Research On Unidirectional Modular DC-DC Converter

With the rapid development of social economy,the AC distribution network is gradually unable to meet people's needs.The DC distribution network has been vigorously developed with its advantages of small power loss,strong power supply capability and high power quality.As a key topology of medium voltage direct current(MVDC),modular multilevel converter(MMC)has become a research hotspot of experts and scholars in various countries.When the MVDC distribution network is connected to those low-voltage DC(LVDC)loads such as industrial facilities,data centers and electrical vehicle charging stations,galvanic isolation is required for safety purpose;And since the energy flow is unidirectional,unidirectional DC-DC converters are the most cost-efficient candidates.Modular Multilevel Converter(MMC)has great application prospects in power distribution applications due to its high modularity,easy expansion,and easy access to energy storage components in modules.Therefore,this paper finally chose the unidirectional isolated DC/DC MMC as the research topology.This paper first introduces various types of converters suitable for medium voltage DC distribution networks,and conducts detailed research on modular multilevel converters,and analyzes its basic working principle,modulation strategy and voltage equalization strategy.Optimization was made to the pre-existing dc-dc converter,which consists of a modular multilevel inverter in the primary side of the high-frequency transformer and an uncontrolled rectifier in the secondary side.The high frequency design of transformer is realized by optimizing the modulation strategy of MMC side.At the same time,the optimal design of the modulation strategy is to control the sub-modules in groups,combined with the current-driven modulation strategy,thus achieving the soft-switching of all semiconductor devices.Moreover,a voltage-balancing strategy,which balances sub-module voltages by constructing auxiliary circuit within group and alternating driven signals between groups,is proposed,and voltage sensors is needless.Based on the analysis of the operational modes and the deduction of the constraints,key parameters of the converter is designed and the experiment results verify the correctness of the theoretical analysis and the feasibility of the design scheme.