Research On Key Technologies And Application Of Modular Multilevel Power Conversion

The rapid development of power electronic conversion technology makes the demand for high voltage and high capacity power conversion system increasing day by day.The multilevel converter breaks down the capacity constraints of the single power device and power module,making it one of the best engineering solutions in the field of high voltage and high power conversion applications.Modular Multilevel Converter(MMC)overcomes the shortcomings of the traditional multilevel converter,is the most promising high-voltage high-capacity power conversion topology.Research on the operating mechanism and control method of different topologies of modular multilevel power conversion in different application fields,is not only to ensure the safe and reliable operation of modular multilevel converter but also directly to enhance the efficiency and stability of the core power conversion system based on modular multilevel converter,for the realization of high efficiency,high reliability,high stability and high-precision power green conversion technology.This study has a very important theoretical research value and practical significance.This thesis is funded by National Key Research and Development Plan-Key technology and equipment for efficient energy transfer and conversion[YS2016YFGX050058]and Hunan Provincial Innovation Foundation for Postgraduate-Key technology research on modular multilevel railway power conditioner[CX2016B127],and also supported by National Energy Conversion and Control Engineering Technology Research Center.It focuses on the circulating current suppression,power fluctuation,energy balance,current control and power condition engineering application of modular multilevel power conversion.The main work and innovation are reflected in the following five aspects:(1)The general circulation model and suppression method of half-bridge MMC are proposed,which solves the problem of low damping characteristics,high currentstress and low running efficiency.The direct modulation method is used to establish the transient model involving the capacitor voltages and circulating current,and the weak damping characteristic of half-bridge MMC is discussed.The low-frequency oscillation phenomenon and its causes are analyzed,and the influencing factors of low-frequency oscillation frequency are deduced.The amplitude and phase of the double frequency circulating current are analyzed quantitatively,and its resonant mechanism is studied.On this basis,a general circulating current model including low-frequency oscillation circulating current and double frequency circulating current is established.The method of power feedforward active damping circulating current suppression is proposed and the selection principle of active damping parameters is given.The simulation and experimental results show that the proposed general circulating current model can accurately describe the low frequency and double frequency circulating currents within the half-bridge MMC.The integrated circulating current suppression method can effectively suppress the transient low-frequency oscillation circulating current and the steady double frequency circulating current,which greatly enhances the system stability and improves the power conversion efficiency.(2)The unified low-frequency control method for half-bridge MMC is proposed,which provides a new method for half-bridge MMC motor driving full frequency range operation.The phase impedance of the half-bridge MMC is changed by adding the resonant filter circuit.The phase impedance parallel resonance and series resonance phenomena introduced by the resonant filter circuit are analyzed.The resonant filter parameters are properly designed to achieve the double frequency circulating current at rated frequency operation and the lowest circulating current control voltage at low-frequency operation.The feasibility of controlling the inter-phase circulating currents to suppress the low-frequency power fluctuation is analyzed,and the multi-solution characteristics of high-frequency circulating current are found.Combined with the resonant characteristics of the resonant filter circuit,a single-frequency circulating current injection method and its PIR control method are proposed,and the parameters of the controller are designed.Simulation and experimental results show that the proposed low-frequency control method of half-bridge MMC can effectively increase the modulation ratio of common-mode voltage,and accurately offset the reactive power fluctuations,so as to suppress the low-frequency fluctuation of the module capacitor voltage and maintain its stability and ensure the reliability of MMC at low-frequency operation.(3)The stratified voltage balance control method of single-phase full-bridge MMC system is proposed,and the power balance operation mechanism of full-bridge MMC used as the AC/AC converter is analyzed.The topology of the single-phase full-bridge MMC is analyzed,and its integrated circuit model and AC-DC decoupling model are established.The energy balance state of the arm is analyzed,and the influences of reactive power compensation on the arm power balance are revealed.The feasibility of the power redistribution and the energy rebalancing among the arms by controlling the internal circulating current of the full-bridge MMC is analyzed.On this basis,a stratified control strategy for capacitor voltage balance is proposed,which can be divided into the system level,arms inter-group level,and arms intra-group level.The design principle of the arm inductance,the module capacitance and the stratified controller of the full-bridge MMC is clarified,which provides an important reference for the system stability analysis and design.Simulation and experimental results show that the proposed method can achieve the decoupling of capacitor voltage balance control with full-bridge MMC so that the stratified control parameters can be selected independently,which can simplify the design of the control system and improve the integrated stability of the whole system.(4)The direct power robust current control method of the three-phase full-bridge MMC system is proposed,which breaks through the limitation of the traditional control strategy to the dynamic response speed and tracking accuracy.The equivalent circuit model of three-phase full-bridge MMC and its three-phase three-wire input currents and three-phase four-wire circulating currents are introduced.According to the instantaneous reactive power theory,the discrete mathematical model of input currents,output current and circulating currents control in a??0 coordinate system of three-phase full-bridge MMC is established.The parameters self-adaptive robust deadbeat current control method is designed to inhibit the impact of parameter perturbation.On this basis,the energy balance mechanism of the arms of the three-phase full-bridge MMC is analyzed,and the direct power control method for arm energy balance is proposed.The circulating current instructions for average energy balance,horizontal energy balance and vertical energy balance in ??0 coordinate system are also deduced.Simulation and experimental results show that the proposed direct power robust current control method does not need the phase-locked loop,and there is no interactive coupling between the control variables,which can greatly improve the current control precision and enhances the robustness of the control system without deteriorating the reliability and dynamic response speed of the full-bridge MMC.(5)The internal differences between the half-bridge MMC and the full-bridge MMC are summarized,and the interrelationship between the modular multilevel power conversion technology is expounded.The power condition mechanism of three-phase half-bridge MMC and two-phase full-bridge MMC in V/V and SCOTT traction power supply system is studied,and the equivalent circuit and mathematical model are established.Quantitative analysis and comparison of the power switching voltage and current stress,power devices and capacitor requirements within the half bridge MMC and full bridge MMC are carried out.The power loss differences of three-phase half-bridge MMC and two-phase full-bridge MMC are analyzed by numerical and simulative analytical method,namely power loss curve fitting,Infineon power loss calculation and PSIM power loss simulation.The results show that the performance of the three-phase half-bridge MMC is better in the V/V traction power supply system,while the two-phase full-bridge MMC has obvious advantages in the SCOTT traction power supply system.The proposed numerical and simulation analysis method is a preliminary exploration of the functionality and applicability of the engineering application of modular multilevel power converter.To a certain extent,it has enriched and developed the engineering technology research content of high voltage and high power modular multilevel power conversionIn this thesis,the key scientific and technical problems of modular multilevel power conversion in high voltage and high power field are studied.The theory and method of modular multilevel power conversion topology and control are deeply studied from the circulating current suppression in DC transmission field and the low frequency power suppression in motor drive field of half-bridge MMC,to the capacitor voltage balance control in power flow control field and the robust current control in special power supply field of full-bridge MMC,as well as the power conversion engineering application comparison between half-bridge MMC and full-bridge MMC.The research scope covers the working mechanism,system model,control method,parameter design and engineering optimization of modular multilevel power conversion topology.which forms relatively comprehensive modular multilevel power conversion theory analysis methods and power conversion control technical solutions.The analysis and modeling methods,engineering design and optimization ideas,and power conversion control strategies in this thesis can provide some theoretical and engineering significance for the research on high-voltage large-capacity power conversion topology and control,design and control of power converter in dc distribution network and modular multilevel power converter stability and reliability.