| In recent years,with the development of flexible high voltage direct current(HVDC),flexible HVDC has been widely used in large-scale offshore wind farms for medium and longdistance connection,regional grid interconnection or other occasions.Compared to the traditional two-level or three-level converter,modular multilevel converter(MMC)has many advantages such as high modularity and flexibility,low harmonic level,strong fault tolerance due to the cascading structure of submodules,so MMC has been considered as the promising converter in the HVDC systems.However,the DC short-circuit fault and grid-connected stability issues have affected the safe and stable operation of the MMC based HVDC systems.In order to overcome the two major problems,the DC short-circuit fault protection and ridethrough strategies,AC and DC side impedance models of MMC are investigated in this paper.The main contents of the paper are as follow:Firstly,on the basis of considering various internal control,the AC and DC side impedance theoretical models of MMC are respectively derived by the method of small-signal impedance modeling,and the impact of circulating current control on the port impedance models is analyzed.The accuracy of the theoretical port impedance models are verified by the method of small-disturbance voltage injection in the time-domain simulation model with the same parameter.In addition,the fault characteristics of MMC under DC bipolar short-circuit fault are analyzed,and the analytical formulas of DC fault current and submodule capacitor voltage under fault are deduced,which lays the foundation for researching on MMC’s DC shortcircuit fault protection and ride-through.Secondly,the blocking DC short-circuit fault protection and ride-through strategy for hybrid MMC is studied.A blocking DC short-circuit fault protection ride-through strategy is proposed based on the traditional blocking strategy,which owns the advantages of reactive power compensation and submodules voltage balancing during the DC fault ride-through.The corresponding simulation model and experimental platform are built,and the results verify the effectiveness of the proposed strategy.Furthermore,the strategy can be extended to solve the DC short-circuit fault issue of MMC based multi-terminal HVDC(MMC-MTDC)transmission systems.The simulation resuls of MMC-MTDC model also verify the feasibility and effectiveness of the expansion strategy applied to solve and ride through the DC fault of MMCMTDC.Finally,the non-blocking DC short-circuit fault protection and ride-through technology for hybrid MMC is investigated.The two typical non-blocking DC short-circuit fault ride-through strategie are compared and analyzed in terms of control methods,fault ride-through effects,device costs and power losses,which indicates the respective characteristics and advantages of the two non-locking strategies.In addition,for the hybrid MMC based on the half-bridge submodule(HBSM)and the full-bridge submodule(FBSM),using the negative voltage output of FBSMs,the hybrid MMC can not only operate on overmodulation state under normal condition,but also ride through DC short-circuit fault.Under the premise of considering overmodulation operation and non-blocking DC short-circuit ride-through capability,a optimal design method of the hybrid submodules’ proportion is proposed with the condition of high power transmission capacity and low ratio of FBSM,meanwhile an improved carrier diaposition pulse width modulation(CD-PWM)and capacitor voltage balancing strategy is also proposed to adapt the optimized solution.The proposed optimized scheme not only realizes the DC short-circuit fault protection and ride-through capability,but also improves the economy of the hybrid MMC system.Simultaneously,simulation and experimental research have been carried out to verify the correctness and effectiveness of the proposed optimized scheme. |
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