An Analysis of the Grounding Strategy for Mixed AC/DC Areas

The aim of this paper is to analyze mixed electrified areas where there is a mix of AC/DC electrification. The grounding strategy is different. DC electrification is designed to have the rail isolated from ground and AC usually has the rail and other metallic parts such as structures and OCS poles connected to ground. This approach in mixed areas provokes the corrosion of the elements directly connected to ground, namely structure foundations or metallic rods or even the rail. This mixed environment presents challenges both for safety and corrosion management. On one side it would be a good anticorrosion practice to limit the number of elements that are directly grounded and connect them all by an aerial ground wire. On the safety side it has to be ensured that the voltages in fault condition are compliant with the standards (namely EN 50122). The authors have developed a parametric analysis to understand the impact of different grounding scenarios such as variations of the grounding impedance, impact of the reduction of the grounding impedance of a single element (i.e. the impact of a connection to the grounding mesh of a station or substation), an analysis of the maximum distance between grounding elements along the alignment ensuring that the rail to ground voltages are compliant to EN 50122-1. The methodology proposed is based on the modelling of the line considering electrical elements such as the rail impedance, grounding impedance for different elements (substations, stations, OCS poles) rail to ground impedance and OCS wires characteristics. Once the electrical model is obtained, a parametric analysis for each of the scenarios is performed to determine the impact of a particular variation into the general model obtaining the results of the short circuit analysis along the line. Results of these analyses will be presented as well as the proposed next steps and conclusions.