Feedback Control of Multilevel NPC PWM Rectifier-Multilevel NPC Inverter Cascade

Abstract

Voltage source multilevel inverters have become very attractive for power industries in power electronics applications during last years. The main purposes that have led to the development of the studies about multilevel inverters are the generation of output voltage signals with low harmonic distortion; the reduction of switching frequency. A serious constraint in a multilevel inverter is the capacitor voltage-balancing problem. The unbalances of different DC voltage sources of nine-level neutral point clamping (NPC) voltage source inverter (VSI) constitute the major limitation for the use 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 nine-level PWM rectifier-filter-nine-level NPC voltage source inverter (VSI)-PMSM. In the first part, the authors present a topology of nine-level NPC VSI, and then they propose a model of this converter and an optimal PWM strategy to control it using eight bipolar carriers. The characteristics of this strategy are studied and performances of the drive of the PMSM fed by this inverter are presented too. Then in the second part, we study a new high voltage cascade nine-level PWM current rectifier-filtre-nine-level NPC VSI-PMSM. This study shows particularly problem of stability of DC voltages of the inverter and its consequence on the performances of the PMSM speed control. 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. This study allows finding a solution for the instability problem of the input DC voltages of the inverter. The results obtained with this solution confirm the good performances of the proposed solution. This study shows the effect of the stability of the DC voltages on the PMSM performances. The results obtained are full of promise to use the inverter in high voltage and great power applications as electrical traction.