Study of Neutral – Point Voltage Balancing Problem in Seven-Level NPC VSI Cascade A. Talha 1 , F. Bouchafaa 2 , E. M. Berkouk 3 , M. S. Boucherit 4 Abdelaziz Talha 1 , Farid Bouchafaa 2 , University of Sciences and Technology Houari Boumediene, B.O. Box 32 El-Alia 16111 Bab-Ezzouar Algiers, Algeria. E-mail: atalha@hotmail.fr, bouchafa_f@yahoo.fr El-Madjid Berkouk 3 , Mohamed Seghir Boucherit 4 , Laboratoire de Commande des Processus. Département de Génie Electrique. Ecole Nationale Polytechnique d'Alger − 10, rue Hassen Badi, El Harrach, Algiers, Algeria − BP 182. E-mail : emberkouk@yahoo.fr, ms_boucherit@yahoo.fr Abstract: In this paper, the authors present a study of neutral – point voltage balancing problem in the seven- level Neutral Point Clamped (NPC) Voltage Source Inverter (VSI). We develop a knowledge model of this converter by using connection functions and a space vector modulation PWM strategy which uses six bipolar carriers to control it. To analysis the instability problem of the input DC voltages of the inverter, we study a cascade constituted by a two-level PWM rectifiers – seven-level NPC VSI. The results obtained show that the input DC voltages of the inverter are not stable. To solve this problem, we propose to use clamping bridge, because this bridge allow improve the input voltage of seven-level inverter. In spite of this solution, the output voltage of rectifier is not constant. To remedy to this problem, we propose a solution which uses enslavement for this cascade. This study has allowed to find a solution for the stability problem of the input DC voltages of the inverter. The results obtained with this solution are full of promise to be used to stabilise the input DC voltages of this converter. Keywords: Multilevel, clamping bridge, feedback, PWM, space vector modulation, current hysteresis. 1. INTRODUCTION The apparition of new power components controllable in the opened and closed states has let to the conception of new and fast converters for high power applications. Thus, the speed variator has seen its cost decreasing considerably. The progress accomplished in the micro-computer tools has let the synthesis of more performant and robust control algorithms for sets of converter-machine. In this paper, the authors propose a new cascade for high voltage and high power applications. This cascade lets to absorb, in network, sinusoidal currents with unity power factor. It constitutes by two two-level PWM rectifiers– seven-level NPC VSI. In the first part, we develop a knowledge model by using connection functions of this converter. In the second part, we propose space vector modulation strategy which uses six bipolar carriers to control this converter. In this part, the inverter is fed by constant input DC voltages. In the last part of this paper, we study the stability problem of the input DC voltages of the inverter. Thus, we study a cascade constituted by a two- level PWM rectifiers – clamping bridge - seven-level NPC VSI. This cascade allows to improve the input voltages of seven - level NPC inverter. For all that solution, the output voltage of the rectifier is not stable. To remedy to this problem, we propose a solution which uses an enslavement for this cascade. This study has allowed to find a solution for the stability problem of the input DC voltages of the inverter. The results obtained with this solution shows that the proposed solution is very efficient to solve the instability problem of the multilevel inverter. 2. MODELLING OF SEVEN-LEVEL NPC VSI 2.1. The three-phase seven-level NPC VSI structure The three-phase seven-level NPC VSI is a new conversion structure used to feed, with variable frequency and voltage, high power alternating current machines. Several structures are possible for seven-level inverters [1] [2] [3]. In this paper, we study the Neutral Point Clamping (NPC) structure (Figure1). This converter is constituted by three arms and six DC voltage sources. N V C V B U C5 U C4 U C1 UC2 U C3 i3 i2 i 1 M I d6 I d5 I d0 Id1 I d2 I d3 V A D 38 D312 D 37 D 311 D 36 DD30 DD 31 D35 D 31 D 32 D 310 D 39 D 33 D 34 T312 T 38 T311 T37 T 36 T 35 T 31 T 310 T 39 T 32 T33 T 34 D 28 D 212 D 27 D 26 DD20 DD 21 D 25 D 21 D 22 D 210 D29 D 23 D 24 T212 T 28 T 211 27 T 26 T 25 T 21 T 210 T 29 T 22 T 23 T 24 D 18 D 112 D D 211 T 17 I d4 D111 D 16 DD 10 DD11 D 15 D 11 D 12 D 110 D 19 D13 D 14 T 112 T 18 T111 T 17 T16 T 15 T 11 T110 T 19 T 12 T 13 T 14 U C6 Fig.1 A seven-level NPC voltage source inverter