Abstract: Online insulation monitoring is an effective means to improve the reliability of inverter-fed machines. Under the excitation of fast PWM switching, the machine winding leakage inductance will resonate with the internal distributed capacitance producing a voltage oscillation of tens to hundreds of kHz, which will threaten the safe and reliable operation of the winding internal insulation. In this paper, the self-excited high-frequency (HF) switching oscillations are utilized for winding internal insulation online condition monitoring. To avoid the multi-mode aliasing of switching oscillations and improve the monitoring sensitivity, a neutral-to-ground voltage HF switching oscillation-based monitoring method is proposed. First, the characteristics of the common mode voltage excitation of an inverter-fed machine are analyzed. Then, the mechanism and characteristics of the HF switching oscillation are studied. The relationship between the oscillation frequency and the winding internal insulation capacitance is deduced theoretically. Next, the online monitoring scheme is devised, including the switching oscillation frequency extraction, and the internal insulation state evaluation. Finally, the feasibility and effectiveness of the method are verified by the accelerated thermal aging and the aging emulation tests, respectively. The results show that the method can effectively monitor the weak change in the internal insulation condition with the advantages of a high signal-to- noise ratio and robustness to working conditions.