Introduction
The contact wire is a very important element of the electric overhead railway system. In order to be installed, this contact wire has to meet a complex system of conditions which impose strict requirements to the manufacturing process. For over a decade, FUX SA has been dealing with the manufacturing of overhead wires and electric conductors, among which the production of the contact wires used in electric traction. FUX SA meets the strict manufacturing conditions imposed thorough continuous development, achieved not only in collaboration with its partners, but also through different ideas and in-house research and development activities. This article presents the outcome of such studies aimed to help the fitters and the operators of the contact wires manufactured in our company.
The contact wire has to meet a series of requirements, both as recently manufactured product and after a long period of exploitation. Regulations make no distinction between the ages of the contact wire, but the exploitation system would not permit this distinction either. However, there is a small, yet significant difference between a recently manufactured wire and one used for a longer period of time. In our development activity, we have tried to reduce this difference that exists between two contact wires of different ages, so as to produce the same higher quality in conformity which the proper requirements imposed by the standards and regulations in force.
In-house development has been very helpful, as we have completed our integrated quality management system with a diagnosis laboratory for wires and conductors. Apart from conventional standard tests, this lab helps us test contact wires in operation conditions with simulated loads. As this article shows, we have greatly relied on this opportunity.
Our idea relies on the fact that the surface of the contact wire deteriorates through the action of corrosion processes. Changing the surface quality changes the heat exchange between the contact wire and the environment. A recently manufactured contact wire, with smooth and bright surface has a heat emission coefficient that varies between 0.1-0.15. A contact wire that has been used has a surface heat emission coefficient of 0.75. Of course, this doesn’t apply to the contact surface, which is a small-surface element, compared to the whole surface of the contact wire.
This difference in the heat emission factor is apparently higher, although it doesn’t influence the operation under load of the contact wire and doesn’t cause a different exploitation on the whole contact wire. However, this difference has implications for the operational part, which could be positively exploited for a certain case. For this reason, our products include the possibility to apply a surface treatment to the contact wire. Our company offers this possibility of treating the surface of all types of contact wires manufactured in conformity with customers’ specifications.
By treating the surface, we reduce the difference between a new contact wire and an older contact wires from two important aspects. One of the aspects is related to the traction capacity of the contact wire, and the other is related to removing ice, frost or rime deposits specific to winter. These deposits are all related to the heat which is developed and conveyed in the contact wire during operation. That is why we only consider the environment and operation conditions which define the nominal electric energy for the contact wire.
The electric load of the contact wire is mainly determined by its heat treatment. Increasing the load power increases the heat and the temperature of the contact wire respectively. In turn, it influences its resistance to mechanical stress. The temperature can only increase to values to which mechanical stress doesn’t produce irreversible, damaging and unwanted changes. In this case, it is very important to observe if the contact wire is loose.
The most economical method of manufacturing contact wires is by cool drawing. During the cool drawing technological process, the break resistance and toughness increase, but the pull decreases, while the electric resistivity slightly increases. Through the cool drawing, due to the structural dislocations produced, metals become more resistant to future form changes. This is apparently beneficial, because a conductor more rigid and more resistant to friction erosion is obtained. On the other hand, the regulatory provisions concerning the rigidity and electric resistance of the contact wire limit the excessive process of cool drawing. Apart from the positive effects obtained in metals after cool drawing formations, unfortunately, the metals thus processed reach a metastable state. The dislocated structure ensures an enlarged enthalpy compared to that before the plastic deformation and remains unchanged only to low temperatures (for example, the ambient temperature) where atomic movement is very lent.
As the temperature increases enough to move the atoms in the metals to significant speeds, the dislocated structure is retransformed and the material break resistance and toughness drop, the drawing increases and the electric resistance is slightly reduced. The softening share depends on the measure of the plastic deformation, but also on temperature. The tender book includes the extent of plastic deformations, but they cannot be considered. In turn, temperature remains a very important parameter for plastic deformation. The higher the temperature is, the faster the comeback process. By reversing the thinking method, we can determine a value of the operation temperature to the contact wires where the factory plastic deformation remains unchanged. In turn, this temperature value determines the nominal electric load in the contact wire. In order to determine the nominal electric load we have to study the heating of the contact wire in different operation regimes. Supposing a balance thermic state, the temperature of the contact wire is determined by the heat issued by the load current through Joule effect, by the sun heat, and by the heat released through radiation and convection. The first two heat components lead to temperature increase, while the other two components reduce the temperature. The heat produced through Joule effect depends on the wire resistivity and on the load current. The main influence is that of the current; the higher the current, the higher the heat generated during the set timeframe.
The heat generated by sun radiations depends on climate conditions and on the irradiated surface of the contact wire. A shiny, reflective surface connects less heat in the wire than a mat, non-reflective surface. Considering the set specific environment conditions, the temperature growth due to sun radiation is not variable, but it can be considered according to those included in the design standards. Therefore, our study on temperature growth can only be reduced to the effect of the Joule heat.
The most important parameters in designing the material of the cool drawing contact wire are: break resistance, electric resistance, extension coefficient and the softening one respectively. The latter coefficient is directly connected to the thermal stability of the material rigidity, which is the T0.5 value of the temperature to which the 1 hour heat treatment of the material reduces by half its mechanical rigidity.
The objective of our surface treatment was to increase the heat emission of the wire and thus to increase the capacity of releasing heat through emission. Thus, we can reduce the load operation temperature of the contact wire, although we can operate to an increased load current in the contact wire to the same T0.5 value of the temperature. With this procedure, while using the same material quality and the same constructive standard, we can obtain a contact wire with broader scale use.
In the course of the surface treatment, our goal is not to change the break resistance, the extension resistance and the electric conductivity. The process has to be developed so as to avoid the deterioration of these parameters.
Of course, this method does not lead to a substantial increase of the load nominal current value, but it can increase the actual traction capacity of the overhead line. Instead, the treated surface contact wire has another advantage. In the case of classical contact wires, during winter, the ice, rime and hoarfrost deposits worsen the quality of current collection or can even deteriorate de contact wire. The ice has to be removed before setting the line operation. The surface treatment is thus conceived so that water would not set on the contact wire in large drops and so that the ice would not form massive frost deposits on its surface. Therefore, large water drops glide more easily off the wire and instead of solid ice, a sort of rime deposits on the wire which can be removed with the locomotive pantograph at smaller risks.
Moreover, increasing the heat release capacity of the contact wire enables the total or partial removal of ice through heat effect caused by a smaller initially pre-set load. This way, the safety of electric traction can be increased and energy consumption can be reduced. The process of contact wire surface treatment developed by our company has imposed these main goals.
We continue by presenting the results of lab tests aimed at proving the utility and efficiency of our surface treatment procedure.
For the presented tests, the Cu-ETP contact wires were manufactured and standardised in conformity with EN 50149 type AC80, AC100 and AC120. The products have been manufactured through cool drawing of a cylinder conductor manufactured through the Properzi procedure. Samples of corresponding dimensions have been cut from the contact wires. Half (right) of the contact wire sample has been treated on its surface, the other half (left) has been left untreated (Figure 1). Tests have been carried out in the laboratories of FUX SA company.
After the surface treatment, the contact wire samples have been submitted to quality controls. Then, they have been heat-treated for one hour in an electric oven with Borel-type convection at temperatures of 200, 250, 300, 350, 400, 450 and 500 °C. After each of the specified heat treatment, the samples have been qualified again to determine the value of the T0.5 temperature.
After these tests, the contact wire segments have been introduced in our load current testing devices, where we have analysed the differences in heat emission to the treated and the non-treated segments with the help of a heat camera.
Surface behaviour of a new contact wire
Each contact wire manufactured by our company is submitted to rigorous quality tests. During these tests, we mainly check the break resistance, the extension and the electric resistivity of the contact wire. For the three types of contact wires, we have carried out these tests with samples from both the surface treated and non-treated segments.
Table1. Includes measurement results, and in Figure 2. These results are compared through column diagrams.
Both Table 1 and Figure 2 show that the mechanic and electric parameters of the same type of contact wire are not different; therefore, the treated surface does not change these parameters. Moreover, we can see that the values observe the relevant conformity standard and that the contact wire with treated surface can perfectly replace the contact wire with non-treated surface. Therefore, the first objective mentioned in the introduction has been achieved.
In order to reach our next objective, the contact wire samples with treated surface have been submitted to heat treatment in an electric oven at temperature varying between 200 and 500°C, in different steps of 50°C, for one hour. Afterwards, the samples have been left outside to cool at ambient temperature. After a one-day resting, they have been submitted to the same testing measurements as the recently manufactured ones.
The main tested parameter has been the break resistance, but extension and electric resistivity have also been measured, so as to have complete comparison data. The results are included in Table 2. The comparison between the softening level to the contact wires with treated and non-treated surface is presented in Figure 3.
Both Table 2 and Figure 3 show that the softening of the same type contact wires is not very much different because of surface treatment. Break resistance and electric resistivity reduce similarly and extension increases with the temperature increase. This means that surface treatment doesn’t change the value of the T0.5 temperature, which in the case of the contact wire AC80 is … °C, for the contact wire AC100 is … °C, and for type AC120 is … °C. We can thus conclude that the second objective has also been achieved and the contact wire with treated surface can be used in design or in exploitation just like the one with the non-treated surface. Therefore, the contact wire with the treated surface can replace the classical contact wire with no changes needed.
In introduction, we have showed that if the heat emission of the contact wire is increased, the heat enlargement per time unit is high and we can manage to obtain a line with smaller exploitation temperature. On the contrary, if this exploitation temperature is given, the contact wire can operate to a larger load current and it can only be achieved by increasing the heat emission capacity. The heat emission is characterised by the emission coefficient which has values ranging between 0.1-0.15 for the contact wires.
Based on theoretical calculation for a limited exploitation temperature, we can find out the dependency of the maximum load current depending on the emission coefficient. (Figure 4)
This is a well-known phenomenon, because the emissivity of the overhead line increases due to the oxidative influence of the environment and can reach up to 0.75. However, this cannot be completely used to increase the load current charging capacity because wear during operation makes the transversal section of the conductor smaller. This also requires that every segment should have the same age. In conclusion, the increase of emission coefficient has to be artificially generated.
The tested contact wire has been installed in a high current circuit of a testing device created by the company. Half the sample of the contact wire had treated surface and the other half had not. During the testing operation, we have waited until the transitory heating regime passed and we have reached a thermal balance. This observation was achieved with the help of a heat camera.
In the heat balance state, we took photographs with the heat camera (Figure 5), as well as heat measurements with a contact thermometer. In this final case, we used an iron thermocouple that we electrically insulated of the contact wire with a thin layer of polymeric.
The heat camera photograph clearly shows that the temperature of the contact wire with treated surface is smaller than the non-treated surface. Both heat measurements made with the heat camera, the thermocouple respectively, showed that for a load current of 480A to which the AC100 contact wire was submitted, the temperature of the segment with treated surface was by almost 10°C smaller than that of the segment with non-treated surface. Similar values were obtained to the other two types of tested contact wires.
Tests were completed in laboratory conditions at an ambient temperature of 22 °C. While calibrating the heat camera with the help of the thermocouple contact thermometer, we managed to determine the heat emission coefficient of the contact wire. Measurements indicated that the surface treatment technology managed to increase to 0.5% the value of the heat emission coefficient. By using this value in the theoretical calculations presented in Figure 3, it results that we can obtain a 3-5% increase in the load current on the same section by using a contact wire of the same type, but with treated surface. The actual value depends, of course, of the real environment conditions. This growth, as previously shown, does not influence the mechanic or electric parameters and therefore it is not necessary to change design parameters. The third objective mentioned in the introduction is also achieved, namely the load current can be increased without changing the important mechanic and electric parameters, but also by increasing the contact wire heat emission coefficient achieved by treating the contact wire surface.
We have also mentioned in the introduction that this phenomenon is not the singular and most important effect. In winter, ice crystallizes on the contact wire surface endangering the quality and the safety of electric energy collection. Ice has to be eliminated during exploitation as it is capable to cause many problems. In extreme situations, it can cause intermittences in the collection of electric power or even fully cutting off power by breaking the overhead line. Both problems cause major risks for the electric traction in ensuring its seamless exploitation. Another problem results from the fact that railway traction stresses the overhead line per sections, by heating the contact wire for the period of time when the section is stressed and then leaving it to cool. This helps ice deposits when it is cold. Eliminating ice is a current problem today and sometimes it is very difficult to solve.
Ice forms on the contact wire in two ways. If the temperature, humidity and wind have values between corresponding limits, rime with a porous texture and with small mechanic resistance sets on the wire surface, which is usually eliminated by friction to the locomotive pantograph. If environment and exploitation conditions change, it is possible that solid ice will form on the contact wire, especially in suspension points. The deposit of solid ice can be so strong that it cannot be simply eliminated with the pantograph. Moreover, an ice layer may form so thick that not even the exploitation heat could melt it.
Our process of surface treatment improves this situation in two ways. The first one is that the air steam on the contact wire surface doesn’t condensate in large water drops, but in smaller water drops. They glide easily and eliminate the occurrence of a thick ice layer. A thin ice layer is formed instead made of small water drops with the structure similar to rime.
To demonstrate, we have divergently sprayed water from a distance of 100 mm on both sides (treated and non-treated) of the contact wire. Figure 6 shows how the sprayed small water drops gather on the non-treated surface of the contact wire, forming large drops. It is easy to imagine how these drops transform into ice at low temperatures and cause the described problems.
Figure 6 shows that large drops don’t appear on the treated surface of the contact wire, the sprayed water drops glide individually off the wire surface. In case of frost, crystallization begins with these small drops, so, on the one hand, the chance of a thick ice layer is small and, on the other hand, the thin ice layer is easier to remove mechanically or thermally.
The second way in which the treatment of the contact wire surface prevents ice is related to the increase of the thermal emission coefficient through which the treated contact wire releases more heat in the environment. The environment can be a direct one or even the ice layer. In case of frost on the contact wire, it can be submitted to a pre-set electric loan which partially or totally melts the thin ice layer which can then be easily removed by the locomotive pantograph. These conclusions come from both the theoretical calculation mentioned in the introduction and from the tests with water sprayed on the contact wire.
Abstract
Our company is committed to the continuous improvement of its products. By developing innovative products and technologies, we propose to rise quality levels and the operational safety of all our products, either contact wire conductors, wire cables for the automotive industry or energy industry or contact wires for railway or road electric transport. In most cases, the development activity is carried out together with our customers.
This paper has presented the results of the tests applied to contact wires which had been previously submitted to a surface treatment process, representing a result of our latest innovative activities. By correspondingly treating the contact wire surface, our aim was to obtain an increase of the nominal electric load on the same section and to increase the resistance of the contact wire to ice, thus increasing the safety of railway traffic.
For measurements, we have used samples made of contact wires with different section using Cu-ETP or CuAg1materials. The contact wires have been manufactured using in-house technology where the fabrication line has been completed with the surface treatment equipment.
The mechanic and electric properties of the contact wires were tested and the conclusion was that the recently manufactured treated contact wires are similar to those non-treated. Moreover, the new contact wires fully meet the standard specifications.
An important design parameter is the T 0.5 value of temperature which determines in fact the thermal stability of the contact wire. This value, in the course of primary mechanic design, determines the maximum exploitation temperature of the overhead line.
To determine the T0.5 value, we made a series of heat treatments in our lab and then we tested the mechanic and electric properties. We noticed that surface treatment has changed neither the value of the T0.5 temperature, nor the electric or mechanic properties. Following these results, we can conclude that the contact wire with treated surface can replace or change the same type of non-treated contact wire anywhere.
We have also introduced the contact wire samples in a special testing device at high load current. After setting the heat balance, we measured the temperature of the wire with a heat camera, namely a contact thermocouple. We noticed that for the same electric load (same current), the temperature of the contact wire with treated surface is smaller than that of the non-treated wire, so the emission of this type of contact wire is higher. With the values provided by the contact thermometer, we managed to calibrate the heat camera and to determine the heat emission coefficient which ranges between 0.1 and 0.15 for the contact wire with non-treated surface, while for the contact wire with treated surface, the value of this coefficient increased to around 0.5. This increase of the heat emission coefficient permits an approximate increase of 3-5% of the nominal load current depending on the environment conditions.
We also analysed the ice deposit by spraying water on the surface of the contact wire. We noticed that while the water sets in large drops on the non-treated surface leading to thick ice layers, small drops form on the treated surface leading to a thin ice layer with small mechanic resistance. Moreover, the heat emission coefficient of the contact wire increased with the surface treatment resulting in the reduction of the defrost current and therefore of the used electric energy and the easier removal of the ice layer off the contact wire.
The contact wire surface treatment process introduced in the final part of the fabrication line didn’t affect productivity. The raw materials were not changed and the treatment process can be applied to all types of standard contact wires manufactured by our company, which makes us capable to provide the above-mentioned advantages for all our products.
References
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2 A. Csoma, Actual questions on loadability of catenary wires.(2nd. part), Hungarian Rail Technology Journal, vol. XVIII/2, 2013, Magyar Közlekedési kiadó Kft.
3 Gy. Nemcsik, P. Barkóczy, Sz. Gyöngyösi, Development of the drawing technology of shaped wires, XIV. Conference on plastic deformation, Miskolc, Hungary, pp. 228-233 (2011)
4 D.A. Porter – K.E. Easterling: Phase transformation in metals and alloys, Chapman & Hall, London, 1996
5 P. Cotterill – P.R. Mould: Recrystallization and Grain Growth in Metals, Surrey University Press, London, 1982
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