Low-frequency dielectric response of asphalt bonded insulation

Low frequency dielectric response measurements were performed on asphalt-mica stator insulation of vertical axis hydro-generators. These measurements, used as a diagnostic test, were conducted using a high voltage dc source and measuring the charge and discharge currents after the application of voltage steps of various magnitudes. The same dielectric tests were also performed on individual coils removed from the winging at different location, i.e. near the neutral end, the middle of the phase and the line end. Both the level of the dielectric loss and the non-linearity factor were found to correlate well with the aging conditions of the coils, which were related to their position in the winding.