Converter utilisation ratio assessment for total harmonic distortion optimisation in cascaded H-bridge multi-level inverters

This study presents a new solution for harmonic optimisation of multi-level inverters using variable DC sources. The new approach is based on converter utilisation ratio consideration for total harmonic distortion (THD) minimisation. In the proposed solution, switch utilisation ratio (SUR) is introduced as an indication of efficient operation of multi-level converters and is maximised in the final THD minimisation objective functions for both phase and line-line voltages. According to the calculated formula, by SUR maximisation, the DC-link voltages of the cells are minimised which is essential for medium- and high-voltage applications. Alterable DC sources are also considered as degrees of freedom in the final target functions which results in feasible solution for the entire range of the modulation indices. In order to investigate the proposed technique, the simulations are performed for 5-, 7-, 9- and 11-level cascaded H-bridge inverters. Finally, experimental results validate the effectiveness of the suggested scheme for five- and seven-level inverters.