Seite - 253 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 616 poweroutput thantheothersource in independentmode. Lowswitchingfrequencyandhighpower differencemethods coulddeterminewhichonewas themajorpower source andswitchover it at anytimetoaccomplishpowerdistribution.Whenthe loadis low, themajorpowersourcecaneven chargeanothersource. Table3 indicates therelationshipsbetween inverterbridges’ switchingstatuses, directionofphasecurrent i, andcurrentflow.Giventhatphasecurrentflowingfromleft to right is positiveinFigure1,wecouldtell thatenergyflowbetweenthetwosourcescouldonlybeaccomplished whenthe inverterbridges’ switchingstatuseswereat twointermediatepotentials (00)and(11). Table3.Relationshipbetweeninverterbridge’s switchingstatesandpowerflowdirection. Direction of Phase Current Inverter Switching States 0i< 0i> Boundary potentials (01) Both power sources discharging Both power sources charging (10) Both power sources charging Both power sources discharging Intermediate potentials (00) Power source 1 discharging Power source 2 charging Power source 1 charging Power source 2 discharging (11) Power source 1 charging Power source 2 discharging Power source 1 discharging Power source 2 charging Thetrigger regulationof inverterbridges’ switchingstatuses in lowswitchingfrequencyandthe highpowerdifferencemethodisdisplayedinTable4. Table4.Dual inverter triggerrulesof twodifferentcurrentmodulationmethods. Modulation Pattern Inverter Switching States Low Switching Frequency Method High Power Difference Method Boundary potentials (01) Δi is in the area of Δ ≥i h Δi is in the area of Δ ≥i h (10) Δi is in the area of Δ ≥i h Δi is in the area of Δ ≥i h Intermediate potentials (00) Δi crossed control line Δ =i d and switching state of inverter bridge on major power source’s side is 0 Δi crossed control line Δ =i d and when power source 1 is major power source: phrase current 0<i ; when power source 2 is major power source: phrase current 0>i (11) Δi crossed control line Δ = −i d and switching state of inverter bridge on major power source’s side is 1 Δi crossed control line Δ = −i d and when power source 1 is major power source: phrase current 0>i ; when power source 2 is major power source: phrase current 0<i These two improvedmethodsaddedtriggerconditionsof two intermediatepotentials,which madetwopotentials, insteadofbeingtriggeredwhenΔi crossingcontrol linesΔi=±d, triggeredat otherspecificconditions.Normallyonlyone intermediatepotentialwas triggeredinonehysteresis period. The lowswitching frequencymethod, needed to confirm the switching status of inverter bridgeonmajorpowersource’s side, remainsunchangedafter switchingwhenΔi crossingcontrol linesΔi=±d. In thiscase, theswitchingstatusesofbothtwoinverters’bridgeswouldnotbechanged simultaneouslywhenΔicrosses thecontrol linesandtheswitchingfrequencyof inverterdevicescould be lowered toaminimum. In thehighpowerdifferencemethod,whenΔi crosses thecontrol lines Δi=±d,weneed todecidewhether to switchbasedon thepresentphase current i’s direction to ensure themajorpowersourcecouldcharge theothersourcewhentheswitchingstatusof inverter bridge isat twointermediatepotentials. Thismethodincreases thedifferencebetweentwosources’ poweroutputsasmuchaspossible. 253