Space Charge Characteristics of Natural Ester Oil-Impregnated Paper With Different Moisture Contents

Mineral oil is the most common dielectric liquid for high-voltage direct current (HVDC) converter transformers. However, natural ester oil receives much attention as an alternative dielectric liquid due to its environmental friendliness and low fire risk. Moisture is one of the most dangerous substances, which accelerates the degradation of oil-paper insulation systems. Thus, this article investigates the impact of moisture on space charge characteristics of the natural ester oil-impregnated paper by comparing it with the mineral oil-impregnated paper. Space charge accumulation is a typical phenomenon under dc conditions, and its presence can lead to electric field distortion and accelerate insulation aging. The space charge profiles in oil-impregnated papers with different moisture contents are obtained by the pulsed electroacoustic (PEA) method. It has been found that the hydrophilic characteristic of natural ester oil leads to a slow process of forming heterocharges in the natural ester oil-impregnated paper. On the other hand, the hydrophobic characteristic of mineral oil results in faster heterocharge formation in the mineral oil-impregnated paper.