Using DFR Measurements for the Condition Assessment of Stator Winding Insulation Systems

This publication presents the results of investigations that were performed to verify if the use of low-voltage dielectric frequency response analysis can be successfully applied to detect the deterioration of stator winding insulation systems. Three newly manufactured stator coils with a rated voltage of 13.8 kV were aged using the thermal cycle testing procedures described in IEEE 1310 and an extended and modified version of voltage endurance testing described in IEEE 1553, until dielectric failure. Throughout the accelerated ageing process, several dielectric measurements were performed, and the results trended over time. The results of the dielectric frequency response measurements were then compared with conventional dissipation factor measurements and partial discharge measurements to assess the sensitivity of dielectric frequency response analysis applied to stator insulation. This paper also discusses the influences of the nonlinear end potential grading material on the measured dielectric losses. Additional experiments are summarized to illustrate how this nonlinear material behaves depending on the applied voltage magnitude and frequency. Some examples of measurements performed on complete stator windings are also shown and discussed.