Overhead line or cable? This aspect of grid expansion is often the subject of controversial debate. As a matter of fact, there are precise legal framework conditions for high and extra high voltage transmission, stipulating when which type has to be used. Moreover, there is an array of other relevant differences between both ways of power transmission.
Generally speaking, new extra high voltage lines to transport the three phase or alternating current that is customary for the grid, are constructed as overhead lines. Nevertheless, so-called pilot projects have been defined in the Federal Requirement Plan Act (BBPIG) and in the Power Grid Expansion Act (EnLAG) that can result in a different outcome. Furthermore, cables are used when an overhead line would not be at all possible. The 380 kV diagonal power link underneath Berlin is such a case - here, an overhead line across the urban area would not be feasible.This is different for direct current: in the Federal Requirement Plan Act (BBPIG), the Bundestag modified the planning basis for extra high voltage direct current projects in late 2015. Other than for three phase / alternating current, underground cables are standard. However, subsections can still be executed as overhead lines in strictly defined exceptions. One such project is the SuedOstLink direct current project from Saxony-Anhalt to Bavaria.Also executed as cable routes, for instance, are the submarine connections to other countries, such as the Kontek direct current cable that crosses the Baltic Sea. The same applies to the connections of offshore wind farms, regardless of whether they are realised using direct or three phase / alternating current technology.
Overhead lines form a large part of the interconnected grid. At the extra high voltage level, these are designed as steel lattice towers built on top of a foundation, one or several earth-wires for lightning protection as well as the live conductor bundles. These are attached to the horizontal crossarms of the tower via insulators. Depending on the type - there are double triangular, single-level and vertical double arrangements - and the number of live systems, the overhead lines can be between 50 and 70 metres high.Underground cables are usually laid in open trenches. The cable trench is dug out, the cable is laid in a sand layer and the trench is filled up again. The original soil is reused and its original layers are restored.The cables are laid at a depth of 1.5 to 2 metres. Depending on the voltage level, cable type and local circumstances, the underground cable is laid in one or two cable trenches. No more than 700 to 1,000 metres of cable can be laid in one piece because of the limited payload, even for heavy transports. The individual cable segments are connected by so-called sleeves before the trench is filled up.
In case of an overhead line, the conductors need to have a minimum clearance to the ground and as such allow for economic activities below the line. Especially agriculture is easily possible. In the area underneath as well as to the side of the line, there are height restrictions that cannot be exceeded, for instance by trees, to avoid short-circuits.
In case of an underground cable, the corridor can also be used for agricultural purposes or planted with vegetation after the construction phase. Only in the area above the trench and about three metres on both sides, no deep-rooted and hard-rooted trees and shrubs are allowed to grow, in order to prevent damage to the line. No buildings are allowed directly above the trench either.
Overhead lines can be created at a lower cost than underground cables. This is a result of the higher costs of the cable, mainly the substantially more expensive excavation works. How much more expensive this is depends on the subsoil and the number of intersections with other infrastructure such as motorways, waterways or railways. The cost aspect is important as the costs of grid development are charged to the electricity consumer via the grid fees.