Capacitor voltage balancing in neutral-point clamped rectifier using modified modulation index technique

Abstract

In this paper, a simplified control technique is proposed to control capacitor voltages of three-phase Neutral-Point Clamped bi-directional rectifier based on the modified modulation index technique. The modified modulation index technique generates new position of the reference vector in the space vector hexagon keeping both capacitor voltages constant. The modified reference vector is made either large or small enough so that the effects of redundant switching patterns become prominent to balance capacitor voltages. Changing commutation state for the adoption of redundant switching patterns is dictated by reference vector. The modulation index is modified in proportion to the mean difference in the capacitor voltages plus calculated modulation index. The calculated modulation index is due to change in duty cycle of redundant switching states of the three nearest reference vectors in the sectors in which reference vector lies. The proposed algorithm maintains minimum neural point potential variations and also keeps source side and load side parameters within acceptable limits.