Although copper is more expensive than aluminium per unit of weight, the cost difference is compensated by savings in equipment manufacturing and installation
Since copper is more expensive than aluminium per unit of weight it is easy to conclude that the same cost difference is carried over to the equipment level as well. In reality, the use of copper conductors enables cost savings in manufacturing and installation as the following examples illustrate:
Distribution Transformers: Since the resistivity of copper is 0.6 times that of aluminium, the cross-section of the aluminium conductor needs to be 1.66 times the section of the copper conductor for the same resistance. For oil filled naturally cooled 3-phase core type transformers without tappings up to 100 kVA typically used in distribution systems, this results in the window area of the transformer core also becoming 1.66 times larger. The marginal effects of the insulation and gaps provided for cooling do not alter the conclusion significantly. For a square-shaped core window, this translates into an increase of average length of the core by the square root of the increased area, i.e. 1.29 times. This means an increase of 29% of the core volume, mass and costs. It also means increased no-load losses.
The increased aluminium conductor cross-section also means a 29% larger coil outer diameter, which increases the length of conductor and therefore the load losses.
To maintain the energy performance and counter the effects of increased no-load losses and load losses, either the flux density needs to be lowered by a further increase in the core section, and/or more expensive lower loss grades of magnetic material need to be used.
The 66% larger volume of the active part means that the transformer tank as well as the oil used for the aluminium design are at least 66% larger than for the copper design.
In practice, the section of the aluminium conductor is required to be even larger than 1.66 times the copper conductor in order to have an equivalent short-circuit performance, and the effects described above are more pronounced. For larger distribution transformers, the principle is similar even if the dimensional differences are less stark.
Thus it is not just the cost of conductor, but also the cost of magnetic steel, tank and oil needed to achieve the specified energy performance level that determines the total transformer manufacturing cost. The cost difference at the conductor level is considerably narrowed and copper is often the cheaper alternative for high efficiency distribution transformers.
Power cables: For the same reasons as in the case of transformers, aluminium conductors need a cross-section that is 66% larger and a diameter that is 29 % larger than that of the equivalent copper conductor for an identical conductance, voltage drop and energy loss. The larger diameter aluminum conductor requires up to 26% more insulation material (which is increasingly significant with voltage grade). Similar considerations apply for sheaths, armors, shields, jackets.
There is a positive impact of copper on installation costs as well. The smaller diameter copper cables require smaller containment systems, in turn requiring lesser space and civil works. The smaller diameter copper cables also require lesser space for termination on switchgear and thus enable the construction of smaller switchgear equipment cubicles and require less space for their foundations.
State of the Art on the Use of Copper and Aluminium Conductors in Distribution Transformers Manufacturing: R. Salustiano & M. L. B. Martínez Federal University of Itajubá– Lat-Efei
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