Online Condition Monitoring of Line-end and Internal Coils Insulation for Inverter-fed Machines Based on Common-mode Current Modes

Addressing cable insulation parameter effects in practical applications, this paper proposes an online method using common-mode current modes to assess line-end and internal coil insulation conditions in inverter-fed machines. Firstly, a common-mode equivalent model incorporating cable insulation parameters is established. Analyzing the machine’s common-mode impedance characteristics reveals dominant modes and paths of common-mode current under common-mode voltage step jumps. This yields the relationship between high-frequency common-mode current oscillation frequency (f_HFCM) with line-end coil insulation, along with the coupling relationship between mid-frequency oscillation frequency (f_MFCM) coupled with both line-end and internal coil insulation under varying cable inductance. To overcome the limitations of classical Fast Fourier Transform (FFT), which is primarily suitable for stationary signals (single-frequency signals), and the Empirical Modal Decomposition (EMD) algorithm is computationally complex and may have frequency aliasing, this paper uses the Variational Mode Decomposition (VMD) algorithm to obtain the f_HFCM and f_MFCM. The monitoring strategy employs: f_HFCM to determine line-end coil insulation degradation; combined with cable inductance and the insulation degradation condition of line-end coil, f_MFCM assesses internal coil insulation degradation. Experiments demonstrate that the proposed method can sensitively detect incipient degradation in both line-end and internal insulation of inverter-fed machine windings.