Modular multi-level flexible direct current system direct current fault overvoltage restraining method

The invention relates to a modular multi-level flexible direct current system direct current fault overvoltage restraining method which is characterized by comprising the following steps of (1) detecting direct current circuit faults, (2) lowering direct current voltage set parameter level after the faults are detected, (3) setting sending end station transmission power inversion, (4) locking a converter valve and (5) switching off a direct current line breaker. By means of the method, overvoltage level under a direct current line single-pole grounding fault can be effectively lowered, and therefore insulation design requirements on direct current field devices and the converter valve are lowered, system insulation manufacturing cost is effectively lowered, and system related properties are optimized.