Research On The Modeling And Control Of The Modular Multilevel Converter (MMC)-Based DC-DC Solid-State Transformer

Large-scale development and utilization of renewable and clean energy is a key to achieving peak carbon emissions and carbon neutrality in China.With DC distribution technology,the power conversion stages in the grid integration of the distributed renewable generations,the energy storage systems,and the DC loads can be reduced.Besides,DC distribution presents remarkable advantages,e.g.,high efficiency,high power quality,and low cost of power cables,and has broad application prospects.A DC solid-state transformer(SST)which can realize the interconnection of different DC grids is a key component in DC distribution systems.DC SST based on modular technology has the capability to handle high voltage ratings in distribution systems,and is easy to realize standardized mass production and redundancy.In view of the development trend of diverse voltage levels and multiple ports in the DC distribution system,the modular structure has good scalability,which is conducive to reducing the design cycle and design cost of the SST.Among various DC SSTs with modular technology,the DC SST based on the modular multilevel converter(MMC)presents many advantages.The lumped ac stage simplifies the insulation design of the ac transformers in SSTs.The independent structure of the submodules enables easy redundancy strategy,and can easily construct multiple DC ports,which is conducive to the access of distributed energy and energy storage devices and realizes multi-port operation.In case of a DC short-circuit in the interconnected DC grid,the arm inductors can suppress the slew rate of the DC fault current,which bridges time for fault detection and protection strategies.Therefore,MMC-based SST is a suitable option for the interconnection of DC grids.In order to meet the requirements of the interconnection of DC grids,the operation analysis and control of the MMC-based DC SST are required.Due to the MMC conversion stage,the MMC-based SST includes various electrical variables with complex ac and DC components which are tightly coupled with each other,and this brings challenges to the modeling and control.There have been numerous efforts on the modeling,and control of MMCs,however,the previous works are mainly focused on MMCs for line-frequency applications.Different from line-frequency applications,the MMC in the MMC-based SST operates at a significantly higher fundamental frequency.Hence,its modulation and voltagebalancing strategy are significantly different.A commonly used modulation method is that the switching frequency of the power semiconductors is equal to the fundamental frequency(hereinafter referred to as fundamental-frequency modulation).With fundamentalfrequency modulation,the control system only performs the sampling and control once in a medium-or high-frequency period(hereinafter referred to as cycle-by-cycle control),and this results in differences according to its operating characteristics,mathematical modeling methods and control methods compared to line-frequency applications.In the existing research,the mathematical model for the MMC-based DC SST mainly focuses on the steady-state model,and the operation and control mainly focus on the system parameter design,soft-switching analysis,voltage-balancing strategy,active power and arm-energy balancing control,etc.,but presents deficiencies in research related to the cycle-by-cycle control.In response to the above issues,this thesis proposes a mathematical modeling and cycle-by-cycle control method for the MMC-based DC SST,which is suitable for mediumto high-frequency operation.The research contents are as follows:(1)A mathematical model is proposed,which is suitable for the analysis and cycleby-cycle control of the medium-to high-frequency operation of the MMC-based SSTThe characteristics of different harmonic components in main variables and power transfer of the MMC-based DC SST under different forms of the fundamental-frequency modulation are analyzed.According to the characteristics of the cycle-by-cycle control under fundamental-frequency modulation,the coefficients of the DC component,the sine and cosine components at the fundamental frequency of the Fourier expansions of variables are utilized as the state variables,and the state-space model,as well as its linearized smallsignal model of the system,are established.The model describes detailed steady-state and dynamic characteristics of the main variables of the system,and the dominant components of the variables are "averaged" in units of one fundamental period.The proposed model can be used as the basis for the analysis and control system design of the MMC-based DC SST with medium-to high-frequency operation.The accuracy of the proposed mathematical model is verified by both simulation and experimental results.(2)A multi-variable control system with cycle-by-cycle control is proposed for the MMC-based SST,which can fulfill multiple control requirements of DC distribution systemsBased on the proposed mathematical model,a cycle-by-cycle control system of the MMC-based SST is designed.According to the application requirements of the DC distribution systems,the terminal characteristics,the internal state,the long-term operation optimization,and fault-protection control are regarded as the control objectives.The dynamic responses between the main variables of the system and multiple control variables are analyzed,and the controller design method is presented.The proposed control system can effectively realize the control of terminal voltages and currents,capacitor voltages of the submodules,arm energy of the MMC as well as the reactive power,and is capable of submodule fault-tolerant operation and DC short-circuit fault ride-through.The simulation and experimental results verify that the proposed control system can fulfill multiple control requirements of the MMC-based DC SST in grid connections with good performance,which is beneficial to the improvement of stability and reliability of the system.(3)An operation and control scheme to expand the number of ports of the MMCbased DC SST is proposedThe port number of the two-port MMC-based DC SST is expanded to three,and a three-port operation scheme and its cycle-by-cycle control scheme are proposed.A dualtransformer-based topology is used as the ac-link topology.The primary-side windings of the two transformers are connected in series and the secondary-side windings are connected to independent conversion stages,which results in a series power transmission loop,whereby reduces the coupling degree between the powers of three ports and is conducive to the independent control of the three ports.The corresponding power regulation method and the DC short-circuit fault ride-through strategy are proposed.The simulation and experimental results verify that the proposed port expansion and control scheme can effectively realize the independent power regulation of three ports and enhance the DC short-circuit fault ride-through capability of each port,which is beneficial to the improvement of the stability and reliability of the multi-port DC system.