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In the flexible HVdc system, fault current limiters (FCLs) need to be configured to reduce the rising rate of fault current. However, the FCL configured according to the bipolar near-end metallic short-circuit faults may cause the fault current to be unnecessarily oversuppressed and consume equipment life. This article develops a method to activate FCLs of the flexible HVdc system adaptively to follow changes in fault severity. Based on analyzing the relationship between the fault current and the transition resistance at the initial moment of the fault, a method to predict the fault condition based on the single-ended instantaneous information is proposed, which realizes the fault severity assessment. Considering mutual coupling of FCLs action sequences, a fast action sequence generate (FASG) method of FCLs based on fault current limiting contribution is proposed, which realizes the coordination between multiterminal FCLs without communication. An FCL self-adapting activation strategy (SAAS) for flexible HVdc systems based on fault severity assessment and FASG is constructed. This strategy matches the severity of the fault by adjusting the FCL action sequence, improving the current limiting effect of severe faults and the life of FCLs. The accuracy of the SAAS is validated on a scaled-down two-terminal system in the laboratory and the simulation.