A capacitance reduction method of hybrid modular multilevel converter based on multi-harmonic voltage injection

In a modular multilevel converter (MMC) system, the sub-module (SM) capacitors account for a large proportion of cost, volume and weight. Increasing modulation index (m) is an effective method to reducing the capacitance. Traditional MMC based on half-bridge SM (HBSM) cannot boosting m while the hybrid MMC composed of HBSMs and full-bridge sub-modules (FBSM) can increase the m. The negative voltage is required when hybrid MMC works at the boosted m, which is only provided by the FBSMs because the HBSMs cannot output negative voltage. Therefore, appropriate control strategies are needed to achieve the normal operation of hybrid MMC under boosted m. This paper presents a control method suitable for hybrid MMC with high m based on multi-harmonic voltage injection. The m is increased to 1.41 with the proposed multi-harmonic voltage injection. Compared with the HBSM-MMC, the proposed method reduces the total capacitance by more than 32%. Boosting m also reduces the RMS and peak value of the arm current by 18%. A comprehensive comparison was presented to demonstrate the cost and effect. Simulation and experiment verified the proposed method.