Copper displays low levels of creep. Under the extreme loading and temperature conditions of distribution transformer windings, creep rates of aluminium can be up to 25 times higher than for copper. This results in aluminium wound distribution transformers having a higher propensity to failure than copper wound ones.
Copper wire terminations are less prone to failure than aluminium wire terminations. A key reason for this is the different behavior of their oxides. Copper oxide is soft, electrically conductive, and breaks down easily. Aluminium oxide is strongly attached, hard to dislodge and electrically insulating. It also prevents non-mechanical connections such as soldering, which is only possible after applying a layer of tin, copper, or nickel.
Copper wires have no galvanic action, as they are the same element as the connectors, which are usually made of copper or brass (a copper alloy). Aluminium loses material through galvanic action, leading to a loss of contact.
Copper is harder, stronger and more ductile than aluminium, expands less and does not flow at terminations. Consequently it does not require periodic inspection and tightening of screws. Aluminium flows away from the termination under pressure.
The use of the right grade of copper is considered the best way to ensure high short-circuit withstand capability in power transformers, due to copper’s outstanding mechanical properties, such as yield strength and modulus of elasticity. Copper is available with a yield strength as high as 280 N/mm2 for heavy-duty transformers with frequent short-circuits such as those used for arc furnaces. External short-circuits can cause significant weakening of a transformer’s active parts, thus reducing its reliability.
Copper wound distribution transformers 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 larger than that of the copper conductor for the same resistance. This results in a larger transformer core and volume, which also leads to a larger transformer tank than for the copper design. While aluminium 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.
Distribution transformers with HV windings made of copper conductors are less susceptible to metal fatigue than aluminium ones. The fatigue life of aluminium HV winding conductors has been found to be much less than those made of copper under similar operating stress conditions. This suggests that after loosening the HV winding conductor, aluminium wound distribution transformers would fail earlier than copper wound ones.
Higher copper content in transformers improves energy performance and consequently lowers lifecycle costs in most cases. A study commissioned by the European Commission showed that the transformer design option that gives the least lifecycle cost has lower energy losses and uses substantially more copper than the respective base case.
Non-linear loads cause additional load losses in power transformers, which are influenced greatly by the transformer geometry, winding configurations, and insulation and conductor materials. In particular, the current distribution is more uniform with copper conductors due to the higher conductivity.
Finally, transformers with copper windings are often less expensive to manufacture than those with aluminium windings. This is because 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.