Coordinated DC Short-Circuit Fault Management in HVDC Systems with Customized Hybrid MMCs and DCCBs with Lower Clamping Voltage

This paper investigates the coordinated operation of Hybrid Modular Multilevel Converters (H-MMCs) and DC Circuit Breakers (CBs) under short-circuit (SC) fault conditions. The H-MMCs in this study are designed with a reduced number of full-bridge submodules (FB-SMs) per arm (e.g., 20%) and are coordinated with CBs featuring lower clamping voltage requirements, facilitating their application in HVDC systems. Upon DC fault detection, the CB triggers the opening of its mechanical switch, while the corresponding H-MMC is temporarily blocked to prevent submodule discharge. After a short delay, during the AC in-feed period, the fault current flows through two MMC arms, the CB branch, and the fault point in a point-to-point configuration. The FB-SMs in the MMC arms inject a negative voltage, mitigating the current rise in both the MMC arms and the CB branch. This injected negative voltage also reduces the required clamping voltage of the CBs, thereby simplifying their design and reducing implementation complexity and costs. Analytical equations are presented to predict the behavior of the DC fault current in the coordinated operation. Simulation results for the H-MMC with a lower FB-SM ratio and the half-bridge MMC (HB-MMC) with CBs at two different clamping voltage levels are provided and compared using PSCAD/EMTDC. Additionally, experimental results from a scaled-down hardware prototype are presented for the proposed mechanism and compared to conventional approaches.