Development & Implementation of 400 kV Eagle Transmission Tower

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.

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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.

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 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’.

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Visual Impact

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.

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Installation of monopile

Public Acceptance

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.

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Foundations

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.

Construction

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.

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Installation

The four sections of the tubular shaft and four cross-arms were very quick to install using a large mobile crane such that at least one tower could be erected each day by one team. New stringing machinery had been produced which made it possible to finish all construction before the end of 2014. Since there is no access to insulators (composite type) and fittings from the towers, all work on them was done from a mobile lift.

Triple conductor:

925 mm2 AAAC TW

Transition to Underground Sections

Transition between overhead line and underground cable was made in compact compounds. The tension tower consisted of two portal structures with similar shape as the rest of the towers. The                  terminations of the underground cables were then placed close to these portal structures. For each overhead line circuit there are two underground cable circuits, i.e. there are 2×3 terminations for each portal. One set of terminations was placed in front of the portal and the other behind it. That gave a very compact compound with limited visual impact. Moreover, the whole plant is now partly hidden by shrubbery and trees.

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Transition between overhead line and underground cable.

Operation

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The transmission line was commissioned in November 2014 as per plan. Up to now, there has been no bad operating experience, apart from a bad storm, which led to critical horizontal movement of the conductors and apparently caused a short-circuit with subsequent auto-reclosure. The conductor has since been tested in a wind tunnel to assess whether high winds or a flying object was behind the short-circuit.

General Experience

Overall, the new line is well accepted by local communities, power industry experts, contractors and also by the owner. The new tower has shown that it is possible to re-think design as well as past approaches to building new overhead lines, and to receive positive feedback in the process. While the costs of the new towers were higher than for lattice towers, the clear benefit is a better-looking line with reduced visual impact of the environment.