The content of polar substances such as water and acids in insulating oil significantly affects its dielectric properties and electrical strength. Terahertz waves exhibit strong penetration through weakly polar substances while being sensitive to absorption by polar substances

making them suitable for detecting trace polar substances in mineral insulating oil. To explore the feasibility of applying terahertz technology to insulating oil tests

this study prepares samples of insulating oil with varying qualities through accelerated thermal aging tests. Both traditional characteristic parameters and terahertz spectra are measured for each sample. The correlations between moisture content

acid value

dielectric loss

breakdown voltage

and terahertz features are analyzed. Results reveal strong correlations between the peak absorption at 2.1 THz

peak absorption at 2.24 THz

and average THz dielectric loss in the THz absorption spectrum with moisture content

acid value

and power-frequency dielectric loss of the insulating oil. This correlation is further validated by testing field-sampled insulating oil. Terahertz spectroscopy offers advantages such as rapid testing speed

no need for oil sampling or pretreatment

and excellent test repeatability. It provides a novel approach for rapid assessment and online monitoring of power transformer insulating oil quality.