Due to the energy demand in many country‘s transmission systems, authorities have been forced to examine technical solutions, in order to effectively develop the transmission capacity of existing lines.
Possible solutions to the transmission systems development include the construction of new lines, which can be attached to existing ones. Alternatively, the most cost effective answer would be to replace pre-existing conductors in existing lines.
This can be achieved by replacing ropes, which have a lower electrical resistance, in order to transmit more energy within termic limits for each conductor using traditional conductors to decrease the resistance.
By using traditional conductors to decrease the resistance, both the section and the weight are supposed to increase, however this leads us to evaluate the whole project.
It would be interesting to increase the current capacity with a conductor whose weight is not very different from one of the traditional ACSR (Aluminium Conductor Steel Reinforced) but this could lead to an unavoidable temperature increase and subsequently, damage to the conductor. As using traditional conductors is not possible, it is necessary to investigate other options.
Following extensive research by Metallurgists, many innovative materials with interesting physical characteristics which are suitable to a high temperature functioning, are now available on the market.
The material at issue is the Alloy Al-Zr. The Alloy Al-Zr is a high conductability material with a maximum stress, which remains unchanged at high temperatures. By using this alloy, it is possible to build a high thermal limit rope, which works at temperatures that are not possible for traditional conductors.
Four varieties of Al-Zr Alloys are available, depending on the Zirconio percentage, which determines the thermic resistance. The most popular Alloys are AT1, called TAL (Thermal Resistant Aluminum Alloy) and the AT3, o ZTAL (Zirconium Ultra Thermal Resistant Aluminum Alloy).
Having a high thermic level rope depends both on the innovative material and on spatial distribution optimization technique, which involves maximum use of the conductive section, by using the free space between the wires of the rope. As opposed to using traditional cylindric wires, this space can be achieved through implementing shaped wire, commonly known as Conci.
Rope featuring this kind of shaped wire will produce a compact rope which is characterised by a lower electric resistance, at the same diameter. The best way to increase the capability in a line is to replace the existing conductor with an innovative heat-resistant low-rise rope. The best performances in terms of thermal expansion and weight and mechanical tightness are achieved by using composite material as load-carrying core for the conductors.
During 2009-2011, a trial using an innovative thermoresistent conductors’ prototype with composite material core in carbon fibre, was undertaken. During the first year a composite conductor’s experimental characterization was undertaken and was developed in a laboratory, according to a test protocol agreed between the grid operator for electricity transmission and TRATOS Spa. This test resulted in the final conductor‘s prototype being installed on a 132 kV line section.
This particular type of research was carried out in order to solve the problems that arise when using non metallic material as constituent parts of ropes, operating at high temperatures, in a field where there are no reference norms. In order to take various aspects into account such as aging, mechanical fatigue, conductor’s bedding, and ecc, a diverse range of mechanical, thermical and electric tests were undertaken at the RSE Laboratories. Once the results were achieved, TRATOS was able to produce a new conductor prototype. This prototype had the same diameter and external jacket made with trapezoid section wires.
A cyclical processor of laboratory tests was then put into effect in order to define the new mechanical characteristics and the structural arrangement of the final composite robe. Finally, mechanical, thermical and electrical tests were performed in the last prototype, which were aimed at the next on-line installation.
The conductor, which was installed by the grid operator for electricity transmission in Autumn 2011, was done on three 132kV and showed no particular difficulties, compared with traditional conductors of that size. The installation of a line at high altitude (2000m) is a severe test for a conductor, in fact, at low temperatures the mechanical load is transferred from the carbon carrying core to the aluminium alloy coats, resulting in a condition of high mechanical stress of the latter.
Whilst researching the ropes with a hybrid carrying member, TRATOS had to base its work on two essential premises; in the presense of humidity, the contact between the carbon fibres and the alluminium may generate an electromotive force that can cause the alluminium’s corrosion; at temperatures higher than 116°C the carbon fibres must be protected to avoid contact with oxygen, as this could degrade the fibres‘ resin, which are not degradeable.
Taking these factors into consideration, Tratos Cavi, in close cooperation with Soficar of Pau (supplier of carbon fires), devised, tested and executed the hybrid composite wires, which consisted of a central core of pure carbon fibres and an outer layer of glass yarns with high modulus, which prevent the contact carbon-aluminum.
In addition, Tratos Cavi developed the technology for the extrusion at high pressure and reduced temperature of the aluminum sheaths on ropes, which consist of hybrid wires, previously buffered with special non-migrant and resistant to high temperature fillers. As a result of this technology, Tratos Cavi has managed to significantly reduce the permanent elongations.
Design, construction and testing of a load-carrying rope 7 x 3.00 mm, with a crown of 12 ʺconciʺ and a 24-alloy TAL wire, with an external diameter of 22.70mm.
Following initial research, in 2008, Tratos set about making a 22.7 mm outside diameter rope, with hybrid wires support, for high thermal limit and reduced thermal expansion at high loads. It was thought that this rope could be used as an alternative to the rope used by the grid operator for electricity transmission, with aluminum supporting. Additional benefits included unitary mass, ohmic resistance and raw materials‘ cost. The conductor designed had a hybrid load-carrying member 7 x 3.00 mm, with two coronas, one with eleven TAL ʺconciʺ and one with twenty four alloy TAL wires.
This rope was subjected to numerous tests in Tratos Cavi, Morsetterie and the Institute of Gorla Erse Laboratories.
Examining the results of all tests, Tratos Cavi discovered the ability to further improve the performance of the rope by replacing the outer layer of twenty four 2.55 mm wires with a layer of 16 ʺconciʺ and increasing the hybrid wires diameter of the load carrying member from 3.00 to 3.30 mm, without changing the external dimensions.
Design, tests and installation of rope with hybrid load-carrying 7 x 3.30 mm, with two alloy-TAL ʺconciʺ coronas (12 +16) (AT1 according to IEC 62004), with an external diameter of 22.70 mm.
Some changes were applied to further improve the performance and evidence confirmed that these changes did not alter the outer dimensions. The advantages obtained were, an extra safety margin to the breaking load (the wires hybrid load-carrying section increased from 49.48 to 59.87 mm2); a further lowering of the ohmic resistance, which passed from 0.0998 Ὠ / Km at 20°C to 0.095Ὠ / Km; a more uniform and loose outer surface of the rope; an increase of the electrical and mechanical sections in comparison to the previous project; and a lowering of the permanent elongation after loading cycles up to 50% of the breaking load.
Tested at Tratos Cavi, Morsetterie and the Institute of Gorla Erse laboratories, under the same conditions as previously, the results were positive and in line with expectations.
Following this research and subsequent meetings with the grid operator for electricity transmission, it was decided that this rope would be installed in a pass in the Alps, on the 132 KV line in October 2011.
In comparison to other conventional conductors of similar size, this installation was relatively simple.
The attached report, submitted by the engineer, Pirovano dell’Erse (Emblematic Products), concludes:
ʺThe installation of the line at high altitude (2000 meters above sea level) is a severe test for the conductor: in fact, at low temperatures, the mechanical load is transferred from the carbon core to aluminum alloy coats, determining a state of high stress for the latter
Design, construction and execution of an experimental rope destined to the 380 KV line ʺLa Spezia Station-Vignole Borberaʺ.
Following on from the experience and achievements gained in previous trials, work was commenced on the design, construction and testing of the experimental rope for the 380 KV line, ʺLa Spezia Station-Vignole Borberaʺ, in January 2012.
The purpose of this rope was to replace existings models such as ʺAnacondaʺ, which was subject to severe and conflicting restrictions. With this in mind, Tratos Cables decided to implement 3 cores, each of which consisted of hybrid wires carrying cable which was buffered and protected by an aluminum sheath and stranded together with 3 aluminum tube fillers.
The tests carried out at the Institute Erse show that due to the particular geometry, the transfer point is placed at a temperature below 35°. Beyond this, the rope thermally expands according to the expansion coefficient of the core, which is 5.8 x 10exp-6. This feature makes it possible to install the rope on the line ʺLa Spezia Station-Vignole Borberaʺ, without making any changes to the existing plyons.
From Arrhenius curves is in fact derived the temperature for the continuos operation function in 40 years. This temperature is higher than 150°C.
According to this, the rope can be used at a temperature of 130°. At this temperature the arrows rope proposed by Tratos Cavi are lower than the ones that the old rope ʺAnacondaʺ showed at 75°C.
The call of the tenders‘ flow rates (2000 A at 30° in Summer and 2300 A at 10°C in Winter) are both reached from the rope at about 89°C. The flow insured to the maximum operating temperature (130°C) are, 2500 A in Summer and 2750 in Winter, calculated with Shurig and Frick formulas, in presence of sunlight.
The rope at issue is reported below:
Design, construction and execution of a experimental rope with supporting hybrid load-carrying 7×4.50 mm with two alloy-TAL ʺconciʺ coronas (12+16), outer diameter 30.00 mm and design of the varying for high loads.
Using the single load-carrying module (7 x 4.50 mm) of the ʺAnacondaʺ rope, which was stringently tested in a variety of applications, Tratos Cavi has designed and built an experimental size of 1200 meters for high thermal limit, with reduced distortion at high loads, which could be used as an alternative to the rope used by the grid operator for electricity transmission.
This version consists of a load-carrying cable with hybrid wires 7 x 4.50 mm, which is buffered and protected by an aluminimum sheath and two alloy – TAL ʺconciʺ coronas (12+16). This has an external diameter of 30.00 mm.
This experimental size has already been successfully tested in the laboratories of Tratos Cavi and Gorl a Morsetterie. It was also sent to the Institute Erse for tests in September 2012.
The actual breaking load was equal to 24,000 daN and in order to provide a solution for higher loads, Tratos Cavi, developed a variation of this rope which involved reinforcing the main core and extending the outer diameter to 31.50 mm.
Compared to the rope with load-carrying ACI and an outer diameter of 31.25 mm, the updated version included many advantages, such as the ohmic resistance, the breaking load and the effort’s transfer point.
These kinds of rope are very interesting as from the temperature of 30-35°C, the coefficient of expansion passes from the global average value of 14.7 x 10 exp-6 to that of the central nucleus of 5.8x10exp-6.
Comparison between the characteristics of high thermal limit and with reduced deformation at high loads hybrid load-carrying rope in ACI 20 SA and the aluminium load carrying traditional rope.
In comparison to the characteristics of hybrid wires carrying-load ropes 7 x 4.50 and 7 x 4.90 mm, with the outer diameter of 30.00 mm and 31.50 mm and the rope with load-carrying in AC1 20 SA 19 x 3.25 mm with an outer diamter of 31.25 mm, it is noted that at a temperature of 130°C, the high load-bearing hybrid rope 7 x 4.90 mm with a diamter of 31.50 mm, has a current capacity of 17% higher than the rope with a 20 SA AC1 load-carrying, in both Summer and Winter. At the same temperature, the rope with the hybrid supporting 7 x 4.50 mm, with a diamter of 30.00 mm, compared to the same AC1 20 SA rope has a current capacity higher than 11% in both Summer and Winter.
In conclusion, the current capacity of the hybrid load-carrying ropes at 130°C are even higher than those with load carrying ropes in ACI 20 SA at 150°C.