An Improved Power Cable Insulation Monitoring Method for Inverter-Fed Machines Using Switching Oscillation Damping Feature

Online cable condition monitoring is a reliability enhancement technique for inverter-fed machine systems, which requires high sensitivity with quantitative assessment capability to detect the incipient insulation degradation as early as possible. This article presents a noncontact cable condition monitoring method by utilizing the inverter's self-excited high-frequency (HF) pulsewidth modulation switching oscillations. As an HF resonance, the switching oscillation is sensitive to the small change of degrading cable insulation capacitance. First, the characteristic mode, i.e., the HF differential-mode, is selected for sensitive cable condition monitoring from the multimodal switching oscillation in an inverter-fed machine system. Then, an online condition monitoring scheme is proposed, including the framework, the noncontact HF oscillation current sensor, and the indicator(s) of cable insulation condition estimated from the amplitude-frequency features of the characteristic oscillation mode. Finally, experimental work was carried out on a 20 m cable-connected 3-kW inverter-fed machine test rig and the results demonstrate the good sensitivity and robustness of the method.