围绕我国亟待提升驾驭交直流复杂大电网安全能力的重大需求，立足多馈入直流系统换相失败带来的电力系统新安全约束，提出了提升直流抵御换相失败能力和减少换相失败对电网安全稳定影响的新思路：通过对直流换相失败时空离散性和交直竞争特性的充分挖掘，改善直流对交流故障“刚性响应”特性以提升直流抵御换相失败的能力；通过改善底层锁相触发控制鲁棒性，构建全新的分相柔性协调控制策略，提升换相失败后续有功输出能力、故障恢复时无功独立可控性和可自平衡能力减少对电网安全稳定的影响。本项目从机理模型、指标方法、底层触发控制、多控制协调、保护优化等全时空应对策略方面展开全面系统地研究，建立交直流混联电网换相失败时空离散、交直竞争的特性边界评估分析体系和层内自治、层间协调的多时间尺度柔性控制技术体系，提出涵盖设计、运行、维护不同层面的换相失败准确判断、全面评估、精准预防、可靠保护和高效恢复的完整解决方案。

AC fault support capability of HVDC (High voltage direct current) have significant influence on the safe operation of system with multiple DC infeed, and there are some problems in HVDC in the case of AC fault, such as poor controllability of commutation, active and reactive power. The basic theories and key technologies related to HVDC controllability in the case of AC fault, such as synchronous phase locked loop and phase control of converter, commutation failure suppression, active power transfer capability, reactive power quantitative control, and protection coordination in AC fault recovery process, will be studied in this project considering HVDC self-performance and space-time characteristic of system with multiple DC infeed. After the success of the project, the continuous commutation failure during AC faults in the HVDC multi-infeed system can be avoided, the active power transmission capacity during AC fault can be enhanced, the decoupling and quantitative control can be realized for active and reactive power during AC fault recovery, and the AC fault support capability of HVDC will be significantly promoted. The key outputs will provide the core technical support for the safe operation of the huge AC/DC hybrid power system in China.