Effects of load cycles on AC extruded cable reliability: Is a reduced pre-qualification test always the right choice?

In this paper, a new method is proposed for the estimation of time to failure percentiles and life fraction lost in the so-called “Extension of pre-Qualification (EQ) test” according to CIGRE TBs 303. The method accounts for the time-varying temperature during the EQ load cycles via an approach that relies on: 1) Miner's law of cumulated aging; 2) an improved voltage-time characteristic that includes the electro-thermal stress; 3) a probabilistic framework based on the Weibull hypothesis. On these grounds, an alternative to the EQ test is proposed, referred to as “Improved Extension of Qualification (IEQ) test”, based on a more strict correlation between cable insulation reliability in design stress conditions and that during the test itself, thereby taking into account the electro-thermal endurance features of a given extruded cable design.