Study on form-wound machine turn insulation subjected to unipolar and bipolar square waves

The paper examines the time-to-failure of enameled form-wound machine turn insulation subjected to unipolar and bipolar square waves at different switching frequencies. Partial discharge under the same square wave parameters are also reported using a point-to-plane geometry from which the measured PD features are related to the time-to-failure results of the turn insulation. The results show that the life of enameled turn insulation is considerably longer under bipolar square waves than for unipolar waves and that turn insulation lifetime is related to the power of the partial discharges. In addition, a frequency threshold in the time-to-failure results is observed corresponding to a decrease in PD power and the development of surface space charge that affects the PD features is discussed.