Abstract: In contrast to conventional synchronous generators, doubly-fed induction generators (DFIGs) are subjected to dynamic stress variations in their rotor windings, leading to accelerated insulation aging. This critical issue demands enhanced attention to insulation condition monitoring. While existing protection methods are limited to fault detection and subsequent machine shutdown during short-circuit events, this study proposes an innovative online monitoring approach that enables proactive prognosis of rotor winding insulation degradation. The common-mode equivalent impedance is measured online using the switching transient harmonics produced by the rotor-side converter and the corresponding response in leakage current harmonics. The theoretical analysis and experimental results prove that the proposed method can detect the change in the rotor winding insulation parameters online, enabling non-intrusive online monitoring for rotor winding insulation degradation.