Research On Self-Blocking And Balancing Sub-module Of Modular Multilevel Converter

Modular multilevel converter has many advantages in the field of high voltage direct current transmission,such as high output waveform quality,high voltage grade and low voltage withstand value of devices.As an important part of modular multilevel converter,sub-module plays an important role in the whole system operation.There are two problems in modular multilevel converter: DC side short circuit fault and sub-module capacitor voltage balance.Aiming at the above two problems,this paper studies the topological structure and control method of sub-module.Firstly,the modular multilevel converter is studied in this paper and the main advantages of modular multilevel converter.And the problems of DC fault blocking and capacitor voltage balancing are described.This paper introduces the development status of MMC,analyzes the existing topological structure of sub-module,and introduces its basic structure and characteristics.The common capacitor voltage equalization control strategies are summarized.Then,a new sub-module topology with voltage equalization capability and DC fault blocking capability is proposed in this paper by combining the idea of full-bridge sub-module and switching capacitor.The topological structure,basic working principle and control strategy of the sub-module are deeply studied,and the two self-balance functions and characteristics of the sub-module are compared.The characteristics of blocking fault suppression and non-blocking fault suppression of the sub-module are analyzed.The blocking and non-blocking fault suppression have certain limiting effect on DC side fault current.This paper introduces the design idea of the self-blocking and balancing composite sub-module controller,the basic principles of the nearest level approximation modulation and sorting algorithm.Based on the self-balance function characteristics of self-blocking and balancing composite sub-module combined with the sorting algorithm to design the corresponding voltage equalizing control method.In PSCAD,a simulation model of the two-terminal power transmission system based on the self-blocking and balancing composite sub-module was built,and the capacitor voltage balance ability,blocking and non-blocking fault suppression function and control strategy in the sub-module were verified.Finally,an experimental prototype is built based on DSP and FPGA to verify the performance of the self-blocking and balancing composite sub-module under two working conditions: resistive load and AC grid-connected.