Neutral current redistribution and differentiated dc bias protection of multiple main transformers in one substation via a refined metro–grid coupling model

Significant differences in neutral currents exist among different grounded main transformers within the same substation. Simplifying the substation as a single conductor fails to resolve the internal distribution of metro-induced stray current, thereby masking potential DC bias risks. To prevent such failures, a refined metro–power system coupling model that considers multiple main transformers is established using actual parameters of the metro system, power grid, and local soil. The model accurately calculates neutral current of different interconnected transformers, enabling early identification of uneven current distribution that may lead to DC bias. Results show that the maximum ratio of absolute neutral currents among transformers reaches 2.76, while the minimum mean absolute current error against field measurements is 3.74%. A reduced-scale physical analogue provides supplementary validation of the model's ability to reproduce current redistribution among multiple transformer grounding branches. Based on this model, the influence of various metro operating conditions on stray current is analyzed to identify failure-prone scenarios. A neutral-point resistance model, combined with global optimization algorithms with verified stability, designs three-level differentiated protection schemes. The resulting schemes keep all transformer neutral currents within specified limits under the actual topology—unlike traditional-model-based schemes—thereby preventing DC bias-induced failures in multiple main transformers within the same substation.