Research On Common High Frequency AC Bus Energy Routing Topology On MMC And Its Three-phase Imbalance Control

With the increase in grid-connected capacity of distributed new energy sources and the development of the energy internet,the demand for power conversion systems with multiple types of ports and multi-directional energy flow capability in the grid has increased,and the research related to MMC-based energy routing suitable for high-power applications has attracted much attention.However,the current MMC-based energy routing has many problems,such as many energy conversion links,large system size and low power density.In this paper,research is conducted to improve the power density of the MMC-based energy routing and realize the light weight of the system.Firstly,the structure of high frequency AC bus is introduced,the advantages of the high frequency AC bus in the number of power conversion links are analysed.By comparing the connection methods between MMC and high frequency AC bus,a common high frequency AC bus energy routing based on MMC structure(ACMMC-ER)is proposed which reduces the number of power conversion links and the size of the system.The synchronous modulation principle,equivalent parallel characteristics,the operating mode of each port,and the generate mechanism and elimination method of fluctuations in the ACMMC-ER system are further analysed.Next,the key system parameters such as sub-module capacitance,bridge arm inductance and low-voltage DC-side capacitance and the control scheme for the whole system are designed based on the operation mode of ACMMC-ER and the operating characteristics of eliminating symmetry fluctuations.A comparative analysis of the control scheme and the volume and cost is carried out to evaluate the proposed scheme against two typical MMC-based power routers,and results are obtained that the scheme has significant advantages in terms of control and volume,further validating the contribution of the proposed scheme to the improvement of power density and control simplification of the energy routing.A simulation model based on the PLECS platform is built to verify the rationality of the parameter design and the correctness of the control scheme design.Then,the system operation under the condition of grid-side voltage imbalance is analysed,the mathematical model of the ACMMC-ER system with grid-side voltage imbalance and no low-voltage DC side access is established,and the fluctuating current of the sub-module capacitor in the MMC is analysed.The analysis focuses on the fluctuating current characteristics and coupling characteristics of the MMC sub-module capacitors under imbalance fault conditions using conventional control methods to suppress negative sequence currents on the AC side.Two solutions are proposed to improve the quality of power conversion under imbalance condition,the control parameter optimisation solution and the capacitor optimisation solution.The proposed schemes are also validated by building simulations in the PLECS platform.Finally,a 4 k W experimental prototype was built to verify the correlation analysis of synchronous switch modulation,the equivalent parallel characteristics and fluctuation elimination characteristics of the ACMMC-ER topology under the grid-side voltage balance and imbalabnce conditions,and verify the operation of each port and the optimization scheme of high-quanlity power conversion under unbalanced working conditions.The experimental results demonstrate the proposed energy routing structure to reduce power conversion links and reduce MMC capacitance has significant advantages,and the proposed capacitance parameter optimisation scheme and control optimisation scheme can ensure the high quality power conversion under imbalance operating conditions.