Rail Potential Calculation: Impact of the Chosen Model on the Safety Analysis

Abstract

In Traction Electrification Systems (TESs), a current flows into the rails both in normal operation and fault conditions. Therefore, in both cases, a voltage between rails and earth, called Rail Potential (RP), occurs. The international Standard EN 50122-1 requires to evaluate the RP on the basis of the voltage drop in the return circuit. In this work, this approach is named Voltage Drop Method (VDM). Usually, in this approach, the rails are considered isolated from ground, the type of interconnection between the negative pole of the converter and the grounding system of the TPS is not taken into account, and the RP in a generic point of the railway is computed multiplying the current flowing in the return path and the longitudinal resistance of the rails up to the Traction Power Substation (TPS). If the RP exceeds the maximum permissible effective touch voltages, function of time, indicated by EN 50122-1, provisions to reduce the electrocution risk shall be applied. Even if the VDM generally provides conservative values for the RP, it cannot be considered completely faithful, due to the simplifying assumptions usually adopted. Therefore, the decision process to evaluate if some measures to reduce the RP shall be adopted can lead to wrong results. In this work, a faithful circuital model of the railways was used to compute the RP for several scenarios; a comparison with the results computed by VDM was carried out. The goal is to evaluate the trustworthiness of the VDM, highlighting the differences with a more faithful model.