Application of Polymer Ageing Models to Cable Geometry and Time-To-Failure Distributions

Polymer ageing models predict the working lifetime of polymeric insulation in terms of the electrical and thermal stresses to which it is subjected. Two such models are investigated in this thesis and are found to be mathematically similar even though they are based on different mechanisms for the way in which an electric field accelerates the ageing process. It is shown that both models can successfully fit characteristic time-to-failure data from ageing experiments involving thin films. A new method is developed to allow the ageing models to be applied to cable insulation, where the field and temperature are not spatially constant. This method is used to apply one of the models to characteristic lifetime data from experiments involving cables. The fits to data are found to be good, and resulting parameter values are used as the basis for a discussion of the possible effects of specimen volume on ageing. The distribution of failure times observed when thin films and cable insulation are aged at a given experimental condition has also been investigated. This has been carried out using distributions of the activation free energy of ageing within one of the ageing models. It is established that small changes in the minimum activation energy from specimen to specimen could be responsible for the observed failure statistics. Changes in the activation energy distributions with ageing condition suggest that ageing may involve conformational re-arrangements of chain segments in the crystalline-amorphous interface. This is in broad agreement with the conclusions of other workers.