Fault Analysis and Voltage Support Strategy Through Equivalent Line Impedances for MMCs in AC/DC Hybrid Distribution Networks Under Asymmetrical Faults

Under asymmetrical fault, modular multilevel converters (MMCs) in AC/DC hybrid distribution network should limit fault currents and inject currents to support grid voltages. The effect of voltage support is related to the line impedances, hence, injecting active-reactive currents based on line impedance parameters is a valuable solution. However, there are some problems remained to be solved: 1) fault network is quite different with the pre-fault network, and equivalent model of fault network should be built accurately; 2) the optimal ratio of injected active-reactive currents (Id/Iq) based on the line impedances in equivalent model should be deduced; 3) impedance parameters in the model should be estimated. Thus, in this manuscript, fault analysis is described via symmetrical component method, and MMC-based interconnected sequence network (ISN) is proposed to illustrated the fault network with fault branch under the steady-state period of fault. Then voltage support strategy based on the equivalent line impedances of ISN model are put forward. Perturbation and observation algorithm is introduced to estimate these impedances. Finally, the proposed ISN model and the voltage support strategy are verified in MATLAB/Simulink.