Research On Mmc And MMC-SST Schemes Based On Sub-module High Frequency Link Interconnection

Submodule（SM）power fluctuation is an inherent problem in modular multilevel converter（MMC）and MMC-based solid state transformer（SST）,which usually requires a larger size capacitor to absorb.SM capacitors usually account for more than 50%of the total volume and more than 70%of the total weight of the MMC system,which significantly reduces the power density of the converter system.In order to reduce the size of the capacitor in the MMC sub-module and further increase the power density,this thesis researches the fluctuating power elimination scheme in the MMC and MMC-based SST system.First of all,the characteristics of SM fluctuating power in traditional MMC are analyzed,and a coupling scheme is proposed.The equivalent mathematical model of MMC is established,the generation mechanism of fluctuating power in MMC is analyzed,and the SM voltage fluctuation characteristics and capacitance parameter constraint,as well as the bridge arm circulating current characteristics and inductance parameter constraint under the influence of fluctuating power are further analyzed.According to the symmetrical characteristics of SM fluctuating power obtained from the analysis,two coupling schemes of vertical and horizontal fluctuating power are proposed,and it is concluded that the horizontal coupling between three phases is the better scheme.The fluctuating power can cancel each other by adding power channels between the horizontal SMs.Secondly,on the basis of the horizontal coupling scheme,a switched-capacitors MMC（SC-MMC）topology scheme based on SM high-frequency links（HFL）interconnection is proposed.The equivalent admittance model of the HFL is established,and the principle of the transmission,coupling of fluctuating power in the model as well as the automatic equalization characteristics of the SM capacitor voltage are analyzed.The characteristic of switched-capacitors is present between SMs under synchronous control.The impedance constraint of the HFL loop is analyzed,and the constraints of the new capacitor parameters and the new inductance parameter after fluctuating power coupling are analyzed.Based on a practical engineering case,the proposed topology and traditional MMC topology are compared and evaluated in terms of volume and cost,and their engineering application prospects are evaluated.A system simulation model is built,and the simulation results verify the correctness and effectiveness of the proposed switched-capacitors MMC topology scheme.And then,based on the SC-MMC,a quad active bridge（QAB）based triple-port MMC-SST（QM²C-SST）topology scheme is further proposed.A hybrid synchronous phase-shifting（HSPS）control strategy of QAB isolation stage is proposed.The primary side of QAB adopts synchronous control to realize the free flow and coupling of fluctuating power between MMC SMs,and the phase shift control between the primary side and the secondary side realizes the transfer of active power.The working principle of QAB,the principle of fluctuating power transmission and magnetic coupling,and the principle of triple-port power transmission are analyzed.The two triple-port SST systems of the traditional structure and the proposed structure are compared and analyzed with a practical engineering case,including the comparison of component parameters,volume and cost.A system simulation model is built,and the simulation results verify the correctness and effectiveness of the proposed triple-port QM²C-SST topology scheme.In the end,experimental verification and system modular design are carried out of the proposed topology scheme.The SC-MMC and QM²C-SST experimental platforms based on TMS320F28335 control are built to verify the effect of the proposed topology on the elimination of SM fluctuating power,and the functions of SM voltage automatic balance,2^nd-order frequency circulation suppression of bridge arm,multiple dynamic switching,etc.With a practical engineering case,the modular design of the triple-port QM²C-SST system is carried out,and the"cross"-shaped combined SM（C-SM）structure is proposed,and the actual engineering 3D model of the combined sub-module and the system is drawn.