Affected by the rapid control of power electronic equipment, the flexible DC transmission system is prone to frequency fluctuations and disturbance signal, which will bring deviation to the electrical quantity measurement and cause the protection system malfunction. The influence of frequency fluctuations on Fourier algorithm in flexible DC transmission is explored, and the corresponding protection simulation model is built based on the principles of different protections. It can be seen that frequency fluctuations will produce varying degrees of interference to the action range and sensitivity of power line protection. A corresponding algorithm based on amplitude and phase calculation during frequency fluctuations in specific line protection is proposed, and a logic judgment method is designed to comprehensively judge the changes of differential current, AC voltage and frequency to determine the operating state of the system to improve reliability. Considering the influence of frequency fluctuation on the protection, by monitoring multiple electrical parameters in the AC area of the flexible HVDC transmission system, the logic judgment device comprehensively judges the changes of differential current, AC-voltage and frequency to determine the operation state of the system. Simulation results show that this method can reduce the impact of frequency fluctuations and interference signals on line protection, and explore a power grid protection method suitable for the dynamic change of frequency, which is of great practical value to ensure the safe and stable operation of flexible DC transmission network.

flexible DC transmission;frequency fluctuations;AC area