Some hints at the combined effects of load cycles and electrothermal stress on HVDC extruded cable reliability in the long term prequalification test

As confirmed by the new CIGRE Technical Brochure (TB) 496, the prequalification (PQ) test of HVDC extruded cable systems is based on a sequence of 360 “24 hours” load cycles, whose duration is based on a voltage-time characteristic with exponent n=10 and on a design life of 40 years at constant rated voltage, without any explicit consideration of thermal and syner-gistic electro-thermal aging effects related to load cycles. Here a method is proposed for estimating failure time percentiles that accounts for the time-varying temperature during the PQ test load cycles via a “two-level of approximation” approach relying on: 1) Miner's law of cumulated aging; 2) an improved voltage-time characteristic that includes the electrothermal stress; 3) a probabilistic framework based on the Weibull hypothesis. The application to a typical HVDC cable design - with consideration of possible field profile inversion effects due to the load - is novel. In this way, a more accurate correlation between cable insulation reliability at 40 years design conditions and that after 360 “24 hours” load cycles according to CIGRE TBs 219 and 496 is provided, thereby taking into account the peculiar electrothermal endurance features of a given HVDC extruded cable design.