Selection of supports used for overhead transmission and distribution lines is based on technical and aesthetic considerations that continue to create problems for electric utilities. Today, the design and material content of overhead line supports is an important feature of any Environmental Impact Statement (EIS) prepared to support applications to planning authorities seeking permission to erect new overhead line circuits.
In addition to environmental aspects, utilities must consider mechanical strength, aging behavior, assembly methods, installation and life-cycle costs. Traditionally, the choice of materials for supports has been limited to wood and steel. Overhead line design engineers have successfully produced many technically and aesthetically acceptable structures with these materials. Alternative materials have been tried and tested, including concrete and fiberglass. Swiss utilities, which have much experience with concrete poles, are now seeking ways to optimize the installation of these structures that are increasingly used by other service industries, including telecommunications.
Concrete Poles in Switzerland
The use of concrete poles, centrifuged or vibrated, reinforced or prestressed, 20 to 40 m (66 to 132 ft) in height, are widely used in Switzerland. A majority of the transmission network in service (for example, some 80% to 90% in the voltage range of 40 kV to 150 kV) has been constructed with this type of support for the past 50 years. Examples of these structures also are found on the 220/400-kV transmission system. Figure 1 shows a 60-kV line supported by a concrete pole.
Minimal maintenance work during the lifetime of concrete poles is perhaps the characteristic most valued by the utilities responsible for overhead lines, considering the alternatives of steel pylons or timber posts. With the growth of environment protection acts, this interest is likely to increase, precluding the use of traditional anticorrosion treatments for metal superstructures and treatments to prevent the deterioration of wood poles.
Furthermore, the problem of securing rights-of-way for new overhead line circuits is eased by using concrete poles that require less land surface than a steel lattice tower, which is generally designed with four widely spaced feet. Also, the visual impact on the landscape is less severe because concrete poles present slender silhouettes — more or less comparable to the trunks of trees. Beyond Switzerland, the use of concrete supports has been minimal. However, in light of their advantages in saline or aggressive atmospheres, evidence suggests their use will become widespread.
Fabrication in a factory under ideal conditions results in the poles having uniform compression strength of 70 to 80 N/mm2. Nevertheless, they have disadvantages. Moving poles from factory to site requires transporting them by road as well as the need for mobile lifting equipment (for example, cranes on wheels or tracks, or a combination of formwork and winches) to assist with installation. The transportation of concrete poles and the cranes used to erect them are often viewed as insurmountable obstacles. The main difficulty with supports is their weight, size and the absence of practicable roads. In isolated areas, temporary roads through fields and forests impact the environment.
Electricity Network Constraints
The region served by Electricité Neuchâteloise SA (ENSA), in northwestern Switzerland, is characterized by the Jura mountain chain, where the population basin of some 170,000 inhabitants is distributed in the valleys and lakeside that border the region in the south.
The topography of this area means that the ENSA high-voltage electricity network is being installed mainly on steeply sloped sites, frequently in the forest. Surveillance, maintenance and fault rectification are extremely difficult in these situation. Although concrete poles seem like an ideal solution, in this difficult terrain, pole placement presented a major problem (Fig. 2).
Faced with the access problem that hindered the use of traditional concrete poles, manufacturers first developed butt joint techniques between the vertical elements of the supports. However, these solutions were more costly and delicate because they have to be applied in assembly locations, under site conditions, to products whose level of quality results from optimal prefabrication in the factory.
Alternative solutions were sought by imagining pole sections of reduced length and weight, following the numerous examples for steel tube pylons. The comparison extended to the idea of using a helicopter, both for the final transport and for assembly of the main elements (experience with the mounting of crossarms having already demonstrated its feasibility). However, this idea had limitations in the useful payload of helicopters, which previously had just 50% of the lifting capability of today's helicopters with a 12-ton load capacity. Encouraged by this increase in the payloads transportable by helicopter, the Swiss company GRAM SA, a manufacturer of poles, resumed its research activities relating to the design of sectional concrete shafts.
GRAM SA's efforts resulted in the implementation of an assembly cone system with a length of approximately two diameters. The first version included a small quantity of filling mortars, but later designs excluded the need for a construction joint. The final and patented form, called HELIGRAM, comprises two elements for assembly — a design that results from high-performance techniques in the sphere of reinforced and prestressed concrete. The fabrication of the patented cone (male part) was at one time considered impossible, while the second piece (female part) perfectly matched and, also made of concrete, represented a major technical feat.
Although this development was not fully completed, this assembly system was used for one of the largest supports installed in recent years on the ENSA high-voltage network. The pole, having a total length of 49 m (161 ft) with 3 m (10 ft) embedded in the ground by the intermediary of an original foundation cast on the site, was made in three vertical sections plus the three crossarms. It was installed using the formwork/winch combination in the middle of a forest, where the use of a concrete pole previously would have been impractical.
The implementation of a new system of assembly for prefabricated concrete poles using tapered elements (patented) proved to be an incomparable innovation (Fig. 3). Today, it is possible to manufacture concrete poles or shafts in sections, transportable by relatively light means and, in some cases, by air. This innovation enables the vertical installation of low weight sections by helicopters (Fig. 4). The sections are interlocked dry without adhesives, welding, bolts or the joining of fittings.
Early developmental applications using classic installation procedures were followed by a world-first demonstration assembly by a SUPER-PUMA helicopter. This helicopter, one of the most powerful in service in Switzerland, is fitted with two 1400-kW turbines that can lift and manipulate useful loads up to 4500 kg (9920 lb) under optimal conditions.
The prototype installation on an ENSA site included two 60-kV supports. The first was a pole fabricated by centrifuging (round section) two sections having a total length of 23 m (75 ft) and a total weight of 6.7 tons. The second support assembled at the demonstration was a pole of prestressed concrete (two octagonal sections) with a total length of 21.50 m (71 ft) with a total weight — including the crossarms — of 7.1 tons. The two assembled poles support six Aldrey 240 mm2 (0.372 in2) conductors and a 242-mm2 (0.375-in2) Aldrey/steel cable with optical fibers.
ENSA took advantage of concrete pole development and helicopters to erect two 3-km (1.9 miles) 60-kV overhead lines. It used the helicopter to transport 50% of the poles to locations without an existing access. The construction methods used on this project and the increased cost of pole fabrication were economically justified. The additional costs associated with the need for temporary access to pole positions and potential damage to crops far outweighed the cost of employing a helicopter (KAMOV KA32 twin-rotor 2×1800-kW machine, see Fig. 5).
Future Development and Outlook
Several overhead lines for the Swiss electricity network have been erected using poles fitted with a new assembly cone. The practical experience acquired from these projects has resulted in continuous improvement in the organization and management of the ground assistance functions, which differ considerably from those associated with traditional overhead line construction practice.
The use of prefabricated concrete poles in Switzerland is expected to increase. Furthermore, applications will be found throughout Europe in coastal areas, in mountainous terrain and in areas that lack adequate roads, thereby taking advantage of the range of benefits offered by using concrete poles to support overhead line conductors. Also, to satisfy the increasing demand for low visibility overhead lines, concrete poles are becoming more attractive because of the absence of corrosion, surface treatment, and need for routine inspection and maintenance.
The first concrete poles were developed to replace the wood poles used on the medium-voltage overhead lines (16 kV to 24 kV), but the potential for application on higher-voltage systems was quickly identified. As a result of the continuous development by the Swiss, who are now regarded as pioneers in the production of concrete masts, some 380-kV lines are now supported on concrete poles and research is ongoing to extend the use of this technology to 750-kV lines.
Philippe Bettens graduated from Ecole Polytechnique de Genève in Switzerland and then joined Energie Versorgung Weser-Ems AG in Germany. With more than 25 years experience in the design of high-voltage transmission lines, including six years in a consulting engineering group, Bettens now heads the high-voltage transmission lines department at the Swiss Utility ENSA-EEF, in Fribourg.