Analysis Of Capacitor Voltage Fluctuation And Capacitance Design For Sub-Modules In Hybrid Modular Multilevel Converter

Modular multilevel converter(MMC)has been preferred in voltage source converter based high voltage DC(VSC-HVDC)transmission systems due to its advantages of high modularity,flexible scalability and excellent quality of waveforms.Hybrid MMC with halfbridge sub-modules(HBSMs)and full-bridge sub-modules(FBSMs)is considered as a promising topology to deal with DC fault due to its negative output from FBSMs.In addition,the application of negative output from FBSMs can be extended to steady state.With a boosted modulation index,AC voltage can be enhanced and remain unchanged under decreased DC voltage.However,the design of sub-module capacitance for such a hybrid MMC becomes different from that in traditional MMC since the impacts of FBSMs and HBSMs should be compromised.So this paper proposes a design method of sub-module capacitance for hybrid MMC.Firstly,the basic operation principles and basic control strategies of hybrid MMC are introduced.The equivalent mathematical model of hybrid MMC is established.In addition,the principle to choose the proportion of FBSMs in hybrid MMC is proposed in terms of high modulation index and DC fault riding through.Based on the mathematical model,the possible phase relationship between arm voltage and arm current is analyzed with power factor angle ranging from-0.5? to 0.5?.According to the zero-crossing points of arm voltage and arm current,a fundamental cycle can be divided into four periods.The charging and discharging process of capacitor in both kinds of sub-module is physically analyzed in detail within each period.Then the characteristics of capacitor voltage fluctuation in HBSMs and FBSMs are summarized and further portrayed by diagrams and formulas.Based on the characteristics,principles to obtain the amplitude of energy variation in both kinds of sub-modules are deduced with power factor angle ranging from-0.5? to 0.5?.Furthermore,the precise quantitative design method of the sub-module capacitance is brought out.Finally,simulations in PSCAD/EMTDC and experiments on a three-phase MMC prototype are conducted to verify the validity of the theoretical analysis and design.