Aluminium wound distribution transformers have a higher propensity to failure than copper wound ones due to the aluminium conductor’s inferior creep behavior under field conditions.
Creep is the plastic deformation (strain) of metal conductors that occurs when these are subjected to external pulling forces (stresses). Creep is irreversible, unlike elastic elongation which reverses as soon as the external force is removed. Creep depends on the stress level, its duration and the temperature, and is different for each metal.
Effect of creep on distribution transformers
Frequent energization of distribution transformers is unavoidable in weak power networks and power deficient areas, due to various reasons such as extension and maintenance works, scheduled and unscheduled load shedding, faults, and the power demand exceeding generation. Distribution transformers experience large magnetic inrush currents when switched on, which can be several times the rated operating current. These currents produce excessive mechanical stresses and intense localized heating (hot spots). Each time this occurs, the winding conductors creep by a small amount. Over a period of years, this repeated creeping causes the winding conductors to bunch closer to each other than the allowable minimum safety clearances specified by standards. If the elongation in the wire is more than the allowable gap between two layers of the winding then an insulation rupture occurs leading to an inter-turn short-circuit and thus to a failure of the transformer.
Why is this a problem for aluminium wound distribution transformers?
Creep rates can be up to 25 times higher for aluminium than for copper under the extreme loading and temperature conditions of distribution transformer windings in the field. In other words, a copper wound distribution transformer has up to 25 times the life of an aluminium wound one in terms of its ability to withstand creep stresses. The propensity of aluminium wound distribution transformers to fail through creep stresses is thus much higher under identical operating conditions.
Creep Life Assessment of Distribution Transformers, N.S. Beniwal, D.K. Dwivedi, H.O. Gupta, Engineering Failure Analysis, volume 17, Issue 5, July 2010, pages 1077-1085.