Calculation of the Electric Field Inside Cavities Found Through Stator Bar Dissection

Quality control tests, including partial discharge (PD) measurements and dissections, were performed on a new production lot of epoxy-mica stator bars for a 13.8-kV hydroelectric generator. The location and the characteristic patterns of PD were identified by scanning the straight portion of the bar with an electromagnetic antenna. A sample from those locations was selected for dissection and another sample without PD was selected as a reference. Microscopic analysis of specimens from the sample with localized PD activity showed cavities in the groundwall insulation close to adjacent copper strands. Numerical simulations of the electric field inside different cavities were conducted to determine if PD activity can be initiated in cavities of different shapes. Results of the average and maximum field were compared with the Paschen curve for air. The analysis showed that with the maximum calculated field, all simulated cavities would result in PD activity regardless of the inner temperature.