One of the consequences of the Danish Energy policy is that the transmission grid has to expand. Fluctuating wind generation places strong requirements on the grid because large amounts of energy have be transported from wind farms to demand centres in another region or from generation in one location to consumers far away.
While expansion of the transmission grid in Denmark will preferably be performed using underground cables, a few overhead lines need to be built according to agreement made with the government. For example, a 166 km 400 kV overhead line with double circuits was built between 2012 and 2014 from a substation (Kassø) in the south of Jutland to another (Tjele) in the middle of the peninsula. This line, which replaced a pre-existing one circuit line, is the backbone of the transmission grid and connects Germany to Norway and Sweden.
One of the conditions in the grid expansion plan was that the new overhead line had to be built with towers of new design, with no use of lattice towers. The idea was to minimize visual impact on the landscape and obtain acceptance from those most affected and also the public at large. In the planning process, the authorities required three line sections (2.5 km, 4.5 km and 1.6 km) be placed underground. The transition from overhead was done in compact open compounds using dead-end towers with a design matching towers on the line. The article, contributed by Transmission Director Sebastian Dollerup of the Danish TSO, Energinet, reviews main elements of this recent project.
The vision for the new line was not to make it invisible but rather to make the line “calm, elegant and light”. The technical system would not have to be a ‘work of art’, but nevertheless fulfil its purpose of appearing elegant. The resulting design towers (named Eagle 2), with conductors at two levels and one circuit on both sides, have cylindrical shafts, much like the towers used for wind turbines. The cross-arms are of a specific rhombus shape, such that two sides reflect the sky and the other two are dark and in shadow. From a distance, the cross-arms seem quite thin. The new design of towers for this line consisted of suspension towers (with four heights), running angle towers (two heights), tension angle towers (18 and 30 degrees with two heights) and dead-end tension towers. All the different types have the same overall ‘visual expression’.
The shafts and cross-arms were made from galvanized steel, although the architect’s preferred materials were CORTEN steel for the shaft and stainless steel for the cross-arms. The underlying idea was that the towers appear as thin sticks placed in specific order with the same distance between them and with almost invisible cross-arms. To further reduce visual impact, towers were placed with the same span length in each section. The maximum span is 360 m. To give the line a ‘calm’ expression, only a small number of angle towers were used – taking into account the neighbours. Four different heights of suspension towers were used so that towers seen from a distance all seem to have the same elevation, despite the terrain profile.
The first public hearing (June 2009) saw different tower types and ideas presented. At the second (March 2010), the final design was shown – but still with some questions left open. The public was asked their opinion regarding specific alternatives. The overall opinion was that the new design was better even though nobody wanted the new line. After start of construction work (January 2012), the neighbours grew to accept the towers and in fact quite a number of them became fascinated and proud of the new design.
Foundations for offshore wind turbines are typically made with monopiles driven into the seabed and the same type of foundations can also be used for transmission towers on land. Experience using such monopiles on the Kassø-Tjele line proved positive and the method was demonstrated fast and effective. Moreover, the footprint of the foundation is small compared with foundations made with concrete plate and that meant less environmental impact. There were no problems with ground water because of limited excavation.
Towers were made of galvanized steel and therefore the shafts had to be separated into four sections. Know-how from the wind turbine industry was used in the construction of these shafts. The cross-arms could be made in one or two sections.