Topology And Control Scheme Of Nine-arm Modular Multilevel Converter

Due to modular structure features,modular multilevel converter(MMC)shows many excellent performances:1)the voltage and power level of the converter can be easily expanded by increasing the number of submodules(SMs);2)the requirements for the AC side filter can be reduced due to the large number of AC output voltage levels;3)the switching frequency of semiconductor devices can be low to reduce the power loss of the converter;4)the arm is composed of SMs in series,which can avoid the direct series connection of semiconductor devices to reduce the difficulty to balance the semiconductor devices voltage;5)the operation reliability of the converter can be improved through the redundant configuration of SMs.Based on the above advantages,MMC has become the most attractive multilevel topology in medium/high-voltage applications,such as high-voltage direct current transmission systems(HVDC),flexible AC transmission systems(FACTS),medium/high-voltage motor drives,and battery energy storage system(BESS).In medium/high-voltage applications,there are some applications including two sets of three-phase terminals,such as medium-voltage dual-motor or six-phase motor drives and unified power flow controller(UPFC),where two conventional MMCs are usually employed.Fortunately,it is possible to integrate two MMCs into one compact nine-arm MMC(9A-MMC)with dual ports to minimize the system footprint and cost.Compared with two MMCs,9A-MMC can save 25%of the required arms and 50%of the arm inductors,and the corresponding SM capacitors,SM capacitor voltage sensors,gate-drive circuits,and arm current sensors can also be reduced.Therefore,this thesis takes 9A-MMC as the research object,focusing on its operation principle and mathematical model,steady-state operation analysis,topology optimization and specific application scenario analysis.The specific research work and innovation are as follows:1)Operation principle and steady-state analysis of 9A-MMC.Firstly,based on the conventional MMC,the topology characteristics and operation principle of 9A-MMC are analyzed.Then,the average mathematical model of 9A-MMC is derived by regarding each SM as the controlled voltage source and the arm voltage as the series combination of these controlled voltage sources.Based on the mathematical model of 9A-MMC,the operating range of 9A-MMC is studied.Besides,according to the coupling relationship among the electrical components of 9A-MMC,the capacitor voltage fluctuation characteristics and circulating current of 9A-MMC under the same-and different-frequency operation condition are analyzed,which are the foundation of the following research2)Configuration method and control scheme of 9A-MMC.Firstly,the configuration method of 9A-MMC that the number of SMs in each arm can be different is proposed.On the one hand,the limitation of 9A-MMC operating range can be eliminated by the proposed configuration method.On the other hand,when the amplitude difference of two sets of AC output phase voltage is large,the number of SMs can be further reduced.Then,to make the best use of the advantages and avoid the disadvantages of 9A-MMC,this thesis compares the number of required SMs,DC-side voltage,current stress of switching devices,capacitor voltage fluctuation amplitude and power loss between conventional 6A-MMC and 9A-MMC when the number of SMs in each arm is different.Besides,the complete control scheme of 9A-MMC including AC output control,capacitor voltage balancing control and circulating current suppression control is given.Finally,the operation characteristics and theoretical analysis of 9A-MMC are verified by MATLAB/Simulink and experimental platform.3)Topology and operation analysis of hybrid 9A-MMC.Firstly,the hybrid 9A-MMC based on half bridge submodule(HBSM)and unidirectional current full bridge submodule(UC-FBSM)is proposed.The upper and lower arms in hybrid 9A-MMC are composed of HBSMs,and the middle arm is composed of UC-FBSM.Compared with the standard 9A-MMC,which only consists of HBSM,the DC side voltage of hybrid 9A-MMC can be reduced by 1/3,and the number of IGBTs is same as that in the standard 9A-MMC.Moreover,the DC offset in the common mode voltage of hybrid 9A-MMC can be eliminated.Then,the middle arm current of the hybrid 9A-MMC under different operating conditions are analyzed,and the maximum operating range of the hybrid 9A-MMC is given.To expand the operating range of hybrid 9A-MMC,a middle arm current reshaping control method is proposed.Finally,simulation and experimental results verified the performance of hybrid 9A-MMC and the effectiveness of the middle arm current reshaping control method.4)Operation analysis of the flexible interconnection device based on nine-arm modular multilevel converter(9AMMC-FID)and the optimization method of SM capacitance.Firstly,the topology and operation principle of flexible interconnection device(FID)are described.Then,the configuration of 9AMMC-FID is given.Based on the mathematical model of 9AMMC-FID,the power flow regulation range and the capacitor voltage fluctuation characteristics of 9AMMC-FID are analyzed in detail.To solve the issue that the fluctuation amplitude of middle arm capacitor voltage in 9AMMC-FID is greater,this thesis also presents an optimization method for the SM capacitance.Finally,the feasibility and performance of 9AMMC-FID are demonstrated by simulation results.To sum up,to realize the compactness and lightness of power electronic equipment,two independent MMCs are integrated into a compact 9A-MMC with two sets of three-phase terminals.Besides,its operation principle,mathematical model,steady-state operation analysis,topology optimization and application technology are studied comprehensively.