Light rail transit (LRT) is an efficient and sustainable method of traversing urban areas. Powered by electricity, LRT systems are generally cost-effective, quick, and reliable. However, the possibility of stray currents from the rails corroding underground metallic structures is cause for concern. Damage to underground pipes or tanks, for example, can lead to costly repairs and replacement. In this blog post, we share an example of how modeling and simulation can be used to explore the effects of stray currents from a train on a nearby steel pipe, leading to improvements in design and corrosion mitigation.
Risks and Rewards of LRT Systems
As a fixture of most city landscapes today, light rail transit consists of electrically-powered light rail vehicles (LRVs). These short trains — evolved from trolleys and streetcars — meander through the streets alongside cars, pedestrians, and bicyclists, obeying traffic lights and signals. They run predominantly above ground at fairly consistent speeds of 10–30 mph on dedicated rights-of-way and typically receive electricity through overhead lines rather than an electrified third rail. “Light” in light rail transit refers to their lighter carrying capacity compared to heavy rail transportation, although they are also lighter in weight due to their smaller size.
A Utah Transit Authority Trax light rail vehicle traveling south on the green line in downtown Salt Lake City. This image is under the Creative Commons Attribution 2.0 Generic license, via Wikimedia Commons.
Because LRVs run on electricity, they are safer and less expensive to build, maintain, and power than nonelectric modes of transportation. Unlike diesel-powered freight trains, for example, LRVs do not need to carry large amounts of fossil fuels, which are expensive, pollutive, and potentially highly explosive. Electric trains have fewer moving parts and produce very low carbon emissions, making them an environmentally-friendly form of transportation.
Despite these considerable advantages, LRVs are not without risks, especially since a majority of them are DC-powered. With the use of DC — in contrast to AC, which is better suited for heavier and faster trains — stray currents are always faradaic and can induce electrochemical processes such as corrosion (see our Alternating Current-Induced Corrosion model for more information). Conversely, for AC traction that normally operates at a frequency of 16 2/3 Hz, currents are predominantly capacitive. Corroding buried metallic structures, such as pipelines, cables sheathed in metal, and storage tanks, are thus risks that DC-powered LRT systems can pose. This corrosion damage could result in costly repairs or even lead to dangerous situations, such as ruptured natural gas pipelines or loss of structural integrity in buildings and infrastructure, even the LRT itself.
What Causes Stray Currents?
The trains in DC LRTs normally operate with current fed from traction substations (TSS) through overhead lines, and the rails serve as conductors for the returning current. The rails are connected in parallel with the soil, and because they sit on ties and/or ballast (gravel) on soil, they can be regarded as being grounded along the entire length of the rail. At certain soil and design conditions, portions of the current can select alternative paths outside the rails. These portions are referred to as stray currents. The potential field created around the rails indicates the current path, with current entering metallic objects near the train position and exiting near the TSS. Reduction, usually oxygen reduction, takes place at the cathodic entry sites, and oxidation/corrosion takes place at the anodic exit sites.
A detailed schematic of stray current corrosion on buried pipeline. Image taken from Ref. 4, which is licensed under Attribution 4.0 International. No changes were made to the image.
How Can Stray Currents be Mitigated?
Some common ways to mitigate stray currents include decreasing the distances along which stray currents can form, for example, by increasing the number of rail connections and TSS. The electrical insulation between the rails and soils can be improved as well, although for this approach to be meaningful, the rails need to be completely disconnected from other metallic installations with low grounding resistance. Stray current drainage is a popular alternative but can be hard to oversee. Sometimes the use of nonmetallic objects, for instance polymer pipes, or repositioning of sensitive infrastructure are the only choices at hand.
Before implementing LRT systems in cities, the effects of possible corrosion from stray currents should be studied extensively. Simulation software enables analysis of this phenomenon without needing costly physical prototypes. Using mathematical models, engineers can simulate many different scenarios to predict, and therefore, avoid corrosion damage when the rail is implemented.
Monitoring Corrosion
The Stray Currents from a Train in a Light Rail Transit System model represents and analyzes the corrosive impact of stray currents on metal. The model simulates two parallel rails sitting on ties and gravel that connect two TSS over a 3D soil profile. The rails are poorly insulated and no other mitigations are present. A steel pipe is situated in the vicinity of the rails.
Around the rail and the pipe are different soil types. Here, the effects of the stray currents on the pipe, and how those effects change with the modification of soil conductivity and pipe position, are investigated when a train is passing.
Representation of the model’s geometry, including the rails, the pipe, and the variety of conductive soil.
Study and Results
In this example, the COMSOL Multiphysics® software enables users to define material related to electrochemical reactions involved in the stray current leaving and entering the rails and pipe. The Corrosion material library offers several material options. For each material in the soil, the user defines the individual electrolyte conductivities. The passing train is considered as a moving current source. The traction substations supply the train with propulsion current that follows a pattern of acceleration, braking, and idle periods. Easily adjustable geometry options make it possible to add, reshape, remove, or reposition objects. This example shows the impact of repositioning the pipe.
The External Current Source settings representing the train and results for the base scenario.
In this model, a parametric sweep was used to investigate three different scenarios: a base scenario, a scenario with sandy clay treated as clay, and a scenario with the pipe repositioned 50 meters.
Simulated potential field and local current density over the 3D soil profile for the base scenario.
The 3D animation shows the crucial train positions when the stray current forms and how the potential field is spread out in the rail proximity. Acceptable potential variations along materials in different types of waters and soils are typically studied to highlight areas that can be particularly harmful for metallic objects.
The pipe is shown to be quite exposed where it lies, with the calculated corrosion rate being highest near the point of train departure and peaking after 54 s. The behavior is expected as the current exits the pipe near the TSS and the current-return route is long.
Comparing the results of the different scenarios reveals that the corrosion of the pipe is decreased if the resistivity in the soil is higher. Rails situated on clayey soil will thus most likely require more stray current mitigation measures. The repositioned pipe is corroding less and shows a sound alternative on how to prolong the pipe’s lifetime.
Pipe corrosion rate for the investigated scenarios when the train is positioned 700 m from TSS 1 (at 54 s in simulation).
Test the Rail
Want to try modeling stray currents from a train in an LRT system yourself? Download the related MPH-file in the Application Gallery:
Further Reading
- Learn more about LRT from these sources:
- Learn more about modeling electric currents and corrosion on the COMSOL Blog:
Reference
- W. von Baeckmann et al., “Handbook of Cathodic Corrosion Protection”, Elsevier Science, 1997; https://shop.elsevier.com/books/handbook-of-cathodic-corrosion-protection/von-baeckmann/978-0-88415-056-5
- D. Teodorović & M. Janić, “Transportation, environment, and society”, Elsevier eBooks, pp. 747–886, 2017; https://doi.org/10.1016/b978-0-323-90813-9.00011-4
- Z. Cai, X. Zhang, and H. Cheng, “Evaluation of DC-Subway Stray Current Corrosion With Integrated Multi-Physical Modeling and Electrochemical Analysis” IEEE Access, vol. 7, 168404, 2019. http://doi.org/10.1109/ACCESS.2019.2953960
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