Evaluation of the effect of moisture on dielectric properties of ester liquids

Four different types of insulation liquids have been investigated: synthetic ester MIDEL 7131, natural esters MIDEL 1204, MIDEL 1215, as well as Nytro Gemini X mineral oil as reference material. The liquids were subjected to different humidity conditions (32% and 75% respectively) and subsequently characterized by means of AC breakdown, DC conductivity, dielectric spectroscopy (for permittivity and loss tangent) as well as KF titration. The effect of moisture on the loss tangent was minimal in ester liquids, even when saturated, while the temperature dependence of esters was more pronounced than that of mineral oil. The expectation was that synthetic esters would show improved AC breakdown results, but this was not the case. Natural ester MIDEL 1215 showed the best breakdown strength, also outperforming the mineral oil, while the synthetic ester had significantly reduced Weibull scale and shape parameters. Contrary to expectations, natural ester MIDEL 1215 performed significantly better than the synthetic ester in terms of AC breakdown strength and temperature dependence of conductivity and loss factor. Synthetic ester was also found to be much more dependent on moisture content than the natural ester