Effects of surface partial discharges on the dielectric strength of ester oil-impregnated pressboard insulation in power transformers

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dc.contributor.authorNdlovu, Bakhekile A
dc.date.accessioned2022-09-29T09:00:54Z
dc.date.available2022-09-29T09:00:54Z
dc.date.issued2021
dc.descriptionA dissertation submitted to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering, 2021en_ZA
dc.description.abstractThe importance of the smooth operation of power transformers to ensure continuity of supply is indisputable. One of the main transformer failure modes is insulation deterioration because the transformer insulation system constitutes approximately 60% of the transformer material. The aim of the present investigation was to determine the effects of surface partial discharges (also referred to as creepage discharges) on the AC breakdown voltage of ester oil-impregnated pressboard compared to the same effects on mineral oil-impregnated pressboard. This was done by exposing the impregnated pressboard to surface discharges for 0, 3 and 7 hours, followed by subjecting the respective pressboard samples to AC breakdown voltage. The phenomena observed on both the mineral oil and ester oil-impregnated pressboard under exposure to surface discharge activity include bubbles that formed, tree-like white marks propagating from the needle-point to the ground plane and carbon-tracks emanating from the needle point. Some of the mineral oil-impregnated pressboard samples exhibited burn marks as a result of high electric stresses which resulted in further insulator deterioration. The subsequent breakdown voltage results showed a general decline in the breakdown voltage of both the mineral oil and ester fluid cases as the time exposure surface discharges increased from 0 hours to 7 hours. However, the ester oil-impregnated pressboard breakdown voltage values were consistently higher than the mineral oil case by an average of 20%. The observed discharged phenomena and obtained breakdown voltage results are explained using various surface discharge mechanism theories. Exposure of pressboard to surface discharges deteriorates its dielectric strength. The surface discharge mechanisms include insulation deterioration due to cracking, oxidation, high electric field stress and the space charge effect. The present investigation demonstrates how surface discharges affect the breakdown voltage of ester fluid and mineral oil-impregnated pressboard and concludes that ester fluid is a viable possible alternative to mineral oil with regards to the ability to withstand the effects of surface PD on solid insulation in oil filled power transformersen_ZA
dc.description.librarianCK2022en_ZA
dc.facultyFaculty of Engineering and the Built Environmenten_ZA
dc.identifier.urihttps://hdl.handle.net/10539/33368
dc.language.isoenen_ZA
dc.titleEffects of surface partial discharges on the dielectric strength of ester oil-impregnated pressboard insulation in power transformersen_ZA
dc.typeThesisen_ZA
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