Pantographs are among the elements assessed by accredited assessment bodies within the scope of approval of rolling stock. Various methods can be used to measure the forces acting between the pantograph carbon strip and the catenary as well as the wear associated with these forces. Methods differ depending on the EU Member State in question and the type of power supply used by its railway system. TÜV SÜD Rail presents the different methods of measurement.
The EU, and Germany in particular, has one of the world's most extensive railway networks. In the EU, rail companies have over 200,000 km of rail tracks at their disposal. Many of these routes are electrified and suitable for electrical rolling stock. The pantographs of electric drive units are in contact with the overhead line, which is also known as the contact wire. The element of the pantograph that makes contact with the catenary is called the carbon strip. When the train is in motion, the carbon strip is subject to wear from both mechanical and electrical factors. To keep mechanical wear to a minimum, the carbon wires do not run in parallel, but in diagonal lines to the tracks, thereby ensuring that use is made of the entire width of the pantograph's carbon strip. This can be seen very clearly when looking down from, say, a bridge to the overhead line.
However, the method of measurement applied in the approval process to assess the interaction between pantographs and catenary refers to the pantograph. The method applied by the test experts also depends on the railway power supply system. The EU Member States use both DC and AC networks. In DC networks, electrical wear caused by arcing is more common. In this case, the inspectors use the electric arc measurement method, which is recognised in some EU Member States. In most EU Member States, such as Germany, the electricity supply is based on AC networks. Here, electric arcs are quite rare, and the electric arc measurement method is not approved for these networks. Given this, the method of contact force measurement was developed. Parallel to applying one of these two measurement methods, the uplift of the catenary is always measured in line with the requirement of European Standard (EN) 50317.
A new method for measuring contact force
Contact force is the force acting between the carbon strip and the catenary. When the vehicle is at a standstill, this force is referred to as ‘static force’ and is almost constant. However, deviations from this value increase significantly in step with the speed of the rail vehicle. This means that mechanical wear increases if the contact force is set too high. Too little contact force, on the other hand, will favour the occurrence of electric arcs, and thus electrical wear.
Contact force is further influenced by the aerodynamic and dynamic characteristics of the drive unit, the number and distance of the uplifted pantographs per vehicle and the design of the overhead line. The coupling of several drive units to form a train also has certain impacts. The vehicles may be coupled in various ways, or pantographs of different designs may be used. Simulation models according to EN 50318 can be used to calculate the various operational variants. TÜV SÜD Rail inspection engineers developed the contact force measurement method to test critical variants.
High voltage, high demands
However, no measurement system that could meet the high demands was available on the market. The mechanical variables to be measured are around 100 Newton, which is very low. The fact that measurement had to be performed on the driving train, i.e. under live high voltage, was in itself a major challenge. There were several unclear aspects, including how electromagnetic fields impacted on the catenary, how the transfer of measurement data worked and how the sensor system is supplied with power. The weight and size of the sensors also plays a major role; they influence the pantograph's dynamics and aerodynamics, and may distort measurement results. A standard carbon strip weighs as little as 2 to 3.5 kg.
The designers finally decided on a conventional measurement system using electrical force sensors. A small lightweight sensor, ideally suited to the range of measurement and offering the required measurement accuracy, was custom-made for this system. While attaching the sensors below the carbon strip involves certain technical disadvantages, these can be compensated for by taking additional upstream measurements and making the necessary mathematical corrections. As the system is currently undergoing further development and improvement, these corrections will no longer be necessary in the near future. Subsequently, numerous tests were carried out on a test rig and the first pantograph was prepared for testing under real-life conditions. Testing was then performed in Norway under extreme climatic conditions at speeds of up to
220 km/h. After all tests were completed successfully, the contact force measurement method was subsequently applied several times and has now received EN 17025 accreditation.
Measurement of the uplift of the catenary
When the pantograph is lifted while the train is travelling, the overhead line oscillates. The maximum oscillation of the contact wire compared to its position when the train is standing is referred to as ‘uplift of the catenary’. The approach used to measure this difference is completely different from the method applied to contact-force measurement. Oscillations of the overhead line are similar to the screeching sound of train wheels that people hear before a train passes. As soon as a train with a lifted pantograph enters a section of the catenary, the contact wire starts to oscillate and will not return to its resting position until the train has left that section of the catenary. The uneven mass of the catenary reflects the forward propagation of the oscillation waves, which thus return to the pantograph. Given this, systems attached to the pantograph do not allow exact measurements. Neither the resting position to be used as reference value nor the maximum uplift can be measured in this manner.
The solution lies in stationary measurement systems, which are realised with the help of cable sensors that measure the differences in uplift at defined stationary points of measurement. However, to install these cable sensors the overhead line needs to be disconnected from the power supply and the relevant track section needs to be closed. Camera systems based on photogrammetry, by contrast, are portable and can thus be used in mobile applications. These systems film the catenary from a stationary point before, during and after the passing of a train. Evaluation of the footage is carried out on site. For this purpose, the movement of the catenary is scanned in terms of pixels per unit of time and then converted into a measure of length. This method has been accredited according to EN 17025. As measurement of the uplift must always be carried out, it is advisable to combine it with contact-force or electric-arc measurement.
How to avoid electric arcs
Electric arcs can occur if the contact force between the pantograph and the contact wire is very low or interrupted. They result in electrical wear and are indications of poor-quality power transmission between pantograph and contact wire. In rolling stock, electric arcs may even cause triggering of the main switch. Measurement must determine the length of the total burn duration of electric arcs in relation to the overall measurement time. This requires both optical recording and correct time measurement.
Here too, the experts had to develop a sensor that could be applied to various rail power systems and was in compliance with the relevant EU standards. Following a test period with a prototype not sensitive to solar light, which could be used at any time of the day and in any weather, this measurement method was also accredited according to EN 17025 and EN 50317. TÜV SÜD Rail thus offers all measurement methods relevant in EU Member States for measuring the interaction between pantograph and overhead line from a single source.
For further information please contact Dr.-Ing. Thomas Noack