| In recent years,modular multilevel converters(MMCs)have received more and more attention in the field of medium or high voltage transmission or distribution and medium or large power inverter drives because of their unique structure and significant advantages.This thesis first analyzes the basic operating principle of MMC,establishes its mathematical model,and then studies and improves the existing MMC carrier phase shift modulation strategy,circulating current suppression strategy and low frequency operation control strategy.The MMC simulation in MATLAB/Simulink is built to verify the proposed modulation and control strategy.Finally,an MMC experimental prototype is designed to verify the improved modulation and control strategy.The following work is carried out:Firstly,the basic working principle of MMC is given by analyzing the different working modes of the half-bridge sub-module(SM),and the specific relationship between the putting in and cutting-off of the SM and output voltage of the sub-module is derived,and the average mathematical model of the MMC is established.The Kirchhoff voltage-current law is used to derive the correlation between voltage and current in the model,which lays a foundation for the research of MMC control strategy.Secondly,the thesis introduces the modulation strategies commonly used in MMC,and focuses on the application of carrier phase-shift modulation in MMC.First of all,the triangular wave carrier phase shift modulation is introduced,and its output voltage harmonic characteristics are quantitatively analyzed by double Fourier transform.On this basis,this paper proposes a hybrid sawtooth carrier phase-shift modulation strategy with output voltage of N+1 level(Nis the number of sub-modules).Compared with the N+1 level triangular wave based carrier phase shift modulation,the hybrid sawtooth carrier phase shift modulation can effectively reduce the harmonic content of the output voltage and current without increasing the complexity of the bridge arm circulation.Thirdly,the mathematical mechanism is used to analyze the generation mechanism of MMC circulating harmonics,and the second harmonic quasi-PR controller commonly used to suppress circulating harmonics is introduced.After that,combined with the practical application of variable frequency driving(VFD),a segmented quasi-PR controller is proposed to cope with the problem of circulating harmonic frequency variation when the MMC operating frequency changes dramatically and a detailed control parameter design method through the analysis of the controller transfer function is given.Fourthly,the thesis analyzes the problem that the amplitude of the sub-module capacitor voltage fluctuation is large when the MMC is running at low frequency,and summarizes and compares the existing high-frequency common-mode voltage and circulating current injection method for suppressing the capacitor voltage fluctuation of the sub-module.Based on this,an improved three-harmonichybrid injection method is proposed to further reduce the amplitude of the bridge arm current.In addition,for the problems of open-loop injection method of high-frequency circulating current,a closed-loop injection method based on quasi-PR controller is used to perform high-frequency circulating injection to reduce the error of circulating current injection.In addition,MMC is used in the three-phase permanent magnet synchronous motor(PMSM)variable frequency speed control system in this paper,and the PMSM sub-band operation control strategy combined with the high frequency injection method to realize the segmentation injection of high frequency components of MMC under different operating frequencies is proposed.Therefore,it is possible to effectively control the fluctuation of the sub-module capacitor voltage in the entire frequency band.Fifthly,a three-phase MMC simulation platform based on MATLAB/Simlink,and a three-phase MMC drived three-phase permanent magnet synchronous motor experimental platform are built to verify the theoretical analysis.Through the simulation and experiment,the correctness and the effectness of modulation strategy and control strategy are verified.Finally,the conclusion of this thesis is given and the future research is proposed as well. |
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