Research On Key Technology Of Multiport Converter For Smart Grid

In recent years,to accelerate the energy transformation and better serve the “Double carbon” strategic objectives,the installed renewable energies are increasing year by year,and the new DC loads are also emerging rapidly,which bring great challenges to conventional AC distribution.Therefore,the AC/DC hybrid distribution has attracted extensive attentions.As the key devices in smart grid,multiport converters have also attracted much attentions,which are responsible for connecting a variety of feeders with different voltage forms and voltage levels,and realizing the flexible control of feeder voltages.However,due to the limitation of withstand voltage level of switching devices,multiport converters widely adopt multiple submodules with cascade connection,resulting in complex structure,large number of submodules,large size and poor economic performance,which limit its wide applications.Therefore,how to improve the economy,flexibility and power density of multiport converters is the key problem for the development of smart grid.According to different application scenarios,multiport converters can be divided into multiport power electronic transformer(PET),multiport soft open point(SOP)and multi-DC-port converter.This thesis will research the key problems of multiport converters from the perspective of topology,control strategy and performance comparison.Moreover,this thesis will indicate the solutions for the improvement of power density and economy of multiport converters,which include simplifying the topology of converter and reducing the number of submodules.Multiport PET is responsible for connecting medium-voltage AC(MVAC),mediumvoltage DC(MVDC),low-voltage AC(LVAC)and low-voltage DC(LVDC)systems.However,the available multiport PETs are with multiple power conversion processes,submodules and MFTs.Therefore,this thesis will study the simplified method of multiport PET topology,and propose a centralized resonant multiport PET(CR-MPET)topology in Chapter II.With the application of multi-frequency modulation strategy,the MMC in CR-MPET can output MVDC and medium-frequency medium-voltage(MFMV)voltages,simultaneously.CR-MPET uses the centralized resonant circuit and MFT to replace the input series output parallel(ISOP)DCDC converter in conventional PET topology,which effectively reduces the power conversion processes,the number of submodules and MFTs of PET.The control strategy and parameter design of CR-MPET are introduced in Chapter II,and the feasibility of CR-MPET is verified by simulation and experimental results.However,the coupling relationship between resonant components and arm inductors in CR-MPET limits the design of resonant parameters.Therefore,a distributed resonant multiport PET(DR-MPET)topology is proposed in Chapter III,where the MMC only outputs MVDC voltage.A set of resonant circuit and MFT are connected in parallel with each MMC arm,replacing the ISOP DC-DC converter in conventional PET topology,which also reduces the power conversion processes,the number of submodules and MFTs of PET.Moreover,DRMPET has higher freedom in the design of resonant parameters than CR-MPET.On this basis,the design method of resonant parameters in DR-MPET is introduced in Chapter III,and the effectiveness of DR-MPET is verified through simulation and experimental results.Finally,the DR-MPET is compared with available multiport PETs,and the comparison results show that DR-MPET has the advantages in fewer submodules and power conversion stages.Multiport SOP is responsible for connecting multiple AC feeders in distribution network and providing LVDC port for energy storages and DC loads.However,the existing multiport SOPs have the problems of large number of submodules and mutual coupling of port voltages.Therefore,a multiport flexible close loop converter(MP-FCLC)topology is proposed in Chapter IV.MP-FCLC realizes the electrical interconnection of two CHB converters by connecting a set of resonant circuit and MFT in parallel with each bridge of CHB converters,and the LVDC port is expanded by connecting few full-bridges with MFT.MP-FCLC uses only few resonant circuits and MFTs replacing the large number of cascaded DC-DC converters between two CHB converters in the conventional SOP topology,which significantly reduces the number of submodules.In Chapter IV,the working principle,control strategy,parameter design method and performance comparison of MP-FCLC are studied and the effectiveness of MP-FCLC is verified by simulation and experimental results.Multi-DC-port converter aims to integrate AC-DC and multiple DC-DC converters into one single converter in distribution network,which can simplify the power conversion process among multiple ports.In view of the shortcomings of existing multi-DC-port converters,such as many power conversion stages,limited port expansion and voltage level configuration,and low reliability,this thesis studies the DC port expansion of MMC in Chapter V,and proposes a multiport arm-shared MMC(MAS-MMC)topology.MAS-MMC realizes the interconnection of MVAC port and multiple MVDC ports with different voltage levels by sharing the common lower arm with multiple upper arms,which significantly reduces the number of submodules.In Chapter V,the working principle,control strategy,parameter design and fault ride through method of MAS-MMC are introduced.Then,the advantages of MAS-MMC in economy,reliability and flexibility are verified by the comparison with existing multi-DC-port converters.Finally,the effectiveness of MAS-MMC topology is verified by simulation and experiment.