A dual-mode modulation technique for controlling the average neutral point current in neutral-point-clamped converters

Abstract

This paper presents a dual-mode modulation technique which aims to control the average current flow into the neutral point (NP) of the neutral-point-clamped (NPC) converter without the need for any additional hardware. The two modes of operation are normal mode and compensating mode. In the normal operation mode, all the three phases switch between two consecutive voltage levels (between the positive or negative dc-rail and the neutral point) in a switching period. In the compensating operation mode, at least one of the phases switches between the positive and negative dc-rails in a switching period. An analytical solution is developed to determine the duration of these two operation modes within each fundamental cycle based on the converter’s operating condition. An advantage of this solution is that it can be generalized for balancing the capacitor voltages in all applications employing NPC converters. The proposed solution also determines the maximum average NP current injection capacity of the NPC converter under dual-mode modulation technique, which indicates the stable operating range of the converter. The performance of the proposed modulation technique is validated experimentally for various loading conditions.