Active-frequency control method and system for flexible direct-current grid connection of communication-free offshore wind farm

The invention discloses an active-frequency control method and system for flexible direct-current grid connection of a communication-free offshore wind plant. Firstly, a frequency signal of a shore alternating current power grid access point is obtained, the frequency signal is converted into a generated direct current voltage to serve as a direct current voltage reference value of a shore converter station, and the shore converter station controls an output direct current voltage value to be equal to the direct current voltage reference value; the offshore converter station detects a direct-current voltage value and converts the direct-current voltage value into frequency to obtain a frequency reference value; the offshore converter station controls the frequency of the offshore common coupling point to be equal to the frequency reference value; the offshore wind farm acquires a second frequency signal of the offshore power grid; and active frequency modulation is performed on the offshore wind power based o