A capacitance reduction modulation approach of hybrid modular multilevel converter with boosted modulation index and circulating current injection

In a modular multilevel converter (MMC) system, the sub-module capacitors account for a large proportion of the cost, volume, and weight. This paper proposed a method to reduce the total capacitance of MMC based on the combination of boosting the modulation index (m) and second-harmonic circulating current injection (SCCI). A small amount of full-bridge sub-modules was added to conventional half-bridge sub-modules (HBSM) based MMC to form the hybrid MMC. The third-harmonic voltage injection technology was used to increase the m of hybrid MMC without the common-mode voltage injected on the ac-side. An optimized SCCI method determined by m was introduced, which limited the root mean square (RMS) of arm current. An improved modulation approach is used to eliminate the coupling effect of third-harmonic voltage injection and SCCI in boosting m, ensuring that the m increases to 1.15 with the optimized SCCI. Compared with conventional HBSM-MMC, the proposed method reduced the total capacitance by approximately half when m = 1.15. A comprehensive comparison of different capacitance reduction methods was presented to demonstrate the cost and effect. Simulation and experiment verified the proposed method.