Research On Circuit Design And Control Method Of A Modular Multilevel Converter With Integrated Energy Storage

A novel modular multilevel converter with integrated energy storage is studied in this paper,which is composed of half bridge submodules and quasi-full bridge submodules.The detailed operating principle and mathematical model in normal mode and DC side short circuit mode are analyzed and derived.The design methods of key parameters are given,such as submodule voltage level and quantity ratio of half bridge submodule and quasi-full bridge submodule,the voltage ratio of battery and quasi-full bridge submodule capacitor.The current fluctuation characteristics of quasi-full bridge submodule energy storage inductor is analyzed,and the design method of the inductor is given.Capacitor voltage fluctuation characteristics of the two submodules are analyzed and compared.The design method of capacitance of the two submodules is given.The strategy of control is proposed.By AC side current control,battery current control,DC side current control,submodule capacitor voltage balance control and circulating current control,the power control of AC side,DC side and battery side can be realized in normal mode.The maximum power absorbed or emitted by the battery is 10% of the AC side rated power.The strategy of active DC fault ride through is proposed.When the DC side short circuit occurs,it can be smoothly switched from normal mode to DC side short circuit mode,during which the maximum power absorbed or emitted by the battery is 35% of the AC side rated power.The optimization method of submodule capacitance is proposed.The capacitance requirement of half bridge submodule and quasi-full bridge submodule can be reduced to 59% and 47% of conventional method respectively.Meanwhile,compared with conventional method,transistor current stress remains unchanged,and the conduction loss of the converter is further reduced.An experimental prototype including three-phase,24 sub modules is built.The design scheme and parameters of the experimental prototype are introduced.The experimental results verify the feasibility of the circuit design and control method.