Electrical behaviour of the wheel-rail contact

Proceedings of ICEC-ICREPEC2012

Electrical behaviour of the wheel-rail contact F. Houzé1, H. Chollet2, P. Testé1, X. Lorang3, F. Loëte1, R. Andlauer1, S. Debucquoi2, F. Lerdu4, M. Antoni5 1

LGEP, UMR CNRS-Supélec 8507, Universités Paris-Sud 11 et UPMC, 11 rue Joliot-Curie, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France. 2 Université Paris Est, IFSTTAR, Boulevard Newton, Champs sur Marne, F-77447 Marne la Vallée Cedex 2, France. 3 SNCF, Direction de l'Innovation et de la Recherche, Département Physique Ferroviaire et Confort, Groupe MDS, 40 Avenue des Terroirs de France, F-75611 Paris Cedex 12, France. 4 SNCF, Laboratoire d’Essais Electriques (IGLE), 9 quai de Seine, F-93584 Saint Ouen , France. 5 SNCF, INFRA, 18 rue de Dunkerque F-75010 Paris, France.

Abstract — In railway signalling systems, a shunting principle is frequently used to detect the presence and position of a train on a track section. In some situations, the detection can become difficult essentially because of the electrical behaviour of the wheel-rail contact. This situation is called a shunting malfunction problem. To be able to fit rolling stock and/or the track circuits, the only knowledge of the various responses of the track circuit voltage receptor does not suffice and a more robust understanding of physical phenomena involved has become necessary. This paper focuses on two experiments dedicated to the study of the electrical wheel-rail contact. The first experiment concerns measurements on a reduced scale bench in laboratory, whereas the second aims to investigate the same parameters in the context of a real site with an instrumented train on a portion of commercially used track. In both cases, static contact and rolling contact have been studied, however only the rolling contact, which is clearly the worse case, will be considered in the paper. Time transient current-voltage characteristics have been measured and analyzed varying some main parameters such as voltage and current range, signal frequency, applied load. The results obtained on the reduced scale bench prove to be in good agreement with those from site tests. A contact voltage saturation phenomenon was observed, which is discussed in the light of recent papers relative to Branly effect. Keywords : wheel-rail contact; shunting malfunction; oxidized steel; voltage saturation; Branly effect.

I.

INTRODUCTION

In many countries, train locating on railway lines is performed using track circuits based on an electrical shunting detection principle. Lines are divided into a succession of sections a few hundred meters long, each section being equipped with an “emitter” (sinewave current generator) at one end and at the other end a “receptor” (residual rail to rail voltage measurement). When a train penetrates a section, its wheels and axles act as a shunt and short out the track circuit, thus modifying the measured voltage signal ; an appropriate triggering is then used to conclude about section occupancy. However the reliability of this train detection system critically depends on the quality of the wheel-rail contacts. A degradation of these contacts resulting from corrosion (rust) and/or contamination by non conductive elements (e.g. dead

leaves) can affect the voltage measured by the track circuit and consequently the triggering operation. In some extreme cases the residual voltage can be locally over the threshold value, inhibiting train detection for a while : this situation is called a shunting malfunction. As such events have potential repercussion on signalling and traffic control, the concerned railway companies have of course developed technological parades to reduce the occurence of this problem, without a total eradication could be obtained. From a modelling viewpoint, an external non-linear electrical model of the wheel-rail contact based on series of tests on two French sites has been proposed in 2006 [1,2]. A quite satisfying agreement between simulations and measurements was obtained but without giving any physical interpretation of the fitting parameters. More recently, the behaviour of the interface layer between wheel and rail, named as “third body”, was investigated by various microscopic analyses and numerical simulations using a discrete element method [3]. This approach underlined the multi-scale and multi-physical aspects of the problem but unfortunately was not carried on further. From a phenomenological viewpoint, a Japanese team pointed out some main parameters influencing the wheel-rail contact resistance : rust thickness, surface roughness, car load, current magnitude [4]. A measurement campaign on multiple lines allowed to quantify how rust thickness depends on environmental conditions (industrial, seaside, rural or mountain sites) and on train density (a high density being defined as 6 trains/hour or more). The influence of the mentioned parameters on the contact resistance was extensively studied on an equipment simulating realistic wheel/rail contact conditions at low speed. The results agree with previously reported tendencies but no theoretical neither practical conclusion can be drawn. Considering still open questions concerning mechanical and electrical phenomena involved in the wheel-rail contact, the present study aims to bring a contribution to the understanding of the shunting malfunction problem, through the original angle of statistical analysis of time transient current-voltage characteristics measured both in the context of a real train on a line site and on a reduced scale bench in laboratory.


II.

EXPERIMENTAL APPROACH

A. Laboratory Tests on a Reduced Scale Bench The scaled test bench used for this study has been originally designed and developed by INRETS (now IFSTTAR) for mechanical investigations and therefore involves similarity laws firstly based on mechanical considerations : geometry reduction of all dimensions (including size of contact area), conservation of contact pressure and materials, thus conservation of translational speeds. A general view of the equipment is presented in Fig. 1. The bench is mainly composed of a steel (R7, untreated) wheelset scaled at 1:4 ratio, where the tread is simplified to a pure 1:10 cone, applied on a roller equipped with two steel (XC48, untreated) discs, representing the two rails with a running surface profile of UIC60 type also scaled at 1:4 ratio. The wheel and rail-disc diameters are 228 mm and 440 mm, respectively. The rotational speed can be adjusted from 0 to 10 rpm (corresponding to a linear speed of a few tens cm/s). The lateral position of the wheelset with respect to the rail discs can be imposed or kept free to settle at the beginning of a rolling phase. In the first case, sensors allow to measure the lateral and longitudinal forces. A normal load representative of a real car weight per axle is applied to the wheelset through a pneumatic jack (the mass of the elements being less than 5% of the total load).

reference of “healthy contact”, rail surface was carefully grinded and brushed until a bright aspect was obtained, whereas the wheel surface remained as such (light natural oxidation). On the other side, intended to play the part of the “degraded contact”, an accelerated rusting up of surfaces was imposed by soaking the rail-disc periphery in a NaCl (4 Mol/l) aquous solution, the liquid being transferred to the wheel surface at the rolling contact interface ; after a few minutes the wheelset and rails were separated and naturally dried out at lab atmosphere for several hours, leading to a brown-orange colored rust layer.

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Figure 2. (a) : General principle of the electrical contact resistance measurements (four probe technique); on the left side, intentionally degraded surfaces (rusted); on the right side, “clean” reference surfaces.

Figure 1. General view of the 1:4 ratio scaled bench.

From this mechanical basis, some modifications were introduced in order to perform contact resistance measurements following the four-probe method. Electrical insulation between the two rail-discs and the frame was ensured using kapton films. For each wheel-rail contact, current feeding and voltage measurements were realized by three copper thread brushes pressed by spring holders against the internal side of the rail discs and wheels (see schematic principle and detailed view in Fig. 2). For the experiments reported in this paper we used an AC power supply acting as current source (maximum peak value 2 A) with voltage limitation (maximum value 10 V). Current intensity value was obtained through the measurement of the voltage drop at a 1 Ω resistor terminals. As the characteristics of the steel external layer is a critical parameter, a particular attention was paid to the preparation of the wheels and rails running surfaces. On one side of the bench, taken as a

(b) Figure 2. (b) : Detailed view of the “clean” reference wheel-rail contact with copper brushes for current feeding (the foreground one) and voltage measurement (the two background ones).

B. Site Tests on Real Train and Railway Line Parallel to lab investigations on the scaled bench, a campaign of electrical measurements has been performed by the SNCF (French railway network company) services, with an instrumented railcar on a site in Normandy countryside (northwestern France). The site consists in a portion of straight track also used commercially, selected on the basis of previous measurements (residual rail to rail voltage has been recorded and analysed during a 6-months period before the tests).


The current feeding was ensured using the track circuit. A specific set-up has been designed by the SNCF engineers to equip the head wheelset of the railcar and perform wheel-rail voltage drop measurements as the train is moved forward (Fig. 3). On each side a pair of copper running wheels is pressed against the train wheel and another one against the rail, in the vicinity of the running surfaces, but without encroaching on them (to avoid any possible modification resulting from these auxiliary contacts). The adjustment of the pressing load is the key point for the success and reliability of these voltage measurements. The current intensity through the axle of the head wheelset is obtained through a Rogowski coil. Successive sets of electrical data were recorded as the train slowly moved (∼ 1 m/s) along a 100 m portion of line. Previously rust and greasy contamination were rubbed off on a section a few meters long in order to have a “clean rail” reference. Figure 4. Typical evolution of voltage drop measured on clean wheel-rail contact members (grey curve) and rusted ones (red curve); the left part of the graph corresponds to the static load application, the rest to rolling conditions.

EXPERIMENTAL RESULTS

For the whole study, both static contact and rolling contact have been considered, as well as various electrical conditions (AC or DC supply, acting as current or voltage source). Nethertheless we will only focus in this paper on the rolling contact situation (which is the most severe from the shunting problem viewpoint) and on experimental parameters close to the mainly encountered track circuit configuration on the French railway (AC 2kHz, current source).

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In Fig. 4 is plotted the typical evolution of the wheel-rail voltage drops U1 (red curve, rusted contact members surfaces) and U2 (light grey curve, clean surfaces) recorded during a run. Firstly the normal load (2.1 kN) was progressively applied in static conditions, then the roller rotation was set in action (left part of the graph); after which about 3 rail-disc revolutions were performed (black arrows indicate guide marks). Unsurprisingly the signal relative to the reference clean contact keeps very low values and exhibits few fluctuations. Conversely the signal corresponding to the rusted elements is erratic and seldom at low values; lots of peaks at the source voltage limitation (10V) are observed, revealing the existence of a succession of transient high contact resistance situations.

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Figure 3. View of the set-up for wheel-rail voltage drop measurement.

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If we zoom in this global record to see the detail of signals waveforms, different situations can be observed. Fig. 5 shows the two extreme behaviours corresponding to a healthy contact (a) and a totally degraded one (b). In this latter case there is no more current flowing and the source voltage limitation (10 V) is reached.

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Figure 5. Detailed view over a few periods showing the two extreme possible behaviours (the blue curve corresponds to current signal, the red curve to wheel-rail voltage). (a) healthy contact, (b) insulating contact.


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Figure 7. Typical plot of the density of occurence probability of I-V values for the rusted wheel-rail contact during a rolling test (in V-1A-1 units).

From this graph two main observations can be done : (i) a majority of I-V curves (red ‘crest’ of the plot) are nearly linear; cases with major signal distorsion, like those shown in Fig.6.b and 6.c, have clearly smaller occurence; (ii) a saturation of the contact voltage drop is evidenced (edge of blue ‘wings’ of the distribution), at a range of 0.4 V - 0.5 V; this phenomenon has been observed for other experimental conditions of current intensity and applied load.

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propose a statistical approach consisting in the discretisation of current and voltage values and the counting of occurences of any (Ip,Vq) couple. Fig. 7 shows a typical plot of the calculated distribution of I-V values, i.e. the density of occurence probability (rendered by a color gradation) versus the current intensity and voltage drop for the rusted wheel-rail contact (on axes), relative to a rolling test performed with conditions I = 1 A peak, f = 2 kHz, F = 10.5 kN. Such a high load level (representative of reality) has proved to lead to a tiny minority of non-conducting situations as the one shown in Fig. 5.(b). The current and voltage increments for data treatment are 22 mA and 18 mV, respectively.

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Between these two extreme situations, a variety of intermediate behaviours can be found, as attested by the three examples given in Fig. 6. Such curves express a more or less increase of contact resistance with respect to the reference case, as well as a more or less pronounced non-linearity.

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Figure 6. Detailed view over a few periods showing three intermediate possible behaviours (the blue curve corresponds to current signal, the red curve to wheel-rail voltage), showing : slight distorsion of sinus waveform (a), more pronounced distorsion with triangular (b) or square (c) shape.

Regarding the huge number of periods for a whole test, such a detailed observation in an exhaustive way is unfeasible. However it seems important to know to what extend are representative the various intermediate behaviours revealed by a few random zooms as those presented above. Therefore we

B. Measurements on the Normandy Site The same statistical treatment has been performed on electrical data recorded by SNCF engineers on the instrumented railcar moving slowly on the 100 m long test portion of line. Track circuit was varied from 700 mA to 5 A rms. Fig. 8 shows the calculated distribution relative to a run with a 1 A rms signal applied. The I-V distribution looks quite similar to those resulting from the lab rolling tests. The only differences are (i) a secondary ‘wing’ of the distribution (of linear shape and lower impedance) corresponding to the few meters of cleaned rails, and (ii) the confining of the main ‘crest’ (again nearly linear) at small values ; when varying the generator signal the trend for the most probable I-V curve is that the higher the current intensity, the lower the equivalent resistance. A saturation of the contact voltage is also observed, in nearly the same range of values (0.4 V, light blue – 0.6 V, dark blue) as for the lab measurements. Lastly, it is worth mentioning that a fine analysis of the foremost signal cycles, just after current is applied, at a very low speed of the train, has revealed a transition from an insulating to a conductive state in the wheel-rail contact interface.


damages the film and leads to wider conductive channels : the current begins to generate a significant Joule heating. When softening temperature is locally reached the mean radius of the microcontacts strongly increases by several orders of magnitude (non linear behaviour), drastically improving conduction. At high enough current this electro-thermal process can lead to the local welding of the microcontacts ; the oxide film is thus pierced in some places where purely metallic bridge are established.

Figure 8. Typical plot of the density of occurence probability of I-V values for the wheel-rail contact during a test on Norman site (in V-1A-1 units).

IV.

DISCUSSION AND COMMENTS

As it has been observed in Figs. 7 and 8, the evolution of the current intensity versus the contact voltage may become nonlinear as soon as the contact is oxidized. Moreover, a saturation of the contact voltage drop occurs for values in the range 0.4 – 0.6 V; this saturation proves not to depend on the current intensity through the contact. Thinking it over, the most pertinent explanation of this phenomenon seems to be searched in the literature relative to electrical transport properties between oxidized metallic bodies. We would particularly suggest a possible connection with the so-called Branly effect [5], an electrical transition from an insulating to a conducting state for a critical (low) applied voltage which has been evidenced at the end of the 19th century in metallic granular media. More recently [6,7] this phenomenon has been studied in two academic situations : (i) a cylinder filled with compressed copper powder having a calibrated density and a controlled grain size; (ii) a cylinder filled with a one dimension chain of steel balls to which a controlled pressure is applied. In both cases the involved electrical contacts are motionless. At low applied current, the I-V characteristics are reversible and ohmic, with a high constant resistance value. As the current is increased, the resistance suddenly and strongly decreases for a bias voltage independent of I and the contact modification proves to be irreversible. For both experiments, the voltage at the cylinder terminals keeps then a constant value. In the case of the linear chain of steel balls, the constant voltage expresses as Nball-ball contacts × Usat , where the saturation voltage Usat was found equal to 0.4 V. It was interpreted as the softening voltage for steel [8] thanks to the Kohlrausch relation between the contact voltage and contact temperature. The explanation proposed by Creyssels et al. may be summarized as follows. If we consider a mechanical contact between two metallic elements covered by a thin oxide film, the interface will consist of a set of microcontacts due to the roughness of surfaces. At low applied current, the high value of the contact resistance corresponds to a more or less complex conduction path found by the electrons through the film within each tiny contact spot. The electron flow progressively

The wheel-rail case presented in this paper is quite different because of the relative motion of the contact elements. The mechanical contact zone perpetually changes as the wheel is rotated and then the dynamic phenomena (evolution of mechanical area, electrical conduction and local heating) should be considered. However, it is a quite outstanding fact that we also observed a saturation voltage in the range 0.4 – 0.6 V for the two experimental scales. In the absence of physical data relative to the steel grades involved in our experiments, it seems hazardous to give a precise interpretation of the saturation voltage value in temperature terms. We will merely point out that the temperature range corresponding to 0.4 – 0.6 V is about 1050 °C – 1500 °C, which on the basis of usual data for steel stands between softening temperature and fusion temperature. As we mentioned at the beginning of the set-up description, the scaled bench used for this study involves similarity laws based on mechanical considerations (conservation of contact pressure and translational speed). With the 1:4 ratio, lengths are multiplied by a factor 1/4 and surfaces by a factor 1/16. The last point of our discussion will be a reflection regarding how the electrical excitations could also be adapted to the device scale in order to make the laboratory bench fully representative of real railcar on track experiments. Actually all depends on the electrical parameter to be considered as crucial : - If the key parameter is the contact voltage drop, then the current intensity in the contact has to be taken 4 times smaller on the bench than in the site experiments (since on the basis of the Hertz-Maxwell law, the contact resistance for the full scale experiment is four times smaller than for the lab bench). - If the key parameter is the power density dissipated in the contact, then in order to keep the same volume Joule heating term ( ρ J² ), the current density has to be the same and thus the current intensity on the lab bench must be taken 16 times smaller than in the site tests. - If the key parameter is the power per surface unit brought in the contact, then the current intensity on the lab bench has to be taken 8 times smaller than in the site tests. The above examples are not restrictive and other choices are still possible according to the physical phenomena that may occur in the contact. V.

CONCLUSION

Measurements of the electrical behaviour of a rolling wheelrail contact have been performed both on a laboratory bench scaled at 1:4 ratio and on an instrumented railcar on a real track. Current and voltage recordings along thousands of signal periods have allowed us to develop a fruitful statistical approach regarding the I-V curves typology. Whatever the


load and speed values considered, nonlinear characteristics have been observed and a saturation of contact voltage drop has been evidenced. A possible interpretation of this phenomenon is suggested in terms of an electro-thermal process similar to the one recently proposed to explain the transition of conduction between oxidized metallic bodies (refered to as Branly effect). Further investigations will focus on the transition between static contact and rolling contact. A particular attention will be paid to very low speeds, allowing to control the fraction of the contact area renewed during a signal period.

[2]

[3]

[4]

[5] [6]

[7]

REFERENCES [1]

L. Oukhellou, “Modèle électrique externe du contact roue/rail identifié à partir des essais de Gault St-Denis (Expertise ZEST)”, INRETS Final Report (in French), 2006.

[8]

P. Aknin, “Modèle électrique externe du contact roue/rail identifié à partir des essais de Plouaret (Expertise ZEST)”, INRETS Final Report (in French), 2006. S. Descartes, M. Renouf, N. Fillot, B. Gautier, A. Descamps, Y. Berthier and Ph. Demanche, “A new mechanical-electrical approach to the wheel-rail contact” , Wear, vol. 265, pp. 1408-1416, 2008. M. Fukuda, N. Terada and T. Ban, “Study of quantifying and reducing electrical resistance between wheels and rails”, QR of RTRI, vol. 49, n°3, pp. 158-162, 2008. E. Branly, “Variations de conductibilité sous diverses influences électriques”, C. R. Acad. Sci. Paris, vol. 111, p. 785, 1890. E. Falcon and B. Castaing, “Electrical conductivity in granular media and Branly’s coherer : a simple experiment”, Am. J. Phys., vol. 73, pp. 302-307, 2005. M. Creyssels, E. Falcon and B. Castaing, “Experiment and theory of the electrical conductivity of a compressed granular metal”, Proc. 6th International Conference on the Micromechanics of Granular Media, pp. 123-126, 2009. M. Creyssels, “Quelques propriétés du transport électrique dans les milieux granulaires”, Ph. D thesis, ENS Lyon (France), 2006.