Temperature and chemical impact on determination of water content in dielectric liquids by capacitive moisture sensors

Paper analyses temperature and physical-chemical factors that influence determination of water content in transformer insulating liquids by capacitive moisture sensors. Water solubility coefficients are used to calculate the water content from moisture sensor's relative saturation and temperature measurements. Absolute water content and water solubility coefficients of various types and ageing degree of mineral, ester and silicon based transformer insulating liquids were determined in controlled laboratory conditions. Results of capacitive moisture sensor measurement, the method under study, and the Karl Fisher coulometric direct titration method as the reference method, are analysed and discussed in the paper. The experimental results confirm that water solubility coefficients of the studied liquids can be basically correlated to the liquids' chemical structure. The impact of the liquid temperature and oil circulation speed on the error of the capacitive moisture sensor measurement is analysed and quantified. The paper discusses the influence of laboratory experimental conditions on determination of water solubility coefficients as well as potential error sources of water content measurements by capacitive moisture sensors in transformers in operation.