<p>A kind of inverter that owns a high level of modularity, robust fault tolerance, and high-power quality is the Modular Multilevel Converter (MMC). The fault of open-circuit in the submodule (SM) diode constitutes one of the significant challenges to the MMC. It can damage critical components in the submodule, which endangers the MMC’s secure operation. The existing research corpus has yet to present a complete fault protection methodology concerning the MMC when an open-circuit fault occurs in the SM diode. Merely focusing on isolating the defective submodule, it fails to concurrently eradicate the fault’s repercussions while keeping the submodule intact. This article scrutinizes the influence of open diode faults regarding the MMC’s performance and proposes a safeguarding scheme for open-circuit faults relying on a voltage Zener diode implanted within the submodule. This protection scheme can quickly address open-circuit faults in the submodule diode, allowing it to operate normally without needing to bypass the submodule, while also minimizing fault protection time. This solution reduces the potential damage of open diode faults to the MMC and improves its operational reliability and economic benefits. Simulation and experimental outcomes illustrate the efficacy of the proffered protection scheme in addressing open-circuit faults within the MMC diode.</p>