50Hertz Transmission GmbH

Overhead line versus buried cable

Overhead lines or buried cable? In this controversial debate, the supposed advantages of laying underground cables as opposed to constructing overhead lines are frequently cited. However, positive ecological side effects from laying 380-kV underground cables are not to be expected. Quite the opposite: This technology involves restrictions for land owners as well as irreversible ecological damage.

Overhead lines make up a large part of the interconnected system. They ensure low-loss transmission at 380-kV extra-high voltage, and thus guarantee reliable energy supply. Cables, in contrast, are predominately used in medium- and low-voltage networks, as well as for power distribution in densely built-up areas with high electricity demand. However, this technology is also becoming increasingly important in rural areas. Nevertheless, underground cables have, in many cases, ecological and legal disadvantages which are frequently neglected in the public debate.

Impact of underground cabling: The ecological perspective

When burying cables, the soil must be exchanged. Furthermore, not only do the cable routes need to be kept free from deeply rooted plants, they may not be built on for any other purpose. In addition, underground cables radiate heat. This has an effect on soil humidity, which, for example, can lead to drainage or drying out of marshes. The laying of underground cable also requires the construction of cable jointing structures every 500 to 700 meters, as well as compensation facilities along the cable route. As a result, existing biotopes are permanently cut up. As long-term experience with underground 380-kV cable is lacking, the effects on the soil of decomposition of the plastic cable sheathing remain unknown.

Legal aspects

An extra-high-voltage power line is a space-consuming structure – regardless of whether it is built as an overhead line or underground cable. Laying an underground cable will affect the rights of the owners and occupants of the land on which the power line is built and used, to a degree similar to the erection of an overhead line.

On principle, easements --rights of use-- are recorded in the land register. The owners receive appropriate compensation. This ensures that the transmission system operator can build the power line and subsequently access it in order to carry out the necessary maintenance and repair works. In the case of underground cable, continual and direct access for maintenance and repairs is only guaranteed when the area above the cable remains free. As a consequence, use of the underground cable route for agricultural purposes is not possible or is subject to restrictions. Furthermore, it is not permitted to build on the route. As a result, underground cabling imposes more restrictions on land owners than overhead lines.

 

Infobox

Overhead line versus cable

The 380-kV extra-high-voltage network in Germany and Europe is currently almost exclusively composed of overhead lines. Buried 380-kV cable is used e.g. in big cities such as Berlin and London. The differences between overhead lines and cables reside mainly in their design, costs, service security and their impact on the environment.
Overhead power lines are exposed to the elements, which under extreme weather conditions can lead to rare interruptions. While the failure rate is lower with buried cable, such failures involve greater expense as buried cables are less accessible, resulting in time-consuming repair and replacement work. Furthermore, it is currently unclear how the decomposition of the plastic cable sheathing affects the soil.
Overhead lines can be used for around 80 years. The lifetime of cables is currently around 40 years. Laying of underground cables requires 4 to 10 times higher construction costs. Laying of cable in tunnels would increase costs by a factor between 30 and 40.