Simplified Method for Fault Transient Analysis of Flexible DC Distribution Lines

Flexible DC distribution network has flexible control, high power quality and can be equipped with new energy distributed generation, but the serious over-current problem after DC line fault is one of the important factors restricting its development. The analysis and calculation of short-circuit fault current is of great significance for the design and selection of system parameters. However, MMC converter station is a highly complex nonlinear system, and its topology is relatively complex for DC distribution network. The load of distributed generation and DC / DC adapter makes the fault analysis of DC distribution system more complex than that of DC transmission system, so it is unable to get the fault current calculation results Therefore, the short-circuit fault current is often obtained by simulation. The research on approximate expression of fault current can only be carried out for two terminal DC system, but it can‘t be done for line short-circuit fault current in other topologies. Based on the theory of modular multilevel converter (MLC), this paper presents a new design of MLC According to the basic characteristics of MMC (converter, MMC) flexible DC power grid system parameters, the approximate analytical solution of line short-circuit fault current is obtained by reasonably simplifying it, which can accurately describe the overall trend of fault current in a period of time after the fault, and provide a certain basis for the selection and setting of DC line protection setting parameters.