Research On Operation Characteristics Of Series Connected Asymmetric Hybrid MMC In DC Transmission System

With the application of flexible DC transmission technology,modular multi-level converters are also widely used.Traditional concatenation MMC is suitable for high-power transmission scenarios,while series MMC is less expensive than a concatenation MMC at the same voltage level,so series MMC can be applied to small power branches in high-power long transmission lines.At present,the research on series connected asymmetric hybrid MMC mainly focuses on the operation characteristics under steady state control strategy and fault control strategy,while the operation characteristics under frequency domain and other control strategies are less studied.This paper mainly studies the operating characteristics of the capacitive voltage of the sub-module capacitor voltage of the series connected asymmetrical hybrid MMC and the characteristics of the DC side impedance in frequency domain.Firstly,the topology and control mode are introduced,on this basis,the internal harmonics are inferred,and the harmonic components of the capacitive voltage of the submodule are obtained,and the peak of the capacitive voltage of the submodule is reduced by using the method of triple frequency harmonic injection according to the harmonic composition,so as to reduce the capacitance of the submodule and further reduce the cost of the series MMC.In addition,in order to analyze the stability of series MMC,DC side impedance modeling is carried out in this paper.Using harmonic state space method,the time domain model of series MMC is converted into a frequency domain model,on which the impedance frequency characteristics are derived.And by further sorting out its frequency model,it is concluded that the influence factors of DC side impedance model are mainly AC impedance,bridge arm impedance and modulation signal,and the specific effect of these factors is analyzed by simulation software.Finally,this paper compares the control strategies and fault characteristics of the concatenum MMC and the series MMC.The difference between the two control strategies is mainly reflected in the suppression control of the ring flow,and in the event of failure,the series MMC has a stronger suppression effect on the fault current.By comparison,it is found that the series MMC is more suitable for small power transmission scenarios.