Electricall con Electrica conductors ductors Stefan Fassbinder Deutsches Kuper-Institut January 2010
Practical application applicationss of electrical conductors conductors
With a conductivity that is about 60% o that o copper, aluminium just ails to make it onto the podium o the best three metallic conductors. Silver takes gold, so to speak, with the silver medal going to copper, and gold coming in third to take bronze. Aluminium ollows a little behind gold to take ourth place, but well ahead o the rest o the feld o metal conductors. Non-metallic conductors (carbon, electrolyte solutions, conducting polymers, superconductors and nanotubes) have their role to play – oten in new applications - and rarely compete with metals.
E l e c t r i c a l c o n d u c t o r s
Practical applications o electrical conductors
Practical applications of electrical conductors 1
Metals: familiar and versatile materials
1.1
Metallic conductors: the choice is limited
With a conductivity that is about 60% o that o copper, aluminium just ails to make it onto the podium o the best three metallic conductors. Silver takes gold, so to speak, with the silver medal going to copper, and gold coming in third to take bronze. Aluminium ollows a little behind gold to take ourth place, but well ahead o the rest o the feld (see Table 1). The high prices o gold and silver makes their use in cables, wires, conductors and electrical machines uneconomical, though they do fnd application as bond wires in integrated circuits where they are used in minute quantities. All other known elements and compounds trail the top our metals in terms o electrical conductivity by some way, with many materials not electrically conducting at all. Alloys, which are mixtures o dierent metals, have much lower electrical conductivity than pure metals. The only two metals thereore oering high electrical conductivity at economically viable prices are aluminium and copper, with the latter setting the benchmark or all other materials. According to documents published by the German Copper Institute (DKI), the conductivity o copper used or the conduction o electricity (Cu-ETP-1, Cu-OF-1 or Cu-OFE) is 58.58 MS/m 1. The IEC standard 6008 was already quoting a value o 58.51 MS/m in 195. This corresponds to 101 % o the value in the International Annealed Copper Standard (IACS), which in 1913 set the standard elec trical conductivity o engineering copper to be 58.00 MS/m, 3 – the benchmark against which other electrically conducting conducting materials must be measured. Ω*m (at 0°C) Silver Copper (99.95%) Gold Aluminium CuCrZr alloy Tungsten Brass (CuZn37) Iron Stainless steel Lead Resistance wire (CuNi44Mn1) Coal, graphite
Ω*m
0.0160 * 10 0.017 * 10-6 0.00 * 10-6 0.083 * 10-6 0.0375 * 10-6 0.0550 * 10-6 0.0645 * 10-6 0.1000 * 10-6 1.0000 * 10-6 0.080 * 10-6 0.4900 * 10-6 40.0000 * 10-6 -6
Sea water Tap water Distilled / demineralized water Ice Garten soil, top soil, peatland soils Porous limestone Wet concrete Dry concrete Sand Gravel, crushed stone Quartzite, weathered limestone Rock
rom to 0,1 1 5 100 000 100000 10000 100000 5 50 30 100 30 100 000 10000 00 500 000 3000 300 1000 1000 10000
Table 1: Resistivity values o selected metallic materials compared to the resistivities o various types o water, soils and rocks, which are oten are treated as ‘conducting’ when discussing earthing systems
Aluminium is a light metal with a density o only about 30% that o copper. Furthermore, the day-to-day trading price o aluminium, which is always quoted per unit weight (strictly, per unit mass), is usually slightly, and sometimes signifcantly,, lower than that o copper. However, signifcantly However, the crucial quantity determining the amount o conducting material required in a particular application is the conductor cross-section. What counts is thereore the volume and not the mass (or weight) o material. Although the better conductivity o copper means that two litres o copper can replace more than three litres o aluminium, copper as conductor material requires twice the mass o aluminium. So why is it then that in Western Europe, or instance, aluminium is hardly ever used in the manuacture o electrical machines? Or why are electrical machines using copper lighter and more compact that aluminium designs (or the same e cie ciency ncy)? )?
Practical applications o electrical conductors
Figure 1: Squirrel-cage rotors cast rom copper were exhibited to the public or the frs t time at the Hanover Trade Fair in 2003
1.
Figure 2: In high-voltag high-voltagee cables the insulating material makes up a greater raction o the total cross-sectional area than the conductor material
Electrical machines
Consider an electric motor in which aluminium rather than copper is used or the motor windings. I this motor is to be technically equivalent to one wound with copper (particularly with respect to eciency), the current densities have to be reduced by 40%, that is the cross-sectional area o the conductor will have to be increased by 64%, thus increasing the size o the laminated core and all other mechanical components. However, the electrical sheet steel used or the laminated core also has its price on the markets, which essentially cancels out the savings made by using aluminium rather than copper in the windings. As a result, work has been underway or a number o years aimed at casting the rotor cage rom copper.4 A number o these new rotors are now commercially available and have already been used in the frst practical applications (Figure 1). The problem o casting the rotor cages rom copper was the much higher melting point o copper (1083 °C) compared to a much more convenient convenient 660 °C or aluminium. This led to a signifcantly higher rate o wear o the casting mould. Fortunately, these problems have now been solved and moulds with economically easible lietimes lietimes are now available.5 Comparison of Dimensions
Cable diametre 13.2 mm
Typical Typi cal fire resistant cable 3 * 1.5mm2 with protective earth Cable comprises sheath and core insulation of organic insulant with glass fibre or mica filler and flame protection foil
Cable diametre 7.2 mm
Mineral-insulated cable 2 * 1.5mm 2 with copper sheath as protective earth Cable comprises copper sheath and mineral insulant, optional outer sheath of LSF plastic
Figure 3: Mineral-insulated cables
1.3
Figure 4: The st strruct ctu ure o ‘freproo’ plastic-c -co oated cable and mineral-insulated cable
Cables and wiring
Space is really a critical criterion when discussing electrical cables and wires. In a low-voltage (LV) plastic-sheathed cable with conductor cross-sections o up to 10 mm2 per conductor (Figure 5) or in high-voltage (HV) cables (Figure ), the lion’s share o the cross-sectional area is occupied by the insulating material. I aluminium rather than copper is used as the conductor material, the additional cross-sectional area required is more or less negligible in comparison.
3
Practical applications o electrical conductors At least that is the situation or conventional plastic-coated cables. Mineral-insulated cables and wires (Figure 3) are not only absolutely freproo 6, they also take up much less space (Figure 4) than conventional plastic-sheathed cables. For a time, these mineral-insulated cables were even equipped with an aluminium sheath, but this never became established and copper sheathing remains the norm.
Figure 5: Even in building installation and service cables, the conductor material still makes a smaller contribution to the total cross-sectional area than the insulating materials
Figure 6: Only in low-voltage high-current cables does the conductor material make up most o the cable’s total crosssectional area
And in most European countries, copper is still used predominantly, i not exclusively, or electrical installation work in buildings. So why is it that most European standards do not permit the use o aluminium conductors with crosssections up to 16 mm2 (or in some cases) up to 10 mm2? There are three main reasons: Although aluminium is quite ductile, it is not as ductile as copper. The ends o sti wires laid in walls e.g. as connections to ush-mounted sockets or wall outlets tend to break ater being repeatedly bent back and orth. This can be problematic i the imminent racture point is located inside the insulating sheath and i the wire continues to be used. In such cases the ault can remain undetected until the wire has to carry a sizeable current (that is one close to its rated maximum current) and although it could be years beore this situation arises, when it does, the conductor material will melt at the racture point and sustained arcing can occur. Aluminium also tends to orm these local constrictions more readily than copper and as it has a lower melting point and a lower coecient o thermal t hermal conductivity than copper, this sort o local melting will occur more readily in wires and cables with aluminium conductors. In the worst case, this can cause the aluminium to catch fre and burn like a use wire.
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When exposed to air, the surace sur ace o aluminium rapidly becomes covered by a hard, durable oxide layer that does not conduct electricity, thus making it harder to ensure electrical contact. contact. The build up o oxide at points where aluminium wires are terminated or connected, can increase the local electrical resistance o the conductor. The increased resistance can cause elevated temperatures with the risk o heat damage to the insulating materials and possibly fre. Copper also undergoes oxidation when exposed to air, but perhaps surprisingly, the oxide layer does not inhibit electrical contact, even though the copper oxides (CuO and CuO) have conductivities some 13 orders o magnitude less than elementary copper and can thereore hardly be described as electrical conductors.
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Aluminium has a propensity to undergo slow material creep. When subjected to high pressures, the material will yield over time. One result o this is that originally tight connections connections may gradually become loose. Connection technology is available that can deal with this problem and it is worth investing the extra cost and eort involved or installations involving involving relatively ew connection points (e.g. HV overhead transmission lines), but not or more complex branched networks such as those ound inside buildings.
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Because o the second o the three problems listed above, above, connections involving the ends o aluminium conductors should always be made as tight screw-astened contacts. Unortunately, the third problem discussed above means that these joints are oten o ten not permanent. Spring contacts contacts can be helpul, but they tend to suer rom the problems associated with the insulating aluminium oxide layers. In both cases, the result is a slow rise in the contact resistance
4
Practical applications o electrical conductors at the connection point and thus to an increased risk o fre. Grandathering regulations continue to protect older aluminium installations in Eastern Germany and in most countries in Eastern Europe, but the only real protection being provided by this sort o regulation is protection rom the threat o improvement! Fortunately, methods are now available or ensuring proper electrical contact between these older ‘protected’ ‘protected’ installations installations and newer electrical systems. These connectors connectors combine spring-loaded contacts with a special contact paste made rom grease and sharp metal particles. When the connection is made the particles penetrate the existing aluminium oxide layer while the grease protects the contact area rom renewed corrosion.7 Copper is also the preerred conductor material in high-voltage cables. Although the use o aluminium would result in only a slight increase in the overall conductor cross-section, the insulating materials and the exterior shielding required or HV cables are expensive and the greater total cross-sectional area o the cable would cancel out the savings made by using the cheaper conductor material – in contrast to the situation with low-voltage power cables (Figure 6). It is also worth remembering that the cable shielding is always made rom copper, because it is the only material suitable or the job. I aluminium is chosen as the t he conductor material, then processing the scrap cable at the end o its (admittedly long) service ser vice lie will involve the additional additional step o separating the two materials. As a material, pure copper has a practically infnite lietime. It can be reprocessed an indefnite number o times without suering any loss o quality. About 45 % o the copper required today is generated rom scrap, and the products or which it is used (cables, transormers, water pipes or roofng) will remain in use or a long time, on average around orty years. However, However, orty years ago, the demand or copper was only about hal o what it is today. It ollows that about 90 % o the copper used at that time is still in use today. This applies equally to aluminium and other metals. Metals are not consumed, they are used.
1.4
Which metal or which job?
Apart rom their electrical conductivity, the other technologically important properties o copper and aluminium dier so signifcantly (density is an obvious example) that their areas o application application are and have always been clearly distinct (Figure 8). And not a lot has changed or is likely to change in that respect. The only really novel development in recent years has been the introduction o cast copper rotor cages (Figure 1). There are really only three, now our, areas in electrical engineering in which aluminium and copper are competing in the same market segments: segments:
New! All telecom wiring
Motors HV and EHV underground cables
Busbars Busba rs Transformers Transform ers
Building wire
Copper
Figure 7: The underground cable in use at Dietlikon power station in Switzerland: a compromise solution that combines the technological properties o copper and the price o aluminium
All high and medium voltage overhead wiring Cast squirrel cage rotors for three-phase induction motors Low and medium voltage underground cables
Aluminium
Figure 8: Practical uses o copper and aluminium in the electrical engineering sector: areas in which both metals can be used are rare
5
Practical applications o electrical conductors Low- and medium-voltage cables: The decision here is which is the lesser o two evils: a greater cable cross-
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section or a higher cable weight? Generally speaking, aluminium cable will be substantially cheaper. However, it is still worth recalling that copper cable is more ductile and less susceptible to electrical contact problems and thus oers a greater margin o saety than a corresponding aluminium cable. Due to its smaller cross-section, the copper cable will also be easier to install as the stiness o the cable depends on the square o the crosssectional area and thus on the ourth power o the diameter! It is also possible to get very small stranded copper cable; stranded aluminium cable is only available at nominal cross-sectional areas o at least 10 mm2 and the individual strands are still very thick compared compared to those in the equivalently sized copper cable. For technical reasons, so-called ‘fnely stranded’ and ‘extra extra fnely stranded’ conductors are only available in copper copper..8 As a result, the fnest aluminium conductors available are signifcantly stier than the fnest copper conductors con ductors and this dierence has on occasion led to some rather costly surprises. On paper, the aluminium conductor may well be cheaper to buy, but that ails to take into account the extra cost and eort involved in installing the less pliable aluminium cables.9 Recently, a combination Cu-Al cable has appeared as a compromise solution and is being used at the Dietlikon power utility in Switzerland as an underground cable in low-voltage distribution networks (Figure 7). A representative rom the Swiss Dietlikon plant gave a presentation on the product and the underlying concept ater being invited to attend meetings o DKE Committee Committee 71 ‘Saety o Inormation Technology Installations including Equipotential Bonding and Earthing’ (DKE: German Commission or Electrical, Electronic and Inormation Technologies). The Dietlikon electricity utility is the frst known distribution network operator that is systematically converting its distribution network to a fve-wire TN-S system – work that it o course only carries out during repairs, network expansions and new installations. In this new cable, the phase conductors have the same cross-section as the neutral conductor, which helps to achieve a symmetrical cable structure. The phase conductors are made o aluminium, while the same-diameter same -diameter neutral conductor is o copper, enabling it to carry a greater current and thus making the cable better suited to coping with the harmonic pollution problems problems that are so commonly discussed today. The protective earth conductor is confgured in this case as a surrounding copper-wire shield, which oers ar higher symmetry and EMC than a conventional conventional fth conductor. Transformers: The problem o winding space is not as acute in transormers as it is in electric motors, which
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is why the use o aluminium can at least be taken into consideration. In act the main leakage channel, i.e. the gap between the HV and LV windings, must have a certain size or the ollowing three reasons: insulation, limiting the short-circuit current, and cooling.10 However However,, a transormer with aluminium windings will be larger i power losses and all other important operational data, such as the short-circuit voltage, are to be kept at the same level as an equivalent transormer with copper windings (ater all, this is what we mean when we say two transormers are equivalent). However, the total weight o the marginally larger transormer with aluminium windings will be slightly lower. Dierences in manuacturing manuacturing costs pretty much cancel each other out and in the opinion o a number o well-respected manuacturing companies, the choice o conductor material is primarily a question o company philosophy. Busbars: In this application, spatial requirements weigh even less heavily in the decision-making process, but still
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remain a actor. Secondly, busbar applications are characterized by a large amount o conducting material and a small quantity o insulating material in a small space. This highlights the dierences in material prices. Thirdly, the large number o electrical connections within this small volume mean that the connectivity problems associated associated with aluminium are more pronounced in such applications. When all these aspects are taken into consideration, we are let with a stalemate and the question o which material to select becomes almost philosophical. However, However, it is important to ensure that prices and costs are not being conused. I price is taken as the main criterion or selection, aluminium generally tends to be preerred. But i all the costs (including operational costs) are taken into account, it usually turns out that aluminium can learn a thing or two rom copper.11 Copper it seems also has the better appearance, because some o the aluminium busbars available are copper-coated – not to improve electrical contact (because drilling, punching and screwing will anyway damage the copper coat), but simply or aesthetic reasons (Figure 9, Figure 10).
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Practical applications o electrical conductors
Figur Fi guree 9: 9: There There are cop copper per bus busbar bars, s, alum alumin inium ium bus busbar barss … •
Figur Fi guree 10: 10: …and …and copp copper er busb busbars ars mad madee o o alumi aluminiu nium m
One new area o application is copper rotor cages (Figure 1): In this application, the crucial actor actor is the greater electrical conductivity per unit volume o copper. This actor alone made it worthwhile tackling all the technical problems associated with the development o these devices. For more inormation, the reader is reerred to descriptions available elsewhere.4, 5
Aluminium’s undisputed domain is that o overhead high-voltage cables,1 where space require requirements ments are o no signifcance but where weight plays a critical role. The lower strength o aluminium aluminium means that the conductor cables need to be reinorced with a steel core but this does not change the act that the cables can be produced at low cost and that the two t wo materials can be readily separated rom one another magnetically when scrapped.
2
Non-metallic conductors – a real alternative?
The so-called semiconductors like germanium and silicon, which in the periodic table o the elements are located between the metals and the nonmetals and which are at the heart o the electronic elec tronic systems that we know today, are a subject in their own right and would go well beyond the scope o the present article. ar ticle. But semiconductors aside, we can ask whether metals are the only other materials capable o carrying an electric current. Or are there other substances that could be useully deployed as conductors?
.1
A material or special purposes: Carbon
We are all amiliar with the graphite electrodes in electric arc urnaces, and graphite electrodes were also used in discharge lamps. In act, the very frst incandescent lamps were produced not with tungsten flaments but with flaments made o carbon. Carbon brushes are still used today to establish electrical contact with the commutator segments in DC machines. They are called brushes because their predecessors were in act made rom braided copper and looked like tiny brushes. But graphite has better lubricating properties than copper. In applications in which the signifcantly lower electrical conductivity o carbon is insucient, sintered graphite-copper composites composites are available (Figure 11). As sintered materials are not alloys they do not suer rom the reduction re duction in conductivity that typically accompanies the alloying process.
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Important in electrochemistry: liquid conductors
Electrolyte solutions are typically made up o ionic salts dissolved in water in which the charge-carrying ions are ree to move. The dissociation o ionic salts in water to yield conducting uids underlies such important processes as electrolysis or the generation o electric power in a battery, and it gives soil its electrical conductivity, albeit one that is very low and strongly weather dependent. In order to show compliance with some (oten seemingly arbitrary) soil resistance resistance limit value, those in the know will carry out the requisite earth resistance measurements measurements ater a heavy downpour o rain. It is worth emphasizing that the resistivity values shown on the let in Table 1 all have the actor 10-6 attached. The resistivity values o metals and those o what we commonly reer to as earth thereore dier by between 6 and 1 orders o magnitude! magnitude!
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Practical applications o electrical conductors .3
Electrically conducting polymers: the material or a new generation o cables?
Plastic materials that are themselves able to conduct electricity (i.e. organic polymers that are ‘intrinsically conducting’) conducting’) are rare. Most electrically conducting polymers (so-called ‘conductive ‘conductive polymers’) are plastics that have been induced to carry electrical elect rical current by adding fllers such as stainless steel akes, steel fbres, silver-coated glass beads, graphite or carbon black. The raction o these additives is usually limited to a ew percent by volume so as to be able to continue to exploit the properties o the organic polymer itsel. As a result, the electrical con conduc ductivities tivities o these materials are at least our orders o magnitude lower than in metal conductors, in some cases the conductivity is reduced by as much as 14 orders o magnitude. These relative relatively ly low conductivities are adequate or even desirable as these materials are predominantly used to discharge or prevent static charging and to shield high-requency electrical or electromagnetic electromagnetic felds and waves. When it comes to potential applications or conductive polymers, power and data transmission cables are hardly top o the list. Although one speaker at a discussion discussion meeting13 on the subject did present a demonstration model in which a torch bulb (estimated current: 50 mA) was connected to a battery via an electrically conducting polymer rod (estimated cross-section: 10 mm2). The current density in the conductor was, however, still about three orders o magnitude lower than would be ound in copper or aluminium. I these sorts o materials are going to replace metals in certain applications, they are likely to be used in the orm o very thin oils or extremely thin layers laid down by vapour deposition that are designed to provide shielding rom electrical felds. In act, such applications are already well-established. I the plastic casing o a device that has to be protected rom emitting radiation or protected against the eects o incoming radiation is already (slightly) conducting, then this type o antistatic coating can be dispensed with. It is o course perectly justifed to ask why one would want to replace a metal casing with a plastic casing, when the metal was anyway better at providing the required screening properties and when the plastic has frst to be made conducting conducting by incorporating metallic additives. The answer lies in the extrudability o the plastic polymers and the greater scope they oer in terms o coloration and design. But who knows? Perhaps metals will fnd a way to close the gap. Intrinsically conducting polymers were discovered about 5 years ago, with one such polymer having an electrical conductivity similar to that o a metallic conductor. The problem with these materials, however, is that they are all inusible, non-ormable and insoluble – making them practically impossible to process. They are also susceptible to attack by oxygen and when exposed exposed to air they airly rapidly lose their conductivity, which it turns out is not only directionally dependent but also varies strongly depending on the manuacturing process used. It is obvious that materials with these properties will never be selected in avour o metal conductors, which exhibit ar superior processability and stability. In a number o instances it has proved possible to improve the properties o intrinsically conductive polymers, but this has always resulted in a reduction in the material’s electrical conductivity by several orders o magnitude. These materials are used in the same limited areas as the conductive flled polymers, namely in the prevention o static charge build-up. One such material is polyethylene dioxythiophene (PEDT), which is used to provide antistatic coatings or photographic flm. Without the PEDT coating, the flm would accumulate static charge during the photographic development process. I allowed to build up, the charge can discharge as a ash o light that would re-expose the flm and ruin the original image. The fnal image would then look like it had been taken in a thunderstorm. It should be mentioned that electrically conducting polymers have been used or some time in power transmission systems, specifcally in HV cables, where so-called so- called ‘semiconducting ‘semiconducting layers’ are are introduced once around the conductor and once between the inner insulation and the outer cable coat, the latter serving to provide ‘feld-strength ‘feld-strength control’. This enables the electric feld to be kept as homogeneous as possible and prevents local spikes in the electric feld that would cause partial discharging and the gradual destruction o the cable’s cable’s insulation. Despite the currently rather limited applications o conductive polymers, one visionary at Kabelwerk Kabelwerk Brugg was not deterred rom publishing an article in an IEC inormation leaet in which he describes a scenario or ‘Power Networks 050’ where each network is composed exclusively o cables that are made rom conductive polymers and that can thereore be manuactured in a single extrusion process. The exceptionally high insulating capacity o the insulating material (around 100 kV/mm) would apparently also enable high-voltages to be used in living areas. Working electricians will no doubt shudder at the thought. The high capacitance o such cables certainly helps to reduce EMC problems, but the calculations on which this vision o the uture is based completely ignore the tripping conditions in these cables and ail to take a number o other important actors into account. The company’s website14 makes no mention o such ights o ancy and an enquiry as to what had become o the idea yielded the inormation that no one knows anything about it and the author o the original article let the company some time ago.
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Practical applications o electrical conductors
Figure 11: Brushes made rom a sintered graphite-copper composite are an alternative to pure carbon brushes
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Figure 12: The structure o a superconductor: Copper is an essential component o superconducting cables
Superconductors
Superconductivity is a physical phenomenon exhibited by certain materials in which at temperatures temperatures below a materialspecifc critical temperature the materials lose their ohmic resistance resistance making them in principle able to conduct electric current without loss. The discovery o high-temperature superconductors a little more than 0 years ago resulted in an astonishing increase in the critical temperature rom around 4 K beore the discovery to around 100 K aterwards. In other words, the distance rom the critical temperature to absolute zero increased by a actor o 5. Roughly put, one could say that the use o superconductors in applications suddenly became became about 5 times easier. For instance, or so-called high-temperature superconductors, the rerigerant medium is liquid nitrogen, which is ar cheaper to produce than the liquid helium previously previously required. But 100 K is still -173 °C and the eort required to maintain this temperature is large. But this eort may well be worthwhile, particularly in applications that exploit another benefcial property o superconductors – their ability to carry carr y current densities approximately one hundred times greater than those in metals, where current densities are limited by thermal eects. Semiconductors are used to generate extremely powerul magnetic felds or research in nuclear physics and or medical diagnostics. They are also used in the construction o lighter machines or applications in which volume or weight are o crucial importance. For a long time many o these highly specialized applications delivered behind-the-scenes benefts that remained remained generally 15 unknown to the wider public. An industry association has now been established in Germany that is working to promote superconducting applications and improve public recognition o these technical developments. Applications include a drive system or a naval vessel and an 8 MW wind turbine. Superconducting short-circuit current limiters also look set to revolutionize power network engineering. Until recently the demands or a vanishingly small network impedance during normal operations and or a suciently large impedance in the event o a short-circuit appeared incompatible and a compromise solution was needed. It now seems that it is possible in principle to meet both demands and a number o systems are currently undergoing practical testing. In addition to the critical temperature another important parameter o any superconductor is its saturation current density, called quench, that is the current density at which superconductivity suddenly collapses just as suddenly in act as it appears. The remarkably remarkably simple solution to this problem involves involves a conventional metallic conductor conductor (usually made o copper) that surrounds the superconductor and that carries the current or the very short period until the short-circuit has ceased with the current limited by the ohmic resistance o the metallic conductor.
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Practical applications o electrical conductors The numerous reports in recent years o the t he potential o superconductors to save energy should, however, however, be viewed with a healthy degree o scepticism. The power network components components that we have been discussing such as extrahigh-voltage underground cables and large transormers already have e ciencies signifcantly above 99 %, in act act a high-power transormer tr ansormer (≈800 (≈8 00 MVA) exhibits an e cienc ciencyy o 99.75 % at ull load loa d and 99.8 % at hal load. In I n grids such as those in Germany, Austria and Switzerland no more than 5 % o the electrical energy is lost along the path between the power generating station and the domestic outlet socket – and most o that 5 % is lost in the heavily branched low-voltage distribution network. Distribution Distribution transormers transormers have h ave eciencie cienciess o ‘only ‘only’’ 98.5 % at ull load 16 and 99.0 % when operating at hal load. Even i copper losses at hal load are a quarter o their value under ull load conditions, the energy needed to cool the transormer down to the cryogenic temperatures o a superconductor remains unchanged. A (relatively large) distribution transormer with a rated output o, say, 1 MVA and losses o 15 kW (or signifcantly less than 5 kW when operating at hal-load) would have to be maintained at a temperature o 100 K in order or any sort o energy savings to be made. And even then, only the copper losses would be eliminated, not the iron losses that actually contribute substantially to the transormer’ transormer’ss lie-cycle lie-c ycle costs. Calculations have shown that or an extra-high-voltage underground cable a positive energy balance would be achieved at transmission powers o 5 GW and above. That corresponds to the total power output rom our nuclear power plant blocks. But a cable o this type does not exist as there is simply no demand or it at present and there is unlikely to be any demand in the uture. The model calculation is thus purely academic and o no real practical utility. There have also been reports o energy savings o ‘up to 50 %’ i the wind turbine mentioned above is ftted with a superconducting generator. First o all, the expression ‘up to’ is usually o no practical worth as it only ever specifes one extremum, while the other extremum in the opposite opposite direction and the average value are never mentioned. Secondly, what is meant here is, o course, a reduction in the losses, which translates to an energy saving o about 1 % o the energy generated. Wind turbines typically operate at ull load or only a relatively ew number o hours per year. It is all the more important then to recall that the copper losses increase with the square o the load, but that the cooling or the superconducting material is a permanent require requirement ment and has to be maintained even during windless periods as the duration o such periods is unpredictable. It is also worth noting that one could also save about 90 % o the power losses using conventional copper conductors were these conductors cooled rom the usual operating temperature to cryogenic temperatures. The temperature dependence o the ohmic resistance o copper would eectively allow us to create a ‘90 % superconductor’ – but nobody would ever do this, because it is simply not worth it. Finally, we note that superconductivity unctions only ully with direct electric current, and is only partially present with alternating currents. Attempts to use superconductors directly to avoid ohmic losses and thus save energy are well suited to newspaper reports or political sound bites, but they tend to be compromised by practical realities. realities. Superconductors do though oer extremely interesting applications in areas where copper and silver conductors cannot be used. Returning to the wind turbine discussed above, the generator can be made smaller and lighter by using superconducting materials and this opens up new perormance categories that would unattainable with a conventional electric generator, as a conventional generator would be so heavy that no crane is currently available that could lit it into place. A act that is generally not mentioned too prominently in the relevant press releases.
Carbon again: Nanotubes
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Some years ago the national papers started to report on something called ‘nanotubes’. As the name suggests, nanotubes are tiny tubes o rolled-up graphite with diameters o around 1 nm. According to these reports, these novel tubules have all sorts sor ts o benefcial properties among them ‘high electrical conductivity ’. But what’s ‘high’? The lowest resistivity value measured so ar is 0.34 Ω·mm2/m – exactly 0 times higher than that or copper. Physicists have also apparently measured extremely high current carrying capacities or these nanotubes, with some measurements claiming ampacities o 1011 A/mm2. How is that possible? The answer lies in the minute size o these tubules, whose diameters are six orders o magnitudes magnitudes smaller than the wires in a typical electrical installation cable, meaning that their cross-sectional areas are twelve orders o magnitude smaller. Relative to the cross-sectional area, a nanotube thereore has 106 times more surace area available than a conventional copper wire over which it can dissipate heat – a similar ratio to that ound between small and large transormers. transormers.17 However, i the nanotubes are bundled together to produce a conductor with a cross-section o 1 mm2, the bundle will not have much more surace area available than a conventional conventional wire, as the ollowing calculation shows: A cube o ‘nanotube material’ with an edge length o 1 m has a resistance o 0.34·10-6 Ω. I it could be made, a ‘nanowire’ ‘nanowire’ 1 m long and with a cross-sectional area o 1 mm2 would have a resistance o 0.34 Ω. At the current density o 1011 A/mm2 mentioned above, the ‘nanowire’ ‘nanowire’ 11 would have to carry a current o 10 A. The power loss in this one-metre-long ‘nanowire’ ‘nanowire’ would thereore be: P
=
10
I R
=
10 A 0.34 ⋅
V A
=
3.4 101W . ⋅
Practical applications o electrical conductors It goes without saying that the nanotubes would be destroyed within a nanosecond. But so ar this question has no real practical relevance because frstly, frstly, no one is seriously thinking about using these materials as electrical conductors (inormation on possible uture uses o nanotubes can be ound on a dedicated website18) and secondly, the longest nanotube created thus ar is only 1 mm. While that may sound pretty modest on its own, relatively speaking it corresponds to a length o 1 km in a conventional wire with a diameter o 1 mm. And we all know how important it is or physicists to see things relatively.
(Endnotes) 1 Inormation leafet i4 ‘Kuper ‘Kuper / Vorkommen, Vorkommen, Gewinnung, Eigenschaten, Eigenschaten, Verarbeitung Verarbeitung,, Verwendung’ Verwendung’ [‘Coppe [‘Copper:r: Deposits, extraction, properties, properties, processing, use’, available rom the German Copper Institute (DKI), Düsseldor, Germany or at: www.kuperinstitut.de 2 www.burde-metall.at/iacs.htm 3 www.copper.org/applications/electrical/building/wire_systems.html 4 www.copper.motor.rotor.org 5 Stean Fassbinder: ‘Eine runde Sache: Kuperrotoren’ [‘Turning to eciency ciency:: Copper rotors’] in de, 20/200 20/2004, 4, p.p. 68 6 Stean Fassbinder: Fassbinder: ‘Brandsichere Kabel und Leitungen’[‘Fireproo cables’] in etz, 1-2/1997, p.p. 48 7 Fritz Hengelhaupt:‘Kontaktv ‘Kontaktverbessernde erbessernde Wirkung von Kontaktpasten Kontaktpasten ür die Elektro-Installation Elektro-Installation’’ [‘The [‘The use o contact pastes in electrical installation installation work’] in de, vol. 15-16/2001, p. 38 8 EN 60228 (VDE 0295):2005-09 9 Stean Fassbinder: Fassbinder: ‘Rationalisierungsmaßnahmen in kommunalen Stromnetzen’[‘Rationalization strategies in local electric power networks’] in de, 5/2001, p. 40 10 Stean Fassbinder: Fassbinder: ‘Verteiltransormat ‘Verteiltransormatoren oren – Teil 3: Betriebsverhalten’ Betriebsverhalten’[‘Distribution transormers – Part 3: Operational behaviour’] in Schweizer Zeitschrit ür angewandte Elektrotechnik, 4/2005 11 alumno: Spanish Spanish or student student or pupil 12 Stean Fassbinder: Fassbinder: ‘Erdkabel ‘Erdkabel kontra Freileitung?’ [‘Undergr [‘Underground ound cable vs. overhead cable?’], in de, 9/2001, appears in DKI reprint ‘Drehstrom, ‘Drehstrom, Gleichstrom, Supraleitung – Energie-Übertragung heute und morgen’ [‘Three-phase AC, DC and superconducting systems – Power transmission now and in the uture’] rom the German Copper Institute (DKI), Düsseldor 13 www.otti.de 14 www.brugg.ch 15 www.ivsupra.de 16 Stean Fassbinder: ‘Verteiltran ‘Verteiltransormatoren sormatoren – Teil Teil 5: Wirkungsgrad von Verteiltra Verteiltransormato nsormatoren’ ren’[‘Distr [‘Distribution ibution transormers transo rmers – Part Part 5: Eciencie cienciess o distributi distribution on transormers’] in Schweizer Zeitschrit ür angewandte Elektrotechnik, Elektrotechnik, 6/2005 17 Stean Fassbinder: ‘Verteiltra ‘Verteiltransormato nsormatoren ren – Teil 1: Warum Warum überhaupt Transormatoren in Versorgungsnetze Versorgungsnetzen?’ n?’ [‘Distri [‘Distribution bution transormers transo rmers – Part Part 1: Why have transormers in distribution networks?’] in Schweizer Zeitschrit ür angewandte Elektrotechnik, 1/2005, p. 79 18 http://www.pa.msu.edu/cmp/csc/nanotube.html
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