Design Method and Verification of the Converter Bridge Arm Reactance for Improving the DC Fault Operation Capability of Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) System

Aiming at the problem that the Half-bridge Modular Multilevel Converter (HBMMC) lacks the ability to block DC faults, this paper proposes an optimized design method for the converter bridge arm reactance based on the Full-bridge Modular Multilevel Converter (FBMMC) topology, aiming to enhance the DC fault operation capability of the flexible high-voltage direct current (HVDC) transmission system. Through theoretical analysis, the current clearing mechanism of FBMMC under DC faults is revealed: after blocking, by utilizing the counter electromotive force of the bridge arm capacitor and the reverse blocking characteristics of the diodes, the fault current is forced to decay rapidly, achieving the self-protection and rapid recovery of the converter. Simulations based on the bipolar short-circuit fault scenario show that after the FBMMC is blocked, the AC feeding current and the DC bus current can rapidly drop to zero, verifying its ability to block DC bus faults. By further comparing the configuration schemes of bridge arm reactance on the AC side, DC side, and AC-DC side, the simulation results show that the configuration scheme on the AC side has the lowest peak value of the fault current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(7.21 \text{kA}$</tex> for the DC bus and 3.16 kA for the bridge arm), and has better tolerance to transient stress. Considering the economy and engineering feasibility, it is finally selected as the optimal configuration. This study provides theoretical support and engineering reference for improving the reliability of the flexible DC transmission system, and is of great value for the fault protection design of high-voltage direct current transmission technology in the context of new energy grid connection.