Elsevier

Electric Power Systems Research

Volume 103, October 2013, Pages 241-247
Electric Power Systems Research

Analysis of the insulation resistances of a high-speed rail transit system viaduct for the assessment of stray current interference. Part 1: Measurement

https://doi.org/10.1016/j.epsr.2013.04.011Get rights and content

Highlights

  • The insulation resistances of an Italian high-speed railway concrete viaduct are investigated during its construction.

  • The insulation resistance measurements are made in three sections of different characteristics of the viaduct “Ticino”.

  • Six sessions of measurements in a period of two and a half year were carried out.

  • The measured insulation resistances show a variation in time, which is related to the progress in viaduct completion.

  • The main requirements for the protective provisions for both low- and high-speed railway lines are detailed.

Abstract

An investigation into the insulation resistances of the concrete structures of a high-speed rail transit system viaduct is presented in this paper. Measurements of the insulation resistances in three sections of different characteristics are carried out during the construction stages of the viaduct, in order to get insights into the relation between insulation resistance changes and viaduct completion. The measurement campaign consists of six sessions of measurement in a period of two and a half years. The measured insulation resistances between a span and its supporting piers and between two adjacent spans show a variation in time and among the viaduct sections. The dispersion of the measurements in the three viaduct sections is highly reduced in the last session of measurements, showing that the insulation resistances are related to the progress in viaduct completion and the characteristics of viaduct sections. The main requirements for protective provisions for both low- and high-speed lines are also detailed.

Introduction

In dc electrified traction systems, such as electrical trains, tram systems and underground trains, the current drawn by the vehicles returns to the traction power substation (TPS) through the running rails which, besides forming part of the signalling circuit for the control of train movements, together with return conductors (if installed) are then used as the current return circuit path. However, owing to the finite longitudinal electric resistance of the rails and their imperfect insulation to ground, some part of the return current leaks out of the running rails and returns to the TPS through the ground [1]. As any underground metallic structure has a lower electrical resistance than the soil, the stray current flows in metallic structures in close proximity to the railway towards the TPS. At the point where the stray current enters the metallic structure, the cathodic part of the corrosion reaction occurs (oxygen reduction) [2]

12O2+H2O+2e2OHwhereas at the point where the stray current flows out of the metallic structure to the electrolyte (e.g., soil), an anodic corrosion reaction takes place, resulting in oxidation (dissolution) of the metal [2]

FeFe2++2e.

A practical limit for considering the corrosion negligible is a current density of jcorr = 2 mA/m2, which corresponds to a corrosion rate

vcorr=kjcorrof 2.34 μm/year for iron (k = 1.17 (μm/year)/(mA/m2)) [3]. As the current density cannot be easily measured, the electric potential is taken into account, and the corrosion rate can be determined by means of potential or longitudinal voltage measurements, as described in [4], [5], respectively. The dc flowing in the soil may interfere with surrounding or crossing metallic structures (such as methane and gas pipelines [6], and cables), as well as with other rail transit systems such as the new ac high-speed lines, especially where there is an extended and very close parallelism between the two systems. The stray current may then be collected by the current return circuit of the ac high-speed line (formed by the earth wire in parallel with the running rails) and flow through all the metallic fixed installations connected to it (such as masts supporting the overhead contact line, sound deadening/proofing barriers, ladders for access to piers and parapets, to name a few). The interference may be particularly significant for steel reinforced concrete transportation infrastructures, i.e., civil structures, such as bridges, viaducts and tunnels, whose service life is indeed limited by the potential corrosion of the reinforcement. If adequate electrical insulation between the metallic fixed installations and the reinforcing bars (rebars) or prestressed bars is not provided, the stray current may reach the rebars flowing through concrete, which in this case is the electrolyte. Cathodic and anodic regions may then be formed in the rebars, with possible risks of corrosion [7]. Generally, the dc currents unlikely produce an actual corrosion on steel in concrete, conversely of what happens to metallic structures buried in soil, where the corrosion attack can have a severe outcome. In fact, concrete with alkaline properties and free of chlorides passivates the steel, thus increasing its resistance to stray currents. However, in particular environments the alkalinity of concrete can be neutralized, e.g., by carbonation (caused by environmental carbon dioxide) or by chloride contamination. Under these particular circumstances, passivity of steel may decay and corrosion can be initiated, although two conditions on the electric field inside the concrete are necessary: it must be strong enough to force the current to flow through the rebars, and it must last long enough to produce acidification and destroy the steel passivity [3].

In order to control the effects of stray currents generated by the operation of dc traction systems, mitigation measures should be introduced at the source of interference and, if these are impractical or ineffective, at the interfered structure [7], [8], [9]. As regards dc traction systems, requirements for protective provisions are specified in the Standard EN 50122-2 [5] where, in particular, the generic need to keep rails and connected metallic structures insulated from earth is mentioned. EN 50122-2 applies to all metallic fixed installations part of the traction system and to any metallic structure buried in the ground. For ac high-speed lines, the adoption of protective provisions to limit the intensity and effects of stray currents is usually specified as a contractual requirement. These protective provisions must be adopted during the design stage of the railway infrastructures in order to avoid subsequent expensive modifications and consist essentially in limiting the stray current path to the reinforcement of a single span only by insulating adjacent spans with insulated joints and the spans from the piers by insulated bearings. The main requirements for protective provisions for both low- and high-speed lines are detailed in Section 2.

This paper, the first of a series of two contributions, shows the measurement results of the insulation resistances of the concrete structures of an Italian high-speed rail transit system viaduct during the various stages of viaduct construction. The viaduct named “Ticino”, on the Italian high-speed railway line between the cities of Milan and Turin, was chosen for the experimental campaign as it was still in the early stages of its construction. The measurements were carried out in three sections of different characteristics, detailed in Section 3. A discussion of the measurement results is given in Section 4. In the second part of this series [10], the measurement results are interpreted by means of an equivalent circuit in order to show the dependence of the insulation resistances on the earth wire to rebar resistances and an assessment of stray current interference is given.

Section snippets

Protective provisions for rail transit systems

Owing to the number of metallic structures bonded to the earth wire (running rails, masts, lightning protection wires, earthing rods, sound proofing/deadening barriers, and electrical equipment), the new ac high-speed rail transit systems may offer low impedance return paths to the rectifier station for dc stray currents. In particular, this occurs when the separate distance between the dc and high-speed rail transit lines is small, i.e., at the junctions between railways, at bridge crossings

Experimental measurements and discussion

The “Ticino” viaduct, shown in Fig. 1 and represented schematically in Fig. 2, is composed of N = 34 spans which lie on 33 support piers. Each span has a length of 34.5 m and is composed of two caissons, each lying on a pier through 2 bearings (4 for each caisson, for a total of 8 bearings for each span). The bearings have then a twofold function, to allow the correct transfer of load and to provide electrical insulation between spans and piers. The abutment numbered 1 in Fig. 2 is towards the

Discussion

In Fig. 7, an equivalent circuit of the first two spans of the viaduct is depicted; the circuit is simplified as the resistances of rebars are neglected and the earth wire and the earth wire to rebar resistances are not represented. Rj is the resistance of the insulated joints, Rbf, Rbfs and Rbsg are the resistances of the bearings at the ends of each span, and Rg is the ground resistance of the piers. The measurement points are numbered 1 (Turin side) and 70 (Milan side) in the abutments, from

Conclusions

The insulation resistances of a viaduct of the Italian high-speed railways are investigated during the various stages of the viaduct construction for the assessment of stray current interference. The measured insulation resistances between a span and its supporting piers and between two adjacent spans in a viaduct section show a variation in time, which is related to the progress in the completion of metallic fixed installations; moreover, the insulation resistances of a measurement session

Acknowledgements

This work was supported by the High-Speed Consortium Turin-Milan “C.A.V.TO.MI.”, Italy, under contract no. 6400004583.

The author thanks Mr. Giovanni Santi of C.S.F.C.E. (Italian Commission for the Study of Electrolytic Corrosion Phenomena), Bologna, Italy, for valuable discussions and useful suggestions and Mr. Marco Landini of the Department of Electrical, Electronic, and Information Engineering “Guglielmo Marconi”, University of Bologna, Bologna, Italy, for taking part in the measurements.

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