An MMC-HVDC Protection Method Based on the Difference in the Ratios of the Voltage-Frequency Components

The rapid failure development of flexible DC transmission systems has introduced significant challenges to the research of DC protections，and the strict requirements of quick motion must enable the research of protections to focus on the fault transient quantity. To optimally fit the actual project，the frequency-dependent characteristics of the line and the traveling wave transmission process cannot be ignored in the high-voltage large-capacity transmission system. Thus，we perform a fault analysis in this study. Aiming at the insufficiency of the single-ended protection in withstanding transition resistance，this paper analyzes the voltage and frequency characteristics of the measurement point when faults occur inside and outside the protected area and uses the transition resistance as a variable to observe the changing trend of the frequency ratio difference under different fault conditions. A protection scheme，based on the difference in the ratios of the voltage-frequency components inside and outside the protection zone，is designed by using the S-transform. Next，the noise identification criterion is provided according to the frequency spectrum characteristics of noise under the S-transform；Finally，a four-terminal flexible DC transmission system model is built on the PSCAD/EMTDC simulation platform to verify the proposed method. The results show obvious differences in the ratios of voltage-frequency components under faults inside and outside the zone. The proposed protection scheme can accurately identify short-circuit faults at different positions of the line and judge the fault pole. Compared with the protection method based on voltage variation and high-frequency energy，the method in this paper demonstrates a higher resistance to transition resistance. The protection method displays obvious discrimination even in the case of long-distance high-resistance faults. Thus，it can perform better in large-capacity long-distance power transmission systems. Concurrently，in the noise environment with different signal-to-noise ratios，the protection method can still accurately identify noise removal and faults. The protection method can complete the fault identification within 2 ms and meet the quick-action requirements of the flexible straight system. Thus，it can be used as the main protection of the system operation. © 2023 Tianjin University. All rights reserved.