Electrical conductor materials
Aluminium is a good conductor of electricity; after all, its conductivity reaches about 65% of the value of copper. However, aluminium just misses the winner’s rostrum of the conductivity of all metals, where silver takes the gold position. The silver medal goes to copper, while gold itself “only” receives the bronze medal. Aluminium follows at some distance in fourth place. After that comes nothing for a while. Due to their high prices, gold and silver are not considered for use in cables, wires, conductors and electrical machines, but at best as connecting wires in integrated circuits, where they are only needed by the milligram. All other known elements and compounds, if electrically conductive at all, follow at some distance behind. Alloys (mixtures) of different metals generally have a considerably lower conductivity than pure metals.
The choice is limited
This leaves copper and aluminium as the only two metals with an economically usable electrical conductivity, whereby copper is the measure of all things. The conductivity of copper for conductive purposes (Cu-ETP-1, Cu-OF-1 or Cu-OFE) is given as 58.58 MS/m. The IEC standard 60028 specified a value of 58.51 MS/m as a standard as early as 1925. This corresponds to 101% of the International Annealed Copper Standard IACS, which has set the standard conductivity of technical copper at 58.00 MS/m since 1913 and against which other conductor materials must also be measured.
Cables and wires
In the case of cables and wires, the argument of space requirements plays only a subordinate role in most cases. The lion’s share of the cross-sectional area of a low-voltage sheathed cable in the range up to about 10 mm² per conductor or of a high-voltage cable is due to the insulating material. The cross-sectional surcharge for aluminium is more or less lost in this. At least this is the case with conventional plastic cables. Mineral-insulated cables and conductors offer a considerable space advantage over ordinary plastic cables in addition to their absolute fire safety. For a time, these mineral insulated cables and conductors were also manufactured with a sheath made of aluminium instead of copper, but that did not catch on either.
In most European countries, copper is predominantly or exclusively used for building installations.
There are three main reasons for this:
- Aluminium is quite ductile (plastically deformable), but not as ductile as copper. The ends of stiff wires laid in walls, e.g. in flush-mounted boxes and on wall outlets, break off after being bent back and forth several times. It becomes problematic if the end of the wire inside the insulating sheath is very close to breaking off and continues to be operated in this state. The fault remains unnoticed until the wire is once loaded with a significant current (close to the rated current), which can take years. Then the constriction melts through, which again can happen much more easily than with copper because of the lower melting point and lower thermal conductivity of aluminium, not to mention the tendency to form such constrictions, and an arc can remain. This can even cause the aluminium to ignite and burn like a fuse.
- When exposed to air, aluminium very quickly becomes coated with a hard, resistant oxide layer that does not conduct electricity and therefore makes contacting difficult. Contact resistance can occur, which in turn ends in a fire risk. Copper also oxidises in air, but this oxide layer strangely does not hinder contacting, although copper oxides (CuO and Cu2O) with their conductivity, which is about 13 orders of magnitude worse than that of the elementary metal, can hardly be considered conductors.
- Aluminium tends to flow for a long time. The material yields over time under strong pressure. Thus, connections that are initially tight can gradually become loose. This problem can be solved with the corresponding extra effort in connection technology, and for lines with relatively few connection points, such as high-voltage overhead lines, this effort is worthwhile, but not in branched networks such as inside buildings.
Because of the second problem described, aluminium conductor ends should always be contacted with tightly tightened screw contacts, but these are often not durable because of the third point. In principle, spring contacts are a remedy, but then the oxide layers become a problem again. In both cases, the contact resistance increases slowly, which in turn leads to a fire hazard. The old aluminium installations in Eastern Germany and most Eastern European countries enjoy “grandfathering”, but this is only an effective protection against possible improvements. However, in order to be able to connect such “protected” installations with new system parts, there are special connectors with spring-loaded contact pressure and a special contact paste made of grease with sharp-edged metal particles. The particles push through the already existing aluminium oxide layer when connecting, and the grease protects against renewed corrosion.
Copper is also preferred as the conductor material in high-voltage cables, because the insulating materials and the outer shielding that come into question there are expensive, and even the small increase in the overall cross-section of the cable that is required in the case of aluminium compensates in turn for the saving in the conductor material – unlike in the case of low-voltage power cables with larger cross-sections. It should also not be forgotten that the shielding is always made of copper because there is no other way, and if aluminium is chosen for the conductors, after the – albeit long – life of such a cable, another work step has to be inserted when scrapping it in order to separate the metals from each other.
The material copper lives not only a long time, but practically forever. It can be recycled as often as desired without any loss of quality. About 45% of the quantity needed today is produced from scrap, and the products made from it, whether cables, transformers, water pipes or roofing, are used for a very long time, perhaps 40 years on average. But 40 years ago, the demand was only about half of what it is today. So about 90% of the material used then is reused today. This fact applies in a similar way to aluminium and other metals. Because: metals are not consumed, but used.
Low and medium voltage cables
In individual cases it must be decided whether a larger cable cross-section or a higher cable weight is the lesser evil. An aluminium cable is usually a lot cheaper. However, it remains to be considered that the copper cable also offers the greater safety reserves here for the reasons of ductility and contactability mentioned above and is much easier to lay due to the smaller cross-section, because the stiffness increases with the square of the cross-section, i.e. with the fourth power to the diameter! Also, very small copper cables are already available as stranded conductors, but aluminium cables are only available from 10 mm² nominal cross-section, and the individual wires are still very thick. So-called “fine-stranded” and “finest-stranded” conductors are only available in copper for technical reasons. This pushes the latter difference to the extreme and has already led to nasty and also expensive surprises, which nevertheless appeared on paper as savings, because on this, being patient, the extra effort in laying did not appear, but the lower procurement price of the material did. A compromise recently appeared in the form of a combined Cu-Al cable, which has recently been used as an underground cable in the low-voltage distribution network at the Dietlikon electricity company.) A representative from there presented this product and the concept behind it after he had been invited as a guest to participate in the meetings of Committee 712 “Safety of Information Technology Installations including Equipotential Bonding and Earthing” of the DKE, where German experts are beginning to run short. E-Werk Dietlikon is the first known distribution network operator to consistently convert the distribution network to the 5-conductor network form TN-S – naturally only for repairs, new buildings and extensions. The outer conductors (pole conductors) here have the same cross-section as the neutral conductor, which accommodates a symmetrical design, but aluminium was chosen for the outer conductors and copper for the neutral conductor, which means that the neutral conductor can withstand higher loads and the cable can thus cope with today’s much-discussed load due to harmonics. The protective conductor is designed as a shield made of copper braiding, which ensures much better symmetry and EMC than a conventional fifth conductor.
Here the argument of space requirements loses further weight, but on the other hand it is still a factor. Furthermore, busbars use a large amount of conductor material together with very little insulating material in a very small space. This makes the differences in material prices more obvious. Thirdly, there are also many connection points in a small space, which makes the contacting problems of aluminium more noticeable. In the sum of all these aspects, a stalemate arises again, which makes the question of material selection one of philosophy. However, care must be taken not to confuse price with cost again. When interpreted in terms of price, aluminium tends to come out of the comparison as more advantageous. In addition, copper apparently enjoys the better reputation or the better appearance, because there are also busbars made of aluminium with copper coating – not for better contacting, because the coating is lost anyway during drilling, punching and screwing, but for aesthetic reasons. You can find more on the subject of busbars in the publication „Copper for Busbars“.
Superconductivity is the physical phenomenon that certain substances suddenly lose all ohmic resistance when the temperature falls below a certain “jump temperature” and are thus able to conduct electricity without loss in principle. With the discovery of high-temperature superconductivity in 1987, the transition temperatures jumped overnight from the 4 K range to the 100 K range. This corresponds to 25 times the distance to absolute zero. Roughly speaking, one could say that this simplified the application and use of such conductors by a factor of 25. Liquid nitrogen, which is much cheaper to produce, can now be used as a coolant instead of liquid helium. Nevertheless, 100 K = -173°C – and the effort for cooling is corresponding. However, this effort is particularly worthwhile in applications that make use of another advantage of some superconductors, which allow current densities in the range of 100 times what metal conductor materials allow due to their heating. This makes it possible to generate extremely strong magnetic fields for nuclear research and medical diagnostics, or to build smaller, lighter machines for applications in which volume or weight are extremely important.There was initial talk of a military ship propulsion system and an 8 MW wind turbine. The superconducting short-circuit current limiter is likely to revolutionise grid operation technology in the future. Until now, the demands for a negligibly small grid impedance in normal operation and a sufficiently large one in the event of a short-circuit were incompatible and had to be bridged by compromises, but this balancing act is now possible in principle and individual systems are being tested in practice. Another important parameter of the superconductor, apart from the transition temperature, is the saturation current density, at which the superconductivity collapses just as suddenly as it came. A conventional metal conductor, usually made of copper and designed as a sheath for the superconductor, then takes over the current for the short moment until it is switched off and limits it through its ohmic resistance. Electrical engineering waited a long time for this Egg of Columbus.
In the meantime, the idea of combining this component with a superconducting transformer has emerged. In this way, it could also become useful, which it is not on its own, since a transformer has too little power loss to save more on it than the cooling consumes.
The grid resources listed here, such as extra-high voltage underground cables and large transformers, have efficiencies well above 99%, a so-called marginal power transformer (≈800 MVA), for example, 99.75% at full load and 99.80% at half load. In electricity grids such as those in Germany, Austria and Switzerland, no more than 5% of the total energy is lost from the power plant to the socket – and most of this is lost in the highly branched low-voltage grid. Distribution transformers, for example, have efficiencies of “only” 98.5% at full load and 99.0% at half load. If the copper losses drop to a quarter at half load, the energy expenditure for cooling to the so-called “cryogenic” temperatures of a superconductor always remains at full level. A (relatively large) distribution transformer of e.g. 1 MVA rated power would therefore have to be kept at 100 K with 15 kW, at half load with considerably less than 5 kW, so that any energy saving remains – and even this would only save the copper losses, not the iron losses which are really expensive in the life cycle cost calculation.
Of course, an “efficiency” for underground and submarine cables must always be related to the respective length, since at the same current and voltage a metre of cable always has the same power loss. According to the latest reports, the newest product in this field has a power loss of < 5% – and that with a transmission capacity of 2.6 GW and a length of 1500 km!
There are also reports of “up to 50% energy savings” from the aforementioned wind turbine with superconducting generator. Apart from the fact that “up to” is always a completely useless statement because it only indicates an extreme value and suppresses the opposite extreme value as well as the average value, of course a reduction of losses by 50% is meant, which corresponds to an energy saving in the range of 1% of the generation. Furthermore, it is particularly relevant here, with the rather few annual full-load hours that are typical for wind power, that the copper losses increase as the square of the load, but the cooling power for the superconductor is constantly required in full, even over the standstill in lulls, as their duration cannot be planned. Incidentally, even with conventional copper conductors, almost 90% (loss) energy could be saved if they were cooled down from the usual operating temperature to cryogenic temperatures. Due to the dependence of the ohmic resistance on the temperature, one would practically already have created a “90% superconductor”, but nobody does that either, because it is not worthwhile. Ultimately, superconductivity only works completely with direct current, but only partially with alternating current, since eddy currents always occur somewhere, even outside the conductor. All attempts to save energy by means of superconductors in a direct way by avoiding ohmic losses may be suitable for presentation in the daily press and in politics, but they are stuck at practical hurdles. In fact, superconductors enable applications that are not possible with conductors made of copper or, if need be, silver. In the present case of the wind turbine, for example, the point is that the generator can be made considerably smaller and lighter, thus making it possible to advance into performance classes that are unattainable with conventional machines, as there is simply no appropriate crane to erect such a turbine. This point, however, is unfortunately rather neglected in the corresponding press releases and is not adequately explained at trade conferences either.
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