An alternative five level NPC converter for medium voltage AC drives and technical issues

This paper presents a study of an alternative five level inverter for medium voltage ac drives based on neutral point clamped concepts. The main advantage of this topology is its higher voltage operation compared to the commercial three level structures but with less mounting complexity when it is compared to a traditional five level neutral-point-clamped converters. Besides the converter is suitable for various low frequency modulation techniques which are able to reduce switching losses and thus increase the converter efficiency. In this line a detailed explanation of the topology is presented along with a cost comparison among the traditional multilevel topologies based on topological analysis. A description of a five level modulation process for this topology is also presented and in this line a discussion of modulation issues in very low switching frequency is also addressed. The differences and similarities of modulation technique in this study from a classical phase-disposition pulse width modulator are discussed. An important feature of this modulator is its ability to improve the converter harmonic spectrum. All this work is supported by results which are aimed to confirm these ideas.