Insulation breakdown in oil-immersed inverted current transformers can result in explosions. In order to solve the problem, six 500kV oil-immersed inverted current transformer tests have been carried out, setting up insulation damage defects and applying overvoltage impulses to them to deepen the defects, recording the parameter changes of current transformers with different degrees of defects under various types of monitoring devices, and then identifying the suitable devices and parameter characteristics that can warn of insulation defects. The test results indicate that the discharge measured by the PD Tester and the Wide Frequency Device exhibit a similar trend, and Wide Frequency Device is more predictive than PD Tester. When the current transformer exhibits obvious partial discharge, the proportion of high-frequency harmonic in the leakage current increases significantly. Therefore, the high-frequency harmonic component can serve as a warning signal before the current transformer breakdown.