By Thierry Capelle, Nexans, Belgium
Nexans is currently working on a multi-million dollar contract for Red de Energia del Peru (REP) to supply 875 km of 220 kV AERO-Z high voltage, bare aluminium, overhead conductors to reinforce the electrical power infrastructure in the north of Peru. REP, a subsidiary of the Columbia-based ISA Group (the largest electrical transmission company in North Latin America) is installing the 455 mm2 cross-section, double Z layer conductors along Peru’s pacific coast to create a new 190 MVA, 290 km, three-phase link between the substations of Zapallal, Paramonga and Chimbote to the north of the capital, Lima. The installation work is currently under way, and is scheduled for completion in early 2008.
The reinforcement of the electrical transmission infrastructure in northern Peru is being driven by the need to meet the growing demand for energy in the country’s industrial and mining region. REP is also constructing new electrical substations to interconnect a new gas fired substation.
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.
A particular difficulty is the complete absence of rainfall – the last time it rained in this area of Peru was 1967, so there is no natural washing of the overhead lines. On conventional stranded conductors with a ridged surface, this can cause substantial layers of dust to build up over time. If this dust is not removed by regular mechanical cleaning it can combine with the grease that tends to seep from the internal structure of the conductor to form ‘sand sleeves’ that accelerate corrosion, as well as act as insulators that result in a loss of electrical performance.
In contrast, AERO-Z has a virtually smooth cylindrical outside surface that reduces the build-up of dust and can be easily cleaned by hand. REP’s asset managers have reported that the introduction of this type of conductor has resulted in a substantial reduction in maintenance costs as the lines are much easier to keep clean than conventional conductors.
AERO-Z features aluminium or aluminium alloy conductor wires wrapped around a core of steel or aluminium alloy. However, unlike conventional round or trapezoidal – Trap – conductors, the AERO-Z conductors feature one or more concentric layers of specially profiled Z-shaped wires, which lock neatly into each other. The outer layer, which is virtually smooth, has small helical grooves created between the upper edges of the Z-shaped wires with carefully chosen geometrical parameters.
AERO-Z conductors have one or more concentric layers of specially profiled Z-shaped wires that lock neatly into each other
During manufacture, all the internal cavities of the cable are filled with grease introduced at a temperature of higher than 120 °C, thus eliminating air and moisture. The large contact area between two Z-wires in the same layer effectively forms a hermetic casing that protects against seepage of this grease towards the outside of the conductor and against the penetration of abrasive particles into the conductor. This improves the internal protection compared with conventional conductors, in which the charge cycles tend to cause the protective grease to migrate towards the outside. In contrast, AERO-Z maintains a constant level of corrosion protection, and this guarantees slower ageing of the wires over time.
Of course, as well as preventing the build up of dust, the AERO-Z’s interlocked structure performs well in conditions where snow and frost accretion might become a problem. The ‘sleeves’ that form tend to detach more rapidly because AERO-Z is twice as rigid in torsion compared with a conventional conductor.
Even if a wire should break, the overlapped and interlocking Z-section of the conductor’s outer layer wires hold it in place under mechanical service stress so that there is less risk of it detaching and touching a phase wire or another near object. Furthermore, this property is preserved until five contiguous wires have been broken. This ‘undestrandability’ offers a key advantage over the only equivalent cable technology, which is the Trap wire. If a Trap wire breaks, its wedge form forces it outside the conductor, so even if only one strand breaks, it tends to unravel and cause a short circuit. Also, in contrast to the smooth AERO-Z conductors, when Trap wires rotate, they slide and produce a noisy corona effect and additional losses.
Drag coefficient & damping
A key technical challenge addressed in the ongoing development and improvement of AERO-Z is the ‘Cx’ factor, which is the effect that wind has on an object in terms of drag coefficient. This can exert terrific strain, not only on the conductor itself, but also on the support structure.
Rigorous wind tunnel and field tests have shown that at certain wind speeds, AERO-Z has a greatly reduced drag coefficient compared with traditional stranded conductors – as much as 40 per cent lower. Similarly, the conductor ‘gallops’ less, and this is an important security factor ,which lowers the strain on both the pylons and the conductor. Numerical simulations have also confirmed that the strains can be reduced by 30 per cent on the hardware in the case of turbulent winds because of the excellent self-damping of the AERO-Z design, increasing the security and the life expectancy of the line.
Blowing in the wind
The University of Liege in Belgium carried out a computer simulation of a violent, turbulent storm to compare AERO-Z with conventional high voltage aerial conductors with round wires. This allowed its performance to be assessed not only in static, but also in dynamic terms. AERO-Z was shown to be more stable and produced far less strain on towers than normal HV conductors.
In countries where high and turbulent winds prevail, this is a distinct advantage because storms often result in the collapse of towers or hardware, rather than broken conductors. This means that AERO-Z can achieve important cost savings in terms of the support structure. The conductors do not need any additional reinforcement, and poles and towers will be subjected to lower strains.
In the case of Peru, REP has asked for the overhead lines to be designed to withstand a relatively moderate maximum wind speed of 42 metres per second.
Solving network congestion
In July of this year, REP energized an earlier overhead line project, to the south of Lima, also featuring AERO-Z. However, while corrosion was still an important concern, the major issue in this case was to relieve network congestion by providing additional transmission capacity on the 53 km bottleneck route between the San Juan and Chilca substations in order to meet the growing demand for power in the south of Peru.
A particular advantage of the AERO-Z design is that it results in more metal within a given cross-section, so for a given diameter, electrical resistance is lower, as are Joule losses, which means that more energy can be carried along the same size of conductor. At the same carrying ampacity, this means a lighter conductor with lower forces on the towers, enabling spans to be increased.
Since the AERO-Z conductors have the same basic principle as traditional conductors, they can be installed without changing the type of fixing accessories, such as suspension, dead-end clamps and insulator chains.
The initial plan for the San Juan to Chilca substation connection was to upgrade the existing overhead lines using conventional all aluminium alloy conductor lines with a 507 mm2 cross-section. However, Nexans persuaded REP to switch to using a double circuit of AERO-Z 301 (with a 301 mm2 cross-section). This approach provides an effective 602 mm2 of current carrying cross-section, resulting in a massive 43 per cent improvement in power capacity with no significant increase in the forces on the support structures, enabling REP to utilize the existing transmission towers. A total of 610 km of AERO-Z 301-2Z has been installed for this project to create a double 350 MVA overhead connection.
The AERO-Z conductors for both of the projects in Peru were designed, developed and manufactured in Nexans’ Elouges plant in Belgium.
Although AERO-Z has a higher initial cost than conventional conductors, when confronted with challenging local climate conditions in Peru, it has helped to solve corrosion problems and to reduce network congestion problems. This, in turn, has enabled REP to benefit from a sound return on its investment through improved network security and capacity.