Analysis of stray current, track-to-earth potentials and substation negative grounding in DC traction electrification system
In the first part of the paper, we look at a stray current model under the ideal conditions with uniformly distributed track-to-earth resistances and develop the track-to-earth potentials and stray current relationships within a segment of track. We then expand the model to include buried metallic structures in the vicinity of the track using field theory to calculate the potential gradients generated due to flow of rail current and show that a potential difference experienced by a crossing or paralleling underground structure that traverses these gradients could cause stray current corrosion. We discuss the limitations of these models with the actual conditions such as nonuniform back-to-earth resistances, nonuniform soil resistivities. In the second part, we look at the stray current model when there are inadvertent breakdowns in the track insulation. The points of rail insulation breakdown may be approximated using the spherical ground electrode model and equipotential lines to illustrate the flow of stray current. Common grounding of the substation negative bus, substation AC ground mat and electric utility neutral are examined and discussed.