Aging of petroleum and synthetic based insulation oils from the view of structural analyses

Insulating system of power transformers is mostly composed of two basic components: insulation oil and cellulose paper (oil-paper system). These components are of an organic nature and thatpsilas why they become very sensitive to operation degradation conditions (temperature, oxidation, moisture etc.). Aging of these components can endanger the failure free operation of the transformers. The main objective of the paper was to analyze the aging process of liquid part of oil-paper insulation system by selected structural analyses. Three petroleum and three synthetic based insulation oils were chosen for the purpose of this experiment. The commercially available cellulose paper formed the solid part of the insulation. All insulation systems were exposed to accelerated thermal aging at increased temperature of 90 degC for the time range of 50 - 3000 hours. The degradation of oils inner structure was analyzed by Fourier transform infrared spectroscopy (specifically by Attenuated Total Reflectance technique). The analysis of insulation oil behaviour during thermal decomposition was the other aim of our experiment. Thermogravimetry and derivative thermogravimetry were applied as the proper tools for this purpose. Results of structural analyses clarify very well the differences between the behaviour of mineral and synthetic insulation liquids at common temperatures as well as during the thermal decomposition.