Abstract: The integrity of the cellulosic insulation in power transformers is considered one of the most relevant parameters that affects their performance and reliability. Electric faults, such as short circuits, have thermal and mechanical effects that degrade the paper and can eventually produce the end-of-life of the transformer. The evolution of the properties of the paper insulation of a commercial continuously transposed conductor due to thermal ageing was characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the paper was analysed macroscopically and microscopically using scanning electron microscope. A finite element numerical mechanical model of the conductor was implemented to reproduce the experiments and to obtain the load level and strain state that produce failure at each ageing state, aiming at developing a failure model for the insulation. This model may contribute to an improvement in manufacturing processes and management of the electrical system.

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