Contrary to popular belief, transformers with copper windings are often less expensive to manufacture than those with aluminium windings
Since aluminium as a metal is cheaper than copper in USD/kg terms, does it necessarily follow that distribution transformers with aluminium windings are cheaper to manufacture? This proposition is too simplistic and ignores the higher costs of the magnetic core, tank and oil accompanying aluminium windings. The reality is that very often, high efficiency transformers with copper windings are actually less expensive to manufacture than those with aluminium windings.
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 tanks as well as the oil used for the aluminium design are at least 66% larger than for the copper design.
In practice, during design 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. Copper is very often the winner as the conductor material.
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