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Abstract—Modern science and technology are developing rapidly, the power industry is also developing at a high speed, the traditional transformer is more and more difficult to content the requirement of the power system. Power electronic transformer (PET) is a kind of composite installation composed of power electronic converter and high frequency transformer. It can realize voltage conversion, electrical isolation and energy transmission functions of traditional transformer, and can also control voltage and current in power system and intelligent management. It is the best choice to realize voltage transformation function in modern power system. Compared with the traditional power electronic transformer, a power electronic transformer with lower device failure loss and better output waveform is proposed, the modular multilevel converter (MMC), which has a highly modular architecture. This essay revolves about the new type of MMC based power electronic transformer in the medium voltage distribution network, which adopts a three stage topology made up of the high voltage input stage, the intermediate isolation stage and the low voltage output stage. The input stage uses the MMC structure, the isolation stage is composed of several input series output parallel (ISOP) double active bridge DC / DC converters, and the output stage uses the traditional three-phase four leg inverter topology. This paper introduces the development process and the development direction of the power electronic transformer in recent years, analyzes and discusses the principle of three parts of power electronic transformer, and expounds the expected experimental results at the end of the paper.

Title: " Design and optimization of modular multilevel DC transformer" author: Guangyu Wang, Huiqing Wen

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