Extensive investigation of symmetrical and asymmetrical cascaded multilevel inverters for electric vehicle applications

摘要

To optimize the conventional power electronics in electric vehicles, many researchers investigate the application of multilevel inverters (MLIs). Thereby, cascaded MLIs are of particular importance. One crucial design parameter of cascaded MLIs is the construction of the individual MLI modules. For example, the selection of the DC source voltage in each module affects the magnitude of any MLI step and as a consequence the symmetry characteristics of the inverter output voltage. Therefore, this paper mathematically highlights symmetrical and asymmetrical cascaded MLIs (SMLIs, AMLIs) in detail. The findings show that for a fixed number of modules, SMLIs typically show a significantly higher redundancy of voltage levels compared to AMLIs. In contrast, an increase of the number of different levels is achieved applying AMLIs. With a special design rule of AMLIs, an identical resolution of the output voltage is produced as with SMLIs while keeping the number of modules low. However, the asymmetric nature of the modules can have a negative impact on the power balance of the inverter. A final comparison regarding the total harmonic distortion demonstrates that lowest distortion is achieved by MLI configurations that provide a high number of voltage levels combined with symmetrical output voltage patterns.