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            11 - Effect of carboxyl groups on transformer board surface properties            
Audrey Bourgeois, Gérard Mortha (EFPG), O. Moreau (EDF R&D)

Site Web de l'EFPGAs part of a research program in partnership with EDF R&D in the field of flow electrification in power transformers, some fundamental studies have been started on the physical chemistry of this phenomenon.

Flow electrification phenomena in power transformers have been studied since the seventies. A lot of failures on high power transformers have occurred between 1970 and 1996 in different countries (Japan, United States, South Africa and France), drawing the industry's attention to this problem. Many research programs have been fulfilled to improve the knowledge on flow electrification and electric charge generation in power transformers. However, the complete process still remains partially unknown.

A high power transformer is composed of different metallic parts electrically insulated one from each other by cellulosic pressboard and oil. The oil is the cooling liquid, flowing past the pressboard. When a liquid is in contact with a solid wall, physical chemical reactions occur and the solid-liquid complex initially neutral becomes polarized. Two space charges appear: one in the oil which can relax in contact with grounded metallic walls, and one accumulated in the pressboard. The charge accumulation in the pressboard can only be limited by a possible chemical saturation of the interface, by leakage currents, or by a discharge along the interface initiated by potential gradients becoming too high versus the pressboard dielectric strength.

The study herein presented focused on the role played by the cellulosic pressboard, i.e. its physical and chemical functionalities, to understand the process of surface electrification. Special manufactured and modified pressboards have thus revealed that the phenomenon is highly related to carboxylic groups. Experiments with demineralised pulps treated by carboxyl complexing agent such as methylene blue have confirmed the important role of ionisable groups in papers. It appeared that some additives could reduce the electrification phenomenon. The effect of the surface morphology was also studied. Therefore, different chemical treatments of the pulp and degree of pulp refining were applied on commercial and hand-made pressboards. It was found that the surface properties (hydrophilic nature, porosity, chemical nature…) also contributed to significant variations in the electrical charge generation.

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