Water Solubility of Aged Mineral Oil and Natural Ester Liquids

Acceptable moisture content limits of in-service transformers vary depending on the type of liquid and the voltage class of the transformer. These limits are often, if not always, defined in terms of maximum water content (in mg/kg) as measured by Karl Fischer titration. Karl Fischer titration is a common test used for measuring the moisture content of transformer liquids but it can only be completed by taking a sample and testing it in a laboratory. In contrast, capacitive probes can be used to measure relative saturation, or water activity, in real time on in-service transformers and that measurement can be used to calculate the water content. One thing that is often not considered when setting the water content limits, or when calculating the water content via a capacitive probe, is how the water solubility of a liquid changes over the life of a transformer as polar oxidation products accumulate in the liquid. This paper investigates the water solubility of seven different liquids, five mineral oils and two natural esters, that underwent an accelerated aging process in sealed tubes at $150^{\circ}\mathrm{C}$ for up to 26.4 weeks. Of most interest is how the water solubility for a particular liquid changes over a range of aging periods under the same conditions as well as how the solubility change is different for different types of liquids. The impact of such changes in water solubility is discussed and how these changes may be utilized to evaluate transformer condition.