An improved method of detecting contamination of HV stator windings in the field

The preferred method of detecting contamination of stator winding insulation is to measure its DC resistance (IR) and polarization index (PI). However, a number of cases have been reported where good IR and PI values have been obtained despite the windings being considerably contaminated. While attempting to compute variations of the polarizing currents in the frequency domain, the author has made of the results obtained in conjunction with low-voltage AC measurements at the supply frequency to detect contaminations of the two windings. A method has been suggested that could help detect contamination in cases of the above-mentioned good IR and PI values by estimating tan delta and capacitance variations with frequency from the charging current curve and comparing the same with measured tan delta values