ABSTRACT

The production of stray currents by DC Light Rail Systems leads to the corrosion of the supporting and third party infrastructure in close proximity to the rail system. This paper focuses on the impact of earthing topologies on the level of corrosion that will be observed on buried structures in the proximity of a transit system. The work describes a modeling technique that can be applied to predict the level of stray current (and hence corrosion damage) in a DC traction system where the soil resistivity varies along the route of the transit system, vertically as well as horizontally. The modeling technique used involves the accurate computation of the shunt and series parameters for use in a resistive type model using a commercially available software package. The results demonstrate the importance that soil resistivity has on the corrosion risk to traction system and third party infrastructure. Such information could ultimately be used to vary the stray current control design across a transit system to provide a match with the external environment and ensure a consistent corrosion lifetime for structures across the whole route.

INTRODUCTION

Current leakage from DC railway systems is an inevitable consequence of the use of the running rails as a mechanical support / guide way and as the return circuit for the traction supply current. Since the rails have a finite longitudinal, or series resistance approximately 25-60 mO /km or 25-60 µO /m per rail, and a poor insulation from earth, typically from 2 to greater than 100 O /km. A proportion of the traction current returning along them will leak, or stray to earth and flow along parallel circuits (either directly through the soil or through buried conductors) before returning onto the rail and the negative terminal of the DC rectifier.

Given that current flow in a metallic conductor is electronic, while that through electrolytes such as the earth, concrete etc. is ionic, it follows that there must be an electron to ion transfer as current leaves the rails to earth. Where current leaves the rail to earth there will be an oxidation, or electron producing reaction:

Chemical equation (available in full paper)

This reaction is visible after time as corrosion damage. For current to return onto the rail there must be a reduction or electron consuming reaction. In an oxygenated environment this will typically be:

Chemical equation (available in full paper)

This stray current can cause significant damage to the running rails, the supporting infrastructure and local third party utilities such as pipes and cables. A key factor in determining the level of threat to the third party utilities and the supporting infrastructure is the resistivity of the surrounding soil. Typically, a uniform soil resistivity model is assumed over the length of a system. In reality, it will change with both depth and system chainage.

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