Transformer Reliability: Failures of Aluminium High Voltage (HV) Windings due to Fatigue

Distribution transformers with High Voltage windings made of aluminium conductors have a higher propensity to failure than copper wound ones due to metal fatigue

Fatigue   

Fatigue is the progressive local damage that occurs when a material is subjected to repeated stresses (pulling, pushing, shearing or twisting forces). Ultimately, after a certain number of repetitions which vary inversely with the intensity of the stresses, the material suffers a fracture. This is akin to what happens when you bend a paper clip back and forth several times with a slight force. Fatigue is the single biggest reason for the fracture of metals. Fatigue life is defined by the number of cycles that a material can withstand a given stress applied repeatedly.

Why is conductor fatigue important for 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 and elongate by a small amount. This is particularly critical for high voltage (HV) windings. These loosened conductors vibrate due to the cyclic electromagnetic stresses from both service and inrush currents, leading to fatigue and ultimately to fracture.

Why is this a problem for distribution transformers with aluminium HV windings?

Researchers have compared the fatigue behavior of comparable sizes of wires made of electric conductor grade aluminium and electrolytic copper which are used in distribution transformer HV windings. 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 fact suggests that after loosening of the HV winding conductor, aluminium wound distribution transformers would fail earlier than copper wound ones due to metal fatigue under identical operating conditions.

Reference

Creep and Fatigue Behaviour of Aluminium and Copper High Voltage Winding Conductors of Distribution Transformers, N. S. Beniwala, D. K. Dwivedib and H. O. Guptaa, Indian Institute of Technology Roorkee, Roorkee, India.

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