Modified hybrid modulation scheme with even switch thermal distribution for H-bridge hybrid cascaded inverters
Hybrid cascaded multilevel inverters have been recognised as an important alternative in the medium-voltage inverter market due to their high-quality output and outstanding availability. To optimise the performance of voltage-source converters, modulation technologies have attracted more and more attention. With conventional modulation technologies, the switches within a cell are not utilised at a uniform switching frequency, which is not benefit for reliability and modularity. To solve this problem, a modified hybrid modulation strategy based on unipolar frequency doubling is proposed for H-bridge hybrid cascaded inverters. The equivalence of different multilevel carrier-based modulation strategies is revealed and corresponding implement methods are provided. Based on the equivalent hybrid modulation strategy, a modified hybrid modulation strategy is presented by using unipolar frequency doubling. Compared with the conventional modulation strategies, the proposed strategy makes a more uniform use of the switches within a cell. The proposed strategy keeps the output quality and efficiency while the maximum switching frequency is decreased by half. The strategy has been successfully substantiated by the simulation and experimental results of a seven-level hybrid cascaded inverter.