Impact of SiC semiconductors switching transition speed on insulation health state monitoring of traction machines

In modern traction propulsion applications, voltage source inverter (VSI) fed traction motors today operate very close to borderline conditions. With new emerging semiconductor technologies, higher inverter switching frequencies will be possible and high inverter dv/dt-rates appear, resulting in transient overvoltages at the machine which increase the stress on the insulation system and lead to insulation degradation. Thus, insulation condition monitoring is getting more and more important to ensure a safe and reliable operation of traction motors in trains and locomotives, trams and so on. This study proposes an online insulation monitoring approach that is able to detect incipient insulation defects by evaluation of the motor transient current response on voltage pulses injected by standard inverter switching. Experimental results of this concept are obtained with tests on a 1.4 MW induction machine for railway application. Additionally, the influence of different dv/dt-rates up to 20 kV/μs on the monitoring performance is verified using a VSI-inverter equipped with SiC semiconductors.