Research On The Topology And Control Optimization Of Interlinked Modular Multilevel Solid-state Transformer

Solid state transformer(SST)integrates power electronic switches and high-frequency magnetic conversion technology,with multiple power ports and intelligent control functions.Modular multilevel converter(MMC)is suitable for medium and high voltage distribution systems with an inherent medium voltage DC(MVDC)port and excellent harmonic and redundancy characteristics.In recent years,the interlinked MMC-SST has become a research hotspot due to the high topology and control flexibilities and excellent MVDC fault ride-through capability.This paper mainly focuses on the defects of interlinked MMC-SST in circuit topology,distribution parameters design of isolation stage and control strategy.By combining theoretical analysis and experimental verification,the topology and control optimization of interlinked MMC-SST is studied.The specific research contents are as follows:(1)For the research on the low-frequency ripples of interlinked MMC-SST sub-module(SM)and its influence and suppression strategy.The formation mechanisms of SM low-frequency ripple voltages and the arm double-frequency circulating currents and the main reason for the large capacitance volume in SM are analyzed.The distribution characteristic of SM low-frequency ripple currents between three phases is explored.The equivalent admittance model of high-frequency interconnecting channels is studied.The transmission of SM low-frequency ripple power to the isolation stage is studied.The common high-frequency bus interlinked MMC-SST topology is proposed.The SM low-frequency ripple currents coupling and the arm double-frequency circulating currents elimination are realized by high-frequency interconnection.The synchronous modulation principle of isolation stage full-bridge circuits is analyzed.The self-clamping characteristics of SM capacitor voltage and low-voltage DC capacitor voltage are studied.The circuit characteristics,key component selection and volume evaluation of conventional interlinked MMC-SST are compared and analyzed to protrude the operating performance and power density advantages of common high-frequency bus interlinked MMC-SST.(2)For the research on the coupling relationship between the operating performances of the high-frequency interconnecting channel and the distribution parameters and the optimization of the distribution parameters of the interconnecting channel in the common high-frequency bus interlinked MMC-SST scheme.The equivalent distribution parameter mathematical model of the synchronous full-bridge simplified circuit is established,the relationship between the active transfer capability and the ratio of the leakage inductance and the distribution resistance is investigated,and the influences of distribution resistance on voltage clamp error and dynamic inrush current are analyzed.The design objective for the distribution parameters of the interconnecting channel could be obtained.The main influence factors of distribution parameters in the design process of synchronous transformers under two types of insulation conditions are analyzed,the distribution parameter analytical models of synchronous transformers are established.Based on the influences of distribution parameters on circuit performances,an optimal design scheme for the distribution parameters of synchronous transformers is proposed.The steady-state performance of the high-frequency interconnected channel could be improved.(3)For the research on the changes in dynamic tracking performance and anti-interference performance due to the reduction of SM capacitance in the common high-frequency bus interlinked MMC-SST scheme.According to the typical control strategies,the double-closed-loop classical controller is designed,and the tracking performance and anti-interference performance difference before and after capacitor lightweight are studied.It is revealed that the tracking performance difference can be fully compensated by the regulator parameters,while the anti-interference is weakened with the reduction of capacitance.Then the dynamic averaging model of the MMC-SST is developed and a hybrid controller based on the discrete prediction model is designed.The proposed hybrid controller can significantly improve the dynamic response speed compared with the conventional controller,thus the anti-interference performance of the system under small capacitance conditions is optimized.Besides,the low-inertia safe operation of the lightweight MMC-SST is also evaluated through three typical scenarios:hybrid AC/DC distribution system,MVDC power transmission system,and photovoltaic storage joint power supply system.The results indicate that the normal operation of the MMC-SST is not affected by low inertia in the power routing mode,while the inertia could be compensated synergistically by the energy storage devices of SST in the power supply mode,which could improve the safe and stable operation performance under low inertia conditions.Finally,the correctness and validity of the proposed common high-frequency bus interlinked MMC-SST topology structure,the distribution parameters optimization process of the high-frequency interconnecting channel,and the dynamic performance improvement scheme of the lightweight system are verified by the established simulation and low-power experiment platform.