Abstract: This article describes the frequent blockage of the security filter at the converter station of the North Channel of the Yu-E back-to-back flexible DC project (420 kV Yichang converter station) since its operation. The causes of the failures were analyzed, which included such as tap water quality, electric valve failure, and excessively long backwashing cycles of carbon and sand filters. Suggestions such as shortening the backwashing cycles of carbon and sand filters, conducting regular functional tests on electric valves, and manually backwashing carbon and sand filters were proposed.