Abstract: In power grid operation, transformers are essential core equipment. They realize the step-up, step-down and cross-regional transmission of electric energy through voltage transformation, and their stable operation directly affects the power supply reliability of regional power grids. The insulating oil filled in the transformer tank insulates windings to avoid insulation breakdown, takes away heat produced during equipment operation, and extinguishes arcs when internal arc faults happen. When local overheating occurs due to failures of internal components, the surrounding insulating oil decomposes under high temperature and generates various fault characteristic gases soluble in oil. In routine substation operation and maintenance, insulating oil samples are commonly collected for chromatographic analysis. Potential hidden faults can be detected early by comparing gas components with historical test data.