Abstract: The authors conduct routine tests on two 220 kV transformers and find that the dielectric loss factors of the high- and medium-voltage bushings significantly exceed the specified limits. Based on data from insulating oil chromatographic analysis, simplified insulating oil tests, and frequency-domain dielectric spectroscopy (FDS), the authors return the high-voltage bushing of one main transformer to the manufacturer for inspection, testing, and disassembly. The analysis shows that uneven compression of the bushing gasket causes internal moisture ingress and subsequent partial discharge, leading to abnormal dielectric loss factors. All identified abnormal bushings are replaced, and preventive control measures are proposed.