Influence of moisture content on dielectric properties of pressboard impregnated in mineral insulating oil and natural ester

Natural ester as a substitute for mineral insulating oil has been used more and more widely in distribution transformers and large power transformers. The reliable operation of the natural ester transformer is essential to the safety and stability of the power grid. And the safe operation of the natural ester transformer relays the good dielectric properties of oil-pressboard insulation. In recent, the effect of moisture content on the dielectric properties of natural ester has been studied, while the research on the influence of moisture content on dielectric properties of pressboard impregnated is not detailed. In this paper, the temperature characteristics of permittivity, dissipation factor and resistivity at power frequency of pressboard impregnated respectively both in mineral insulating oil and natural ester with the moisture content about from 0.2% to 6% were studied. And the authors also studied the AC breakdown characteristic of oil-impregnated pressboard. The results were important for the safe and reliable operation of natural ester transformers.