Fourier-Transform Infrared Spectroscopy Characteristics of Mineral Oil and Ester Liquid under Thermal Ageing

Retrofilling lower voltage power transformers by swapping the mineral oil for ester liquid in the transformer tank has become increasingly popular. Due to the presence of materials that absorb liquid in the transformer and difficulties in fully draining the tank, there will be some residual mineral oil remaining after a retrofill which could affect the fire and other properties of the ester liquid. Fourier-Transform Infrared (FTIR) Spectroscopy can be used to determine the residual mineral oil in ester liquid. In another application, ester liquid, as an identification marker, is added to either tap-changer diverter switch oil or the oil in the selector tank to detect the possibility that acetylene-contaminated oil has migrated to the other compartment or the main transformer tank. FTIR spectroscopy can be used to determine the concentration of ester liquid in mineral oil. Both applications utilize FTIR spectroscopy to quantify cross-contamination. It is however necessary to demonstrate that such methods are not significantly affected by any ageing products of either ester liquid, mineral oil or paper insulation. To investigate this, accelerated thermal ageing experiments using the transformer liquids with paper insulation were conducted. The results indicate that by-products from thermal ageing include carboxylic acids with a C=O double bond, but this can be differentiated from the ester C=O double bond by using FTIR. This assures the validity of using the FTIR technique in the new applications.