Characterisation of Insulation Material Aging and Lifetime using Newly Defined Parameter Partial Discharge Extinction Temperature

Abstract

Inverter-fed machines' insulation is exposed to high frequencies and transient voltage stress. The development of new technologies such as silicon carbide (SiC) with high switching frequencies and dv/dt has further increased these stresses. These factors pose a major challenge to the insulation of the stator winding as they cause and favor partial discharges (PDs). PDs are the primary electrical degradation mechanism of insulation. The international standards define repetitive partial discharge inception voltage (RPDIV) and repetitive partial discharge extinction voltage (RPDEV) as the most important parameters for characterizing electrical stress and insulation strength. Measurement of RPDIV and RPDEV during drive lifetime is however, challenging, as the inverter has to be disconnected and variable pulse voltage source connected instead. As an alternative, this paper proposes using the constant dc link voltage of the drive inverter and modifying the temperature instead. As RPDIV and RPDEV show a clear and strong dependence on temperature, the alternative characterizing ageing parameters partial discharge inception temperature (PDIT) and partial discharge extinction temperature (PDET) can be defined. To verify the properties of these characterizing parameters, a special test procedure was developed. Prefabricated conductors of form wound windings were stressed and aged, using temperature and pulse voltage. Stress was applied using bipolar pulse voltages clearly above RPDIV in combination with increased temperature to establish accelerated ageing. Several times during lifetime, ageing is stopped and measurements of RPDIV, RPDEV and PDET are carried out. This allows the parameters to be monitored and compared with regard to the insulation lifetime. As will be shown, the newly defined PDET is an excellent parameter to characterize ageing and material lifetime.