A modified nearest level control scheme for a double star chopper cell modular multilevel converter for operation at lower values of amplitude modulation index

This paper presents a modified nearest level control (NLC) technique for a double star chopper cell modular multilevel converter (MMC). There are two prominent NLC techniques available in the literature for the MMC, one where the rounding off is done between 0.5 levels, and the other where the rounding off is done between 0.25 levels. However, both of them suffer from a serious drawback that they are unable to generate an output voltage at lower values of the modulation index. This paper presents a modified NLC technique that is capable of generating a satisfactory output voltage for lower values of modulation index while maintaining an acceptable total harmonic distortion in the output voltage waveform where the conventional NLC techniques fail to do so. Simulation results for an MMC with four submodules per arm are presented to verify the proposed modified NLC technique.