Improved AAVM and Small-Signal Model of Hybrid MMC Considering Dynamic Differences Between Full-Bridge and Half-Bridge Submodules

The hybrid modular multilevel converter (MMC), which consists of half-bridge submodules ( HBSMs ) and full-bridge submodules (FBSMs), is the general form of MMC. The conventional arm average value model (AAVM) of hybrid MMC assumes that the dynamics of HBSMs and FBSMs are identical and does not consider capacitor voltage differences between FBSM and HBSM, so it cannot describe the dynamics of hybrid MMC accurately. This article proposes an improved AAVM of a hybrid MMC, which includes an equivalent circuit and an arm modulation signal allocation algorithm: first, considering the dynamic difference between HBSM and FBSM, the equivalent circuit of improved AAVM was derived; second, an allocation algorithm that describes the nonlinear distribution of arm modulation signal between HBSM and FBSM was proposed. Considering the influence of high-order harmonics, the differential equation of the electrical part of hybrid MMC in the
d
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q
frame was derived. Based on the nonlinear allocation algorithm of reference voltage and the piecewise solution method, the expressions of HBSM and FBSM equivalent reference voltages in the
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q
frame were derived. A modular small-signal model of a hybrid MMC in the
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q
frame was established. The correctness and accuracy of the proposed models are verified by simulation and experimental results.