Abstract：Fault ride-through of flexible low-frequency transmission is an important issue that needs to be addressed in future applications. During the fault process, it is necessary to ensure the safe operation of the modular multilevel matrix converters (M3C), prevent overcurrent, overvoltage and overcapacity from triggering protection actions and cutting off the M3C, which would prevent the system from crossing the fault. There is only one recent demonstration project for low-frequency transmission of offshore wind power in China, and there is a lack of operational experience in fault ride-through of flexible low-frequency transmission. However, existing literature studies on fault ride-through of flexible low-frequency transmission mainly focus on the power redundancy and unbalanced bridge arm capacitor voltage during the fault period, but lack a systematic analysis of other problems faced by the system during the fault period. This paper systematically analyzes the possible problems faced during fault ride-through of flexible low-frequency transmission to provide a reference for the design of subsequent protection and control strategies. Firstly, the mathematical model and control strategy of M3C are introduced, and a detailed model of the low-frequency transmission system for offshore wind power is established. Secondly, the transient characteristics of M3C are analyzed through simulation of symmetrical and asymmetrical faults on the fundamental frequency side and low-frequency side of M3C. Finally, based on the transient characteristics of the system under fault conditions, the problems faced by the system in achieving fault ride-through are summarized. The following conclusions can be drawn from the simulation analysis: (1) Under symmetrical faults on the fundamental frequency side, the main problem faced is the over-limitation of M3C capacitor voltage due to power redundancy. Under asymmetrical faults on the fundamental frequency side, the system faces problems of power redundancy and unbalanced bridge arm capacitor voltage during the fault period, as well as the impact of negative sequence components on system control performance during the fault period. Negative sequence component control can be introduced on the fundamental frequency side system to achieve different negative sequence control objectives. (2) Under symmetrical faults on the low-frequency side, during the fault ride-through period, M3C experiences power reversal, and during the recovery period, the low-frequency system experiences overvoltage problems, but the capacitor of M3C does not experience overvoltage during the ride-through period. During the low-frequency fault period, voltage-frequency control switches to vector control, and there is a coupling problem between low-frequency active and reactive power. Under asymmetrical faults on the low-frequency side, during the fault period, there are problems of unbalanced bridge arm capacitor voltage and overvoltage on the non-fault phase during the ride-through period. Moreover, the strategy of simultaneously suppressing the negative sequence to zero on the low-frequency side and the wind farm grid side may lead to overvoltage problems. (3) Under asymmetrical faults, zero-sequence voltage leaks to the opposite side on the valve side of the converter.
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