The combination of electro-thermal stress, load cycling and thermal transients and its effects on the life of high voltage ac cables

The paper proposes a procedure for life estimation of high voltage ac cables in real operating conditions, i.e. subjected to voltage and load cycles, so that electrothermal stress is the dominant aging factor of cable insulation. As possible alternatives for representing the effects of the electrothermal aging of insulation, three life models well known in literature are considered, namely the Zurkov, Crine and Arrhenius-IPM models, each within the probabilistic framework needed for associating time-to-failure with reliability. The cumulation of loss-of-life fractions over load cycles is evaluated through Miner’s law. The thermal transients that affect insulation as a consequence of cyclic current variations are simulated via the CIGRE two-loop thermal network analog. The procedure is applied to XLPE-insulated high voltage ac cables, subjected to two typical stepwise-constant daily load cycles differing as to the load severity. The application shows that cable life is very sensitive to load cycles, thermal transients and electrothermal synergism, aspects that all deserve attention for estimating accurately the life expectancy of high voltage ac cables in service. The three life models employed, though weighing differently the electrothermal synergism as a consequence of their own functional expressions, give a concordant indication about the possible life extension of cables already in service.