Abstract: In order to ensure the stability of China Southern Power Grid （CSG） after the commission and operation of Nuozhadu, Xiluodu DC, the impacts of DC pole blocking fault on the security and stability of Nuozhadu and Xiluodu HVDC transmission systems under AC/DC parallel operation and island operation mode were analyzed. Various critical factors that affected the security and stability of Nuozhadu and Xiluodu HVDC were studied in detail and summarized concisely, such as the sensitivity of stability control measure, the type of DC pole blocking fault, the power flow of AC grid, the maintenance method for 500 kV line and the island operation mode, then the safe and stable control scheme of the system was obtained. Moreover, the design schemes of stability control system for Nuozhadu UHVDC and Xiluodu HVDC were put forward, including control objects, control factors, configuration scheme for security and stability control device, main function, as well as the information communicated between different stability control equipment.

Key words: Nuozhadu, Xiluodu, HVDC transmission, security and stability control