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Power electronics



Introduction

Powerelectronicequipmentisadevicecomposedofvariouspowerelectroniccircuits.Usedfortheconversionandcontrolofhigh-powerelectricenergy.Alsoknownasconverterdevice.Itincludesrectifiers,inverters,DCconverters,ACconverters,variouspowersuppliesandswitches,motorspeedcontroldevices,DCpowertransmissiondevices,inductionheatingdevices,reactivepowercompensationdevices,electroplatingandelectrolysisdevices,andconvertersforhouseholdappliances.Devices,etc.

Amongthem,theDCpowersupplycanbecomposedofarectifieroraDCconverter,whichisusedforthepowersupplyofDCmotorspeedregulation,charging(rechargeablepowersupply),electroplatingandscientificinstruments.TheACpowersupplycanbecomposedofafrequencyconverter(seeACconversioncircuit).Itisdividedintovariablefrequencyvariablevoltagepowersupply(usedforACcageasynchronousmotorspeedregulation),constantfrequencyconstantvoltagepowersupply(usedtoformACuninterruptedpowersupply),ACstabilizedpowersupply,intermediatefrequencyinductionheatingpowersupply(poweroutputfrequencyupto8kHz,Usedforinductionheatingandquenching),high-frequencyheatingpowersupply(poweroutputfrequencyishigherthan8kHz,usedforquenchingandwelding)andsoon.Utilizingthefastswitchingperformanceofpowerelectronicdevices,astaticnon-contacthigh-powerswitchcanbeconstructedtoreplacethetraditionalelectromagnetichigh-powerswitchwithcontact.

Deviceprotection

Powerelectronicdevicesareaffectedbytheperformanceofthedevicesused,andtheirabilitytowithstandovervoltageandovercurrentisrelativelypoor.Forexample,motors,transformers,etc.usuallycanworkforseveralsecondsorminutesatseveraltimestheratedcurrent,whilepowerelectronicdevicescanbedamagedin0.1secondorlessunderthesameconditions.Therefore,inadditiontothereasonableselectionofthevoltageandcurrentcapacityofthedevicewhendesigningthepowerelectronicdevice,itisalsonecessarytotakesomespecialprotectivemeasurestopreventthedevicesinthedevicefrombeingdamagedduetoovercurrentandovervoltage.Somepowerelectronicdeviceshavespecialrequirementsforenvironmentalconditions(suchastemperature,coolingwaterpressure,windspeed,etc.),andtheseconditionsneedtobemonitoredtoensurereliableoperationofthedevice.

Overcurrentprotection

Overcurrentwillquicklyheatupthedevice.Iftheovercurrentisnotcutofforlimitedintime,thedevicewillsoonbedamaged.Thegreatertheovercurrent,theshorterthetimethedevicecanwithstandtheovercurrent.Commonlyusedovercurrentprotectionmeasuresare:①Usingfastfuse.Thefusingtimeisusuallywithin20milliseconds.However,thefastfuseisexpensiveandtroublesometoreplace,andisoftenusedasthelastmeasureofavarietyofovercurrentprotectionmeasures.②SetupanACcircuitbreaker.Theactiontimeislonger,rangingfrom0.1to0.2seconds.ItismainlyusedtocutofftheconnectionbetweentheACcircuitandtheACpowersupplytopreventfurtherexpansionofovercurrent.③InstallafastDCswitch.Itsactiontimeisabout10-20milliseconds,whichcanprotectthethyristorandothercomponentswithoutthefastfusebeingblown.InstalledontheDCendofACcircuits,usedforlargeandmedium-capacitypowerelectronicdevices.④Addafastshort-circuitdevice.Theactiontimeisabout2to3milliseconds.Whenanovercurrentoccurs,itcausesthepowertransformertobedirectlyshort-circuitedviaafastshort-circuitdevicetopreventtheovercurrentfromenteringthepowerelectronicdevice.⑤Usingelectroniccircuitsforover-currentdetectionandprotection.

Overvoltageprotection

Overvoltagewilldamagetheinsulationofthedeviceandmakeitunabletowork.Commonlyusedover-voltageprotectionmeasuresare:①Usingresistance-capacitanceabsorptioncircuit.Itisformedbyaseriesconnectionofacapacitorandaresistor.Thecapacitorisusedtoabsorbtheovervoltageinthepeakstate,andtheresistanceinserieswiththecapacitorisusedtoconsumetheenergyoftheovervoltage,therebysuppressingtheoscillationofthecircuit.②Selectnon-linearresistancedevices.Utilizethevolt-amperecharacteristicofthiskindofdeviceclosetoZenertubeandthecharacteristicthatitscharacteristiccanberestoredautomaticallyafterbreakdown,realizetheovervoltageprotectionofthepowerelectronicdevice.Commonlyuseddevicesareseleniumstackovervoltagesuppressorandvaristor.③Usingelectroniccircuitsforover-voltagedetectionandprotection.

Anti-interferencetechnology

Powerelectronicdevicesarehigh-powerelectricenergyconversionequipment,andseriouselectromagneticinterferenceisinevitableattheinstallationandusesite.Inadditiontodirectlycausingthepowerelectronicdevicestomalfunctiondirectly,thestronginterferencesignalenteringthedevicemainlyinterfereswiththecontrolcircuitofthedeviceandcausesthedevicetomalfunction.Commonanti-jammingtechnicalmeasuresinclude:①Suppressthestrengthoftheinterferencesignal.Forexample,ACrelays,ACcontactorcontactsandparallelcapacitorscanreducetheelectricsparkinterferencewhenthecontactsareonandoff.②Reasonablewiringtoreducethedistributedcapacitance,distributedinductanceandmutualinductancebetweenthewirestopreventinterferencesignalsfromentering.③Implementelectromagneticshielding.④Choosethegroundingpointcorrectlyandgrounditwell.⑤Strengthentheanti-interferenceabilityofthecircuititself.Selectelectroniccomponentswithgoodanti-interferenceperformancewhendesigning.⑥WhenthecontrolcircuitsharesaDCstabilizedpowersupply,eachpartofthecircuitshouldbeledseparately.⑦Trytoreducetheinputandoutputimpedanceofeachunitcircuitwhenthesignalstrengthallows.⑧Usingisolationmeanssuchastransformercouplingtoisolatethecontrolcircuitfromthesignalsourceorthemaincircuitofthepowerelectronicdeviceonthecircuittopreventinterferencesignalsfromentering.

Application

Thepowersystemisthemaincarrierofenergyutilization,transmissionanddistribution,anditplaysanimportantroleinthesocialeconomy.Thecrisisoffossilenergyandclimaticenvironmenthascausedthepowersystemtobetransformedfromlarge-scaledevelopmenttosustainabledevelopmentandintelligentization.Large-scaleaccessofdistributedpowersourcesandenergystoragedevices,coordinationoflocalpowergrids,microgridsandbackbonepowergrids,efficientandflexiblepowertransmissionmethods,intelligenttwo-wayinteractionofpowerdistributionandpowerconsumption,andimprovementofpowersupplyqualityandreliability.Itisacharacteristicofpowersystemtransformation.

Inthepowersystem,grid-connectedpowergenerationofrenewableenergy,powerconversionofenergystoragedevices,flexibleinterconnectionofACandDCpowergrids,two-wayflowofdistributedpower,dynamiccompensationofreactivepowerandharmonicsAllneedtorelyonpowerelectronicdevicestoachieve.Withthedevelopmentofhigh-voltageandhigh-powerpowerelectronicdevices,theimprovementofthemodularization,unitizationandintelligenceofconverters,theimprovementoftheperformanceofcontrolstrategiesandmodulationstrategies,powerelectronicdeviceswillplayagreaterroleinthepowersystem.

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