What are the typical data for a paper-synthetic ester insulating system?

FIG 1: Moisture equilibrium curves for cellulose paper and synthetic ester, Midel 7131

A higher temperature will ensure that a shorter time is needed to reach a moisture equilibrium state. At lower temperatures, this can take much longer to achieve. A single measurement of water content in esters through Karl Fischer titration does not enable the assessment of the dynamics of water migration in the insulation system. A Karl Fischer titration test result without an indication of the oil temperature at sampling will be of no value at all! An alternative is a capacitive sensor, this will give a more accurate indication of the real state of the moisture content in the insulation system. The aging rate of both cellulose and insulation systems needs to be considered to get to the actual state of the cellulose paper moisture content. 

TABLE I: The guidelines for interpreting the condition of cellulose insulation on the basis of water content in synthetic ester [1]

The impregnation of Solid Insulation with Synthetic Esters

Seeing that the kinematic viscosity, the number of open pores in the insulation material, diameter of the pores, inside and outside pressure of the material will determine the rate of liquid penetration into the insulating material. Impregnation for synthetic esters at 60⁰ C is the same as for mineral oil at 20⁰ C. It should be noted that the impregnation of pressboard with synthetic esters take approximately 1.5 times longer than mineral oil.

The breakdown characteristics of solid insulation components impregnated with synthetic esters

According to experimental data, has a much higher electrical permittivity (3.2) than mineral oil (2.2) This causes the pressboard to be impregnated with this liquid and it also has a greater electrical permittivity, this result in a decreased value of the electrical field and an increase in the electrical strength. 
Further experiments revealed however that pressboard impregnated with esters are more susceptible to the influence of partial discharge, which is not an ideal scenario in insulating structures. [2]

Electrical field distribution in synthetic ester insulating systems

A cellulosic paper and synthetic ester system has been studied as well as the cellulosic paper and mineral oil system. It was found that the intensity of the electrical field is higher in the paper with the synthetic ester dielectric liquid, this is the electrical field in the paper. However for the mineral oil system, the electrical field in the paper is lower, but in the oil, it is higher. The fact that the higher electrical permittivity of synthetic esters with that of mineral oils causes a decrease in the electrical field in the liquid, but an increase in the electrical field stress of the solid insulation. The difference between the distinctive regions are lower in the case of ester liquids, this acts to their benefit. [3]
1. IEC 60422. Mineral Insulating Oils in Electrical Equipment—Supervision and Maintenance Guidance; International Electrotechnical Commission (IEC): Geneva, Switzerland, 2013
2.Rozga P, Using the three-parameter Weibull distribution in the assessment of threshold strength of pressboard impregnated by different liquid dielectrics. IET Sci Meas. Technol 2016, 10, 665-670 [CrossRef]
3. Sbravati A, Rapp K, Schmitt P, Krause C, Transformer insulation structure for dielectric liquids with higher permittivity. In Proceedings of the 2017  IEEE19th International Conference on Dielectric Liquids(ICDL) Manchester, UK, 25-29 June 2017 [Crossref]
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