| High-voltage and high-power DC/DC converters are widely used in high-voltage DC(HVDC)transmission and medium-voltage DC(MVDC)distribution networks because of their important roles in power grid voltage matching,power flow regulation and fault isolation.Due to its high modularity characteristic and extensible feature,modular multilevel converter(MMC)is often used in topology research of high-voltage and high-power DC/DC converter.Taking this kind of converter as an object,this paper proposes a LLC modular multilevel converter(LLC-MMC)topology,and gives deeply analysis and research on the working principle,circuit characteristics,parameter design and circuit advantages of this converter.This paper analyzes the clamping characteristics of half bridge submodule(SM)in MMC,and deduces a series of high-voltage and high-power DC/DC converter topologies according to the middle and low-voltage DC/DC converter topologies,and conducts detailed analysis and research on the proposed LLC-MMC.The specific research work is as follows:(1)The DC and AC equivalent circuits of LLC-MMC are analyzed,and the simplified equivalent circuit of LLC-MMC is deduced.Then the circuit characteristics of LLC-MMC are analyzed through fundamental harmonics approximation(FHA).According to the circuit characteristics and control requirements of LLC-MMC,this paper gives the design method of circuit parameters.According to the control method of LLC-MMC,every mode of LLC-MMC in the working process and the process of realizing the whole ZVS are introduced in detail.Aiming at the problem that the capacitor voltage may be unbalanced after the half-bridge submodule is introduced,a reasonable and effective capacitor voltage balance control strategy is designed.(2)The working principle of DAB-MMC is introduced,and based on the equivalent circuit of DAB-MMC,the steady state working waveform of the circuit is given.Then the expressions of voltage and current in steady state operation are deduced,and the expressions of transmitted power is given.By analyzing the conditions of DAB-MMC to realize the whole ZVS,this paper deduces the power condition expression,and based on the control method and circuit characteristics of DAB-MMC,the reason why DAB-MMC cannot work normally under light load condition is explained.On this basis,the principle that LLC-MMC can operate normally under light load is analyzed based on the circuit characteristics of LLC-MMC,and the advantages of LLC-MMC compared with DAB-MMC in light load is determined.(3)The circuit principle,the whole soft switching process of LLC-MMC and capacitor voltage balance strategy mentioned in the paper are simulated and verified.In view of the light load advantage of LLC-MMC over DAB-MMC,the loss simulation of the circuits of DABMMC and LLC-MMC is given and compared.Finally,the structure design and system control ideas of LLC-MMC experimental platform are introduced,and the circuit principle and the advantages of light load of LLC-MMC are verified with abundant experimental results. |
PDF Full Text Request