Train-earth-grid coupling DC magnetic bias model for stray current diffusion

•By utilizing Green's formula, the earth potential under multi-train operating conditions is calculated.•Two stray current intrusion paths from the substation grid and armored cable are considered.•The main intrusion path of stray current is the metal protection layer of the armored cable.•Change the grounding strategy of the metal protection layer and use BCD to control DCBMC. The DC magnetic bias phenomenon in transformers can result in increased noise and harmonic components. This study focuses on the DC magnetic bias generated by stray currents in the DC traction power supply system of neutral grounding transformers under multiple train operation conditions. A two-layer soil model is utilized to calculate geodetic potential, with rail potentials from both mainline and depot serving as sources for stray current within the model. Two paths of stray current intrusion into the transformer via the grounding network and cable armor layer from the Main substation have been modeled and analyzed. A simulation test has been designed to verify that the maximum error between experimental data and model calculation data is 8.87%. The implementation of a blocking connection device between the depot and mainline, as well as a change in grounding strategy for the metal protective layer, can effectively reduce DC bias magnetic current in the substation.