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Cellulose-Based Electrical Insulation Materials Dielectric and Mechanical Properties REBECCA HOLLERTZ

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Cellulose-Based Electrical Insulation Materials Dielectric and Mechanical Properties REBECCA HOLLERTZ Cellulose-based electrical insulation materials Dielectric and mechanical properties REBECCA HOLLERTZ Doctoral thesis KTH Royal Institute of Technology School of Chemical Science and Engineering Department of Fibre and Polymer Technology ISBN 978-91-7729-327-9 TRITA – CHE-report 2017:21 ISSN 1654-1081 Tryck: US-AB, Stockholm, 2017 The following papers are reprinted with permission from: Paper I: Springer Papers II, III, VII and VIII: IEEE Transactions on Dielectric and Electrical Insulation AKADEMISK AVHANDLING Som med tillstånd av Kungliga Tekniska högskolan i Stockholm framläggs till offentlig granskning för avläggande av teknologie doktorsexamen fredagen den 12 maj 2017, kl. 10.00 i F3, Lindstedtsvägen 26, KTH, Stockholm. Avhandlingen försvaras på engelska. Fakultetsopponent: Professor Markus Biesalski, TU Darmstadt, Tyskland Copyright© Rebecca Hollertz, 2017 ABSTRACT The reliability of the generation and distribution of electricity is highly dependent on electrical insulation and is essential for the prosperity of our society and a ubiquitous part of our everyday life. The present study shows how some important material properties affect the electrical properties of cellulose-based electrical insulation systems which are used together with mineral oil in high-voltage transformers. Among other things, the effects of paper density and of the lignin content of the fibres on the dielectric response and charge transport of the papers have been studied. The underlying mechanisms of the inception and propagation of streamers, responsible for the most costly failures in transformers, at the oil-solid interface have been investigated and the important role of paper morphology on streamer propagation has been demonstrated. With papers, in contrast to films of synthetic polymers and microfibrillated cellulose, the branching of streamers increased and the length of slow negative streamers decreased. It was also shown that for polymers with permittivities close to that of the oil, the inception voltage was higher than with polymers with higher permittivities. This was explained by a different distribution of the electric field at the interface, and it means that with a solid restricting the initiation volume and with a permittivity of the solid matching the permittivity of the liquid, there is a lower risk for streamer inception, in negative polarity. Fibres were also modified prior to paper sheet preparation in attempts to improve the mechanical and dielectric properties. The properties of papers containing cellulosic micro- and nanofibrils and SiO2 and ZnO nanoparticles indicate that these additives can indeed be used to improve both the mechanical and dielectric properties. For example, the tensile strength index of papers with periodate-oxidised carboxymethylated CNFs increased by 56 % and by 89 % with a 2 wt% and 15 wt% addition, respectively, of the fibrils, when used together with different retention aids. A three-layered structure with two papers laminated together with a thin layer of microfibrillated cellulose also showed an increased DC breakdown strength by 47 % compared to a single-layer paper with a similar thickness. SAMMANFATTNING En god elektrisk isolering, i olika utrustningar i distributionskedjan, är i hög grad väsentligt för en tillförlitlig överföring och distribution av elektricitet. Detta är i sin tur en förutsättning för dagens välstånd och är av stor betydelse i vår vardag. Resultaten från doktorandprojektet visar hur flera materialegenskaper påverkar dielektriska egenskaper hos papper som används i kombination med mineralolja som elektriskt isoleringsmaterial i högspänningstransformatorer. Bland annat har inverkan av såväl papperets densitet som fibrernas ligninhalt på både laddningstransport och elektriska förluster studerats och kvantifierats. Dessutom har mekanismerna för uppkomsten och utbredning av streamers, som är den mest kostsamma orsaken till transformatorhaveri, i gränsytan mellan olja och fast fas undersökts. Resultaten visar att det finns ett klart samband mellan papperets morfologi och utbredningen av streamers. För långsamma negativa streamers på papper, jämfört med på filmer av syntetiska polymerer och mikrofibrillerad cellulosa, ses en ökad förgrening som vidare leder till kortare streamers. Genom att jämföra uppkomsten och utbredningen av streamers på olika polymerer studerades också inverkan av isoleringsmaterialets permittivitet. Med polymerer med en permittivitet som matchar oljepermittiviteten krävdes en högre elektrisk initieringsspänning för streamers. Detta innebär att risken för uppkomst av streamers i negativ polaritet minskas i gränsytan när permittiviteten hos det fasta materialet överensstämmer med oljans. Baserat på dessa resultat genomfördes olika fibermodifieringar för att förbättra de mekaniska och dielektriska egenskaperna. Resultat för papper som innehåller fibrer som modifierats med mikro- och nanofibriller och speciellt utvalda nanopartiklar visar att dessa kan användas som tillsatsmedel för att förbättra båda dessa egenskaper. Dragstyrkeindex ökade med 56 % och 89 % med 2 vikt% respektive 15 vikt% nanofibriller (karboxymetylerade fibriller som perjodat oxiderats) tillsatta i kombination med lämpliga retentionsmedel. Dessutom ökade DC genomslagsstyrkan med 47 % med en tre-skikts konstruktion med två omgivande papper som laminerats med ett skikt av mikrofibrillerad cellulosa jämfört med ett homogent papper av samma tjocklek. LIST OF PUBLICATIONS I. Dielectric properties of lignin and glucomannan as determined by spectroscopic ellipsometry and Lifshitz estimates of the non-retarded Hamaker constants Rebecca Hollertz, Hans Arwin, Bertrand Faure, Yujia Zhang, Lennart Bergström, Lars Wågberg, Cellulose, 20, p. 639-1648, August 2013 II. Effect of Composition and Morphology on the Dielectric Response of Cellulose-based Electrical Insulation Rebecca Hollertz, Claire Pitois, Lars Wågberg, IEEE Transactions on Dielectrics and Electrical Insulation, 22, p 2239-2248, August 2015 III. First mode negative streamers at mineral oil-solid interfaces David Ariza, Rebecca Hollertz, Marley Beccera, Claire Pitois, Lars Wågberg, Accepted for publication in IEEE Transactions on Dielectrics and Electrical Insulation IV. Inception of first mode negative streamers at mineral oil-solid interfaces David Ariza, Marley Beccera, Rebecca Hollertz, Ralf Methling, Lars Wågberg, Sergey Gortschakow Manuscript V. Influence of paper properties on streamers creeping in mineral oil David Ariza, Marley Beccera, Rebecca Hollertz, Lars Wågberg, Ralf Methling, Sergey Gortschakow, Accepted for publication in the Proceedings of the 2017 International Conference on Dielectric Liquids VI. Chemically modified cellulose micro- and nanofibrils as paper- strength additives Rebecca Hollertz, Verónica López Durán, Per Larsson, Lars Wågberg Submitted VII. Dielectric Response of Kraft Papers from Fibres Modified by Silica Nanoparticles Rebecca Hollertz, David Ariza, Claire Pitois, Lars Wågberg 2015 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, p 459-462 VIII. Comparison of Oil-impregnated Papers with SiO2 and ZnO Nanoparticles or High Lignin Content, for the Effect of Superimposed Impulse Voltage on AC Surface PD Roya Nikjoo, Hans Edin, Nathaniel Taylor, Rebecca Hollertz, Martin Wåhlander, Lars Wågberg, Accepted for publication in IEEE Transactions on Dielectrics and Electrical Insulation CONTRIBUTIONS TO PUBLICATIONS I. Main author*. Hollertz prepared the purified lignin and spin-coated the surfaces of lignin and hemicellulose. The purified hemicellulose was prepared by Dr Yujia Zhang. The spectroscopic ellipsometry measurements were performed with the instruction and supervision of Professor Hans Arwin. The calculations of the non-retarded Hamaker constants were made by Dr Bertrand Faure. All the co- authors were engaged in writing the manuscript. II. Main author*. Hollertz prepared all the samples and performed all the measurements. Dr Uno Gäfvert assisted in the interpretation of the data. III. Co-author. Hollertz took part in planning the experiments and gave input into the sample preparation. Hollertz prepared the paper samples and characterized the papers and polymers. The streamer tests were performed and the data analysed by David Ariza. Hollertz wrote the descriptions of the materials and took an active part in the full manuscripts, with analysis and writing. IV. See III. V. See III. VI. Main author*. The fibrils were prepared by PhD-Student Veronica Lopez Duran. Hollertz prepared all the paper sheets, while the characterization was performed together with Veronica Lopez Duran. Although Hollertz had the main responsibility for the analysis of the data and the paper writing, it was done together with the other co-authors. VII. Main author*. Prepared all the paper samples and characterized them. VIII. Co-author. Hollertz prepared the paper samples and characterized the material properties, besides their dielectric response and behaviour under discharges, and took part in writing the manuscript. The ZnO nanoparticles were surface-modified and characterized by Martin Wåhlander. *Main author embodies the work of formulating hypotheses, planning the experimental work and having the main responsibility for the manuscript. At the beginning of the PhD project, these tasks were undertaken together with Professor Lars Wågberg, but later more independently under his supervision. Conference papers (peer-reviewed) not appended to this thesis: Novel cellulose nanomaterials – towards use in electrical
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