FEEDBACK CONTROL OF DC LINK VOLTAGE OF THE BACK-TO-BACK PWM MULTILEVEL CONVERTER

Farid Bouchafaa - El Mad-Jid Berkouk - Mohamed Seghir Boucherit

   A serious constraint in a multilevel inverter is the capacitor voltage-balancing problem. The unbalance of the different DC voltage sources of the nine-level neutral point clamping (NPC) voltage source inverter (VSI) constituted the major limitation for the uses of this new power converter. In order to stabilize these DC voltages, we propose in this paper to study the cascade constituted by three phases three-level PWM rectifier-clamping bridge filter-nine-level NPC voltage source inverter (VSI). In the first part, the authors present a topology of nine-level NPC, then they propose a model of this converter and the algebraic PWM strategy to control it. Then in the second part we study the three-level PWM rectifier controlled by hysteresis strategy. In the last part of this paper, the authors study the stability problem of the input DC voltages of the inverter. To remedy to this problem, the authors propose a solution which uses a feedback control for this cascade. The results obtained are full of promise to use the inverter in high voltage and great power applications as electrical traction.

Keywords: NPC inverter, clamping bridge, rectifier multilevel, current hysteresis, PWM strategy, input DC voltages, power factor unit

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