Seite - 252 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 616 3.2. CurrentModulationMethod in IndependentMode Different from star and trianglemodes, independentmode is powered by two sources and controlledbytwoinverters,whichmeanseachwinding’selectricpotential is controlledbybothof the twoinverters’bridgesandhas fourstatuses: (10), (01), (11), and(00). The first number of switching status stands for the electric potential of inverter 1’s bridge. Thesecondnumberofswitchingstatusstandsfortheelectricpotentialof inverter2’sbridge. Thestatus of the inverter bridge upper arm (on) and lower arm (off) obtained high potential, which was substitutedwith number “1”. The status of the inverter bridge upper arm (off) and lower arm (on)obtainedlowpotential,whichwassubstitutedwithnumber“0”. Because the twosourceswere insulated, themid-point voltage difference of the two sources unm variedwith different inverter switching statuses, which brought about diverse phase voltages in different switching statuses, especiallywhenVdc1 =Vdc2.Whenthemid-pointvoltageof the twosourceswasunm=0, thephase voltagewasthemid-pointphasevoltage.Whentheswitchingstatuseswere (10), (01), (11), and(00), thecorrespondingmid-pointphasevoltageswere (Vdc1+Vdc2)/2,−(Vdc1+Vdc2)/2, (Vdc1−Vdc2)/2, and−(Vdc1−Vdc2)/2. Hence, whenVdc1 = Vdc2, we could get fourmid-point voltages via dual inverters’ control. WhenVdc1 = Vdc2, we could get threemid-point voltages and themid-point voltageswithswitchingstatuses (11)and(00)werebothzero. Thetraditionalhysteresiscurrentmodulationalgorithmhas twopotentialsandcorresponding twotriggerareas:Δi≥ handΔi≤−h.WhenVdc1 =Vdc2, thereare fourpotentials in independent modeof thehysteresis currentmodulationalgorithm.Apart fromtwotriggerareasusedforactivating switchingstatuses (01)and(10), twointermediate trigger lineswereneededforactivatingswitching statuses (00) and (11)—two intermediate potentials. Provided the two intermediate trigger lines wereΔi= dandΔi=−d,withdas the linear currentdeviationof the intermediate line,wecould getEquation (23) basedon theprinciple thatmid-pointvoltage isproportional to current errorΔi whentriggered: h : Vdc1+Vdc2 2 = d : Vdc1−Vdc2 2 . (23) Equation(23)couldbe transformedto: d= Vdc1−Vdc2 Vdc1+Vdc2 h. (24) Itwasobvious thatwhenVdc1>Vdc2,d> 0;whenVdc1<Vdc2,d< 0;whenVdc1=Vdc2,d= 0, andat this time, two intermediate trigger linescoincidedatΔi= 0. Ithas tobepointedout that in eachwinding’s fourpotentials in independentmode, twoboundarypotentials are triggeredbyΔi being in thecorrespondingarea toensure thatΔi candepart fromthat triggeredareaswiftly in the relativelygreaterphasevoltagegeneratedbyboundarypotentialsas longasΔi is in thecorresponding areaandrestrictedtohysteresisband [−h,h]. Twointermediatepotentialswere triggeredbycrossing thecorrespondingintermediate trigger lineswithoutconsideringthecrossingdirection; thiscontrol logicmainlymatteredinslowingfluctuationsofΔidowntoloadsmallervoltage.Arathersmallphase voltagewouldbe loadedifΔi is close to thecenter line (Δi=0) toavoiddrasticfluctuations frequently hitting thehysteresis boundary if constantly loadedwith largephasevoltage. Whenappliedwith thismodulationmethod, the changing ratewoulddecreaseafterΔi crosses the control lineΔi= d orΔi=−duntil ithits theboundaryagain. Twoadditionalpotentialswouldeffectively retard the fluctuationspeedofΔi tomakecurrentchangesmilderandreduce the inverters’ switchingfrequency. Twoimprovedmulti-levelhysteresismodulationmethodswereproposedonthefoundationof theabovemulti-levelhysteresiscurrentmodulationstrategy. Theywere the lowswitchingfrequency methodandhighpowerdifferencemethod. These twomethodswereachievedbyaddingthe trigger conditionsof inverterbridges’ switchingstatuses (00)and(11) in independentmode. Due to independentmode being powered by two sources, power distribution between two sourceswas involved. Thus,amajorpowersourcewasproposedandwasexpectedtohavegreater 252