Improved AAVM and Small-Signal Model of Hybrid MMC Considering Dynamic Differences Between Full-bridge and Half-bridge Sub-modules

Hybrid modular multilevel converter (MMC), which consists of half-bridge sub-module (HBSM) and full-bridge submodule (FBSM), is the general form of MMC. The conventional arm average value model (AAVM) of hybrid MMC assumes the dynamics of HBSMs and FBSMs are identical and does not consider capacitor voltage differences between FBSM and HBSM, so it cannot describe dynamics of hybrid MMC accurately. This paper proposes an improved AAVM of hybrid MMC, which includes equivalent circuit and arm modulation signal allocation algorithm: firstly, considering the dynamic difference between HBSM and FBSM, the equivalent circuit of improved AAVM was derived; secondly, an allocation algorithm which 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 electrical part of hybrid MMC in the d-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 d-q frame were derived. A modular smallsignal model of hybrid MMC in d-q frame was established. The correctness and accuracy of the proposed models is verified by simulation and experimental results.