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Copper wound transformers are a boon for space constrained urban power distribution systems
Power distribution utilities have to manage within the limited space available for sub-stations in densely populated urban areas, thus placing a premium on smaller sub-station equipment including distribution transformers. Outside of the sub-stations, distribution transformers are usually hoisted on poles where lower weight is important.
Both these requirements of lower volume and lower weight are fulfilled by copper wound distribution transformers which are invariably smaller and lighter than aluminium wound ones of an equivalent capacity and energy performance.
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 and mass. 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.
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, 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, thus leading to a larger volume for the aluminium wound distribution transformer. For larger distribution transformers, the principle is similar even if the dimensional differences are less stark.
While aluminium as a metal is lighter than copper of an equal volume, in the case of distribution transformers, this advantage is nullified by the increased volume (and thus weight) of the conductor, steel core, tank and oil. This often results in the equivalent copper wound transformer actually being lighter.
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