The Application of Polarization and Depolarization Current on the Large Turbo-generators

This paper represents the application of the polarization and depolarization current (PDC) measurement of the stator winding insulation of two large turbo-generators, which have the same rate of 50.8 MVA, and 11.5 kV. The PDC measurements were conducted on the stator winding insulation of both generators by using two different test circuits i.e., (individual) phase-to-ground insulation and interphase insulation circuits. Then, the PDC results of the two investigated generators were compared. To evaluate the phase-to-ground insulation condition of the stator winding insulation of both generators, the conventional dielectric parameters in the time domain, such as polarization index and insulation resistance, were also studied. Moreover, the PDC results in the time domain were converted into the dielectric dissipation factor in the frequency domain to allow the analyzing the phase-to-ground insulation of the stator winding of the generators. The PDC results of the phase-to-ground insulation circuits of both investigated generators illustrate the bent shape of PDC results which were affected by the influence of the stress control coating resulting in a high value of the Polarization index. Besides, the PDC results of the interphase insulation circuits of the first investigated generator illustrate the polarity reversal (or Anomalous current) of both PDC, while the second investigated generator illustrates the non-polarity reversal of both PDC. The polarity reversal of the PDC results of the interphase insulation was due to the dielectric loss of the interphase insulation is a lower influence when compared with that of phase-to-ground insulation.