Cellulosic materials to improve power transformers

With the invention of the lightbulb and the development of electrical machinery, the industrial world developed an appetite for an ever-increasing electrical power consumption. To answer this demand power grids were built that could transport electricity from producers to consumers. Central to the power grid are transformers, they are passive electrical devices which can rise and lower the voltage in electrical grids, in order to transport electricity with minimum losses. Power Transformer (PT) are particular types of transformer found at the beginning of electrical networks. PT require an insulation system with excellent thermal and dielectric properties. A combination of mineral oil and cellulosic insulation (Kraft paper and pressboard immersed in the oil) is used in power transformers. Oil acts as the main insulation and dissipates heat while cellulosic insulation breaks the stress in the oil and provides a mechanical support. Paper-oil insulation in PT will degrade over time and, while oil can be periodically renewed, cellulosic insulation cannot be replaced. In normal working conditions, the first component of a PT that will fail is the paper insulation. Therefore, assessment methods of PT focus on evaluating the state of the cellulosic insulation.

This thesis focuses on the ageing behavior of PT insulation, focused on Kraft paper, with the overall idea of developing new methods for assessing the insulation state in real equipment.
The presented research is divided in four chapters.

In the first chapter, a general bibliographic survey provides the necessary information to present the global topic. This chapter starts by a general introduction of transformers history, with an overview of transformer insulation, its structure and function. The chemistry of insulation materials, paper and oil, is also presented with a description of the general ageing behavior of Kraft insulation.

The second chapter focuses on the use of methanol as an ageing marker for cellulose degradation in PT. This is an indirect method that relates the dosage of a dissolved substance (methanol) to the state of degradation of the cellulose insulation inside a PT. A review of the relevant literature on the subject and description of the experimental methods which have been used to validate methanol as an ageing marker are presented. In our study, a particular focus was brought on the chemical role of paper main components (cellulose, hemicelluloses, lignin) in the production of methanol during ageing. The origin of methanol generated as an ageing marker of cellulose depolymerization, and the role of each paper component in methanol production, were investigated through the study of model molecules. Lignin is a major contribution to methanol release due to the presence of methoxyl groups.

In the third and fourth chapter, the same set of samples was studied. It was selected to represent the possible variations in the organic and inorganic composition of paper. Variation in organic composition were achieved by simply using paper samples of different types. Variations in inorganic composition was achieved by ion exchange, to substitute some cations present in Kraft pulps, and enrich the paper with other metal ions. Once ageing starts, lignin protects the pulp from oxidation, not only by reducing overall value of the oxidative damage but also the proportion of acid components in the oxidation. This correlated with permittivity and depolymerization, in both cases, samples with an acid lead oxidation corresponded to the fastest depolymerization and loss of dielectric properties. Main metals (even in ppms concentration) showed to have an important effect on depolymerization and oxidation. Calcium, magnesium, and to a lesser extent sodium, showed a lower proportion of acid lead oxidation that correlates to slower kinetics of depolymerization while iron and copper showed an important acid oxidation that correlate to fast depolymerization kinetics.

Thesis available on: https://cnrs.hal.science/LGP2/tel-04210645v1