1.NR Electric Co., Ltd., Nanjing 211102, China;2.State Grid Economic and Technological Research Institute Co., Ltd., Beijing 102209, China

The flexible DC transmission system is susceptible to oscillations with different frequencies under various operation conditions due to the impact of the characteristics of power electronic equipment and control systems. As a determining factor for achieving equipment safety and stable operation of the power grid, the protection system should achieve accurate monitoring of system harmonics and reliable operation. Based on the cumulative effect of current energy during the high-frequency oscillation, a current-type harmonic protection algorithm based on inverse time action is proposed. Based on the characteristics of high-frequency harmonic voltage, the zero-crossing monitoring protection algorithm, the harmonic distortion rate protection algorithm, and the high-frequency harmonic protection algorithm are provided. Each protection algorithm cooperates with each other to achieve comprehensive protection for different voltage oscillation types. Finally, the high-frequency harmonic protection configured in domestic flexible DC engineering practice is presented, and the operation characteristics of the system after high-frequency oscillation are analyzed based on the protection principle, verifying the effectiveness and accuracy of high-frequency harmonic protection.