Underground cabling costs

Underground cabling construction. Photo: ABB.

Without doubt: It is more expensive to lay extra high voltage cables underground than to build overhead power lines. The costs depend upon, amongst other things, ground structure. Costs are inevitably much higher in rocky terrain than in marshland or sandy terrain.
On the other hand, operating costs for underground cabling are lower. This cannot, however, compensate for the high level of investment required at the 380 kV level. It is very difficult to put a figure on the extra costs for underground cabling. There is neither in Germany, nor internationally, sufficient experience upon which to base such an estimation.

Underground cable technology

Wheat field. Photo: Pixelquelle.de

At low and medium voltage levels, the German distribution network is mainly underground. At these voltage levels, underground cabling is the most advanced technology. Overhead power lines are standard at the high voltage level 110 kV and the extra high voltage level 380 kV.

Unlike overhead power lines, underground cables need robust insulation. The copper or aluminium core is encased in plastic sheathing that protects it from animals or chemicals in the soil. Additionally, the cables are embedded in a layer of sand. Laying multiple cables means that capacity can be increased to any level.

The whole installation must be buried deeply enough into the ground to prevent any contact with, for instance, agricultural machinery.

Within the power grid underground cables behave quite differently to overhead power lines. For instance, reactive power which is non-usable but nevertheless necessary for transmission, needs to be compensated in underground cables. This takes place at small stations located at 5 to 40 kilometre intervals along the underground cabling route.

Nature and the environment

Migrating cranes. Photo: Pixelquelle.de

In contrast to overhead power lines, underground cabling projects face much less criticism of  their impact. This is undoubtedly the main advantage of underground cabling. However, although wildlife is hardly affected by underground cables, the impact on vegetation has not yet been fully researched.

Whether underground cables have any serious ecological impact at all depends very much upon the respective topography. In mountainous areas, construction work will disturb the existing ecosystem. In wetlands and moorland, heat generated by the underground cables can change the natural habitat around the cables to a small extent, and thus drive out natural fauna and flora.

Laying cables underground often means cutting through an intact and cohesive habitat. To what extent moors and wetlands may dry out in parts because of the underground cables, needs to be investigated in every individual case.  Extra high voltage underground cables heat the soil as well. This may also have an impact on the growth of some individual plant species.

  • Not dangerous to birds
  • Apart from the actual transmission route which needs to be kept free of trees, the landscape is not adversely affected
  • Underground cables are not vulnerable to lightning or ice and snow storms
  • The magnetic field directly above the underground cabling is in fact higher than that directly below an overhead line. This does, however, fall more rapidly laterally.

 More information on the costs and technology of partially undergrounding extra high voltage lines: Joint Paper (ENTSO-E/ Europacable) on the feasibility and technical aspects pf partial undergrounding