Underground Electric Power Transmission: Applications, Challenges, and Benefits

Electric power transmission can indeed be placed below ground; however, such installations are not always practical due to various factors including cost, heat dissipation, and maintenance. This article explores the feasibility of underground transmission, its applications, challenges, and benefits, offering insights into why underground options are often preferred in densely populated areas or specific geographical settings.

Applications of Underground Transmission

Underground transmission lines are commonly used in densely populated urban areas and underwater locations such as rivers, lakes, and even the sea. These installations can offer several advantages, including aesthetic benefits, reduced visual obstruction, and protection against weather and environmental elements. For instance, in urban settings, underground cables can help maintain the city's aesthetic appeal, free from the clutter of overhead power lines.

Challenges of Underground Transmission

Despite the numerous benefits, underground transmission faces significant challenges, including higher installation costs and more complex engineering requirements. The high cost of insulation materials is a major factor. Since underground cables are exposed to less heat dissipation, the insulation material requires more robust construction to ensure safety and reliability. In some cases, heat dissipation can be a limiting factor, necessitating deeper burial depths and larger conductors to meet safety standards.

Additionally, the strict distance limits for underground AC cables (around 100 km) and the high costs associated with DC conversions make these installations less practical over long distances. Under water, the insulation requirements are even more stringent, and the installation process can be even more challenging and costly.

Trade-offs and Best Practices

While underground transmission is feasible for short runs in front of airports and in densely populated areas, the decision to use underground or overhead transmission often depends on the specific needs of the power distribution system. Transmission lines are generally at very high voltages (thousands of volts) for long distances, whereas distribution lines are at lower voltages (a few thousand volts) for shorter distances, typically the last few miles of the power grid.

For distribution purposes, only the final leg of the transmission often becomes underground, particularly in newer subdivisions. This practice ensures that there are no visible overhead wires and integrates seamlessly with the urban infrastructure, but it comes with significant financial implications. The use of such underground cables involves many layers of expensive insulation, making them significantly more costly than overhead lines, which can be constructed with bare aluminum conductors and rely on air for cooling.

It is also important to note that overhead transmission lines tend to have lower capacitance between conductors, which can lead to less energy loss. Underground cables, on the other hand, introduce additional capacitance, increasing losses. This is particularly noticeable in DC systems, where the use of buried cables can be more feasible due to technological advancements in insulation materials and cooling mechanisms.

Historical and Technological Trends

The feasibility of underground transmission has been a subject of exploration since the 1960s. Major cities have implemented 345kV lines underground, although these installations are still quite expensive. Between the cost of insulation and the added labor of burying the cables, the price differential can be substantial, potentially ranging from 10 to 100 times the cost of overhead installations. Despite the challenges, continuous advancements in technology and material science may further reduce these costs in the future.

Conclusion

While underground electric power transmission offers several advantages, such as aesthetic improvements and protection from environmental factors, it is still a complex and costly endeavor that requires careful consideration of the specific application context. For densely populated urban areas and water bodies, underground cables remain a viable option, although overhead transmission lines often remain the more practical choice for longer distances and wider geographical areas.