Inverter-fed drive stator insulation monitoring based on reflection phenomena stimulated by voltage step excitation

Reflected wave transient phenomena in voltages and currents of inverter-fed drives caused by voltage source inverters (VSI) operating with pulse width modulation (PWM) with high dv/dt rates are well known and analyzed in many studies. The reflection phenomena occurring at the machine terminals, resulting in high overvoltages are one reason for the stress and aging mechanism of AC machines insulation system. The motivation of this work arises out of the need for a continuous monitoring of the insulation health state for traction drives systems in order to ensure high reliability of the drive over many years of operation. With a voltage step initiated by a switching transition, the inverter elicits a response in the drive system. By using the information of the resulting transient effects, conclusions can be drawn on basis of the oscillation behavior which are indicative for a change in the insulation system. Investigations in this work show that an alteration in the shape of the system transients, in peak values as well as frequency components, correlates with a dielectric insulation capability of the machine winding insulation. Measurements on a small, low voltage, random wound, induction machine (5.5kW) and a medium voltage induction machine (1.4MW) with form-wound coil based stator system are performed. Artificially induced insulation aging is realized by accelerated thermal aging to demonstrate the effect. Additionally, two different inverter types are used, with standard IGBT modules (dv/dt ~2-4kV/μs) and new SiC semiconductor inverter technology (dv/dt up to 20kV/μs), to analyze the influence of the shape of the excitation voltage step.