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All-aluminium alloys conductors: cutting overhead costs in Brazil

Brazilian transmission line operator EBTE is constructing five new 230 kV overhead transmission lines with a total length of 775 km in the Brazil state of Mato Grosso. Sidnei Ueda, overhead line manager for Nexans in Brazil, explains why an all-aluminium alloy conductor has been selected for this vital project.

Sidnei Ueda, Nexans, Brazil

Mato Grosso is Brazil’s third largest state with an area of around 900 000 km2. It is situated in midwest Brazil bordering Bolivia. With the Amazon Basin to the northeast of the state, Mato Grosso is an important contributor to the current 100 GW of installed hydroelectric power (HEP) that provides some 85 per cent of the country’s total power needs, making Brazil second only to China as a user of HEP.

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The largest HEP schemes in Mato Grosso include the 176 MW Ponte de Pedra plant on the Correntes River and the 160 MW plant on the Itiquira River. There are also many much smaller hydropower plants, ranging from a few megawatts to dozens of megawatts in capacity.

In 2008 the Alupar company EBTE was the winner of the public auction held by ANEEL, the Brazilian electricity regulatory authority, for a project to reinforce the power infrastructure in Mato Grosso.

The project (ANEEL 04/2008 Group D) will provide additional transmission capacity to handle the output of a number of new power generation projects that will contribute to Mato Grosso’s economic development as well as increase Brazil’s national power generation capacity by strengthening its interconnected national system.

The project comprises five new 230 kV, 300 MW overhead power transmission lines with a total length of 775 km. EBTE plans to begin operation in the first half of 2010.

Conductor selection

When designing high-voltage overhead transmission systems, a number of simultaneous requirements need to be satisfied, such as good conductivity to reduce losses, safe clearance above the ground, sufficient strength for the applied loads and practical costs for the long lengths of conductor to be installed.

Pure aluminium provides good conductivity (around 61 per cent that of copper), and thanks to its low density (2.7 g/cm3 compared with 8.9 g/cm3 for copper) is clearly the most desirable low-loss conductor material for overhead lines.

The drawback to using pure aluminium is its limited strength, as this means that only relatively short spans can be achieved using all-aluminium conductors (AACs). The traditional answer has been to use an aluminium conductor steel reinforced (ACSR) design in which a galvanized steel core supports the load and an outer layer of high purity aluminium strands carries the current. ACSR is widely used throughout the world, due to its established reputation for reliability and performance.

However, for the Mato Grosso project, Nexans suggested the attractive, cost-saving alternative offered by its AAAC design. This concentric stranded conductor is manufactured from a heat-treated 6201 aluminium-magnesium-silicon alloy that offers good strength to weight performance, so that the aluminium cable is able to support its own weight as well as carry the current.

AAAC conductors offer a number of practical advantages compared with ACSR. Its lower weight per unit length combined with equivalent mechanical strength means reduced sagging, so tower spans can be increased. Furthermore, corrosio n resistance is improved in environments that might cause galvanic corrosion, while power losses are reduced since the inductive effect of the steel core is eliminated.

Although relatively new in South America, AAAC has been used with great success in many projects throughout the world, especially in Belgium, France and the UK.

Reduced Tower numbers

For a long distance high-voltage overhead line project the total cost breaks down as: one-third for the conductors, one-third for the towers and their foundations and one-third for other elements such as accessories.

There is no cost penalty for using AAAC as the conductor, since its cost is broadly similar to ACSR. It does, though, have a major impact on the tower cost element, as the capability to increase the span between towers enables a number of towers to be eliminated from the scheme.

A photograph (left) and a cross-section (right) showing the latest design of overhead line ACCC with an aluminium conductor and composite core.
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The total 775 km length of transmission lines to be installed in Mato Grosso will require 1550 towers à‚— an average span of 500 m. The use of AAAC has reduced the number of towers needed by some 10 per cent, or around 150 towers. This has had a significant impact on the overall project cost.

In addition to the in-place cost saving, Nexans’ AAAC offered a further advantage for EBTE, as it is manufactured in Brazil at the Lorena plant in the state of Sao Paulo. Each overhead line will comprise three phases so, with around 2350 km of 455 mm2 round wire conductor required, local manufacture offered considerable savings in delivery and logistics costs compared with sourcing the conductor from Europe.

Future developments with Aero-Z

The environmental conditions for the Mato Grosso project are relatively benign. The average temperature is 30 à‚°C and the maximum 40 à‚°C. The area is also generally not subject to the extreme hurricane force winds that can cause destructive galloping of the overhead lines, so it is well suited to the conventional round wire design of overhead cable. The new lines are designed to last at least 30 years, but normally will not need replacement for 40 years or more.

However, some future projects in Brazil might benefit from a more advanced conductor such as Nexans’ AERO-Z that features a compact design with Z-shaped fully interlocking wires. It reduces drag (pressure on lines due to strong winds), minimizes galloping, lowers corrosion and snow accretion, and raises ampacity by 10 per cent in an equivalent diameter, or reduces Joule losses by 15 per cent at the same ampacity. This makes AERO-Z of particular interest to all power producers and the utilities that are planning to install lines in areas subject to extreme weather conditions.

Aero-Z has equivalent accessories, and can be installed in the same way and with the same equipment as conventional conductors.

An installation of Aero-Z overhead lines in Niger, Africa.
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Red de Energia del Peru (REP) has been using AERO-Z on various projects since 1999 after deciding that it was the most reliable and resistant conductor for the specific local climate conditions in Peru, where overhead line installations are highly challenged by corrosion due to the proximity of the Pacific Ocean.

Most recently, nearly 900 km of AAAC internally greased Aero-Z has been installed to help resolve corrosion and corona loss issues in an area of Peru where there is a complete absence of rainfall, so there is no natural washing of the overhead lines. REP’s asset managers have reported that the introduction of AERO-Z has resulted in a substantial reduction in maintenance costs, as the lines are so much easier to keep clean than conventional conductors.

In Africa, Nexans has also provided AERO-Z high-voltage overhead conductors as part of a contract to upgrade the capacity of the 264 km power line between Birnin-Kebbi (Nigeria) and Niamey (Niger). Together with capacitive compensation, the upgrade has increased the power transmission capacity by at least 75 per cent à‚— to 70 MW in the worst summertime conditions and up to 80 MW in winter.

Local experience is essential in convincing utility companies to make a design change on long-tem assets. So Nexans is currently carrying out some experimental installations in Brazil to demonstrate the advantages of Aero-Z. At present though, Aero-Z is not manufactured by Nexans in South America and has to be transported from factories in Europe.

Aluminium Conductor Composite Core

Looking at the longer-term future for overhead cables, a particularly exciting development is aluminium conductor composite core (ACCC). This innovative, lightweight conductor features a composite core comprising carbon fibres embedded in an epoxy resin matrix surrounded by strands of annealed aluminium Z-shaped wires.

The composite core carries the majority of the mechanical load on the conductor, while the soft annealed aluminium carries the current. The compact core enables more aluminium to be packed into the same overall envelope as an ACSR, thereby increasing its current carrying capacity. The result is an efficient and corrosion-free conductor with high tensile strength and low linear expansion and extremely low sag.

Its high-breaking load allows longer spans between pylons of up to 2.5 km, so it could present the ideal solution for some of the river crossing challenges that will face future transmission projects in Brazil, especially in the Amazon Basin.

ACCC can be installed using the same techniques as traditional conductors and can double the current carrying capacity compared with conventional ACSR. So in addition to its benefits in longer spans, ACCC is therefore an almost universal replacement conductor.

Cat-1 line monitoring

Rather than regarding overhead transmission lines as a static commodity, they should be viewed as an active system that changes its operating behaviour minute by minute as wind, weather and load conditions vary. To operate this system to maximum effect it is useful to have access to real-time information that shows exactly how the transmission lines are responding to these variations. It is then possible to make them work smarter and harder without compromising safety margins.

The CAT-1 monitoring system.
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One especially useful technique is Nexans’ CAT-1 real-time transmission line monitoring system that can be very quickly installed on existing or new lines. The system measures weather parameters and collects line-tension information. This vital information is then computed and forwarded in real time to the operator’s control system (EMS/SCADA). This enables the actual line rating to be calculated to establish the true additional capacity available as well as the sag that the line can safely accept within the conductor’s design temperature.

By providing advanced warnings when approaching limit conditions, CAT-1 helps to increase the reliability of the network and gives information to quickly react and avoid severe problems. In some cases, the capability to drive the network harder to provide additional capacity may even delay or possibly prevent the need to invest in an infrastructure upgrade.

The first CAT-1 transmission line monitoring system was installed by Virginia Power in the US in 1991. Since then, over 300 transmission line monitoring systems have been installed over 100 utilities in more than 20 countries on five continents.

Over two-thirds of the 30 largest utilities in North America have CAT-1 systems, and over half of those utilities operate CAT-1 systems in real-time to provide accurate real-time ratings to utilities’ EMS/SCADA.