电气工程及其自动化毕设之文献翻译.docx
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电气工程及其自动化毕设之文献翻译.docx
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电气工程及其自动化毕设之文献翻译
文献翻译
学院名称电气工程学院
指导教师职称教授班级学号学生姓名
ElectricPowerSystems
IntroductionofElectricPowerSystems
Asthepowerindustrygrows,sodotheeconomicandengineeringproblemsconnectedwiththegenerating,transmissionanddistributionsystemsusedtoproduceandhandlethevastquantitiesofelectricalenergyconsumedtoday.Thesesystemstogetherformanelectricalpowersystem.
Itisimportanttonotethattheindustrythatproduceselectricalenergyisuniqueinthatitmanufacturesitsproductattheveryinstantthatitisrequiredbythecustomer.Energyforthegenerationofelectricitycanbestoredintheformofcoalandoil,andofwaterinreservoirsandlakes,tomeetfuturerequirements,butthisdoesnotdecreasetheneedforgeneratorcapacitytomeetthecustomers’demands.
Itisobviousthattheproblemofthecontinuityofserviceisveryimportantforanelectricalpowersystem.Noservicecanbecompletelyprotectedfromthepossibilityoffailureandclearlythecostofthesystemwilldependonitsrequirements.Howeveranetreliabilitygainisobtainedbyemployingacertainnumberofgeneratingunitsandbyusingautomaticbreakersfortheseparationintosectionsofthebusbarsingeneratingstationsandofthetransmissionlinesinanationalorinternationalgridsystem.Infactalargesystemcomprisesnumerousgeneratingstationsandloadsinterconnectedbyhigh-capacitytransmissionlines.Anindividualunitofgenerationorsetoftransmission‘linescanusuallyceasetofunctionwithoutinterruptingthegeneralservice.
Themostusualsystemtodayforgenerationandforthegeneraltransmissionofpoweristhethree-phasesystem.Infavorofthissystemareitssimplicityanditssimplicityanditssavingwithrespecttoothera.c.system.Inparticular,foragivenvoltagebetweenconductors,withagivenpowertransmitted,withagivendistance,andwithagivelineloss,thethree-phasesystemrequiresonly75percentofthecopperoraluminiumneededinthesingle-phasesystem.Anotherimportantadvantageofthethree-phasesystemisthatthree-phasemotorsaremoreefficientthansingle-phaseones.Thesourcesofenergyofrlarge-scaleelectricitygenerationare:
1.steamobtainedbymeansofaconventionalfuel(coal,oilornaturalgas),thecombustionofcityrefuseortheemploymentofnuclearfuel;
2.water;
3.dieselpowerfromoil.
Thereareotherpossiblesourcesofenergysuchasdirectsolarheat,windpower,tidalpower,etc.,butnoneofthesehasyetgonebeyondthepilot-plantstage.
Inlargesteampowerplants,thethermalenergystoredinsteamisconvertedintoworkbymeansofturbines.Aturbineconsistsessentiallyofashaftorrotorfixedinbearingsandenclosedinacylindricalcasing.Therotorismadetoturnsmoothlybymeansofjetsofsteamfromnozzlesaroundtheperipheryoftheturbinecylinder.Thesesteamjetsstrikebladesattachedtotheshaft.Centralpowerstationsemploycondensingturbinesinwhichthesteampassesintoacondenserafterleavingtheturbine.Condensationiseffectedbythecirculationoflargequantitiesofcoldwaterthroughthetubesofthecondenser,thusincreasingtheexpansionratioofthesteamandtheconsequentefficiencyandworkoutputoftheturbine.Theturbinesareconnecteddirectlytolargeelectricitygenerators.
Inturbinestheactionofthesteamiskinetic.Thereisprogressiveexpansionofthesteamfromthehighpressureandrelativelysmallvolumeatwhichitleaves.
Steamismadebyheatingwaterinaboiler.Theusualboilerhasafurnaceinwhichfuelisburned,andtheheatgivenoffduringcombustionisconductedthroughthemetalwallsoftheboilertogeneratesteamatapressurewithintheboilervessel.Innuclearplants,steamisgeneratedwiththeaidofareactorinwhichthecontrolledfissionofuraniumorplutoniumsuppliesthenecessaryheatforthevaporizationofwater.Thusthereactorrep;acesthesteamgeneratorofconventionalplants.
Useismadeoftheenergypossessedbywaterinhydroelectricstations.Inordertotransformthisenergyintowork,hydraulicturbinesareused.Modernhydraulicturbinesmaybedividedintotwoclasses:
impulseturbinesandpressureorreactionturbines.Oftheformer,thePeltonwheelistheonlytypeusedinimportantinstallations;ofthelatter,theFrancisturbineoroneofitsmodificationsisuniversallyemployed.
Inanimpulseturbine,thewholeheadofwaterisconvertedintokineticenergybeforethewheelisreached,asthewaterissuppliedtothewheelthroughanozzle.Inthepressureorreactionturbinethewheel(orrunner)isprovidedwithvanesintowhichwaterisdirectedbymeansofaseriesofguidevanesaroundthewholeperiphery.Thewaterleavingtheseguidevanesisunderpressureandsuppliesenergypartlyinthekineticformandpartlyinthepressureform.
Thedieselengineisanexcellentprimemoverforelectricitygenerationinplantbelowabout10,000KVA.Ithastheadvantageoflowfuelcost,abriefwarming-upperiodandlowstandinglosses.Moreoveritrequireslittlecoolingwater.Dieselgenerationisgenerallychosenforsmallpowerrequirementsbymunicipalities,hotelsandfactories;hospitalsoftenkeepandindependentdieselgeneratorforemergencysupply.
Thetransmissionofelectricalenergybymeansoflinesisagreatprobleminelectricalpowersystems.Transmissionlinesareessentialforthreepurposes:
1.Totransmitpowerfromahydroelectricsitetoaloadcenterperhapsaconsiderabledistanceaway;
2.Forthebulksupplyofpowerfromsteamstationstoloadcentersarelativelyshortdistanceaway;
3.Forinterconnectionpurposestotransferenergyfromonesystemtoanotherincaseofemergency.
Thetransmissionvoltageisdeterminedlargelybyeconomicfactors.Infact,inatransmissionline,ifthedistance,thepowerandthepowerlossarefixed,thetotalweightoftheconductorvariesinverselyasthesquareofthetransmissionvoltage.Fortheeconomictransmissionofpoweroverconsiderabledistancesthevoltagemustthereforebehigh.Naturallywithhighervoltagestheinsulationcostalsorisesandtofindtheoptimumvoltagewemuststrikeabalancebetweenthiscostandthesavingthroughthereductionofthecross-sectionoftheconductors.
Forhighvoltages,overhead-lineconstructionisgenerallyusedwithsuspension-typeinsulators.Steeltower,calledpylonsservetocarrytheinsulators,witheachconductorsuspendedfromthebottomofagrouporstringofinsulatorunits.Thefollowingtypesofconductorarethosemostcommonlyused:
strandedcopperconductors,hollowcopperconductorsandACSR(aluminumcable,steelreinforced)conductors.
Distributionincludesallthepartsoftheelectricitysystembetweenthepowersubstationssuppliedfromhigh-voltagetransmissionlinesandtheconsumer’sswitch.Electricpowerisreceivedfromsubstationsanddistributedtotheconsumersatthevoltagelevelsandwiththedegreeofcontinuitythatareacceptabletothevarioustypesofconsumer.Inlargemetropolitansystemsbothoverheadandundergrounddistributionmethodsareused.Althoughundergrounddistributionismoreexpensivethananoverheadsystem,itisvirtuallyanecessaryinheavilyurbanizedareas.Insmallertownsandinthelesscongesteddistrictsoflargecities,theentiredistributionsystemisusuallyoverhead.
ElectricSystemMonitoring
Amodernelectricpowersystemisanassemblyofmanycomponentseachofwhichinfluencesthebehaviorofeveryotherpart.Properfunctioningofthesystemasawholemakesitnecessarytomonitorconditionsexistionatmanydifferentpointsonthesysteminordertoassureoptimumoperation.
Theconcernofthecustomersisprimarilythatthefrequencyandvoltageofthesupplyareheldwithincertainrathernarrowlimits.Sincefrequencyofthesystemisthesameeverywhere,itmaybemonitoredbyasinglefrequencymeterlocatedatanyconvenientpoint.Incontrastthevoltageofthesystemmaybequitedifferentatdifferentpoints.Consequently,itisnecessarytomakecontinuousobservationofthevoltageatcertainkeypointsonthesysteminordertoprovideacceptableservice.
Efficientoperationofthesystemisobtainedbyassigningproperloadschedulestoeachofthegeneratorsonthesystem.Newerplants,althoughindividuallymoreefficient,maybelocatedatpointsonthesystemwheretheirloadingoccasionslargesystemlosses.Itisdesirabletooperatewithadivisionoftheloadbetweengeneratorssothatthetotalcostoffuelconsumedisminimized.Toprovidereliabilityofthepowersupplyintileeventofunexpectedconditions,itisdesirabletohavethetotalkilowattratingofallmachinesinoperationsomewhatgreaterthanthetotalloadpluslosses.
Thisexcessofgeneration,knownasspinningreserve,isthenavailableforpickingupsuddenlyappliedcustomerloadsortopickuptheloaddroppedbyageneratorthatmustberemovedfromserviceforemergencymaintenance.
InstrumentationisnecessarytopermitbillingofCustomersforenergyused.Manyinterconnectionsexistbetweendifferentpowersystems.Instrumentsmustbeprovidedatinterchangepointstopermitbillingforenergytransferredfromonesystemtoanother.Thecontinuousmonitoringofenergytransferisnecessarytoassurethatinterchangedpoweriswithinthelimitsofcontractagreements.
Thecontinuousmeasurementofconditionsonmajorpiecesofequipmentisnecessarytoavoiddamageduetooverload.Asloadincreasesfrommonthtomonth,pointsatwhichadditionalcapacityofequipmentisrequiredmayberecognizedandprovisionmadefortheinstallationofadditionalequipment.Thusinstrum
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