电气工程及其自动化毕设之文献翻译.docx

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