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APPLICATION OF IR SPECTROSCOPY FOR QUALITY CONTROL OF LIQUID DIELECTRIC

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A method based on IR spectroscopy for quality control of dielectric liquids (insulating mineral oil, ester dielectric liquid Midel 7131, as well as oil mixtures with an ether content of 10, 20 and 30% vol.) during their aging, are developed and tested. IR spectra are recorded on an FT-801 IR-Fourier spectrometer manufactured by Simeks Research and Production Company. Analysis of oil samples and ester-based mixtures is carried out without prior preparation on the MFTIR (multiple frustrated total internal reflection) attachment with a zinc selenide ZnSe element and a built-in visualization system on an external monitor. Results of the research show that as synthetic ester is added to the oil, the IR spectrum of the mixture demonstrates an increase in the intensity of absorption bands caused by stretching vibrations of the bonds CO–C, C–O, C–O–C, C(=O)–O–C. The intensity of the peak characteristic of esters at a frequency of 1161 cm–1 (C(=O)–O–C) changes especially noticeably. It is revealed that during the essential oil mixtures oxidation, the intensity of characteristic bands of the carbonyl group C=O (1747 cm–1) and the ester group C(=O)–O–C (1161 cm–1) decreases, while the absorption intensity in the frequency range of the CH group (2920 cm–1) remains practically unchanged. It is proposed to evaluate the degree of decomposition of mixtures of ester dielectric liquid, as well as essential oil mixtures, using the “destruction index”, which is calculated using a formula that takes into account the change in optical density in the band of the C=O and C(=O)–O–C groups. This indicator is anticipated to allow for quantitative control of the quality of alternative ester liquids and essential oil mixtures during their service.

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