Overhead line in the national grid – how they are built
The national grid is the highway of the electricity supply system. Almost the entire national grid is made up of overhead power lines. It is both reliable and cost effective to use overhead high voltage power lines to transmit energy over long distances.
Careful preparation is required
Building power lines has historically been a time consuming exercise. Today, it goes a lot faster. But it still takes several years for planning and building. Contractors are in charge of the building process.
A lot of planning is required to identify the optimal power line route. The process includes environmental studies, permits and consultations with authorities and landowners. Before construction begins, the right to use the land must be secured.
The area around the power lines is wide enough to avoid damage caused by falling trees.
The pylons are installed and the lines are deployed
The pylons are put together on site or in a central location and then transported to the final place. Insulator sets are attached to the pylons − otherwise the pylons would become electrically conductive.
The phase conductors are attached with the help of a pilot line. This is done by a machine at each end, one that pulls and one that brakes. Spacers are fit between the lines to keep them apart. A fitter´s car is being used. The power lines are attached in sections of five kilometres and then welded together by an explosive charge. Optical fibre cables for telecommunications are installed at the same time as the conductors.
In a substation, the lines are connected to other power lines. These could for example be lines in the regional grid, the voltage level below the national grid.
The expansion continues
Today, there are 15,000 kilometres of power lines in the national grid and the expansion continues in order to meet the demand for a secure and flexible national grid with high capacity, well in compliance with environmental requirements.
Underground cable in the national grid – how it is done
Today, the national grid is nearly to 100 percent made up of overhead power lines. It is only in rare circumstances that underground cables are used.
A long process
It takes several years to plan and build a power line in the national grid. It is a long process, that also includes environmental studies, permits and consultations with authorities and landowners.
The cables are laid out and spliced
The width of the working area is about 10 – 15 metres and the width of the cable trench varies between 2.5 and 5 metres. The cables are transported on cable drums by trailers to the site. Before installation of the cables, the trench is filled with special thermal sand. The cables are rolled out in the trench. At least six parallel cables are required for a normal connection in the national grid.
When the cables have been installed in the trench, the ends must be spliced. Splicing is a complicated task that requires trained experts.
The cable trench is filled
As a final step, the cable trench is filled with thermal sand and suitable soil. Approximately 30 centimeters underground, or deeper for farmland, a special cable warning net is used to prevent accidental damage to the cables.
Most of the vegetation returns, but the cable route is still visible. The cables must always be accessible. Large trees and bushes are removed. No buildings are allowed to be built near the cable route, but farming is permitted.