A serious constraint in a multilevel inverter is the capacitor voltage-balancing. In order to stabilize these DC voltages, we propose to study the cascade constituted by a three phase three-level PWM rectifier, a clamping bridge and a nine level NPC voltage source inverter (VSI). In the first part, the authors present the topology of the nine-level NPC and 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, which is controlled by a hysteresis strategy. In the last part of this paper, the authors study the stability problem of the input DC voltages of the inverter. Clamping bridges are used to control each DC capacitor voltage. To control the full DC bus voltage a closed loop control is implemented inside the control system of the rectifier. The obtained results are full of promise to use the inverter in high voltage and great power applications as for electrical traction.
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