| In recent years,with the rapid development of modern power electronics technology,the demand for high-voltage large-capacity power conversion systems has been increasing.With its structural advantages,multilevel converters have broken through the capacity limitations of power devices and power modules,and are currently one of the ideal solutions for high-voltage large-capacity power conversion engineering applications.Due to its significant advantages over traditional multilevel converters,Modular Multilevel Converter(MMC)has aroused widespread concern in academic and industrial circles at home and abroad,and is the most attractive multilevel conversion topology in the current high-voltage large-capacity power conversion system.Therefore,this thesis has conducted the thorough careful research on four aspects of modular multi-level conversion system including multi-objective coordinated control technology,low harmonic output control technology,multilayer energy balance technology and low-frequency ripple control technology,and provides theoretical guidance and technical reference for the engineering application of MMC.This thesis is funded by the National Key Research and Development Plan-Key‘Technology and Equipment for Efficient Energy Transfer and Conversion'[YS2016YFGX050058],the National Natural Science Foundation Key Project of China ‘Research on High-power and High-efficiency Electro-acoustic Transduction Mechanism and Control Method' [51837005],and Hunan Provincial Innovation Foundation for Postgraduate ‘Control method of MMC with integrated split energy storage [CX2018B169],and also supported by the National Electric Power Conversion and Control Engineering Technology Research Center.It focuses on the basic theories of the modular multilevel power conversion system including multi-objective modeling and control,external output characteristics,internal energy balance,and low-frequency input ripple and the key technical problems under different engineering applications.The main innovations are giving as follows:(1)Aiming at the problems of mutual coupling and difficult coordination control among output current,circulating current,capacitor voltage of MMC,a circulating current optimization control method based on hierarchical prediction is proposed for MMC.Firstly,the continuous state equations of output current,circulating current,and capacitor voltage are deduced according to the basic circuit law,and the effect of the number of inserted submodule in arm put on control target of MMC is explored.The front stage control objectives of hierarchical model predictive control(MPC)includes output current,circulating current and capacitor voltage,while the back stage control objective of hierarchical MPC only contains the circulating current,which aims to achieve the optimized control of the circulating current with the ensured the multi-objective control of MMC.In addition,by analyzing the impact of changes in the number of inserted submodule per phase on the increasing prediction value of output current and circulating current,a simplified rolling optimization method and an optimized search space design method are simultaneously proposed to further reduce the hierarchical MPC calculation.(2)Aiming at the high power and high fidelity requirements of MMC used in marine power amplification applications,a modified modular multilevel switch-mode power amplifier topology and its high fidelity control method are proposed.Considering the characteristics of high-power output in MMC and the low-harmonic output in the high-frequency full-bridge converter and combining the advantages of these two different topologies,a modified modular multilevel switch-mode power amplifier is proposed to ensure the topology basis for the high-power and high-fidelity energy conversion.The mechanism of the influence of the operating characteristics of the different switching frequency of the modified topology on the output performance is explored.So a hybrid model predictive control method is proposed,which combines the multi-objective processing ability of the FCS-MPC method and the good tracking ability of the MMPC method,and ensures the control basis for the high-power and high-fidelity energy conversion.(3)Aiming at the problem of instantaneous energy absorption and feedback resulted from locomotive startup/braking in traction power supply system impacting on the power grid,an MMC with integrated split energy storage topology and its internal power coordinated control method are proposed.Firstly,the mathematical model including MMC and bidirectional energy storage converter is established.On this basis,the form of internal power flow of MMC under negative sequence compensation and energy storage operating is explored,and the calculation formula of reference circulating current of MMC under different operations.Furthermore,the mechanism of the imbalance of submodule capacitor voltage and energy storage SOC under different operations is analyzed.So a hierarchical balance control method is proposed for MMC with integrated split energy storage,which realizes the balance of submodule capacitor voltage and energy storage SOC within the same arm,among the different arms,and among the different phases of the MMC with integrated split energy storage.(4)Aiming at the problem that the submodule in MMC with integrated split energy storage suffers from the low-frequency ac current flowing into the energy storage unit,a low-frequency ripple current suppression method based on multistep MPC is proposed.Considering the influence of the switching action of the DC-DC converter and the full-bridge submodule on the current ripple that flows into the energy storage unit,a state-space equation including the inductor current,capacitor voltage,and output current is established,and a multi-objective evaluation function is constructed and it ensures the coordinated control among the different control objectives;Then,the impact of the dynamic response capability of the controlled variables on steady-state tracking error is analyzed,and a multi-step MPC method is proposed to reduce the steady-state tracking error of capacitor voltage.Then by converting the multi-step optimization problem into the least squares optimization problem,the computational complexity of the multi-step MPC method is reduced by the sphere encoder algorithm.The simulation results verify the rationality and effectiveness of the proposed method for low-frequency ac current suppression and computation reduction. |
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