In-Service Microstructural Modifications caused by Fatigue Phenomena leading to the Failure of a High-Voltage Cable

Abstract

Abstract This study analyses the causes leading to the failure of a high-voltage cable in regions close to it “mooring” at the end near to one of the high-voltage towers. The horizontal span of the cable between moorings was over 1040 m, with a difference in elevation between high-voltage towers of approximately 300 m. The conductor was made up of 30 aluminium strands with a 7 strands eutectoid steel core. The aluminium filaments were distributed in 2 layers, an outer layer with 18 filaments and an inner layer with 12. The steel filaments were distributed in a layer composed of 6 filaments surrounding a single central filament. All the filaments had a diameter of 2.6 mm. Cold drawn and twisted aluminium and the possible presence of recovery substructures in regions close to the fracture were analysed by means of optical microscopy. The steel was similarly studied, verifying its pearlitic nature and assessing the existence of softening phenomena due to recovery or recrystallization. The thicknesses of the zinc coating on the steel filaments were determined. The fracture faces of the aluminium and steel filaments were analysed by scanning electron microscopy and the fused remains resulting from possible arcs between the aluminium filaments were examined. In order to assess the deterioration of the filaments, tensile tests were performed on all the individual filament in regions adjacent to and at a certain distance from the fracture. The failure of the conductor had its origin in the outer aluminium filaments due to causes imputable to fatigue phenomena. The reduction in the cross-section of the conductor led to an increase in temperature, leading to the weakening of the remaining aluminium filaments and their subsequent failure, the electrical current being subsequently transferred to the steel core. The increase in electrical resistivity characteristic of steel led to a significant increase in temperature that drastically decreased the mechanical strength of the steel cables.