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B2-305

Session 2004
CIGR

New type of Tower for Overhead Lines

Henning Oebro1 Eltra2

Erik Bystrup Bystrup Architects and Industrial Designers

K. Krogh and M. H. Foder RAMBOLL

Summary The paper presents a new type of tower for a 400 kV overhead line. The type is new in several respects: - Layout of route: The route for this line is composed of shallow bendings resulting in a route following the contours of the landscape better than a conventional line composed of straight lines. - Design of tower: The tower design consists of two elements; a shaft and a lattice structure tower top with very few members, all tubular profiles. - Materials: The tower top is made of stainless steel tubes connected by cast joints in stainless steel, and the tower shaft of hot dip galvanised steel. - Foundation: The foundations are produced as 7 to11m long steel tubes with a diameter of 1,5 m driven into the ground by a large pile driver.

Table of Contents for the Paper 1. Introduction and background. 2. Design of tower. 3. Construction and technical experiences. 4. Economy. 5. Future development

Member of SC B2 Eltra amba, Fjordvejen 1-11, DK-7000 Fredericia, Denmark, e-mail: hoe@eltra.dk

1.1.1. Introduction and Background Eltra is system operator for the western part of Denmark and owner the 400 kV grid. In March 2001, Eltra received a license to construct a new 400 kV connection between the cities of Aarhus and Aalborg, finalising a major 400 kV ring. This connection consists of overhead lines (117 km) with intermediate sections of underground cables (14 km). A minor part of the connection (27 km, shown on the map below in green) is placed going through a rural area with few technical installations. For this part the Danish Minister for Energy decided to demand a new type of tower based on a design competition. The wining entry, selected from among 48 entries, was based on a tower with a cylindrical shaft of weathering steel and a tower top constructed as a lattice structure built of very few tubular members, all in stainless steel.

Wining entry

New 400kV line

Transition from Donau- to design-towers

2. Design of Tower The overall design strategy for the new 400kV tower was to design a tower which, unlike the existing Donau towers, did not add visual noise or interference to the landscape. A tower consisting of few elements and simplicity in the design. A tower that would be read as aesthetic calm and repetitive elements strolling through the landscape contours. To achieve this a hierarchy on how a technical installation are perceived in the landscape was developed: -At a distance, only the body of the poles is visible, standing quietly measuring out the landscape -A little closer the insulators, the high voltage strings and the lattice top become visible -Close on the tower the details of the lattice top, the joints, the connections and the electrical components become visible

Shafts in the landscape

The lattice top

To support this hierarchy a choice of materials for the different elements was suggested: Weathering steel for the shaft and Stainless Steel for the lattice top. Thus in colour and texture the weathering steel becoming to the soil and the Stainless Steel lattice top becoming part of the sky. Together these materials would strengthen each other giving character to both elements and underline the design strategy. Due to local public opposition the weathering steel for the shaft was abandoned and replaced with hot dip galvanised steel. The new 400 kV tower accomplishes a variety of assignments in one unique design: Creating a continuous visual appearance, thus Minimising the visual impact on the landscape Angle tower capabilities allow a harmonious, bent alignment of overhead line and landscape Due to a straight-forward and inexpensive foundation method, the tower leaves a minimal footprint Fitting lattice tubes with cast stainless steel joints allows faster in situ assembling The overall design allows a reduction of the magnetic fields

One of the main adjustments of the wining entry was to change the vertical strings to V-strings in order to be able to use the same tower as a suspension tower and as a running angle tower for minor angles, to lower magnetic fields and to achieve a narrow right of way. The tower head is constructed as a lattice structure of stainless steel tubes welded together. The joints of the tubes are cast in stainless steel. Developing the moulds for the castings and execution of the castings were a difficult task due to the complicated geometry.

A prototype of the suspension tower was constructed and tested mechanically to 105% of the maximum design loads.

Prototype

V-string

Angle tower

A tower family with 3 types of towers was developed: - Suspension tower - Running angle tower for max. 5o bending - Angle tension tower for 5 to 45o line angle 3. Construction work and Technical Experiences The line was erected during 2003. The main part of the foundation work was made in April-August, the erection of towers took place in June-August, and the stringing of conductors took place in July- September.

Steel pile

Connection

Ready for pole

3.1 Foundation The foundation is a large steel pile (a pipe 1,5 m in diameter, 22 mm thickness, 7 to11 m long) driven into the ground by using a hydro hammer (9,2 tons). This method was suitable for soil with the actual occurrence of sand and clay on this line. The working time for this operation is approximately 30 minutes per pile.

A 2 m base section of the shaft is overlapping the upper part of the pile on a length of 1,5 m. The necessary strength connection between shaft and pile is established by concreting this volume with concrete with a compressive strength of min. 60 MPa. Together with the ribs in both the base and the pile it forms a shear lock, which is able to transfer the forces between the elements. This connection principle allows rather big tolerance between pile and shaft. Finaly a concrete plinth is concreted outside the base section. This process includes correct alignment of the shaft as well as a fine corrosion protection of the vulnerable steel part through terrain surface. The corrosion protection of the pile itself is based on extra wall thickness as corrosion allowance. 3.2 Erection of Towers The new concept for design of shaft and foundation was very advantageous concerning erection of tower systems. Each section of the shaft is supplied with heavy flanges for bolting shaft sections together. Therefore each shaft section is put on top of the adjacent section and on top of the base section at the foundation by a mobile crane and the flanges are bolted together. After that the assembled tower top together with insulators and stringing accessories are lifted up by the mobile crane and connected to the shaft by flange bolting. This whole operation is done in a few hours.

Erection of shaft

Stringing

Lifting lattice top

3.3 Technical Experiences During the design and construction process many issues had to be addressed. A few examples are given below. 3.3.1 Overhead Line with "Bendings" The route of the line was designed with many small bendings in order to adapt the route to the landscape and the farms in the area. Towers influenced by these bendings (angles up to 5o)

will have increased forces in tower top, shaft and foundation and the shafts and foundations for these towers were strengthened compared to the normal suspension towers. This was judged beneficial for the line expression in spite of the increased weight of the structures. As V-chains were used everywhere, the appearance from tower to tower did not change even if the line angle varied from tower to tower.

Overhead line with bendings

The line in the landscape

3.4.2 Materials Used Structural steel used for shafts is of the type S 355 J2G3 (EN 10025). This was because of strength, but also because of chemical composition of this steel. As the whole shaft is hot dipgalvanised, and the zinc thickness should be at least 200 m for 50 years service time without any maintenance, this steel is the most suitable. The tubular stainless steel in the tower head is of quality 1.4307 (EN 10088). This is the cheapest type, which is enough for corrosion stability, and a low carbon type because of easy welding qualities. Stainless steel castings are of quality 1.4308 (prEN 10213), which is a proper compromise between cheap steel, but with reasonably strength, corrosion stability and easy welding. All bolts are of type 8.8, hot dip galvanised, which is sufficient because all bolt are placed inside the tower. 3.4.3 Casting of Stainless Steel Joints The moulds for the castings were manufactured as part of the development project and used for the prototype as well as for the production of the remaining tower tops. For the final delivery tenders were invited on basis of the Directives of the Europe Union and the moulds were sent to the chosen producer. It was difficult to deliver castings in the required quality, but repairs by welding were rather easy and were done to the extent necessary.

Moulds

Casting

Stainless steel joints

3.4.4 Vibration of Tower Head Elements Vortex shedding around the tubular members in the tower head was foreseen as a potential problem and theoretical analyses showed that some members were exposed more than could be accepted. They might be damaged by fatigue due to the vortex-induced vibrations. The problem was overcome in different ways. The outside diameter of one member was reduced in order to change its dynamic properties and behaviour. After the prototype of the tower head was produced structural damping of the tubular members was measured, and the result was as expected very low values: logarithmic decrement below 1%. The most susceptible members were cured for vortex shedding by making holes at the front and back of the central part of the members. The favourable effect of the introduced air leakage from the pressure side to the suction side of the members was confirmed by wind tunnel tests as well as subsequent measurements on the erected prototype. By using the results of the measurements in the theoretical analysis the improvement in design lifetime for fatigue was shown to be increased by a factor of more than 30, which was fully satisfactory. 4. Economy The costs of the line with the new towers (excl. development costs) are compared to the estimated costs of a conventional line with lattice towers placed in the same right of way. Costs in EUR 1.000 per km of line Conductors: Towers Foundation Insulators, hardware Assembly of towers Stringing of conductors: New line 38 215 76 28 14 31 Conventional line 38 70 120 20 31 31

Compensations, etc. Planning, etc. Total:

144 16 562

144 16 470

Development costs, incl. full- scale tests: EUR 2 million. 5. Future Development In the current design there is still potential for improvements. A further optimisation of the construction may result in a reduction of the tower price and a better logistic work process both at the factory and in the field. The successful design signal has resulted in a proposal for a new 2x400 kV tower, based on the same design principle and using the experiences obtained in the development of the 1x400 kV tower. The illustrations below exemplify designs that are investigated for the time being.

dragonfly

eagle