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Lowdown on overhead line innovations

French transmission operator RTE is trialling Lo-Sag in the Limousin region
Credit: Nexans

Technology is helping utilities upgrade their overhead line assets to carry significantly more power, writes Francis Debladis.

Utilities worldwide are under ever-increasing pressure to deliver more electrical power to support economic growth, especially in large urban areas.

However, creating new power infrastructure in regions with a high population density can present major challenges, both in land and equipment costs and the extended timescales required to obtain the necessary permits and rights of way.

There is now an effective alternative approach based on recent innovations in overhead line (OHL) design and materials technology that enables existing power transmission routes to be upgraded to carry significantly more power within existing rights of way.

The Aero-Z advanced conductor
Credit: Nexans

There are essentially four options for increasing OHL capacity:

ࢀ¢ Increased conductor cross-section: The greater the conductor cross-section then the more power it can carry. However, the stresses on the conductor, due to its own weight and increased wind pressure etc, require the towers to be changed or at least reinforced;

ࢀ¢ Increased voltage: Increased voltage at the same operating current will deliver more power. This calls for a greater distance between the phases and therefore tower changes and modifications;

ࢀ¢ Monitor the line: Rather than regarding OHLs 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 dynamic line rating (DLR) 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;

ࢀ¢ Increase the operating temperature: The higher the allowable operating temperature then the more current, and consequently power, that the OHL can carry. This rating is limited by the material used to make the conductors. For most systems today, the temperature rating is around 80-90à‚°C. And this is determined by the coefficient of thermal expansion that rules the sag.

Conductor evolution

Before we look in detail at the latest high-temperature conductor technology, it is useful to review some of the steps in the evolution of OHL design, which have needed to satisfy a number of simultaneous requirements: good conductivity (to reduce losses); safe clearance above the ground; sufficient strength for the applied loads; and affordable costs for the long lengths of conductor to be installed.

Copper is, of course, a natural choice for underground cables. But pure aluminium appears to be the most desirable conductor for overhead lines thanks to its good conductivity (around 61 per cent of copper) and low density (2.7 g/cm3 compared with 8.9 g/cm3 for copper).

There is an important drawback to pure aluminium – its limited strength means only relatively short spans between towers can be achieved using all aluminium conductors (AAC). 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.

There is now an attractive, cost-saving alternative offered by the all aluminium alloy conductor (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 carrying the current.

AAAC conductors offer several practical advantages over with ACSR:

ࢀ¢ Lower weight per unit length combined with equivalent mechanical strength means reduced sagging so tower spans can be increased;

ࢀ¢ Corrosion resistance is improved in environments that might cause galvanic corrosion;

ࢀ¢ Power losses are reduced since the inductive effect of the steel core is eliminated.

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

The RTE trial marks Lo-Sag’s European debut. It has previously demonstrated success in Brazil
Credit: Nexans

Tower numbers reduced

For a long distance high voltage overhead line project, the total cost breaks down as around 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.

As an example, Brazil’s Mato Grosso project – 775 km of 230 kV, 300 MW transmission lines to connect new hydropower schemes to the country’s Interconnected National System – required around 1550 towers, an average span of 500 metres using AAAC. The number of towers needed was reduced by some 10 per cent, or around 150 towers, making a significant impact on the overall project cost.

In areas where environmental conditions are relatively benign – no major extremes of temperature or extreme hurricane force winds that can cause destructive galloping of the overhead lines – the conventional round wire design of overhead cable can be a sensible solution.

However, some projects will benefit from a more advanced conductor such as the Aero-Z, which features a compact design with Z-shaped fully interlocking wires. It reduces drag (i.e., 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 utilities 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. Recently, nearly 900 km of 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.

Increasing operating temperature

In installations where there is a desire to increase the operating temperature, then the longest established conductor is the aluminium core steel supported (ACSS) design. It can operate at over 200à‚°C but does suffer from significant sag.

An alternative is the design with a core of Invar (a nickel-iron alloy). Again this can reach over 200à‚°C but it has a low strength/mass ratio and cannot accept icing conditions. Gap type conductors feature a special construction that creates a small gap between the steel core and thermal-resistant aluminium alloy layer. This design offers good sag and current carrying characteristics and can reach 180à‚°C, but does require specialized installation procedures.

An exciting new development is the Lo-Sag conductor, the result of a five-year development programme involving Nexans’ technology centres in France and Belgium. This overhead line technology consists of a thermal resistant aluminium conductor wrapped around a composite carbon core, When compared with the ACSR conductor, based on a steel core, Lo-Sag’s composite carbon core of the same diameter is much lighter and 50 per cent stronger.

The use of high tensile strength carbon fibres in an epoxy matrix to carry most of the conductor’s mechanical loads offers a number of advantages: high glass transition temperature; high breaking load; high elastic modulus and good creep properties.

Most importantly, the carbon core’s coefficient of thermal expansion is roughly one tenth of that of steel, so it expands (and therefore sags) much less when heated by the high current flowing in the conductor, enabling vital safety clearances to be maintained between the conductor and the ground, even at high operating temperatures. These properties enable the new overhead lines to be driven much harder by an electricity utility, running at significantly higher temperatures to carry around twice as much power.

As an example, we can consider a typical 400 metre span between towers with a 322 mm2 conductor cross-section. A conventional ACSR conductor, at a typical safe limit, might operate at 90à‚°C with a sag of 15.1 metres, while Lo-Sag could operate at 150à‚°C with a sag of just 13.4 metres.

Lo-Sag is therefore perfect for upgrading existing routes to higher powers, using established routes and existing cable towers with minimal modifications, reducing both project delivery times and costs. Lo-Sag is also expected to have a lifetime of at least 40 years, similar to or even better than conventional conductors.

The Lo-Sag conductor’s composite carbon core
Credit: Nexans

Installations and trials

The world’s first Lo-Sag installation was carried out for Light, Rio de Janeiro’s electric utility. It has been operating in the field for around two years on a 138 kV transmission line that connects Cascadura to Sàƒ£o Jose in Pavuna district, one of the areas of Brazil with a specific need for more electricity, where it has been proved to increase the power transmission capacity by over 70 per cent.

In this particular exercise, Lo-Sag has been used to reconductor an existing 23.5 mm diameter ACSR line, normally operated at 70à‚°C to provide an ampacity of 690 A and carrying 165 MW with a 9.3 metre sag. With the new Lo-Sag conductors, the lines now operate at 150à‚°C, with a 1300 A ampacity, carrying 310 MW with a 9.1 metre sag.

RTE, France’s electricity transmission system operator, is currently trialling Lo-Sag in Haute-Vienne, in the Limousin region. This field trial involves about 1 km of high voltage, 90 kV overhead line being upgraded between Bellac and Saint-Martin-Terressus. This is the first time that Lo-Sag has been deployed in Europe. Comprehensive testing of the Lo-Sag technology will take place throughout the course of the year, with the objective of checking compatibility with the existing grid as well as reviewing its mechanical properties and the capability to withstand France’s variable climate, including combinations of wind, ice and snow. The aim is also to evaluate the aging of Lo-Sag compared to a ‘conventional’ conductor.

While Lo-Sag has been developed to meet the particular needs of power transmission upgrading projects, the concept also offers important advantages in the construction of new transmission lines. In particular for the long spans, over 1 km, required for river crossings, Lo-Sag could help to reduce the height of the towers by up to 30 per cent.

Francis Debladis is Nexans’ overhead lines corporate technical manager. For more information, visit www.nexans.com.

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