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.”
A track circuit tracks trains. 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. A track is unoccupied if the circuit remains uninterrupted. A train is present if there is a short in the circuit. Figure 1 shows a simple drawing of how it works.
Fig. 1. Simple rendering of how track circuits work.
Track Dangers
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, 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. Figure 2 shows three of these designs. Other factors that add to induction are phase conductor geometry, current imbalance, and ground conductivity.
Fig. 2. Diagram showing three types of conductor configurations.
Knowing the inherent characteristics of a new transmission or power line that shares right of way with a rail circuit is vital when planning it to ensure all trains are accounted for and crossing signals are fully functioning. When it comes to determining these characteristics, EMA has the necessary experience. 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.
Simulating Risks
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 helps to better understand the issues that new lines or existing lines experience. 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.
Taking Measurements
EMA will travel to take on-site measurements to have accurate inputs in the model. At a site, EMA will take three main measurements: 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. Experts have set limits for how high these measurements can be. For the insulating joint and rail-to-ground that number is 50 volts and 5 volts for rail-to-rail.
Another piece being investigated at this time is the type of material being used for the ballast, or the foundation, just below the tracks. Rock serving as a ballast, as shown in Figure 4.
“The ballast is very highly resistive which has a tendency to increase your rail-to-ground voltage,” DiNicola said.
To achieve proper ground conductivity for track circuits, railroad companies use ballast. EMA measures the conductivity to ensure that the ballast is compatible with the overhead power lines.
Fig. 4. Rock is used as ballast under a railroad.
Soil Conductivity
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. Soil moisture content affects the soil’s ability to conduct an electrical current, according to the Natural Resources Conservation Service.
We will also measure tower bonds. The counterpose, a device that grounds transmission lines, is associated with tower bonds. 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.”
The EMC Plus model incorporates all of these measurements to create a more accurate simulation and makes only necessary mitigations.
EMA Experience
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.
Contact our consultants by clicking here and let them solve your problems.
Ansys sells EMC Plus exclusively. To learn more, click here.
