Effects of the combination of electro-thermal stress, load cycling and thermal transients on polymer-insulated high voltage ac cable life

In the paper, the effects of the combination of electro-thermal stress, load cycles and thermal transients on polymer-insulated cable life are analyzed on the basis of Miner´s cumulative damage law. Loss-of-life fractions within each portion of a stepwise-constant load cycle are calculated resorting to an electro-thermal life model holding for cable insulation subjected to voltage and temperature, considered within a probabilistic framework for associating life with residual reliability and failure probability. Thermal transients are treated by means of a well-known approach leading to a simplified but analytical expression for the temperature response of cable layers and soil to each current step. The application, relevant to high voltage ac XLPE-insulated cables with a stepwise-constant daily load cycle, shows that life is actually sensitive to load cycles, as well as to chosen life model and thermal transients, that should be considered for accurate cable life inference.