| In recent years,the application of Modular Multilevel Converter(MMC)in the field of electric drive has attracted more attention.This paper takes MMC as the research object,and studies the fluctuation of the capacitor voltage of the sub-module under low-frequency operating conditions and its solution when it is applied in the field of electric drive.The main contents are as follows:The working principle of the MMC topology is analyzed in detail.The influence of the switching state of the sub-module on the module capacitor voltage is analyzed in depth.The overall control strategy and pre-charging process are introduced.The modulation strategies of several multilevel converters are studied,and the carrier phase shift is selected as the modulation method of MMC.For MMC,the choice of phase shift angle has a great influence on the output voltage performance.Different carrier phase shift modulation methods corresponding to different phase shift angles are studied,and the output level and output voltage harmonic performance of different carrier phase shifting modes are compared.Considering that the number of sub-modules is large,the difference of the output voltage levels of different modulation methods is small and No.1 carrier modulation method is most suitable.The problem of capacitor voltage fluctuation in the field of electric drive is studied,and the reason of voltage fluctuation is theoretically analyzed.When the running frequency is low,the period is long,and the energy of the upper and lower arms is in a one-way transmission state,which causes the modular multi-level converter to fluctuate over the safe range of the capacitor when the low-frequency operation is performed.The fluctuation formula of the sub-module capacitor voltage is derived.The analysis concludes that the operating frequency is inversely proportional to the sub-module capacitor voltage fluctuation.The principle of high-frequency injection method to eliminate the fluctuation of capacitor voltage is studied.The suppression effects of double-sine wave injection method,two-wave method,double-third harmonic injection method and square wave plus third harmonic injection method are studied respectively.And compared several high frequency injection methods.The experimental platform of three-phase MMC was built,and the research content of this paper was verified on the platform.The results show that the high frequency injection control strategy suppresses the capacitor voltage fluctuation of the MMC.Finally,the drive of the asynchronous motor is realized by the MMC,which verifies the correctness and feasibility of the control strategy. |
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