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PassageoneDCcircuit
Thecircuitelementusedtomodelthecurrent-resistingbehaviorofamaterialisthe1.Itisthesimplest2element.GeorgSimonOhmiscreditedwithformulatingthe3relationshipforaresistorbasedonexperimentsperformedin1826.ThisrelationshipisknownasOhm'sLaw.Ohm'sLawstatesthatthevoltageacrossaresistoris4thecurrentflowingthroughtheresistor.Theconstantofproportionalityistheresistancevalueoftheresistorinohms.ThecircuitsymbolfortheresistorisshowninFig.1.Forthecurrentandvoltageshown,Ohm'sLawis
u(t)=Ri(t)
(1)
whereR≥0istheresistanceinohms.
SinceRisconstant,Eq.
(1)istheequationofastraightline.Forthisreason,theresistoriscalledalinearresistor.Agraphofu(t)versusi(t)foralinearresistorisalinepassingthroughtheoriginwithaslopeofR.
Resistorswhoseresistancesdonotremainconstantfordifferentterminalcurrentareknownas5resistors.Forsucharesistor,theresistanceisafunctionofthecurrentflowinginthedevice.Inreality,allpracticalresistorsarenonlinearbecausetheelectricalcharacteristicsofallconductorsareaffectedbyenvironmentalfactorssuchastemperature.Manymaterials,however,closelyapproximateanideallinearresistoroveradesiredoperatingregion.
Inductorsreactagainstchangesincurrentbydroppingvoltageinthepolaritynecessarytoopposethechange.Whenaninductorisfacedwithanincreasingcurrent,itactsasaload:
droppingvoltageasitabsorbsenergy(negativeonthecurrententrysideandpositiveonthecurrentexitside,likearesistor).Theabilityofaninductortostoreenergyintheformofamagneticfield(andconsequentlytoopposechangesincurrent)iscalledinductance.ItismeasuredintheunitoftheHenry(H).
Inductorsusedtobecommonlyknownbyanotherterm:
choke.Inlargepowerapplications,theyaresometimesreferredtoasreactors.
Capacitorsreactagainstchangesinvoltagebysupplyingordrawingcurrentinthedirectionnecessarytoopposethechange.Whenacapacitorisfacedwithanincreasingvoltage,itactsasaload:
drawingcurrentasitabsorbsenergy(currentgoinginthenegativesideandoutthepositiveside,likearesistor).Whenacapacitorisfacedwithadecreasingvoltage,itactsasasource:
supplyingcurrentasitreleasesstoredenergy(currentgoingoutthenegativesideandinthepositiveside,likeabattery).Theabilityofacapacitortostoreenergyintheformofanelectricfield(andconsequentlytoopposechangesinvoltage)iscalledcapacitance.ItismeasuredintheunitoftheFarad(F).Capacitorsusedtobecommonlyknownbyanotherterm:
condenser
Choosethebestanswerintotheblanks.
1A.capacitorB.inductor
C.resistorD.source
2A.passiveB.active
C.linearD.nonlinear
3A.current-currentB.current-voltage
C.voltage-powerD.current-power
4A.equaltoB.directlyproportionalto
C.inverselyproportionaltoD.differentfrom
5A.linearB.constant
C.nonlinearD.time-varying
PassagetwoACcircuit
TheeffectivevalueofvoltageandcurrentisthecommonmethodofexpressingtheValueofAC.ThisisalsoknownastheRMSvalue.TheeffectivevalueofACisdefinedintermsofanequivalentheatingeffectwhencomparedtoDC.OneRMSampereofcurrentflowingthrougharesistancewillproduceheatatthesamerateasaDCampere.Forpurposeofcircuitdesign,thepeakvaluemayalsobeneeded.Forexample,insulationmustbedesignedtowithstandthepeakvalue,notjusttheeffectivevalue.Itmaybethatonlytheeffectivevalueisknown.Tocalculatethepeakvalue,multiplytheeffectivevalueby1.41.
6.Whichequationistrue?
()
A.Vpeak×1.41=VrmsB.Vpeak÷0.707=VrmsC.Vpeak×0.707=Vrms
7.Accordingtothepassage,iftheeffectivevalueis100V,thepeakvalueis().
A141B70.9C70.7D14.1
InordertosuccessfullyanalyzeACcircuits,weneedtoworkwithmathematicalobjectsandtechniquescapableofrepresentingthesemulti-dimensionalquantities.Hereiswhereweneedtoabandonscalarnumbersforsomethingbettersuited:
complexnumbers.Justliketheexampleofgivingdirectionsfromonecitytoanother,ACquantitiesinasingle-frequencycircuithavebothamplitude(analogy:
distance)andphaseshift(analogy:
direction).Acomplexnumberisasinglemathematicalquantityabletoexpressthesetwodimensionsofamplitudeandphaseshiftatonce.
PassagethreeElectricPowerNetwork
Theconveyance1ofelectricpowerfromageneratingstation2toconsumersisknownaselectricpowersystem.Theelectricpowernetworkisthepartoftheelectricpowersystembutthegenerators3andtheconsumers,includingsubstations4,transmissionlines,anddistributionnetworks5.
Thefunctionofanelectricpowernetworkistoconnectthegeneratingstationswiththecustomersandwiththeminimumdisturbance6totransferthispowerovertransmissionlinesanddistributionnetworkswiththemaximumefficiency7andreliability8fordeliverytoconsumersatvirtuallyfixedvoltageandfrequency.Electricpowersystemsaredesignedtoprovideconsumerswithelectricalenergyofacertainstandardorquality.
Fourfactorsdeterminethequalityofsupply:
1.Constancyoffrequency(inourcountry,thestandardfrequencyis50Hz),
2.Constancyofvoltage,
3.Levelofreliability,
4.Purityofsinusoidalwaveform9.
Theprimaryconcernoftheelectricpowersystemengineersistomaintainaconstantoperating10frequency.Thisjobisnormallyassignedtoanautomaticcontrolsystemthatmaintainsatalltimesrealpower11balanceinthesystem.Amismatchinrealpowerresultsinafrequencydeviation.Thenextmostimportantjobistomaintainaproperprofilethroughoutthesystem.Thisisaccomplishedbyproperflowsofreactivepower12onthevariouslines.
PowerNetworkStructure
Thewiringbetweenthegeneratingstationandthefinaldistributionpointiscalledtheprimarydistributionsystem13.Thereareseveralmethodsusedfortransmittingthepowerbetweenthesestwopoints.Thetwomostcommonmethodsaretheradialsystemandtheloopsystem.
Thelargenetworkcanbebroadlydividedintotwoparts.i.e.transmissionsystemanddistributionsystem.Inaddition,thenetworkshouldyetincludesubstations.Transmissionlineshavetwotypes:
overheadline14andundergroundcable15.Overheadlinesmainlyconsistofsteeltowers16,conductorwiresandinsulators17.
1.Conductors,usuallythreeforasingle-circuitlineandsixforadouble-circuitline.Theusualmaterialisaluminum18reinforcedwithsteel.
2.Step-upandstep-downtransformers.
3.Lineinsulators,whichmechanicallysupportthelineconductorsandisolatethemelectricallyfromtheground.
4.Supporters,whicharegenerallysteeltowerandprovidesupporttotheconductors.
5.Protectivedevices,suchasgroundwires,lightningarrestors19,circuitbreakers20,relays21,etc.
6.Voltageregulating22devices,whichmaintainthevoltageatreceivingendwithinpermissiblelimits.
Operation
TheelectricpowernetworkcanbeclassifiedintoDCandACnetwork.Nowadays,3-phase,3-wireACnetworkisuniversallyadoptedforelectricpowernetworkasaneconomicalproposition.However,distributionofelectricpowerisdoneby3-phase4-wireACnetwork.TheelectricpowercanbetransmittedeitherbymeansofDCorAC.Eachmeanshasitsownmeritsanddemerits.ComparingHVDCtransmission,
TheHVACtransmissionhasthefollowingadvantages.
1.TheACpowercanbegeneratedathighvoltage.
2.ThemaintenanceofACsubstationiseasyandcheaper.
3.TheACvoltagecanbesteppedupandsteppeddownbytransformerswitheaseandefficiency.Thispermitstotransmitpowerathighvoltagesanddistributeitatsafepotential23.
ThehighvoltageDCtransmissionhasthefollowingadvantagesoverhighvoltageACtransmission.
1.ItrequiresonlytwoconductorsascomparedtothreeforACtransmission.
2.Thereisnoinductivereactance24,capacitivereactance25,phasedisplacementandsurge26problemsinDCtransmission.
3.Duetotheabsenceofinductivereactance,thevoltagedropinaDCtransmissionlineislessthantheAClineforthesameloadandsendingendvoltage.ForthisreasonaDCtransmissionlinehasbettervoltageregulation.
4.Thereisnoskineffect27inaDCsystem.Therefore,entirecross-sectionofthelineconductorisutilized.
5.Forthesameworkingvoltage,thepotentialstressontheinsulationislessincaseofDCthanthatinAC.Therefore,aDClinerequireslessinsulation.
6.ADClinehaslesscorona28lossandreducedinterferencewithcommunicationcircuits.
7.ThehighvoltageDCtransmissionisfreefromthedielectric29losses,particularlyinthecaseofcables.
8.InDCtherearenostabilityproblemsandsynchronizing30difficulties.Althoughatpresent,transmissionofelectricpowerismostlycarriedbyACthereisanincreasinginterestinDCtransmission.Theintroductionofrectifiers31andthyratrons32hasmadeitpossibletoconvertACforgenerationanddistributionintohighvoltageDCfortransmission.
Nowadays,ACvoltagestandardclassesinelectricpowernetworkare0.4,6,10,35,66,110,220,330,500kV,inthefuture,750kV,1000kVlevelsandmorewillbedevelopedinourcountry.Voltageabove220kVareusuallyreferredtoas“extra-highvoltage”(EHV)33andthoseabove800kVarereferredtoas“ultra-highvoltage”(UHV)34.UHVtransmissioncanhaveimportantadvantagessuchasaveryhighpowercapabilityforeachright-of-way35andareductioninthenumberoftransmissionlinesrequired.
Exercise.
[Ex1]Choosetherightstatementsaccordingtothepassage.
1.Generatoristhepartofelectricpowernetwork.()
2.T
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