Investigation of Early Effects of Moderate Warm-Ups on Transformer Oils: Electric and Spectroscopic Analysis

This article is devoted to studying, under in-service conditions, the probable effects induced by moderate heating on the electrical behavior of transformer oil. The experimental tools used are the polarization/depolarization current (PDC) technique and the Fourier transform infra red (FTIR) spectroscopy. The aim is to highlight and understand how the insulating oil is affected by the frequent warm-ups into in service power transformer. The experiments, carried out under dc voltages and room temperature, are conducted on two kinds of oil samples: The first is virgin and it was not submitted to any thermal treatment or electrical stresses. Samples of the second family were subjected, before each measurement, to a specific heating at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$75~^{\circ }$ </tex-math></inline-formula>C for 1 h. The PDC technique allows qualitative evidence (under dc electric fields and in a rest situation) of the electrostatic effect of the moderate thermal heating. The measurements indicate a fully space charge conduction mechanism for the previously heated oil, while the conduction measured in the unheated oil does not follow any specific mechanism. After being supplemented by FTIR spectroscopy, the electrical current features are interpreted as trapping/detrapping processes that are, respectively, correlated to the dehydrogenation/reticulation phenomena in transformer oil.