Calculation Of DC Short Circuit Fault Current In MMC-HVDC Grid And Analysis Of Its Influencing Factors

High voltage direct current based on modular multilevel converters(MMC-HVDC)grid has the advantages of flexible operation capability and independent power adjustment,which can solve the large-scale grid connection problem of intermittent renewable energy power generation,and will play a backbone in the global energy transformation.However,when a DC short-circuit fault occurs in the MMC-HVDC grid,the fault current rises quickly,which easily causes problems such as overcurrent and overvoltage of the sub-modules,and the fault causes great harm to the safety of the power grid.The DC short-circuit fault restricts the development of MMC-HVDC grid technology.This article revolves around the analysis of DC short circuit faults in MMCHVDC grid,and focuses on the fault current calculation methods and the fault current influencing factors of MMC-HVDC grid.The main work and innovations of this paper are as follows:(1)Based on the current characteristic quantities,the differences in the evolution of the fault current between pole to ground short circuit fault and pole to pole short circuit fault are compared.The research shows that neutral line impedance and ground resistance are the reasons for the differences in the evolution of fault currents of different DC fault types.Based on the transient energy characteristic quantity,the fault energy transfer mechanism of DC short circuit fault in MMC-HVDC grid is analyzed,and the difference in power transmission between the AC side and the DC side of the sending-end converter station and the receiving-end converter station after a fault is compared.The above research has laid the foundation for the simplified calculation of DC short-circuit fault current.(2)Based on the principle of MMC equivalent circuit,the DC short-circuit of MMCHVDC grid is analyzed,simplified and decoupled,and the calculation expression of DC short-circuit fault current is derived,and the calculation method is verified by the simulation model of the four-terminal MMC-HVDC grid built on PSCAD/EMTDC.The maximum error of all verification cases is 7.12%,and the average error is 2.22%.The verification results show that the calculation method has high accuracy.(3)Aiming at the calculation of the fault current with the input of the fault current limiter,a method for calculating the fault current of MMC-HVDC grid with the input of fault current limiter is proposed,which expands the calculation of the DC short-circuit fault current of MMC-HVDC grid.The fault characteristics of the action of the fault current limiter are analyzed,and the evolution of the fault current including the fault current limiter is divided into three stages,and the equivalent model of the fault current limiter in the transition phase is established.Then the fault current expressions of each phase are deduced respectively,and the proposed calculation method is verified by the simulation model of four-terminal MMC-HVDC grid with fault current limiter built on PSCAD/EMTDC verification.The verification results show that error of the calculation method is less than 3%,which has high accuracy and has certain engineering practical value.(4)Based on the fault current suppression efficiency indicators,the influence of arm reactors,neutral line reactors,current limiting reactors and fault current limiters on the fault current evolution in MMC-HVDC grid is quantitatively compared and evaluated.Research shows that the arm reactor has the worst suppression efficiency of fault current,which is far weaker than other components.The fault current limiter has the best suppression efficiency on fault current,and the neutral line reactors and the current limiting reactors have basically the same suppression efficiency on fault current.