Effect of Moisture on Particles Accumulation and Oil Breakdown Characteristics in Mineral Oil and Natural Ester Under Non-Uniform DC Electrical Field

Natural ester is a good substitute for mineral oil. It has been successfully applied in power transformers, and its application scale will continue to increase. With the aging of oil-paper insulation in transformers, moisture and impurity particles will inevitably be generated. In this paper, dry and wet mineral oil and natural ester samples, as well as dry and wet cellulose particles, were prepared. The accumulation behavior of the cellulose particles and the conductivity current in mineral oil and natural ester under a non-uniform dc electrical field were compared by using eight combinations of dry or wet cellulose particles and dry or wet insulation oils. The results show that the speed of the cellulose particles and the growth of impurity bridges are faster in wet mineral oil and natural ester, especially when the cellulose particles are also wet. The increase of electrical field strength accelerates the appearance of this phenomenon. The cellulose particle bridging phenomenon in the mineral oil is more significant than that in the natural ester. Under the same dc voltage, the saturated conductivity current in every mineral oil sample decreases as the electrode distance increases; however, the opposite phenomenon is observed in the natural ester samples. Moreover, the oil moisture content increase has a more significant influence on the saturated current and the dc breakdown voltage than the particles' moisture content increase. For the same particle concentration level, the natural ester has a lower conductivity current and a larger dc breakdown voltage than the mineral oil. The natural ester presents better resistance to particle pollution and better insulation properties than the mineral oil.