Abstract
Oil-immersed paper insulation and paper pressboards for structural support are widely used in electrical power transformers. Cellulose thus fulfills an essential task for the smooth power supply of our societies. However, the prevailing temperatures in such equipment, combined with a targeted service life of several decades, pose a serious challenge to the long-term integrity of cellulosic paper insulation. Therefore, numerous studies have been conducted to obtain kinetic data on the degradation processes that contribute to the thermally induced decomposition of cellulose. These studies usually rely on the assessment of the average degree of polymerization by viscosity measurements. In this work, we applied and optimized more advanced methods for the characterization of cellulosic materials based on gel permeation chromatography for the special case of thermally stressed unbleached Kraft paper samples. This allowed studying the molar mass distributions of paper polymers upon exposure to heat, as well as the investigation of changes in their conformation in solution and the observation of thermally induced cross-linking. In combination with group-selective fluorescence labeling, it was possible to track over time the changes in molar mass-dependent profiles of carbonyl and carboxyl groups of authentic Kraft insulator paper samples under thermal stress. In addition, changes of the hemicellulose composition were quantified. We hope that this analytical approach to the in-depth characterization of thermally stressed insulator paper will prove useful for future studies of this important cellulose product, and that our findings will contribute to a better understanding of the thermal decomposition of paper in general.Graphical abstract
Highlights
Cellulosic materials have been employed as electrical insulators in cables and all kind of electrical devices since the earliest days of electrical engineering
Oil-immersed Kraft paper (UKP) and Whatman paper (WP) samples were subjected to accelerated aging at 170 °C for up to 14 days
While our results clearly demonstrate that thermal aging under conditions simulating long-term service in power transformers causes crosslinking, we are at present not able to confirm the presence of any particular crosslinking mechanism or rule out another based on the available data
Summary
Cellulosic materials have been employed as electrical insulators in cables and all kind of electrical devices since the earliest days of electrical engineering. Oilimmersed paper insulators are still widely used in electrical power transformers and high voltage power cables (Prevost and Oommen 2006), essentially because cellulosic materials feature excellent electrical insulation properties in dry state combined with high availability and a relatively low price. Due to good mechanical properties at elevated temperatures compared to synthetic and more expensive polymer alternatives, paper pressboards serve as mechanical support in oil-filled power transformers. For such purposes, unbleached Kraft pulp derived from softwood species is primarily used as the raw material (Krause 2012). Long-term exposure of certain parts of the paper insulation to 100 °C and more is not uncommon
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