Transformer Oil Dehydration By Using Relative Saturation Method
Prescon have imported a brand new European oil dehydration machine (6000L/H) with built in moisture content sensor. Machine is best for field oil dehydration and filters under vacuum with oil heating. Machine is fitted with moisture sensor to efficiently remove moisture and client will not be fooled by instantaneous BDV improvement. Mostly we find degraded BDV and moisture soon after return to service as most moisture is in paper and wood which returns under service to oil. It’s dangerous even in paper causing hydrolysis and reducing life of paper.
Water types in transformer
Water can exist in several different states within the transform i.e Dissolved water is hydrogen bonded to the hydrocarbon molecules of which oil is composed Emulsified water is supersaturated in solution but has not yet totally separated from the oil. It usually gives oil a milky appearance. Free water is also supersaturated in solution but in a high enough concentration to form water droplets and separate from the oil.
RS Technique inside view
Solubility of Water in Oil
is defined as the total amount of water than can be dissolved in the oil at a specific temperature. The solubility of water is not constant in oil but changes due to temperature. As the temperature increases, the amount of water that can be dissolved in oil also in-creases. The increase is not linear but exponential in function.
Relative Saturation of water in oil (RS)
Is the actual amount of water measured in the oil in relation to the solubility level at that temperature.
The correlation between the water content in new, filtered, mineral oils at room temperature and the dielectric breakdown voltage using ASTM method D 1816 (0.04 inch gap) (water content, ppm) is shown in the tabular figure.
Taking the same dielectric breakdown voltage data and converting it to RS (Figure 1, %RS graph) provides a much straighter curve except at the extremes. It is evident that there is a better correlation between RS and dielectric breakdown voltage than with moisture concentration and dielectric breakdown voltage.
Conclusion: Transformers are more complicated systems. BDV is remains a function of the relative saturation of water in the oil. During the cool-down cycle of a thermal transient in a transformer some of the moisture returns to the paper and some of the moisture remains in the oil. The relative saturation of water remaining in the oil will influence its dielectric breakdown voltage.