Abstract: The 10 kV low-resistance grounding system primarily relies on zero-sequence overcurrent protection to isolate single-phase ground faults. However, with its relatively high setting values, this protection fails to operate during high-resistance ground faults. Therefore, this paper establishes a series RLC equivalent model for the line. From the time-domain perspective, it investigates the constraint relationship between zero-sequence currents of faulty/non-faulty lines and the bus zero-sequence voltage. Based on the single-loop characteristics of non-faulty lines, a virtual differential current index is constructed. Analysis reveals that the virtual differential current of non-faulty lines is approximately zero, while that of faulty lines is significantly greater than zero. Accordingly, a highly sensitive ground fault protection method based on virtual differential current is proposed for low-resistance grounding systems. The proposed method requires only zero-sequence current and voltage information of a single line to detect faults, with a maximum tolerance for transient resistance reaching 3000 Ω. Finally, its effectiveness was validated through PSCAD simulations and RTDS-based device testing.