Comparison of the Electrical Ageing Under Sinusoidal and Square-Wave Stresses

The problematics of ageing in the Electrical Insulation Systems (EIS), especially in machines fed by adjustable speed drives, have been studied in numerous scientific publications, and even more with the introduction of wide-band gaps materials such as SiC. These fast-switching components induce over-voltages, which allow partial discharges to incept and lead to an erosion of the insulation materials. Various ageing mechanisms exist and depend on multiple parameters, but the influence of voltage slope, and therefore rise time, is a controversial subject which could require some further examination. Thus, it may be interesting to determine if these parameters have a direct impact on the ageing of EIS or if they only contribute to it indirectly, while their own effect would be less significant. To clarify this point, accelerated tests were performed and aimed at the comparison between the ageing of samples supplied by sinusoidal and square-wave stresses. The amplitude and frequency of the signals were the same, with a duty cycle of 50% in order to study the influence of the voltage slope only. Several levels of stress and frequencies were selected for the analysis and with additional purpose of reduction of tests duration. Lifetimes of samples were recorded to determine if there were any differences and to what extent.