tns tncs tnc tt it等接地系统的区别Word文档格式.docx
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tns tncs tnc tt it等接地系统的区别Word文档格式.docx
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I.powersupplysystemforengineeringconstruction
Thebasicpowersupplysystemofpowersupplyforconstructionisthree-phase(),threewireand(380/220V)three-phasefourwiresystem,butthesetermsarenotverystrict.TheInternationalElectrotechnicalCommission(IEC)hasmadeuniformregulations,whicharecalledTTsystem,TNsystemandITsystem.Amongthem,TNsystemisdividedintoTN-C,TN-SandTN-C-Ssystems.Hereisabriefanalysisofthevariouspowersupplysystemsmentionedabove.
(1)thebasicmodeofengineeringpowersupply;
AccordingtothevariousprotectionmodesandtermsdefinedbyIEC,thelow-voltagedistributionsystemisdividedintothreekindsaccordingtothegroundingmode,namely,TT,TNandITsystems,asfollows.
(1)TTmodepowersupplysystemTTmeansthedirectprotectionsystemofthemetalcasingoftheelectricalequipment,calledtheprotectiveearthingsystem,alsoknownastheTTsystem.ThefirstsymbolTdenotesthedirectconnectionoftheneutralpointofthepowersystem;
thesecondsymbolTrepresentstheloadequipment;
themetalcaseandthenormalunchargedmetalpartaredirectlyconnectedtotheearthandareindependentofhowthesystemisgrounded.
1)whenthemetalcasingoftheelectricequipmentischarged(thephaselinemeetstheshellortheequipmentisdamagedandtheelectricityisoutofcirculation),becauseoftheearthingprotection,theriskofelectricshockcanbegreatlyreduced.However,thelow-voltagecircuitbreaker(automaticswitch)doesnotnecessarilytrip,resultinginleakageofequipmentoftheearth'
ssurfacevoltageishigherthanthesafetyvoltage,belongingtothedangerousvoltage.
2)whentheleakagecurrentisrelativelysmall,evenifthefusedoesnotnecessarilyfuse,sotheleakageprotectorisneededforprotection,sotheTTsystemisnotsuitabletobeusedinthe380/220Vpowersupplysystem.
3)TTsystemgroundingdeviceofsteelconsumption,anddifficultrecovery,laborandmaterialcosts.
NowsomeconstructionunitsistheuseofTTsystem,theconstructionunitspeciallyinstalledagroundingdevice,leadstoaspecialgroundingwire,groundingdevicetoreducetheamountofsteel,asshowninfigure1-2.
Figure1-2
SeparatethespecialprotectionlineofthenewlyaddedPElineandzerolineNwork,itscharacteristicsare:
A.commongroundandthezerolinecontactwithoutelectricity;
thenormaloperationoftheB.,thezerolineofworkcanbecurrent,andnospecialprotectionlinecurrent;
C.TTsystemsuitableforelectricalequipmentandverysmallcapacitydispersionsituation.
(2)TNmodepowersupplysystem.Thispowersupplysystemisaprotectivesystemwhichconnectsthemetalshelloftheelectricalequipmentwiththenormalunchargedmetalpartandtheworkingzeroline,calledthezeroprotectionsystem,andisrepresentedbyTN.Itsfeaturesareasfollows.
1)oncetheequipmentshellcharged,zeroconnectionprotectionsystemcanbeincreasedastheleakagecurrent(220V)short-circuitcurrent,thiscurrentislarge,manytimesoftheTTsystem,isactuallyasinglerelativelyshortcircuitfault,thefusefusewillfuse,releaseoflowvoltagecircuitbreakerwillactimmediatelyandmakefaulttrippingpowerequipment,safety.
2)TNsystemsavesmaterialandworkinghours.Itiswidelyusedinourcountryandmanyothercountries.ItcanbeseenthatithasmoreadvantagesthanTTsystem.IntheTNmodepowersupplysystem,isdividedintotwokinds,suchasTN-CandTN-S,accordingtowhethertheprotectivezerolineisseparatedfromtheworkzeroline.
(3)TN-Cpowersupplysystemwhichisusedasaworkingzerolinezeroconnectionprotectionline,canbecalledtheprotectionofneutralline,NPE)duetotheunbalanceofthree-phaseload,workingzerolineunbalancedcurrent,haveacertainpotentialdifferenceontheline,sothemetalelectricalequipmentconnectedwiththelineprotectionthevoltageontheearth.
2)iftheworkzerolineisbroken,thenprotectthezeroconnectedleakageequipmentshellcharged(totheground220V!
).
3)ifthelineofpowertouchestheground,theouterpotentialofthedevicerises,causingthepotentialdangertospreadontheneutralline.
4)whenusingtheearthleakageprotectoronthemainlineoftheTN-Csystem,alltherepeatedearthingbehindtheleakageprotectormustberemoved,otherwisetheleakageswitchcannotbeclosed.Moreover,thezerolineoftheworkshallnotbedisconnectedunderanycircumstances.Therefore,theutilitylinecanonlymakerepeatedearthingontheuppersideoftheleakageprotector.
5)theTN-Cmodepowersupplysystemisonlysuitableforthethree-phaseloadbasicbalance(no220Vload).
(4)TN-Smodepowersupplysystem,whichisthepowersupplysystemwhichseparatestheworkzerolineNandthespecialprotectionlinePEstrictly,iscalledtheTN-Spowersupplysystem.AsshowninFigure1-4,thecharacteristicsoftheTN-Spowersupplysystemareasfollows.
1)whenthesystemisrunningnormally,thereisnocurrentonthespecialprotectionline,butthereisunbalancedcurrentontheworkingzeroline.ThePElinehasnovoltagetotheground,sothemetalcasingoftheelectricequipmentisconnectedwithzero,andtheprotectionisconnectedtothespecialprotectivelinePE.Thesafetyisreliable.
2)theworkingzerolineisonlyusedasthesingle-phaselightingloadloop.
3)specialprotectivelinePEshallnotbedisconnected,norshalltheleakageswitchbeallowedtoworkasazeroline.
4)useleakageprotectorline,thereshallbenorepeatgroundingleakageprotector,andthePElinehasrepeatedlygrounding,butwithoutleakageprotector,sotheTN-Ssystemofpowersupplylinescanalsoinstallleakageprotection.
5)TN-Smodepowersupplysystemissafeandreliable.Itissuitableforlowvoltagepowersupplysystem,suchasindustrialandcivilbuilding.Beforetheconstructionoftheproject,"
threelinksandoneleveling"
(electric,water,roadandhorizon)mustbepoweredbytheTN-Smode.
(5)TN-C-Spowersupplysystemintheconstructionofthetemporaryuseofelectricity,ifthefirstpartis(no220Vload)TN-Cmodeofpowersupply,andtheconstructionspecificationconstructionsitemustadoptTN-Spowersupplysystem,canbeinthesystemafterthepartofthesitedistributionboxfromthePEline,asshowninfigure1-5.ThesystemiscalledtheTN-C-Spowersupplysystem.ThefeaturesoftheTN-C-Ssystemareasfollows.
Figure1-5TN-C-Smodepowersupplysystem
1)workzerolineNandspecialprotectionlinePEUnicom,asshowninFigure1-5afterthemainswitchboxunbalancedlinecurrentisrelativelylarge,theelectricalequipmentzeroconnectionprotectionbyzerolinepotentialinfluence.ThegeneralswitchboxbehindthePElinewithoutcurrent,thesectionofwirenovoltagedrop,therefore,theTN-C-Ssystemcanreducethevoltageoftheelectricalequipmentshell,butcannotcompletelyeliminatethevoltage,thevoltagedependsontheloadontheNlineofunbalancedcurrentsizeandtheNlineintheswitchboxfrontlinethelengthof.ThehighertheloadimbalancecurrentandthelongertheNlineis,thegreaterthedeviationoftheequipmentenclosuretothegroundvoltage.Therefore,theloadimbalancecurrentshouldnotbetoolarge,andrepeatedearthingshouldbedoneonthePEline.
2)PElineinanycasecanentertheleakageprotector,becauseleakageprotectionactionwillmakebeforetheendofthelineleakageprotectiontrippingcausedbylarge-scalepoweroutages,specification:
zerolinezeroconnectionprotectionshallnotbeconnectedtoanyswitchandfuse.
3)thePElinemustnotbeconnectedwiththeNlineexceptinthemainbox,andtheotherpartsarenotallowedtoconnecttheNlinewiththePEline.NoswitchesandfusesareallowedonthePEline,andtheconnectionmustbefirm.
Throughtheaboveanalysis,theTN-C-SpowersupplysystemisatemporaryadaptationontheTN-Csystem.Whenthethree-phasepowertransformerworkswellandthethree-phaseloadisbalanced,theTN-C-Ssystemisfeasibleinthepracticeofconstructionpowerconsumption.However,intheunbalancedthree-phaseloadandtheconstructionsitehasdedicatedpowertransformers,TN-Smodepowersupplysystemmustbeadopted.
(6)theITmodepowersupplysystemIindicatesthatthepowersupplysidehasnoworkgroundorisconnectedtothegroundthroughhighimpedance.ThesecondletterTrepresentstheloadsideelectricalequipmentforgroundprotection,asshowninfigure1-6.
TheITmodepowersupplysystemhashighreliabilityandgoodsecuritywhenthepowersupplydistanceisnotverylong.Itisgenerallyusedinplaceswherepowercutsarenotpermitted,orplacesrequiringstrictcontinuouspowersupply,suchascontinuousproductionunits,operationroominlargehospitals,undergroundminesandsoon.Thepowersupplyconditioninundergroundmineispoor,andthecableiseasytodamp.TheuseofITpowersupplysystem,evenifthepowersupplyisneutral,oncetheequipmentleakage,singlephaseleakagecurrentisstillsmall,willnotdamagethebalanceofpowersupplyvoltage,sothesystemgroundingpowerthantheneutralpointissafe.
However,ifthepowersupplydistanceisverylong,thedistributioncapacitanceofthepowerlinetotheearthcannotbeignored.AscanbeseenfromFig.1-6,whentheloadisshortcircuitedortheelectricleakagecausestheequipmentcasetobeelectrified,theleakagecurrentformsaloopthroughtheearth,andtheprotectiondevicedoesnotnecessarilyact,
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