Fault Transmission Modeling and Non-Bypass Fault Tolerance Control of Cascaded H-Bridge Modular Multi-Level Converter Fiber Optic Communication System

The existing fault control strategy for the fiber-optic communication system in modular multilevel converters involves directly bypassing faulty submodules. When redundancy is depleted, the equipment will shut down. This paper investigates the propagation model of such faults and proposes a non-bypass fault-tolerant control method. This approach minimizes the need to bypass faulty submodules, thereby improving device reliability. First, a mathematical model of fault propagation is established, incorporating boundary conditions for downlink command loss and uplink status interruption. The model reveals the positive correlation between modulation ratio deviation and current fluctuations during downlink faults, and deduces the coupling mechanism between switching states and DC voltage fluctuations during uplink faults. Based on this mechanism, a non-bypass fault-tolerant control method is designed to maintain the submodule corresponding to the faulty fiber in a constant voltage state, thereby minimizing the use of redundancy. Finally, using a three-phase cascaded static synchronous compensator as the test platform, the propagation mechanism of fiber-optic communication system faults is validated, achieving transient fault fluctuations of less than 0.1% and permanent fault DC voltage fluctuations within ±1%. This paper proposes a novel approach to ensure the continuous operation of cascaded energy storage converters. It reduces unnecessary bypass actions triggered by transient communication faults, thereby extending the reserve time of redundant submodules and enhancing the system's continuous operation capability between two planned maintenance intervals.