電氣工程專業(yè)英語課件lecture13-2012

1各位老師：上午好！Lecture13SpecializedEnglishforElectricalEngineering2Lecture13ReadingandTranslation

PowerQualityandItsImprovementPart1FamiliarPowerQualityProblem

Powerqualityisthecombinationofvoltagequalityandcurrentquality.Thuspowerqualityisconcernedwith

deviationsofvoltageand/orcurrentfromtheideal.Notethatpowerqualityhasnothingtodowithdeviationoftheproductofvoltageandcurrent(thepower)fromanyidealshape.Powerqualityhasduringrecentyearsachievedanincreasinginterest.Theconceptofpowerqualityincludesthequalityofthesupplyingvoltagewithrespectto,forinstance,interruptions,voltagedips,voltagesag/swell,imbalance,harmonicdistortion,voltagefluctuationandflickerandsome3Lecture13kindsofhighfrequencytransients,etc.Seriesandshuntactivepower

conditionershavebeenproposedandusedforcompensatingpurpose.Beforefurtherdescriptionofdefinitepowerqualityproblems,itisnecessarytointroducesomerelativeconcepts.Apointofcommoncoupling(PCC)isaverycommoniteminpowerqualitystudies.Bydefinition,aPCCisthepointofthepublicsupplynetwork,electricallynearesttoaparticularconsumer’sinstallation,andatwhichotherconsumer’sinstallationare,ormaybe,connected.ThefeedersterminateatsomecertainPCCs.Asfordescribingthedegreeofdistortionofacurrentorvoltage,thereareseveralways.Twothatarefrequentlyusedarecrestfactorandtotalharmonicdistortion(THD).

4Lecture13Bydefinition,thecrestfactorofavoltageisequaltothepeakvaluedividedbytheeffectivevalueoralsocalledrootmeansquare(rms)valueInthecaseofasinusoidalvoltage(whichevidentlyhasnodistortion)thecrestfactoris.Awavehavingacrestfactorlessthantendstobeflat-topped.Ontheotherhand,acrestfactorgreaterthanindicatesavoltagethattendstobepointy.1.Notethatpowerquality

hasnothingtodowithdeviation

oftheproductofvoltageandcurrent(thepower)

fromanyidealshape.

注意，電能質(zhì)量和電壓、電流乘積（功率）與任何理想形態(tài)的偏差無關。

5Lecture132.Theconceptofpowerqualityincludesthequalityofthesupplyingvoltage

withrespectto,forinstance,interruptions,voltagedips,voltagesag/swell,imbalance,harmonicdistortion,voltagefluctuationandflickerandsomekindsofhighfrequencytransients,etc.電能質(zhì)量的概念包括供電電壓質(zhì)量，例如，關于斷電、電壓跌落、電壓暫降與暫升、不平衡、諧波畸變、電壓波動與閃變以及某些高頻暫態(tài)現(xiàn)象等。3.Bydefinition,aPCCisthepointofthepublicsupplynetwork,electricallynearesttoaparticularconsumer’sinstallation,andatwhichotherconsumer’sinstallationare,ormaybe,connected.

按照定義，PCC是公共供電網(wǎng)絡在電氣上距某一特定用戶設備最近的點，其他用戶的設備也連接或者可以連接到那里。6Lecture13NewWordsandExpressions

powerquality電能質(zhì)量

deviation背離，偏移dip跌落，下垂sag

凹陷，暫降

swell暫升distortion

畸變fluctuation

波動flicker

閃變conditioner調(diào)節(jié)裝置crestfactor波峰系數(shù)PointofCommonCoupling(PCC)公共連接點RootMeanSquare(rms)均方根TotalHarmonicDistortion(THD)總諧波畸變7Lecture13Part2HarmonicsandNonlinearLoads

Thevoltagesandcurrentsinapowercircuitarefrequentlynotpuresinewaves.Thelinevoltagesusuallyhaveasatisfactorywaveshapebutthecurrentsaresometimesbadlydistorted,asshowninFig.21.1.Thisdistortioncanbeproduced,forinstance,bymagneticsaturationinthecoresoftransformersorbytheswitchingactionofthyristorofIGBTsinelectronicdrives.Thedistortionofavoltageorcurrentcanbetracedtotheharmonicsitcontains.Aharmonicisanyvoltageorcurrentwhosefrequencyisanintegralmultipleof(2,3,4,etc.,times)thelinefrequency.Considerasetofsinewavesinwhichthelowestfrequencyis8Lecture13f,andallotherfrequenciesareintegralmultiplesoff.Bydefinition,thesinewavehavingthelowestfrequencyiscalledthefundamentalandtheotherwavesarecalledharmonics.Forexample,asetofsinewaveswhosefrequenciesare20,40,100,and380Hzissaidtopossessthefollowingcomponents.fundamental:20Hz(thelowestfrequency)secondharmonic:40Hz(2×20Hz)fifthharmonic:100Hz(5×20Hz)nineteenthharmonic:380Hz(19×20Hz)Thus,thesumofafundamentalvoltageandaharmonicvoltageyieldsanonsinusoidalwaveformwhosedegreeofdistortiondependsuponthemagnitudeoftheharmonic(orharmonics)itcontains.9Lecture13Wecanproduceaperiodicvoltageorcurrentofanyconceivableshape.Allwehavetodoistoaddtogetherafundamentalcomponentandanarbitrarysetofharmoniccomponents.Asquarewaveisthuscomposedofafundamentalwaveandaninfinitenumberofharmonics.Thehigherharmonicshavesmallerandsmalleramplitudes,andtheyareconsequentlylessimportant.However,thesehigh-frequencyharmonicsproducethesteepsidesandpointycornersofthesquarewave.Inpractice,squarewavesarenotproducedbyaddingsinewaves,buttheexampledoesshowthatanywaveshapecanbebuiltupfromafundamentalwaveandanappropriatenumberofharmonics.Conversely,wecandecomposeadistortedperiodicwaveintoitsfundamentalandharmoniccomponents.Harmonicvoltagesandcurrentsareusuallyundesirable,butinsomeaccircuitstheyarealsounavoidable.10Lecture13Thewidespreaduseofnonlinearloadsisleadingtoavarietyofundesirablephenomenaintheoperationofpowersystems.Theharmoniccomponentsincurrentandvoltagewaveformsarethemostimportantamongthese.Manyharmonicsarecreatedbynonlinearloads,suchaselectricarcsandsaturatedmagneticcircuits.Theyarealsoproducedwhenevervoltagesandcurrentsareperiodicallyswitched,suchasinpowerelectroniccircuits.Allthesecircuitsproducedistortedwaveshapesthatarerichinharmonics.

Considerasinusoidalvoltageconnectedtoanonlinearload.Theloadmaybeasaturablereactance,arectifier,orasetofmechanicalswitchesthatopenandcloseperiodically.Onaccountofnonlinearity,thecurrentwillnotbesinusoidal.Itwillcontainafundamentalcomponentandharmoniccomponents.Fundamentalcomponentisproducedbythesinusoidalvoltage,buttheharmoniccomponentsaregeneratedbytheload.11Lecture13Clearly,theharmoniccurrentsflowinthesinusoidalsourceaswellasintheload.Asfarasthefundamentalcomponentofcurrentisconcerned,itcanlag,lead,orbeinphasewithsourcevoltage.Thus,wecanattributetraditionalactiveandreactivepowerstothisnonlinearload.Inaccircuitsthefundamentalcurrentandfundamentalvoltagetogetherproducesfundamentalpower.Thisfundamentalpoweristheusefulpowerthatcausesamotortorotateandafurnacetoheatup.Theproductofaharmonicvoltagetimesthecorrespondingharmoniccurrentalsoproducesaharmonicpower.Thelatterisusuallydissipatedasheatintheaccircuitand,consequently,doesnousefulwork.Harmoniccurrentsandvoltagesshouldthereforebekeptassmallaspossible.Itshouldbenotedthattheproductofafundamentalvoltageandaharmoniccurrentyieldszeronetpower.12Lecture13

Passivefiltershavebeenusedtoeliminatelinecurrentharmonics.However,theyintroduceresonanceinthepowersystemandtendtobe

bulky.Withtheimprovedperformanceofpowerandcontrolcircuits,activepowerfilters(APFs)havegraduallybeenrecognizedasaviablealternativetopassivefilters.13Lecture13NewWordsandExpressions

flat-topped平頂?shù)?/p>

pointy尖的saturation飽和integralmultiple

整倍數(shù)

fundamental基頻的conceivable

可能的，想象到的arbitrary

任意的squarewave

方波steepside陡坡，陡沿pointycorner尖角decompose分解magneticcircuit

磁路

saturable可飽和的furnace

爐子dissipate

散失，消耗usefulwork

有用功netpower凈功率resonance諧振viablealternative可行的替換物14Lecture13Part3VoltageFluctuationandFlicker

Therearemanydevicesintheelectricpowersystemwhichimposerapidandfrequentchangesofload,withcorrespondinglyrapidvoltagechangessuchasvoltagefluctuationsandflickers.Someofthemostcommondevicesareinductionand

synchronousmotors,motor-drivenreciprocatingloads,motor-drivenintermittentloads,electricarcfurnaces(EAF),electricweldersandmiscellaneousinstallationssuchaselectricshovelsandrollingmills.Therapidandfrequentvoltagechanges,especiallyflickers,causedbythesedevicescanbeperceivedbyandbeobjectionabletonearbycustomersandoftenresultincomplaintstothepowercompanies.Flickerisunderstoodtobethesensationthatisexperiencedbyhumanswhensubjectedtochangesintheilluminationintensity.Thehumanmaximumsensitivitytoillumination15Lecture13changesisinthefrequencyrangefromabout5to15Hz.Thefluctuatingilluminationiscausedbyamplitudemodulationofthefeedingalternatingvoltage.Tobeabletomeasurethelevelofflicker,ameterisneeded.Therearedifferentstandardsexistingdescribingflickermeters.TheflickermeterusedthemostoftenistheonethatisdescribedbytheInternationalElectrotechnicalCommission(IEC),sometimesreferredtoasthePSTmeter.“ST”standsforshorttime,whichinthiscasereferstomeasurementsduring10minutes.UnityoutputfromthePSTmetercorrespondstoaflickerlevelcausing50%ofthepersonsinareferencegrouptobedisturbedbytheflicker.Largeindustrialloads,suchasEAFs,usedformeltingforexamplescrapwithelectricenergy,causevoltagedistortionlikeharmonicsandvoltagefluctuationinthefeedingACsystem.An16Lecture13arcfurnaceisprobablyoneofthelargestexistingend-usersofelectricenergywitharatedpoweruptotheorderof100MVA.Tolimittheeffectsofthesedisturbingloads,compensationdeviceshaveusuallytobeconnected.Manycorrectivepiecesofequipmentandprocedureshavebeenusedtominimizevoltageflicker.Thefollowingconventionalremedialmeasuresarecommonlyconsideredforapplicationsindistributionsystems:i)Motorstarters/Adjustablespeeddrives(ASD).ii)Shuntcapacitors.iii)Voltageregulators.iv)Synchronouscondensers.v)Systemchanges.Onlysomeoftheconventionalcorrectivemeasuresdiscussedaboveareeffectiveincontrollingvoltageflickers.The17Lecture13ASDisnotageneralizedsolution.Thealternativesofsynchronouscondensersandsystemchangescanbeeffectivebutgenerallyareexpensiveand/orimpractical.Utilitiessometimesrefusetoaddnewcustomerswithequipmentwhichwillcauseseverevoltageflickersontheelectricsystembecausenoeffectiveand/oreconomicalsolutioncanbefound.

WiththeadventofcompactFACTSdevices,distributionengineershaveanotherviableoptionavailabletosolvemanyofthevoltagefluct