituationbecausetheygenerallydon"thaveanexternalgroundconnection.)
Moralofthestory:playitsafe,playitsmart,useaseparatecircuit,oraddone,foryoursensitiveelectronicequipment.
Positivevoltagevariationscanbeevenmoretroublingthannegativeones.Ifpowerfulenough,theycandestroycomponentsinsensitiveelectronicequipment.Suchpositivevariations,or"transientvoltagesurges"asthey''recalled,canariseoutsideyourhome.Lightningstrikingpowerlinesisafrequentcause,asisloadswitching(re-routingpoweraroundthegrid)byyourutility.Voltagesurgescanalsobecausedbyequipmentinyourhome.Refrigeratormotors,airconditioners,vacuumcleanersandotherelectricalloadscangeneratevoltagesurgesandelectricalnoise.
Thebestdefenseagainstvoltagesurgesistoinstallatransientvoltagesurgesuppressor(TVSS)betweenyourequipmentandthewalloutlet.Aswithuninterruptiblepowersupplies,surgesuppressorsarecheapandwidelyavailableinavarietyofstylesandsizes.Youmayalreadyhaveone:it''sthe"powerwand"containingabouthalfadozenoutletsintowhichyouconnectyoucomputer,monitorandotherequipment.CheckthelabeltoseeifithasTVSSprotection.IfyouneedtopurchaseaTVSSmakesureitisUL1449listed.Thisassurestheproducthasbeentestedandmeetsindustrystandards.
Noamountofadditionalcopperhousewiringwillprotectyouagainstvoltagesurges,althoughagoodlightningprotectionandgroundingsystemwillisolateyoursensitiveelectronicequipment-alongwiththerestofyourhome-fromnearbylightningstrikes.We''lltalkaboutgroundingandlightningprotectioninafuturearticle.
InNorthAmerica,householdelectriccurrentandvoltagealternate(it''sthe"a"inac)orswitchbackandforthbetweenpositiveandnegativepolesatafrequencyofexactly60alternationseachsecond.Engineerscallthatthefundamentalfrequency;it''susuallyexpressedas60cyclespersecond(cps)or60Hertz(Hz).Inmostotherpartsoftheworld,thefundamentalfrequencyis50Hz.
Alternatingvoltage/currentalternationstaketheformofasinewave.Thatis,ifyoucouldseethecurrentchangingoveroneaccycle(whichtakes1/60thofasecondor16.7milliseconds)theshapeofthealternations,thesinusoidalwaveform,wouldlooklikethis:
Voltageandcurrentbegineachcycleatzero(thehorizontalline),risetoamaximuminone-quarterofacycle,fallbacktozeroatthehalf-cyclepoint,thenreversepolarityandfalltoamaximumnegativevalueatthe3/4cyclepointbeforerisingtozeroagain.Allofthisrepeatseverysixtiethofasecond.Ifyou''veevertouchedaliveelectricline,thevibratingshockyoufeelisthissixty-time-per-secondvoltagereversal.
Inordinaryhousecurrent,120voltsiswhatengineerscalltheeffectivevoltage,whichisactuallyasortofaverage(therootmeansquarevalue)ofthevaryingvoltagedepictedbythesinewave.Theactualpeakvoltageina120-Vcircuitcanbeashighasnearly170V.Exceptforsimplethingslikeincandescentlightbulbs,mostoftheelectricalandelectronicdevicesweusetodayaredesignedforacpowerthatfollowsthesinewavecurve.Tothesedevices,therightvoltageplusaperfectsinusoidalwaveformadduptoidealpowerquality.Bythesametoken,anythingthatdisturbseitherthepower''sfrequencyoritswaveformdetractsfrompowerquality,andinsodoing,reducestheabilityofelectricaldevices-especiallysensitiveelectronicequipment-tooperateproperly.
Whatcandisturbthefrequency?Notmuch,sofaraspowersuppliedbytheutilityisconcerned.Infact,electricutilitiesgotogreatlengthstomakesurethattheirgeneratorsoperateatprecisely60Hzandthatthey''reallsynchronizedascloselyaspossible.WhenthecurrentproducedbyapowerstationinBangor,Maineisattheexactmaximumpointofthesinewave,currentcomingoutofapowerstationinLosAngeles-andoutofyourwalloutlet-isthere,too.
Mostfrequencydisturbancesactuallyoriginateinsidebuildings.Ironically,itismainlythesensitiveelectronicequipmentthatisatfault.Rememberthosedcpowersupplieswekeepmentioning,theonesthatpowermostofour"electronic"equipment?Thesedays,mostsuchpowersuppliesareofatype(called"switchedmodepowersupplies")thatdrawscurrentinsharppulsesratherthaninasmoothlyflowingsinewave.Astheydoso,theyechothosepulsesbackintothepowerline,wheretheyaresuperimposedonthenormalsine-wavecurrentflowingthere.Othercommonhouseholdsourcesoftheseerraticdisturbancesinclude:
Normally,waveformdisturbancesarenotaprobleminahome-officesituation.Reallyseriousproblemsariseinofficesandcommercialbuildingswheretheremaybehundredsofcomputers,electroniclightingballastsandsimilarequipmentonthesamefloor.Thepowerdisturbancesgeneratedbyallthoseswitchedmodepowersuppliescanadduptowreakhavocwithcomputerreliability,nottomentionfirehazardsandotherproblems.
Thenumberofcomputersandperipheralsinstalledinahomeofficeisfartosmalltocausethatlevelofdisturbance.Ontheotherhand,thereisarealpossibilitythatbyconcentratingallofyoursensitiveelectronicsononebranchcircuit,youcandegradepowerqualityenoughtoprovoketheoccasionalelectronichiccupandcrashyourcomputer.
Themaincauseofwaveformdistortioninmodern"electronics"-loadedcircuits,includingtheonesinyourhomeoffice,isanelectricalphenomenonknownasharmonics.Allcommercialpowerisbasedonapuresinewavewaveform(rememberphysicsclass?).Theutilitygenerates60-Hzsinewavepowerandtransmitsittoyourhome.Yourhomewiringdistributesthesinewavepowertoyourwalloutletandyourcomputerpowersupplyisdesignedtorunonit.Theassumptionisthatthepowermaintainsthecorrectsinewaveshapefromgenerationtoutilization.Thisassumptionisnotalwayscorrect.Itisnowunderstoodthattheverypowersupplythatisinyourlaserprinterandcomputercancauseadistortionofthevoltagewaveform,superimposingelectricaljigglesthatcanbeexpressedasmultiplesofthestandard60-Hzfrequency,i.e.,120Hz,180Hz,240Hz…etc.Thesejigglesarecalledharmonics.
Theamountofdistortionismeasuredintotalharmonicdistortion,orTHD.TheIEEEhasissuedguidelinesforrecommendedlevelsofTHD.Generallyanythingover5voltageTHDisunacceptable.
Harmonicshavecreatedseriousproblemsinofficesthatcontainahighdensityofcomputerequipment.WeroutinelyfindahighlevelofvoltageTHDinofficebuildings.Itfarlesslikelythatyouwouldhaveaharmonicprobleminyourhome.However,ifyouhaveundersizedwiringoroverloadedcircuits,youcouldhavethisproblem.
Whatproblemsdoharmonicscause?Themostcommonproblemisaflat-toppingofthevoltagewaveform.Thiscausesareductionofthepeakvoltagelevel,whichinturncanstarveyourcomputer''spowersupplyofenergy.Toputitinsimpleterms,runningyourcomputeronadistorted140-volt(insteadoftheexpected170-v)peakvoltageislikerunningitonabout95voltsAC.Youmaynotseetheproblem,butyourcomputerdoes!Ittakesspecialequipmenttomeasureharmonicssotheproblemmayverylikelygounnoticed.
Ifyou''reconsideringbuildinganewhomeorthinkingaboutremodelingyourcurrentone,youshouldbeawarethatthereamanyadvantagestobringingyourwiringsystemuptocurrentpractices.Thisisespeciallytrueifyouplantoaddmoresensitiveelectronicequipment(whichislikely),maintainahomeofficeorcreatealocalareanetwork(LAN)tolinkyourhomecomputers.
Considerthis:
TheNationalElectricCode®(NEC)governsthewaywewireourhomes,officesandcommercialbuildings.TheNECpermitsupto13outletsona20-ampbranchcircuit,thetypecommonlyfoundinhomes.Thislimitationisbasedonsafety:limitingthenumberofoutletseffectivelylimitsthetotalload(totalcurrentflow)onthebranchcircuitandtherebyeliminatingoverloadingandcircuitbreakertripping.Recommendedpracticesforhomeofficeswithanumberoftechnologysystemsinclude:
Forthemostpart,theserecommendationswillnotinvolvemajorexpensesinhomesthatalreadyconformtopracticespermittedundertheNationalElectricalCode.Theyareespeciallyinexpensivewhenyoucomparetheircosttothecostoflosingyoursensitiveelectronicequipmentandallofthedatayou''vestoredinit.
Youcandosomethingaboutpowerquality.ThinkaboutitthenexttimeyourPCstartsactingup.
Thecompanyhasspecializedinthefieldofpowerquality,powermonitoring,harmonics,grounding,sitesurveys,andtestingforover18years.Ourin-the-fieldexperiencetellsusthattherapidevolutionoftechnologyhasbroughtforthamultitudeofpowerandgroundingproblemswidelyaffectingeveryindustrysegment.Inlightofthesegrowingneeds,wehavedevelopedthefollowingmissionstatement.
Ourmissionistoassistcompaniesin"MakingTechnologyWork™"byinsuringthattheclient''sinfrastructurewillsupporttheirtechnologyneeds.
Thecompanyiscurrentlyfocusedinthefollowingareas:
1.Providingconsultingandtechnicalservicestoidentify,diagnose,andcorrectpowerqualityproblems.
2.Providingconsultingservicesforthedesign/buildoftechnicalfacilitiessuchascallcenters,computerrooms,andchannelassemblyfacilities.Weworkwiththeclientthroughtheentireprojectfrominitialconcepttodesignthenimplementationandacceptance.
3.Providingeducationalservicesthroughpowerqualityandgroundingseminarsandtechnicalpapersandarticles.
Industrysegmentshaveincludedfinancial,insurance,healthcare,educational,manufacturing,utility,communications,broadcasting,andgovernment.
Formoreinformation,pleasecontactJillGrebeatjgrebe@cpccorp.com.Additionalinformationisavailableonourwebsitewww.cpccorp.com.
IfyouhaveaCaseHistorytoshareorwouldliketoreceiveadditionalinformation,contact:
DavidBrender
NationalProgramManager,Electrical
Phone:(212)251-7206
Fax:(212)251-7234
E-mail:dbrender@cda.copper.org
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