The Influence of Voltage Form on the Insulation Resilience of Inverter-fed Low Voltage Traction Machines with Hairpin Windings

In modern automotive traction drives, the insulation system of the electric machine is exposed to an increased electrical stress level due to optimizations of the system parameters of the power electronics (PE). This leads to an increase of the DC link voltage from 400 to 800 V as well as to an increase of the switching slew rate ((dv)/(dt)) using wide band gap (WBG) semiconductor devices like silicon carbide (SiC). The increased stress favors partial discharges (PD), which are highly damaging to Type I insulation systems of low voltage machines. Therefore, it is mandatory for the reliability of the machine to be able to determine the resilience of the insulation system. Standardized PD offline tests with surge voltages are used to characterize the insulation resilience by means of the repetitive partial discharge inception voltage (RPDIV). However, the machine is stressed during operation with PWM voltage waveforms. Consequently, in this article, the PD behavior of both types of voltages is investigated. Contrary to previous literature, where round wire twisted pairs are used, the tests are performed with hairpin wire pairs. Measurements were carried out with an ultra-high frequency (UHF) antenna and a highfrequency current transformer (HFCT). The comparison of the experimental results shows that the time and frequency behavior of the PDs differs significantly between the two voltage forms and therefore has a significant effect on the determination of the insulation resilience. In addition, a procedure is described how the HFCT measurements can be calibrated and thus the apparent charge can be obtained.