An Improved Online Stator Insulation Monitoring Method Based on Common-Mode Impedance Spectrum Considering the Effect of Aging Position

Online stator winding insulation monitoring is becoming an important topic in recent years and many online monitoring methods have been proposed. In these methods, the online common-mode impedance spectrum method can monitor the condition of the ground-to-wall insulation accurately based on the switching harmonics and no additional signal injection equipment is needed. However, the existing method has the limit that it can only detect the overall insulation aging condition. The coils at the line end are subjected to a higher voltage and their insulation degrades faster than the part closer to the neutral end. It is a common practice to change connections to reverse the line and neutral ends to extend the insulation life for Y-connection stator winding machines. In this condition, the existing method would give false alarm because it estimates the severity of aging at different positions of the winding as a whole unit. In this article, the proposed method monitors the parallel capacitance in the frequency range both below and above the resonance frequency. Using multifrequency information, the proposed method can distinguish the insulation aging at different locations, which improves the performance of this monitoring method. The offline and online experimental results both validate the proposed method.