A Portugal utility embraces the benefits of overhead covered conductor systems on the medium-voltage network.
EDP Distribuição has been subject to increasing pressure to address health and environmental issues and customer pressure to improve the quality of supply and network reliability. This has prompted the distribution network operator (DNO) to seek new solutions for overhead line networks deemed responsible for the failings in its network reliability.
A company of Energias de Portugal Group, EDP Distribuição maintains an overhead line distribution network that extends 168,000 km (104,390 miles), supplying 6 million customers in Portugal. In order to introduce new techniques in the design and construction of medium-voltage overhead lines, it was necessary to obtain special dispensation for a pilot project from the Portuguese electrical authorities, Direcção-Geral de Energia e Geologia (DGEG) and Direcção Regional do Centro do Ministério da Economia (DRCME), as the existing regulations only permit the use of bare and insulated overhead line conductors. Therefore, the DNO decided to erect a new and relatively small 15-kV network using a spacer cable system of covered conductors.
Spacer Cable System
EDP Distribuição selected covered conductors supplied by Hendrix Wire & Cable Co., which manufactures these conductors for applications in the voltage range of 5 kV to 69 kV. The spacer cable system incorporates a grounded messenger or earth wire that affords lightning protection and supports the three-phase covered conductors strung in a close triangular configuration. The overhead line design has the mechanical properties to withstand severe weather conditions as well as the electrical insulation to prevent faults due to phase-to-phase and phase-to-ground contact and faults involving contact with trees and animals.
The messenger wire of the spacer cable system provides the mechanical strength to support the loads imposed by the wind and phase conductors. Two types of messenger wire are available, Alumoweld-Aluminum and Alumoweld, which is anchored to the line supports by steel or aluminum alloy brackets. Pole hardware such as tangent, angle, deadend, tap and pole-top extension brackets are available for the spacer cable system. All hardware is designed to be compatible with the Hendrix roll-by installation equipment.
The covered conductors used in the spacer cable system are suspended by high-density polyethylene spacers from the messenger. The diamond-shaped spacers, designed to separate, compact and maintain the conductors in the triangular configuration, are positioned at 10-m (33-ft) intervals.
The hard-drawn stranded aluminum conductors are covered with one, two or three extruded layers, depending on the voltage level, so the thickness of the covering is specific for each voltage. Black- and gray-covered conductors are available; the choice depends on the on-site visual appearance.
For applications that have circuit ratings of 25 kV and above, the conductors have a black semiconducting shield; this also is recommended for 15-kV applications when there is a probability of tree contact. These conductors also include an inner layer of low-density polyethylene for high dielectric strength and easy stripping characteristics, and an outer layer of high-density polyethylene for resistance to tracking, abrasion and ultraviolet degradation. As covered conductors do not have an insulated outer shield, they are considered unshielded cables, which, for safety reasons, must be treated as bare conductors.
High-density polyethylene insulators are recommended for use on covered cable overhead lines, as they avoid the problem of increased corona and the resulting erosion of the covering that can occur if porcelain insulators are used. They also are recommended for use in areas where vandalism is prevalent and in areas of high pollution because of their long leakage distance and washing characteristics.
Pilot Project Construction
The EDP Distribuição pilot project comprised construction of two 15-kV overhead lines connecting two substation feeders in Mangualde in central Portugal. Among the key design considerations to comply with national regulations, was the mechanical design of the spacer cable hardware to both fit the concrete support poles and the combined tension load of the messenger wire (10 kg/sq mm) and phase conductors (1 kg/sq mm).
The installation of the spacer cable system, constructed by EDP Distribuição staff and supported by technical assistance from Hendrix, proved to be a simple four-step process, pole by pole, requiring minimal field crew training:
Poles are equipped with the materials and stringing equipment.
The messenger wire is strung and tensioned.
The phase conductors are strung and tensioned.
Stringing equipment is removed and the spacers are installed.
However, for this project EDP Distribuição chose to use the roll-by method of installing the spacer cable. This method erects the medium-voltage system between deadends; the messenger and the phase conductors are installed along this section. A reel trailer or stand is required to hold the three conductor reels. Starting with the messenger wire, followed by the three spacer conductors, the speed of this operation is performed by the lineman, who has to climb successive poles to connect the conductors to the blocks and tag lines. Once strung, the messenger and spacer cables can be sagged easily.
The recommended procedure for conductor stripping for splicing and installing connections should be a manual procedure using appropriate tools, as automatic splices can reduce the tension. All splices are covered with a cold-shrink cover.
Surge arresters must be installed on spacer cable systems in all positions where it is connected to a bare conductor system, because lightning surges have the potential to cause conductor burn down at this transition. It was decided to take the opportunity of using in the same poles the spacer cable system, a low-voltage overhead line and streetlighting. The design of these concrete poles provides adequate strength to support the additional load.
EDP Distribuição's decision to erect a spacer cable system was based on several reasons and objectives:
Improved system quality (system average interruption frequency index [SAIFI] and interruption time equivalent to installed capacity [TIEPI])
Network reliability in reducing the number of faults
Simplified construction involving fewer linemen
Environmental benefits in terms of limiting tree trimming to an 80-cm (31-inch) radius around the conductors
Reduced number of bird collisions and electrocutions
Reduction in the electromagnetic field
Concrete poles that are able to support several voltage levels and services.
To minimize the deposition of aggressive agents on the covered conductors, spacer cable systems for the medium-voltage overhead line system are not suitable for use in highly polluted areas and within 300 m (984 ft) from the coast.
Alternative Overhead Line Solutions
For the past 20 years, an alternative to bare conductor overhead lines, the aerial bundled conductor (ABC) circuits, have provided satisfactory service. They have a greater impact on visual amenities, material costs are high and span lengths are limited to 40 m (131 ft). Use of ABC for medium-voltage overhead lines has proven to be an expensive solution, so applications are limited to confined areas where bare or covered conductors cannot satisfy clearance problems and underground cables are not a practical alternative.
In 2007, a pilot 30-kV scheme using the Hendrix Wire & Cable tree-wire system was erected through a wildlife protection zone that included small trees. The system was based on covered conductors — aluminum, aluminum alloy or aluminum conductor steel reinforced (ACSR) — similar to the spacer cable system but without a messenger wire. Using standard overhead line components, the average span length exceeded 100 m (328 ft).
Theoretically, there are no limits on span length for spacer cable systems, provided a larger messenger wire with greater ultimate tensile strength is used. Hendrix regularly uses span lengths of 300 m to 400 m (984 ft to 1310 ft), but for the EDP Distribuição project, span lengths were only 40 m, because the low-voltage network and public lighting were installed on the same poles.
Another 15-kV overhead line project using 50-sq mm (0.078-sq inch) ACSR conductor with a single-layer cover was launched in 2008. In addition to using the EDP Distribuição standard overhead line components, the use of this system — formerly used in wildlife protection zones — is an ideal solution for circuits erected in flat terrain.
Wide Range of Options
EDP Distribuição now has a suite of medium-voltage overhead line designs to suit different applications. Its latest design, the spacer cable system offers an economic compact network that now satisfies many of the current challenges of improving the quality of supply and system reliability.
The medium-voltage ABC design offers a solution in situations where bare and covered conductors cannot meet clearance requirements, and the tree-wire system is an option when the investment in a spacer cable system cannot be justified.
At present, EDP Distribuição is still assessing the results of several projects in order to establish the design policies for regular use, but it is not easy at this stage to compare the cost of these pilot projects with traditional solutions.
This new and unique form of medium-voltage overhead line construction has the potential for wide-scale implementation in Portugal and, ultimately, inclusion in national and European overhead line regulations.
António Cardoso (email@example.com) received his degree in electrical engineering from the University of Coimbra, Portugal, in 1998. From 1999 to 2001, he taught electrical engineering in the Polytechnic Institute of Viseu. Since 2001, at EDP Distribuição, Cardoso has been involved in technological innovation activities and currently works in the power-quality activities group.
José Cardoso (firstname.lastname@example.org) was awarded his electrical engineering degree by the University of Coimbra in 1999 and joined the project department at EDP Distribuição in 2000. Cardoso worked on medium-/low-voltage overhead lines and underground cables, namely spacer cable, until 2007 when he moved to the operational department to manage the construction of electrical infrastructures.
José Figueiredo (email@example.com) earned his degree of electrical engineer from the IST-Technical University of Lisbon in 1984 and was a trainee engineer from 1985 to 1986. Since 1986, Figueiredo has been responsible for several technical departments at Electricidade de Portugal and later EDP Distribuição. He is currently adjunct director of the Mondego network and clients direction.
Nuno Mendes (firstname.lastname@example.org) earned his degree of electrical engineer from the IST-Technical University of Lisbon in 1984 and later completed an advanced studies course in safety engineering. In 1986, he joined Electricidade de Portugal and later for EDP Distribuição. He has worked in the operational department, within the safety and health central department, managing low-voltage live working crews. Since 1998, he has worked in the technology department, elaborating on live working activity support documents and technical specifications.
EDP Distribuição www.edpdistribuicao.pt
Hendrix Wire & Cable Co. www.hendrix-wc.com