Investigation of Changes in Partial Discharge Properties of Insulation Material over Lifetime under Accelerated Aging Conditions

Machines fed by inverters are stressed by high dv/dt as well as high switching frequencies. With the introduction of power switches, based on wide bandgap technology like silicon carbide (SiC) technology dv/dt as well as switching frequencies are again significantly increased. With the application of SiC inverters into industrial applications, lifetime considerations of insulation material become critical. At the same time there are almost no investigations and very few literature available, on the reaction of insulation materials to this new boundary conditions. Primary electrical degradation mechanism of insulation material is partial discharge (PD) and international standards define repetitive partial discharge inception voltage (RPDIV) as well as repetitive partial discharge extinction voltage (RPDEV) as one of the key parameters for characterizing electrical stress and resistance of materials to this electrical stress. In this paper the parameters RPDIV/RPDEV characterizing the partial discharge properties as well as their development over lifetime of the material are investigated under accelerated aging conditions. Specimen of insulation material used in form wound windings is stressed beyond specification to reduce lifetime and thus test durations. Pulse voltage stress is applied bipolar under defined frequency and duty cycle. Aging is periodically interrupted to perform RPDIV/RPDEV tests. As a result, it is possible to trace these parameters over lifetime, depending on the stress level applied. While the overall lifetime shows a clear and well-known trend with increasing stress level, the observed trend in PD properties over lifetime obviously change depending on stress level.