Online Condition Monitoring of Line-End Coil Insulation for Inverter-Fed Machine by Switching Oscillation Mode Decomposition

Online condition monitoring of winding insulation has been proved to be an effective means to improve the reliability of an inverter-fed machine, but little work has been done on the insulation of line-end coil subjected to much higher transient voltage stress with a higher risk of premature insulation break down. In this article, the inverter self-excited high-frequency switching oscillations are utilized for the line-end coil insulation condition monitoring. To improve the condition monitoring sensitivity and its robustness to dynamic operating conditions, a switching oscillation mode decomposition method is proposed. First, the condition-sensitive mode, i.e., high-frequency common-mode (HFCM) current is selected and separated by the variation mode decomposition algorithm among three dominant modes in switching oscillation current. Then, the amplitude-frequency features of HFCM current are extracted for condition monitoring. Finally, the overall insulation condition of three-phase line-end coils and the degree of asymmetric degradation are quantitatively evaluated by the frequency and amplitude features respectively. Experimental work was carried out and the results demonstrate the good performance of high sensitivity, good robustness, and non-contract safety.

I. Introduction

Inverter-fed machine has been widely applied in many areas, e.g., electric vehicle drive, high-speed train traction, and offshore wind energy generation, where reliability is critical for system safe and economic operation. However, the transient overvoltage induced by the fast and frequent switching of an inverter at high dv/dt will accelerate the aging of the stator winding insulation [1]–[3]. The stator winding insulation is one of the most fragile components in a drive system accounting for around 40% of failures [3], [4], which degrades rapidly once an incipient failure occurs [5]. In particular, the line-end coil of stator winding has a much higher risk of premature insulation breakdown [6]. As shown in Fig. 1, the line-end coil is defined as the first coil of phase winding that enters the machine [7], which is exposed to the inverter-induced high voltage stress [5]–[8]. It is therefore highly desired to monitor the insulation condition of line-end coil for an inverter-fed machine system at an early stage to reduce the cost of maintenance with increasing reliability. Fig. 1.

Line-end coil in stator winding.