1.College of New Energy, China University of Petroleum (East China), Qingdao 266580, China;2.Power Dispatching Control Center of Guangxi Power Grid, Nanning 530023, China;3.Research Center for Smart Grid, Shandong University of Technology, Zibo 255049, China

When a high-resistor grounding fault occurs in a flexible grounding system, the protection during the parallel stage of neutral point small resistor often refuses to activate, so it is still necessary to consider the impact of the transient characteristics of small resistor exit on the existing transient faulty-line selection methods. The exit of the parallel small resistor is accompanied by a certain transient transition process. For this process, a transient equivalent circuit for a single-phase grounding fault in a flexible grounding system is established. The characteristics of transient electrical quantities of the system when the parallel small resistor exits, the impact of different fault conditions on the amplitude of transient electrical quantities, and the constraint relationship between transient zero-sequence current and transient zero-sequence voltage are analyzed. The impact of transient quantities after the parallel small resistor exits on the stability of the transient faulty-line selection methods is clarified. When the small resistor exits during a high-resistor grounding fault in a flexible grounding system, the generated transient electrical quantities will cause misoperation in the transient amplitude comparison method, transient polarity selection method, and transient power direction method, but the transient zero-sequence current projection method for the line selection is correct. The simulation verifies the accuracy of the results.