Experience with DC polarization-depolarization measurements on stator winding insulation

DC insulation resistance (IR) and polarization index (PI) testing is very widely used to assess the condition of the rotor and stator windings in motors and generators. The IR/PI tests are good at identifying contaminated windings; however, such issues as thermal deterioration or abrasion of the insulation due to coil vibration, are not detected by the IR/PI test in modern insulation. The polarization/depolarization current (PDC) test, where the charging current is measured for several minutes and then the test object is grounded and the discharging current is measured for several minutes, may provide more diagnostic information. This paper presents PDC data from a stator as well as individual stator coils and bars and compares it to results from other off-line tests including the low- and high-frequency PD, dissipation factor tip-up, and DC ramp tests. It seems that partial discharge tests are more sensitive to some insulation problems than PDC, however, more results are needed.