Overhead lines and underground cabling

The modernization and extension of infrastructure are necessary for Switzerland's sustainable energy future. This includes the currently prevalent UHV grids and overhead lines. The use of underground cabling in the transmission grid is relatively recent. Overhead and underground lines offer advantages and disadvantages in terms of project design, construction, operation and maintenance. Swissgrid must therefore evaluate all of the possible variants of overhead and underground lines for each grid construction. The results of these studies are used by decision-making bodies when choosing technologies and corridors.
 

Masten_Grenzwertrichtwerte_250px_en

An overhead line consists of the totality of all phase and earth conductors on a supporting structure and is installed above ground. Overhead lines are used almost exclusively worldwide for energy transmission at high voltage levels, such as 220 and 380 kilovolts. The air surrounding the conductor serves as insulation. The heat which is created by the electricity flow in the conductor can easily be transferred to the environment. More electricity can be transmitted in winter than in summer due to the lower ambient air temperature. In Switzerland, Aldrey cables are primarily used; an alloy of aluminium, magnesium and silicon.

The framework conditions (e.g. power of transmission, length of corridor, line load, soil conditions, etc.) play an essential role in technical design as well as influencing the number of conductors, their section or the choice of materials used for them. There are additional requirements in terms of availability and redundancy. How should underground cables be designed in order to guarantee supply security? Each extension of the transmission grid must be clarified on a case-to-case basis, taking into account the reliability of the operation.

Structural types

Tube block Tunnel construction Directional drilling
Concrete tube blocks are laid in open-air at approx. 1 to 2 meters deep. The width of the wiring trench is approx. 5 meters. A minimum width of approx. 25 meters is required for the construction. To install underground cables without trenches, tunnels of approx. 3 to 4 meters in diameter are constructed in the form of galleries or microtunnelling. The cables are placed on specific cable holders. A drill head to control the digging direction under the obstacles of the surface. To prevent them from collapsing, a stabilizing liquid is squirted behind the drill head and the empty tubes for the cables are laid. This type of construction is suitable for short distances, for example, passages under rivers or roads.

Underground cables are very widespread for the transmission of electricity in regional distribution grids, and are increasingly used in the national distribution grid up to 150 kV. Underground cables have proven themselves in terms of technique and operation in these areas. In the 380 kV and 220 kV transmission grid, very few underground cables of more than 1 km are used; empirical values resulting from operation over a long period of time are therefore lacking.

  Underground cables Overhead lines

Extra high voltage: 380 kV and 220 kV 8 km 6 750 km

High voltage: 36 kV to 150 kV 2 031 km 7 158 km

Medium voltage: 1 kV to 36 kV 33 544 km 10 914 km

Low voltage: 0.4 kV to 1 kV 76 311 km 9 719 km

Each grid construction project is developed as part of a multi-stage federal planning and authorization process involving federal agencies, cantonal authorities, environmental protection associations, grid operators and persons who are directly affected. While the authorities define transmission technology and the project corridor during the sector plan procedure, persons who are directly affected can submit their requests during the procedure for approval of the plans. The «Transmission Line Evaluation Model» developed by the Swiss Federal Office of Energy (SFOE), the Federal Office for the Environment (FOEN) and the Federal Office for Spatial Development (ARE) as well as the Federal Electricity Commission (ElCom) is used for the decisions. The aim is to ensure solutions with a high level of acceptance.

Evaluation scheme for transmission lines

All four evaluation categories flow equally into the technology decision.

* Cost comparison

The costs form part of the decision-making basis. The lifecycle costs are taken into consideration. These include construction, operation, maintenance and electrical energy losses over the entire service life. Depending on the technology and the local circumstances, the costs of underground cabling are higher by a specific factor than an overhead line. In cases with favourable conditions for underground cable and unfavourable conditions for overhead lines, underground cable can be about twice as expensive. With reverse conditions, the factor can be up to 10 and more.

Every Swissgrid grid project, whether overhead line or underground cable, must be evaluated by the ElCom regulator from an economic perspective since the costs are transferred to the consumers’ electricity bill.

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