Measurements of polarization/depolarization currents for modern epoxy-mica bars in different conditions

DC polarization and depolarization current (PDC) measurements are commonly used in the field in order to assess the quality of stator winding insulating systems of large synchronous rotating machines. Results from the measurement of polarization/depolarization currents on modern epoxy mica bars in various conditions are presented in this paper. The measurements were conducted for DC voltages ranging from 1 to 20 kV. Unaged epoxy-mica spare bar with either iron oxide or silicon carbide stress grading coatings were compared in laboratory with bars of similar technology after aging and with bars with incomplete curing. The use of a two active electrodes set up with a guarded electrode allowed separating the contribution of the ground wall insulation to the measured current from the contribution of the end winding portions of the bars.