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Mineral oil is widely used as a dielectric liquid in high voltage direct-current (HVDC) converter transformers because of its excellent electrical insulating and cooling properties. Due to the low biodegradability and high fire risk of mineral oil, natural ester oil is gaining considerable interest as a potential alternative for HVDC converter transformers. This requires more information thorough understanding of the electrical behaviours of natural ester oil under various conditions. This paper compares the impact of thermal ageing on the space charge characteristics of natural ester oil-impregnated paper to mineral oilimpregnated paper. During thermal ageing, although the increased acidity caused by the natural ester oil led to more charge accumulation in natural ester oil-impregnated paper compared to the mineral oil, the natural oil-impregnated paper showed a higher DC breakdown voltage.

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