Virtual-Flux Direct Power Control forMains Connected Three-LevelNPC Inverter Systems

Thispaperproposes acontrol strategy which extends theVirtual-Flux Direct PowerControl traditionally employed fortheconventional two-level VSItoathree-level NPC inverter. Thistopology generates ahigher numberof output voltage levels, increasing theflexibility forselecting anappropriate voltage vector. Themid-point potential is controlled according tothedirection ofthemid-point current andthesignofthemid-point voltage deviation. The methodisadapted tobeusedwithanLCL outputfilter, wheresome undesirable characteristics, suchas filter resonance, havetobecompensated. Further investigation concerning thedependency ofthemid-point voltage withthe currentphase-shift isgiven.Theoretical analysis is presented andtheperformance oftheproposed methodis verified bysimulation. IndexTerms-Direct PowerControl, GridConnected, Three-Level NPC.

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