To address the problem that the voltage monitor has difficulty accurately locating ground faults on the low-voltage side of the distribution transformer, this paper proposed an optimized voltage dip (Vdip) method based on negative sequence voltage changes. First, the asymmetric fault location probability calculation method was used to compare the negative sequence current of the feeder substation and the negative sequence voltage on the low-voltage side of the distribution transformer to determine the probability of asymmetric fault location. Then, based on this probability, the ground fault location was confirmed. Finally, to address issues such as frequency deviation and inter-harmonics, an inter-harmonic elimination method based on sliding window averaging was proposed. Through simulation experiment on the short-circuited auxiliary resistance of the resonant grounding distribution network, and verified in a 22 kV resonant grounding network of a certain regional power grid, the average ground fault location error was 1.06 km. Experimental results showed that the proposed method could achieve a fault location accuracy of up to 98.7% while avoiding inter-harmonic interference, and the model performance metrics were better than existing methods.