The push is on to build new, often long electrical transmission lines across the U.S., connecting renewable sources like wind farms and solar installations to the grid and bringing that power to population centers. Amid this building boom, it can be challenging for utilities and other developers to obtain right-of-way for the power lines. One of the paths of least resistance — minimizing new private easements and leveraging the presence of existing easements — is for the transmission route to share or follow existing railroad rights-of-way.
Although this strategy often helps streamline the routing approval process, it comes with a significant potential complication. When power lines parallel segments of linear metallic infrastructure — like rails or pipelines — for some distance, it introduces a risk for high-voltage alternating current (AC) induction. That means electrical current on the power line induces voltage on the rail, posing a safety risk to the public and rail workers. It also poses a risk of rail communication system interference or equipment damage, which could lead to incorrect operation of barriers and signaling at crossings.
Although AC current is the primary factor with induction to rails, higher-voltage lines are more susceptible to induction due to their larger phase spacing. Standards from the American Railway Engineering and Maintenance-of-Way Association (AREMA) set safety limits for voltage on rails. If the voltage exceeds limits, that could pose a hazard to anyone who touches that segment of rail.
Voltage buildup also can pose issues for equipment used along the railroad track. Communication and control equipment along the tracks can be interfered with or damaged by excessive voltages, especially under power line fault conditions. Railroads often maintain bungalows near crossings, each containing equipment at risk of damage. These bungalows include arresters to protect the equipment inside, but it is better to mitigate such conditions from the start if possible.
Addressing Induced Voltage
Mitigating the risk of induced voltage is a joint responsibility of the railroads and the utilities. Railroad operators need dependable communication systems to know where their trains are, and to do that their equipment must work properly, free from outside interference. Furthermore, railroads strive to maintain safe crossings for the traveling public and a safe work environment for their employees.
When a utility wishes to build transmission lines paralleling existing rail easements, the railroad company typically requires the utility to perform a study identifying the potential risks of high-voltage induction. The utility is responsible for doing all it can to mitigate the issues. But there is an important caveat: Often railroads will not allow the mitigation measure to be close to or touch the tracks themselves. This can have significant implications for which mitigation options are available.
To understand the induction dynamics and rail voltages within the collocated corridor, railroad and utility service providers leverage a complex and expensive suite of software and tools to assess existing and proposed conditions. These tools are extremely powerful and can be challenging to use because of the high degree of detail to which scenarios can be modeled. The results are very detailed and very accurate, but it takes experience to leverage the tools’ full capabilities.
Options for Mitigating Risk
From a design perspective, there are several mitigation options utilities can pursue to minimize induction without having to move routes away from the tracks:
- The phasing can be arranged in a way that minimizes induction from a three-phase transmission line. This option may have limited impact and only help if the amount of induction barely exceeds the limits or if multiple circuits are present.
- The transmission line phases can be transposed evenly along the parallel segment. This transposition process helps to counteract the induced voltage on the rail.
- An aerial sacrificial conductor is a grounded conductor, located under the phases, along the side of the railroad track. These are most often installed and supported on the transmission line structures using davit arms, which extend toward the tracks while maintaining all required code or specified clearances. Induced current on this grounded conductor generates a counter-electromotive force opposing the original field, thereby mitigating the net effect of induced voltage on the rail tracks.
- When using an aerial sacrificial conductor is not possible or if additional mitigation is needed, a buried conductor can be used. This buried conductor works exactly like the aerial conductor but can be positioned even closer to the rail, thereby having even more impact in reducing induced voltages. While the aerial option can be easier for new transmission lines, it could be problematic for existing lines. One advantage of the buried option is that it doesn’t have the same clearance requirements and is often installed 5 to 10 feet away from the edge of ballast.
From the railroad side of the equation, there are two other options. First, the rails themselves can be grounded at certain intervals, taking induced voltage to ground. Although this is a good way to mitigate the voltage, it is problematic for railroads using the rail itself for communication signals. Those signals cannot be interfered with, so filters must be applied to the grounds, allowing the 60-Hz AC voltage to pass through but not the rail signal. The second option is to add isolation joints along the parallel segment of rail. This option can be expensive and require longer rail outage times and is generally a last-resort solution.
Additional techniques offering incremental mitigation, especially against fault conditions, can include requiring additional ballast rock around areas where people might cross the rails; improving grounding in bungalows; and beefing up the arresters protecting equipment. Railroads could benefit from an added source of revenue by leasing right-of-way for power corridors and mitigation solutions.
Because there is no letup in sight for demand for new electrical transmission — especially in regions rich with wind and solar development activity — high-voltage induction onto railroad tracks will continue to be a point of concern. Urban and rural communities alike will need to mitigate the risk. Partnering with professionals who are experienced equally in rail transportation and power delivery is a strong step toward resolution and safe operations of collocated transmission lines and railroads.
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