Measuring Low Molecular Weight Acids in Mineral and Ester Transformer Liquids

Low molecular-weight acids (LMA) are by-products of and catalysts to transformer paper ageing. Their concentration can be well below 0.1 mg KOH/g. There has been no standard procedure to measure and control these acids at low acidity levels. In the present work, a technique using water to separate an LMA at low acidity from two types of transformer liquids, a mineral oil and a synthetic ester, was proposed and its effectiveness was studied. Two vessel configurations, Erlenmeyer flask (EF) and gas-tight syringe (GTS), were tested for the important step of separating the LMA from the liquids. Results showed that both vessels, which can be sealed tight, enable high percentage extraction of LMA (>80%) at acidities of 0.017 mg KOH/g and 0.06 mg KOH/g. The EF proved a preferred choice due to its ease of handling. Further validation of its suitability is confirmed by the high percentage extraction of LMA from a mixture of LMA and high molecular-weight acids (HMA) both added to the transformer liquids. The effective range of the total volume of transformer liquid and water has also been identified.