A comprehensive overview about the topology structure of grounding mode in project which have been built and are building in China, the typical protection configuration strategy is described. Then a new AC bus differential protection is proposed against the topological structure and measuring point of delta reactance via resistance grounding mode on valve side of converter transformer. The current of the virtual measuring point is measured based on other measuring point, so the judgement method of AC bus differential protection is complete and effective. The closed-loop simulation test system of RT_LAB and control and protection system is established based on parameters of project. The new judge method proposed have passed comprehensive tests, which verifies its feasibility and reliability. This work is supported by Science and Technology Project of State Grid Corporation of China “Key Techniques Research on ±500 kV Flexible DC Power Grid Utra High-speed Protection”.