17 Сентября 2019 | вторник | 07:02
Energoterra

Innovations to the core

Марта 24, 2014

Lately, power engineers have been setting more challenging requirements to cabling and wiring products. Present-day overhead and underground power cable lines must have higher conductivity without increase in weight and dimensions, and must ensure reliable operation under the most extreme conditions. How to achieve that? State-of-the-art conductive materials come to the rescue.

Copper vs aluminum
Historically, aluminum and electrical steel have been used as conductors for cables of overhead lines (OL). As to power cables, aluminum still competes with copper, and according to most experts, copper is an absolute leader here. It can be explained by such advantages of the latter as high specific electrical conductivity, durability, ease in welding and soldering. There is a good reason for cable makers’ current thinking that there is nothing like “good old” copper used as conductor.
However, copper has quite a substantial disadvantage which is its high price that keeps rising at a fast pace compared to aluminum. As a result, the latter consolidates its market position little by little.
Evgeny Uvarov, Deputy General Manager of Electrocable International Association, notes that for the last six years, aluminum consumption has outrun the growth rate of copper consumption in the cable industry in Russia and CIS countries (139% and 109.5% respectively). As a result, advantage of copper over aluminum in terms of consumption got reduced in some way. One of the reasons is outstripping growth rate of overhead wires output. However, according to experts, even without account of aluminum used for production of uninsulated wires, copper-to-aluminum consumption ratio reduced from 2.36:1 in 2008 to 2.35:1 in 2011, and this trend is surely will keep going.
“The main reason for wide expansion of aluminum for cables and wires manufacturing is deficit of copper wires and a higher cost of copper which is 3 or 4 times more expensive than aluminum”, says Mikhail Kladov, Head of Innovation and Technological Development Department of Kamsky Kabel LLC. “Compared to copper, conductivity of aluminum is lower by 32%. But speaking about its conductivity per unit weight, it is twice as much as copper due to aluminum’s low density.
Compared to copper, aluminum has a few disadvantages: low mechanical strength and lower contact properties. An important drawback of aluminum is its high oxidability when exposed to air. As a result, a refractory foil is generated on the surface. Its melting temperature is about 200 °С. Such foil has weak conductivity, and it has a detrimental effect on establishing a good contact. To make it up, aluminum alloys are used.
 
Overhead transmission lines with composite core (photo provided by 3M Russia)
 
Course free for alloys
One of the trends is using a zirconium-based aluminum alloy. The main advantage of this alloy is its ability to withstand high operating temperatures, and its high conductivity. 
“Al-Zr alloy has the same strength as standard aluminum 1350-H19, but its microstructure is formed in such a way to keep that strength under high temperatures”, explains Konstantin Belousov, Head of Electrical Equipment Department of 3M Russia. “If standard aluminum gets annealed at 120–150 °C and loses its strength dramatically, the Al-Zr alloy retains its properties up to 210 °C, with account of peak loads of up to 240 °C”.
An example of using aluminum-zirconium alloy is AST cables manufactured by Kirskabel (Kirov region, a member of Unkomtekh Group). Operating temperature of those cables reaches 210 °С, allowable short-circuit temperature is 300 °С. Aluminum-zirconium wire rod is used for gap-type high-temperature cables GZTACSR manufactured by Sim-Ross-Lamifild LLC (Uglich, Yaroslavl region). The manufacturer guarantees operating temperature up to 230 °С, with account of peak loads of up to 310 °С.
“Our enterprise uses aluminum-zirconium alloys when making serial products”, explains Mikhail Kladov. “Those are high-temperature AS compact wires for overhead power transmission lines and XLPE-insulated cables rated for 6-35kV with aluminum metal shield. 
Ferro aluminum alloy (ABE) opens up very interesting possibilities, too. It is used for production of self-supporting insulated conductors, ASp wires for overhead lines, and aluminum wire armored cables.
Nowadays, when production of aluminum alloys is commercialized by Russian and foreign companies, it may safely be said that the future of this practice in the cable industry will be very successful”.
 
Strength of composites
Not that long ago, composite core cables that had already shown themselves to good advantage in the western countries, have come to the Russian market. A composite core contains reinforcing elements used to ensure mechanical strength, and binding material. The interesting point is that the same aluminum alloy wires are used in composite cores, but their combination with other elements provides a conceptually new quality. 
“More than 10 years ago, ACCR (Aluminum Conductor Composite Reinforced) cable was invented by 3M company”, says Konstantin Belousov. “That cable is based on “matrix band” technology. As a result, we have a 2-4.5 mm diameter aluminum wire armored with a few thousand of aluminum oxide fibers. Those wires are twisted into a cable core using traditional cable production methods. Over the core, one or more layers of Al-Zr alloy wires are applied.
Aluminum oxide is actually a ceramic material so it is resistant to high temperature and is not conductive. Being the basis of the matrix, aluminum is practically not exposed to most environmental factors such as humidity, ultraviolet, ozone, etc., or behaves not any worse than the reputable aluminum 1350-Н19. At the same time, being a metal, aluminum protects the fibers against plastic bending such as crushing and offsetting which are the main causes of damaging longitudinally-oriented fiberlike structures. Also, the load is equally distributed in the multiple-wire core along all the wires, which makes the cable more strong”. 
 
Kamkabel production floors (photo provided by Kamsky Kabel)
 
So far, the ACCR cables are imported to Russia, but in 2014, 3M is planning to localize production in the Kaluga region on the basis of Liudinovo Cable plant. From 2011, preliminary works have been performed at the plant for production of aluminum- zirconium wire for ACCR cables.
Another approach to production of a composite core cable was offered by CTC company (USA) some time ago. Their ACCC cable contains a core made with the use of carbon fiber. A conductor of such cable has low thermal expansion factor and low weight which allows using more aluminum without affecting the total weight of the cable. As of today, ACCC cable is introduced to the Russian market by Sim-Ross-Lamifil.
Soon, domestic developments will see the light as well. For example, R&D works for cables with composite core made of carbon and basalt fibers are being performed by Unkomtech along with FGC UES JSC, FGC UES Research and Development Center JSC, and IDGC of Center and Volga region.
 
Looking forward
We asked the experts to make an estimate as to which conducting materials have the most promising future.
“It is a matter of argument, to which conducting materials the future belongs”, says Mikhail Kladov. “All depends on what goal you have. Copper cables have better physical properties and conductivity compared to aluminum ones. But the copper value is higher. Aluminum alloys have better mechanical and electrical properties and are cheaper than copper which is the best solution when making a choice between aluminum and copper. And in case there are any restrictions in terms of dimensions of cabling and wiring products, only silver may compete with copper.”
“Most probably, we will see different types of composite conducting materials in the future which would combine, let’s say, dielectric and current conducting material”, suggests Konstantin Belousov. “Such material will be much in demand for electrical equipment and will be launched by 3M in the near future.
 
From top to bottom: АSp and АS compact wires (photo provided by Kamsky Kabel)
 
Another direction is conducting adhesives, sealants, compounds which may be used for filling up cavities and substitution of heated soldering in cables and connectors.
By the way, even today, tapes with conductive adhesive layer are used for solution of different technical tasks encountered during production of cabling and wiring products. Those tapes contain acid-fee corrosion-resistant acrylic resin with conductive areas covered with a fine coat of silver. Those conductive particles equally distributed within the adhesive create lots of interconnections with low resistance between the basis and the screened surface.” 
Researchers work on creation of other materials with electrical conductivity which would be the order of magnitude greater compared to traditional metals and alloys. Here, nanotechnologies might help. On the Russian market, nanostructural conductors based on copper- niobium, copper-vanadium, and copper-iron alloys are offered by Research and Production Enterprise “Nanoelektro” which is a joint venture of ROSNANO and All-Russian Scientific Research Institute for Inorganic Materials named after A. A. Bochvar. The developments have been already commercialized but for special use only – magnetic flash installations, aerospace industry, microelectronics, etc. However, in the long view, they speak about using nanoconductors in power cables and wires. 
Foreign researchers work on development of quantum wires where carbon nanotubes would be used.
Of course, specialists put high hopes on development of wires and cables based on superconductivity phenomenon, i.e. materials capacity to have zero electric resistance when reaching the temperature lower than a certain preset value. Certainly, this product will not appear today but maybe tomorrow or the day after. Look for more articles in our next issues. 
 
Ekaterina Zubkova, EnergyLand.info

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