Investigation of stator winding insulation condition of MV motors based on dielectric response measurement

Abstract

This paper presents an investigation of the stator winding insulation condition of medium voltage (MV) motors operated in a power plant. The investigated motors have the same voltage rate of 6.6 kV. Such motors continuously encounter various stresses during service which alter the dielectric properties of the insulation system. Thus, the dielectric response measurement (DRM) was utilized in this research to evaluate the integrity of insulation of stator winding insulation, whether the bulk insulation e.g., moisture absorption, or the surface insulation e.g., existing conductive contamination over- or even moisture adsorption along the endwiding surface. In addition, polarization and depolarization current (PDC) and frequency domain spectroscopy (FDS), as techniques based on DRM, were employed to allow the analysis in both time and frequency domains. Such techniques were performed on the whole MV motors during the shutdown for time-based maintenance, both before and after cleaning and drying for comparison purposes. The visual inspection of such motors evidences that there has considerable dust over the endwinding surface. Such motors were thus thoroughly cleaned to remove such visible dust over the surface of the endwinding. Then, the DRM test results before and after cleaning, were comparably analyzed to characterize the dielectric properties due to such contaminant. Therefore, the effect of the contaminants on the dielectric properties of the stator winding insulation system is introduced and discussed.