Topological and Modulation Design of Three-Level Z-Source Inverters

This paper presents the development of two threelevel cascaded Z-source inverters, whose output voltage can be stepped down or up unlike a traditional buck three-level inverter. The proposed inverters are designed using two three-phase voltage-source inverter bridges, supplied by two uniquely designed Z-source impedance networks. These three-phase bridges can either be cascaded at their de sides to form a dc-link-cascaded Z-source inverter or at their ac outputs using single-phase transformers to form a dual Z-source inverter. The dc-link-cascaded inverter has the advantages of not using any clamping diodes and transformers, but does not have redundant switching states within a phase leg for equalizing switching losses among the power devices. This constraint limits the modulation options for the dc-link-cascaded inverter, and indeed, it can only be controlled using the modified carrier disposition technique with appropriate "Z-source shoot-through" states inserted for achieving balanced voltage boosting and optimal "nearest-three-vectors" switching. On the other hand, the dual Z-source inverter with transformer isolation can be controlled using different modulation approaches due to the presence of redundant switching states within a phase leg. Particularly, using a modified phase-shifted-carrier (PSC) scheme with shoot-through states inserted, it is shown that the dual inverter can be implemented using only a single Z-source network, while still achieving the correct volt-sec average and switching loss equalization. This represents a significant reduction in cost, and can more than compensate for the slightly degraded spectral characteristics of the PSC scheme. To verify the theoretical concepts discussed, experimental testing has been performed with the captured results presented in a later section of the paper.