Research On Medium Voltage MMC Circulation Suppression And Capacitance Voltage Balance Method

In recent years,with development of long-distance and high-voltage power transmission technology,MMC have gradually become the mainstream topology of converters used in HVDC.The modular structure of submodules cascade gives MMC advantages such as low switching frequency,low loss,high energy conversion efficiency,low harmonic content,good output waveform and flexible expansion.This new topological structure is widely concerned not only in HVDC,but also in many medium-voltage occasions,such as speed regulating motor drive,unified power flow controller,DC melting ice,solid state transformer and medium-voltage DC distribution network.Compared with DC transmission applications,MMC has a lower voltage level and dozens of sub-modules.At present,its modulation,circulating current suppression and energy balance still mainly follow the methods of MMC in HVDC.In order to optimize the performance of MMC in medium-voltage applications,the main work in this paper is:Firstly,by analyzing the circuit structure and modulation strategy of MMC,the Fourier expressions of MMC arm voltage and output voltage under phase-shifted carrier are given.When circulating current suppression is considered,it is revealed that there are a series of even harmonics in the arm voltage reference signal,which will affect the harmonic of MMC output voltage.On this basis,the coupling between MMC circulating current suppression and phase-shifted carrier is clarified through mathematical analysis and verified by simulation.Secondly,in order to avoid the influence of circulating current suppression on the harmonic of output voltage,a decoupling modulation strategy of MMC is proposed to independently modalize the circulating current suppression signal.On this basis,delay time is introduced to save switching action as much as possible,reduce the additional switching times caused by the independent modulation of circulating current suppression,and obtain the maximum allowable delay time according to the ripple range of circulating current.The proposed decoupled modulation strategy is verified by simulation under four different delay time conditions.Thirdly,an improved energy balance control strategy is proposed in this paper.When the arm balance control is carried out on one phase,a certain fundamental frequency circulating current is injected into the other two phases so as not to affect the DC side current of MMC.A method of capacitor voltage balance of submodules is proposed to reduce the switching frequency,in which the input submodules and the bypass ones are ordered separately to avoid unnecessary switching.On this basis,the real-time voltage threshold is set to limit the submodule capacitor voltage within a certain range to avoid the divergent capacitor voltage value which cause adverse effect on the MMC system.The proposed energy balance control strategy is verified by simulation.Finally,based on the established 600V/10 k W three-phase MMC experimental prototype,the coupling between circulating current suppression and phase-shifted carrier modulation,and the proposed decoupling modulation strategy is verified.