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High-voltage power electronic devices are the core components of new energy power generation and the Smart Grid. However, due to the voltage and power capacity limits of power electronic devices, the research work of medium-voltage and high-voltage converter topology arouses many institutes’attention in recent years. Modular multi-level converter (MMC) presents itself a promising circuit, adopting structure of cascade, which avoids the voltage and power capacity limits of power electronic devices.The main task of this paper is to design hardware and software of Sub-module in MMC. According to MMC work principles, like the bridge arm current waveforms, selecting the switching devices. Then calculates DC-side capacitance value of sub-module, analyses voltage ripples of DC-side capacitor and designs the structure of capacitor group,which is verified by simulation and experment.According to the features of MMC, this paper designs and develops the control circuit of sub-module, including the control circuit and the software architecture. Additionally, as mentioned in the literature, the two different constructions of sub-modules are compared. Finally, the construction plan of sub-module adopted by this paper is verified by experiments.Then by applying modulation of "selection mechanism" and with the assumption that all the sub-modules share the same loss when the converter works in stable., the paper gives out the loss calculation method and presents the loss distribution in sub-module, including the DC side capacitance.What is more, the paper compares the loss distribution in sub-module under different power factors.Finally, from the perspective of heat transfer, the paper analyzes the thermal resistance model of heat sink, and designs the heat sink of sub-module, based on the loss distribution of 500kVA MMC with three phases, which is compared with the simulation results of Ansys. At last, the design of heat sink is verified by experiment.
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