Aging Estimation of Liquid Insulation Using Relaxation Frequency Distribution Analysis

This article analyzes the effects of aging on the dielectric properties of mineral oil (MO), natural ester (NE), and synthetic ester (SE), with artificially acid-induced changes estimated through the analysis of the relaxation frequency distribution (RFD) spectrum. For this purpose, liquid insulation samples having equivalent acid-induced aging periods of 0, 5, 10, 15, and 20 years are emulated by adding formic, acetic, and levulinic acids to the mineral and ester oils. Thereafter, Fourier transform infrared (FTIR) spectroscopy and relaxation current measurement are conducted on these aged samples to investigate the impact of aging on physicochemical changes and different polarization characteristics of the insulation liquid, respectively. The RFD spectrum is derived from the relaxation current for different-aged samples. The analysis of the RFD spectrum reveals different dipolar relaxation-sensitive zones. Every zone of the RFD spectrum carries signature information about the dipolar characteristics relating the aging state with respect to the acid content of the liquid insulation. From the RFD spectrum, four aging-sensitive parameters (peaks and corresponding frequencies of the RFD spectrum) are identified, and based on these parameters, two empirical relations are derived. Experimental investigation shows that the four parameters can be good indicators of the aging status of liquid insulation.