FTIR-Spectroscopic and Electrical Analysis of Thermally Stressed Liquid Dielectrics

Mineral oils are employed as liquid dielectric insulators in many high voltage transformers. It is recognised that the dielectric properties of these oils degrade over time with continued exposure to high voltages and thermal stress. This breakdown can be monitored using FTIR spectroscopy or electrical techniques. In this work the mineral oils have been thermally stressed by exposure to 120°C over a prolonged time period with changes to the electrical breakdown voltage and FTIR spectra measured at frequent intervals. Different samples of oil were prepared with some exposed to Kraft paper, copper or a combination of the two. The ratio of oil: copper: Kraft paper was consistent with that commonly found in transformers. Results show that the growth of the carboxylic acid peak can be correlated with a total acid number (TAN) through a calibration process which is less time consuming, won't overestimate TAN and is an alternative to titration.