Abstract
Million tons of cellulosic paper have been used for insulating coils in oil-filled electrical power transformers, thereby assuring the electricity supply for our societies. The high working temperatures in transformers constantly degrade paper insulators throughout their service life of up to 40 years. We approached the structural changes in oil-immersed cellulosic paper samples upon thermal stress in a study that compared unbleached softwood Kraft paper used as insulator paper with pure cotton cellulose paper. The model experiments used a thermal treatment in transformer oil at 170 °C for up to 14 days. The samples were characterized by means of 13C CP/MAS NMR spectroscopy, mainly based on deconvolution of the C4 resonance. An automated, fast, and reproducible C4 resonance deconvolution employing the “Peak Analyzer” tool of OriginPro 2020 (OriginLab Corporation, USA) was developed and used to exploit 13C CP/MAS NMR spectroscopy for the characterization of thermally stressed paper samples. Our results show that thermally induced structural changes depend heavily on the composition of paper, that hornification and coalescence of fibrils take place, and that the allomorph composition of cellulose crystallites is altered under the given conditions.Graphical abstract
Highlights
A sound understanding of the complex processes that occur during degradation of cellulose that has been exposed to elevated temperatures is of importance to ensure the electricity supply for our societies
Solid-state 13C cross-polarization/magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy is an excellent method to characterize the supramolecular assembly of cellulose polymer chains (French et al 2018; Zugenmaier 2021)
The relative intensity of the hemicellulose C4 signal (HC) signal contributor was Deconvolution of the C4 resonance of 13C CP/MAS NMR spectra of thermally aged paper samples clearly showed that hornification and coalescence of cellulose fibrils are the consequences of an unintentional temperature rise in insulator papers in electrical power transformers
Summary
A sound understanding of the complex processes that occur during degradation of cellulose that has been exposed to elevated temperatures is of importance to ensure the electricity supply for our societies. This is because engineers have been relying since decades on oil-immersed cellulosic paper as insulators in high voltage power cables, condenser bushings, and, most importantly, electrical power transformers (Prevost and Oommen 2006). Partial discharges even may cause locally restricted short-time temperature peaks of 1000 °C (Li et al 2020) Despite these harsh conditions and if operated properly, oil-immersed paper insulation systems last for up to 40 years without replacement (Jalbert et al 2014). Despite these harsh conditions and if operated properly, oil-immersed paper insulation systems last for up to 40 years without replacement (Jalbert et al 2014). and are an important pillar that our societies’ electrical energy supply is resting on
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