风电场中直驱式风力发电机组稳定性评估

Fig.3.Frequency-converterwithgenerator-sidediode-recti erandgrid-sidePWM-converter

referenceframe(d-axis:activecurrent,q-axis:reactivecurrent).ThisfastcurrentcontrolloopscanbefoundinalmostallgridconnectedPWM-inverters.

Inaslowercontrolloop,activeandreactivepowerarecontrolled.Thepowerreferenceisusuallyde nedbyaspeed-powercharacteristicthatdrivesthewind-generatorautomaticallyintheoperatingpointofhighestef ciency(MaximumPowerTrackingstrategy).

Butcontrollingactivepowerisjustoneofseveralpossibilitiesforrealizingtheouter,d-axiscontrolloop.Controllingtorqueinsteadofpowerisanotheralternative.Inmanydesigns,generatorspeedinsteadofpowerortorquede nestheactive(d-axis)currentreference.

TheDC-boosterstabilizesthevoltageattheDC-terminalofthein-verter.Thecontrollerisrealizedbyafastcurrentcontrollersetbyanoutervoltagecontrolloop.

Figure4showsanotherfrequencyconverterconceptthatisverycommon,especiallyforpermanentmagnetgenerators.Here,bothcon-verters,thegenerator-sideandthegrid-sideconverterarerealizedbyself-commutatedPWMconverters.

Thegrid-sidecontrolcanberealizedinexactlythesamewayasintheconceptaccordingto gure3,includingalloptionswithregardtotorqueorspeedcontrol.

Thegenerator-siderecti erregulatesDC-voltagethroughthed-axiscurrentandAC-voltagethroughtheq-axiscurrentcontroller.

IncaseofelectricallyexcitedgeneratorshavingAVRs,reactivepowercouldbecontrolledinsteadofAC-voltage.

III.ELECTRICALCOMPONENTMODELING

A.SynchronousGenerator

Synchronousmachinemodelsforpowersystemanalysisareusuallybasedontheassumptionthatthemagnetic uxdistributionintherotorissinusoidal(e.g.[10]).Withthisassumption,the uxcanentirelybedescribedbyavector.

Theresultingsynchronousmachinemodel,alsonamedthe”Park”-modelcanbestbedescribedinarotor-orientedd-q-referenceframe.Figures5and6showthed-axisandq-axisequivalentcircuitdi-agramsofthesynchronousgeneratormodelofseventhorder.Themodelconsidersstatorwindings,theexcitationwindinginthed-axisanddamperwindingsinthed-andq-axis.Theinductancesassociatedtodamperwindingscaneitherrepresentrealwindingsorjustmodeleddycurrentlossesintherotor.Hence,thedescribedmodelcanbeusedforsynchronousgeneratorswithorwithoutdamperwindings.

2

Fig.4.Frequency-converterwithtwoPWM-convertersandintermediateDC-voltagecircuit

Fig.5.d-axissynchronousmachinemodel

Accordingtotheequivalentcircuitsof gures5and6thestator-voltageequationscanbeexpressedasfollows:

(1)

Therotor-voltageequationsare:

(2)

Thesynchronousmachinemodeliscompletedbythemechanicalequation:

(3)

Thereisthefollowingrelationshipbetweenthemechanicalvariables

andandthecorrespondingelectricalvaluesthatareusedin(1):