MMC-Based Soild State Transformer Compact Design And Control Technology Research

Under the goal of "carbon peaking and carbon neutrality",the power distribution system has two major features:(1)extensive access to new energy sources mainly distributed photovoltaic;(2)transformation of energy electrification,and the construction of massive charging piles and 5G base stations to further increase the power load density.The existing distribution grid adopts the radial architecture of "closed-loop design and open-loop operation" and lacks flexible power transfer channels between distribution lines.Therefore,the wide access of fluctuating new energy and high-power diversified loads in different geographic locations will lead to a series of problems such as tidal blockage in distribution lines,node voltage crossing limits,and insufficient system stability margin.The AC-DC hybrid distribution network can realize flexible interconnection between distribution networks and flexible inter-network tidal regulation,and can provide DC ports at the same time to realize direct grid connection of DC loads and distributed energy sources,which is one of the solutions to the above problems.The modular multilevel solid-state transformer(iMMC-SST),which is composed of highpower power electronics and high-frequency transformers,has high-frequency electrical isolation,multiple AC-DC voltage levels,flexible inter-port power control,reactive power compensation and other functions,and is the core networking equipment for ACDC hybrid distribution networks.However,the ripple power generated by multi-port energy cross-coupling needs to be borne by each sub-module capacitor in iMMC-SST,which makes the cost volume of sub-module capacitors remain high and thus leads to low power density of the device,which affects its further promotion and application in distribution networks.Based on the strong controllability of each power module in iMMC-SST,this paper actively regulates the flow path of ripple power inside the device through the port impedance design of cascaded modules,so as to significantly reduce the ripple power flowing into the capacitors of sub-modules and realize the optimal selection of capacitors and compact design of the device,the main research contents are as follows:Firstly,the topology of iMMC-SST is described,its basic working principle is analyzed,the basic operation control strategy of iMMC-SST is designed by constructing a small-signal mathematical model,and the correctness of the built topology and the designed control strategy is verified by simulation.Next,the iMMC-SST ripple power generation mechanism is analyzed,the influence of converter impedance on the ripple power flow path is studied,the input impedance model of iMMC-SST post-stage module is established,and the influence of the iMMCSST main circuit and control circuit parameter adjustment to realize the module input impedance design on the ripple power flow path and device operation performance is analyzed,and the correctness of the mechanism analysis is verified by simulation.Then proposing an iMMC-SST ripple power transfer scheme based on parallel virtual impedance to reduce the ripple power to be borne by the module capacitor and to achieve capacitor selection optimization.The method reduces the equivalent impedance of the rear DC-DC circuit by parallel virtual impedance,transfers the ripple power to the rear DC-DC circuit,and self-balances on the low-voltage DC side to achieve the purpose of reducing the ripple power of the sub-module capacitor and optimizing the capacitor capacitance.By building a simulation model,the theoretical and actual values of submodule capacitor voltage ripple rejection rate are compared,and the feasibility of the proposed method is verified by simulation.Finally,considering that the parallel virtual impedance method will trigger the interport disturbance transfer problem after the iMMC-SST capacitance optimization,a second-order band-pass filtered parallel virtual impedance design scheme is proposed to achieve low-frequency ripple power transfer while actively suppressing the disturbance transfer between low-and medium-voltage ports,so as to improve the transient operation performance of the iMMC-SST under the compact design with small capacitance,and the feasibility and effectiveness of this impedance optimization design scheme are verified by simulation.