3 ELECTRICAL CONDUCTORS
3.1 ELECTRICAL CONDUCTORS METALS USED The metals that are most commonly used as conductors in electric cables are COPPER and ALUMINIUM.
3.1.1 COPPER COPPER is obtained impurely by scorching sulphurous minerals that contain it and at high levels of purity by electrolytic procedures – REQUIRES RE-WORDING. Making use of its high flexibility, it can be manufactured into very fine wires. This process hardens the copper and limits its flexibility, which is then recovered by subjecting the copper conductor to an annealing process. The conductor stretching operation is called wire drawing, and a number of wires or bunches of wires are wound around a central one to form a conductor referred to as a cord. The flexibility of the cord will depend on the fineness of the wires. The total cross-section is the sum of the areas of each one of the individual wires. In spite of its numerous qualities, copper does have a number of drawbacks, such as its high density and cost, which from time to time requires its substitution by another metal of lower density and cost in order to reduce the cable material and ultimately, the manufacturing costs.
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3.1.2 ALUMINIUM When coming into contact with air, ALUMINIUM is covered with a layer of oxide called alumina, which protects it from subsequent oxidation. Nevertheless, this has the drawback of being an insulator, and hinders their connection with terminations. When in contact with harder metals, such as iron, copper or alloys, and when in the presence of moisture, aluminium produces a galvanic reaction which over a period of time, slowly corrodes resulting in total degeneration. In spite of these drawbacks, which can also be avoided, the fact is that aluminium has qualities that often make it irreplaceable for overhead power line cabling, large sectioned cable for high or low voltage circuits, etc. Given its low density (a third that of copper) and by establishing a relation between their corresponding resistivities, one half of the weight in copper can be replaced by aluminium without adversely altering the electrical resistance. The mechanical properties of aluminium are substantially inferior to those of copper, and as its resistance to traction is low, hence it is used in alloy or with steel wires with the aim of endowing it with high tensile strength for overhead lines. The standard resistivity of aluminium is: 1 35,38
= 0,028264 ohm • mm2/m a 20ºC
3.2 CONDUCTOR CHARACTERISTICS Internationally, the manufacturing of conductors for insulated electric cables is in keeping with the requirements of the Standards IEC 60228 and EN 60228, which are in compliance with UNE-EN 60228. The configuration of a conductor is defined by the number of wires that make it up, and their nominal diameter. The degree of rigidity or flexibility is defined by the different classes that are indicated in the aforementioned Standards, with Class 1 “Solid” corresponding to those conductors comprising only • 21 •
one wire. Class 2 “Stranded” corresponds to conductors comprising several wires laid up together, where the minimum number that each individual cross sectional area should have is specified. Classes 5 a n d 6 “ F l ex i b l e a n d E x t ra Flexible”correspond to flexible conductors, where the Standard makes special mention of the maximum diameter of wires, but not to the number of them, which is left to the judgment of each manufacturer. The difference between Class 5 and Class 6 lies in the fact that in the latter the wires are smaller for greater flexibility. Regardless of the other requirements expressed in the Standard, “the determining factor that must be fulfilled is the maximum electrical resistance of each conductor", as is indicated in the Standard UNE–EN 60228 (coinciding with IEC 60228).
WHAT IS THE GEOMETRIC SECTION? The geometric section of a conductor is the cross section of a wire or the sum of the cross sections of each of the wires, in the case of cords, expressed in mm2. WHAT IS THE NOMINAL SECTION? The rounded-off value that is close to the geometric section, and which is used for the cable name, expressed in mm2. WHAT IS THE ELECTRIC SECTION? This is the maximum resistance value set by the Standard, in Ohm/km at 20ºC, and is thus the only one that guarantees the appropriate behaviour of the conductor with regard to energy transmission. WHAT IS ELECTRICAL RESISTANCE? Electrical resistance is the greater or lesser difficulty with which an electric current runs through a conductor. In accordance with the theory of electrons, an electrical current is no more than the movement of electrons from one body to another. In their movement, electrons have to overcome the nuclei of the atoms making up the conductive material, colliding with them, which results in a certain degree of difficulty. This reasoning explains why bodies have different electrical resistances; this is logical, considering that their atomic makeup is different. • 22 •
3.2.1 STANDARD FOR MEASURING RESISTIVITY Only electrolytically refined copper is used for electrical uses, as the slightest impurity is sufficient to increase resistivity considerably. The resistivity of 100% pure copper is: 1/58 = 0, 017241 Ohm. mm2 / m a 20º C This value is taken as the international standard for measuring resistivity. For electric cables the minimum value of resistivity is 98%.
3.2.2 TYPES OF COPPER FOR ELECTRIC CONDUCTORS Hard copper: used for over-head lines, or in those cases where greater mechanical resistance is required. It has a breaking load of between 35 and 50 kg/ mm2 and a stretch at break of between 0.5 and 3%. A minimum electric conductivity of 97% with regard to the international standard is required. Annealed copper: always used in insulated conductors. It has a breaking load of between 20 and 30 kg/ mm2 and a stretch at break of between 25 and 30%. A minimum electric conductivity of 98% with regard to the international standard is required.
ELECTRICAL EQUIVALENCE BETWEEN CU AND AL Resistivity of Cu 0,017241 Ohm•mm2/m a 20ºC = 0,61 Resistivity of Al 0,028264 Ohm•mm2/m a 20ºC E.g. Section of Al
95mm2 x 0,61 = 57,95mm2 => cable 70mm2 Cu
E.g. Section of Cu
95mm2 : 0,61 = 155,74mm2 => cable 185mm2 Al
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3.2.4 HOW IS THE ELECTRICAL RESISTANCE VALUE OF A CONDUCTOR DETERMINED? In order to determine the electrical resistance value for a conductor, the cable has to be kept at an ambient temperature of between 10 and 30º'baC for a minimum of 12 hours. A Wheatstone bridge or a Thomson bridge is used, depending of the length and section of the conductor. The value, in Ohms, of the reading from the bridge divided by the length of the cable in km will give the resistance of the conductor in Ohms/km. If the measurement has been carried out at a temperature T other than 20º'baC, it will have been corrected by means of the following formula: (*) RT = R20 [ 1 + α20 (T−20)] EXAMPLE: Cable ENERGY–FOC, RV-K 0.6 /1 kV 2 X 1.5 mm2, length: 2,350 m Black conductor reading (Wheatstone bridge): 32.17 Ohm Temperature of the cable: 30º C
32,17 = -------------2,35
= 13,69 Ω / Km a 30º C
RT 13,69 =------------------------------------ = -------------= 13,17Ω / Km a 20º C [ 1 + α20 (T-20)] 1,0393
Maximum value of UNE-EN 60228 = 13,3 Ω / Km a 20º C
(*) RT = Resistance at temperature R20 = Resistance at 20º C α20 = Coefficient of resistivity variation at 20º C
Cu = 0,00393 Al = 0,00403
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