Effect of voltage rise time on time-to-failure of form-wound stator coil enamelled turn insulation

The paper presents the results of aging of medium voltage stator coil enamelled turn insulation under square waves having rise times of 270 ns and 4.5 µs. Unipolar square waves having a peak voltage of 2.5 kV and a switching frequency of 3 kHz were applied to enamelled wire samples while measuring online partial discharge. A two-parameter Weibull distribution was employed for analysis of the effect of rise time on insulation life. The tests were repeated at 6 kV peak with duty cycles of 15% and 50% for both rise times. The analysis indicates that insulation life is dependent on voltage rise time only when the DC component of the wave is short.