Protecting Rail from Unwanted EM Effects
Steel may seem like it’s indestructible, making it perfect for railroads. However, there is an unseen threat: electromagnetic (EM) effects. This makes protecting rail an important part of the design process.
EM effects pose significant risks to both rail performance and safety. Lightning, radio frequency interference (RFI), and electromagnetic interference (EMI) can produce safety hazards, reduce operating capacity and performance, and damage components and systems.
“It’s crucially important that the rail system works as intended because we need to know where the trains are at any given time and the way we do that is through voltages and signals that are sent up and down tracks,” EMA Staff Scientist Casey Peirano said. “We have to make sure that we’re within the compliance limits or else we might lose track of where trains are.”
Trains are tracked using a track circuit. They work by proving the absence of a train on the tracks. Track circuits work by running a circuit in a small block through the rails to a power source. If the circuit is not interrupted, then the track is unoccupied. A train is present if there is a short in the circuit. A simple drawing of how it works can be seen in Figure 1.
Fig. 1. Simple rendering of how track circuits work.
Commonly railroad companies and power distributors will share the same right of way, putting tracks and power lines in close proximity. Transmission and power lines can produce an electric field that induces voltage in the track circuits running parallel to them. This is also known as inductive interference.
Two of the biggest factors that create unwanted induced voltage are the physical layout of the power poles and phase conductors in relation to the rails and the level of current running through the conductors. Three of these designs can be seen in Figure 2. Other factors that add to induction are phase conductor geometry, current imbalance, and ground conductivity.
Fig. 2. Diagram showing three types of conductor configurations.
When planning a new transmission or power line that shares right of way with a rail circuit it is vital to know its inherent characteristics to ensure all trains are accounted for and crossing signals are fully functioning. EMA is especially suited for and experienced when it comes to determining these characteristics. Our combination of measurement and simulation creates a more accurate view of potential problems, reducing unnecessary mitigations and saving money.
“We help to mitigate those interference risks by installing things like counterpoise or track protection devices and we can help you to determine what those devices need to look like and what exactly the interference is going to look like,” Peirano said.
Fig. 3. Example of a rail simulation in Ansys EMC Plus.
EMA has developed the powerful simulation tool Ansys EMC Plus to help predict and model induction effects on track circuits from overhead power and transmission lines. This approach can be used for either planning new lines or modeling existing lines to better understand issues being experienced. EMC Plus uses Carson’s equations for wires over a homogenous ground. EMC Plus stands out from other simulation software because of its ability to model miles of track and lines.
“We can tell you approximately what those voltages are going to look like and how they relate to the safety thresholds,” Peirano said. “We have to make a lot of assumptions when we’re simulating, just approximations to real life, because we can’t model everything explicitly.”
Once initial simulations have been made, EMA will go to a site and take measurements to make sure that the results are in the proper realm. This is done to be sure that the recommended mitigations are actually going to work.
“We can help save customers money by simulating first before they build the counterpoise or upgrade the transmission lines because we can tell them exactly what they need to be doing in order to mitigate risks and we can save them from having to do unnecessary mitigations if the risk is very low,” Peirano said.
EMA will travel to take on-site measurements to have accurate inputs in the model. There are three main measurements that are taken at a site: voltage, soil conductivity, and tower bonds.
“We go to a site and we’re going to measure those voltages to see what they are to determine if in fact the transmission lines are causing interference,” EMA Principal Scientist I John DiNicola said. “We check the proximity of the transmission lines to the rails, how close are they, how far above the ground are the transmission lines with respect to the tracks.”
During voltage testing there are three measurements that are the most important: the insulating joint, rail-to-ground, and rail-to-rail. Limits have been set for how high these measurements can be. For the insulating joint and rail-to-ground that number is 50 volts. For rail-to-rail is has been set at 5 volts.
Another piece being investigated at this time is the type of material being used for the ballast, or the foundation, just below the tracks. Figure 4 shows rock being used as a ballast.
“The ballast is very highly resistive which has a tendency to increase your rail-to-ground voltage,” DiNicola said.
Ballast is used to help achieve proper ground conductivity for track circuits. This needs to be measured to ensure that the conductivity selected or that inherently exists is compatible with the overhead power lines.
Fig. 4. Rock is used as ballast under a railroad.
Calculating the conductivity of the soil is the second phase of on-site measurements.
“We’re going to measure the soil resistivity because the soil conductivity is very important for how voltages build up on the rails,” Peirano said.
Soil electrical conductivity (EC) measures the ability of soil water to carry an electrical current. Factors that influence EC include porosity, soil texture, soil moisture, and soil temperatures. The Natural Resources Conservation Service says it is generally accepted that the higher the soil moisture content the more likely soil will conduct an electrical current.
Tower bonds will also be measured. These are associated with the counterpoise, a device that grounds transmission lines. This happens at each and every pole along the track.
“We need to make sure that they’re number one uniform or relatively uniform,” Peirano said. “Also, we need to make sure that they’re low enough that the counterpoise can generate enough current to mitigate that inductive interference.”
All of these measurements are then put back into the EMC Plus model to create a more accurate simulation and only necessary mitigations are made.
EMA has decades of experience mitigating inductive interference and other issues related to the performance and safety of rail systems. Some of the companies that EMA has worked with include CSX, Burlington Northern Santa Fe, Norfolk Southern, Canadian National Railway, Canadian Pacific Railway, Long Island Railroad, and Riverline.
“Our country always needs more power and since these transmission lines run along in the same corridor as the rails the only way to get more power is to one, either increase your voltage, or two, lower your load, which is going to give you more power,” DiNicola said. “So, it’s constantly evolving and EMA’s in the forefront of it.”
You can find an in-depth look at how we use simulation to predict the effects of overhead transmission lines by clicking here.
EMA also has experience protecting rail from lightning. You can learn more about how we do it by clicking here.
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EMC Plus is sold exclusively through Ansys. To learn more, click here.