Novel investigations on ageing behaviour of thermally upgraded papers for power transformers

In this paper, two studies are presented whose main objective is to go further on the comprehension of thermally upgraded paper ageing. The first one is about the accelerated ageing of standard (STD) and thermally upgraded (TU) Kraft paper in mineral oil. After plotting the viscometric degree of polymerization (DPv) according to the kinetic model proposed by Calvini in 2005 [1], the model is validated for STD paper but not for T U paper. A modification of this model for T U paper is suggested to improve the correlation with experimental data. In the second part, a study is presented on the investigation of the effect of a nitrogen-based additive (DICY) concentration on the paper ageing in oil. The DPv results confirm the protective effect of the DICY on the cellulose chains and show that this effect depends on the nitrogen content, but not linearly. The pH of aqueous extracts and surface FTIR analysis brought new support for a hypothesis regarding the functioning mechanisms of the DICY that are reported in the literature.