Modeling of Partial Discharge Processes in Winding Insulation of Low-Voltage Electrical Machines Supplied by High $\mathrm{du}/\mathrm{dt}$ Inverters

Increasing DC-link voltages and high inverter slew rates in electrical traction drives may be a serious risk for conventionally designed insulation systems. Resulting overvoltages can cause partial discharges (PD) if the critical voltage inside the insulation system is exceeded. This applies in particular to the winding insulation, which is predicted to be the weakest point. The critical voltage can be determined using the Paschen curve. The partial discharge inception voltage (PDIV) is not only depending on the amplitude of the electric field but also on the timely sequence. Modeling partial discharge processes is an instrument to predict the electrical load on insulation systems and to evaluate possible new design criteria for insulation systems. The presented model uses the volume-time-theory to calculate the PDIV of twisted pairs of enameled wires. The results of the simulation are compared to PDIV measurements for bipolar voltage pulses with short rise times.

I. Introduction

In low-voltage traction drives, the DC-link voltages are increasing to achieve a faster battery charging process keeping the current level constant. Additionally, SiC-MOSFETs are used to increase the switching frequency and reduce power electronic switching losses [1]. The use of SiC-MOSFETs causes higher inverter slew rates compared to conventional Si-IGBTs. The resulting overvoltages lead to higher electric fields inside the machine and thus to higher electrical loads on the insulation system [2]–[4]. If the critical voltage inside the insulation system is exceeded, partial discharges can occur. Low-voltage electrical machines are designed to be PD-free at any operation point by standard [5]. The critical voltage can be increased by applying a thicker insulation which decreases the copper filling factor of the machine. If the insulation is exposed to partial discharge, this leads to electrical aging and erosion. After a certain time, depending on parameters such as switching frequency, voltage and material data, the insulation can not longer withstand the electrical load resulting in the breakdown of the insulation [6]. It is known from literature that the winding insulation is usually the weakest element [2]. The expected lifetime for conventional enameled wires reduces to a few hours or less. Another possibility of facing the higher demand on the insulation is to use corona-resistant insulation materials which can withstand electrical loads significantly longer.