School of Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, China

In flexible grounded systems, single-phase grounding protection based on steady-state fault characteristics generally has low resistance to transition resistance, and this problem is further exacerbated after the connection of parallel small resistors. This paper studies the relationship between the ratio of the transient DC component in the low-frequency transient zero-sequence current of faulty and non-faulty lines after the connection of parallel small resistors, and finds that the transient DC component ratio is only related to the capacitance to ground. The relationship between the ratio of low-frequency transient zero-sequence current distortion rates of faulty and non-faulty lines is analyzed, and a method for line fault selection based on low-frequency transient zero-sequence current distortion rates in flexible grounded systems with high-resistance grounding faults is further proposed. Fault line selection is achieved by comparing the magnitude of low-frequency transient zero-sequence current distortion rates of each feeder within a specific time window, and busbar faults and outgoing line faults can be accurately distinguished. This method can avoid the impact of zero-sequence voltage transformer disconnection and zero-sequence current transformer polarity reversal, and is less affected by fault distance. At the same time, this method uses transient fault characteristics to form a line selection criterion, which is beneficial for overcoming the weak steady-state fault component after connection of parallel small resistors, and has low requirements for instrumental transformer accuracy. Finally, the accuracy and effectiveness of the method are verified through real-time digital simulation and experimental platforms.