In order to investigate insulation degradation of high voltage traction drives, series of measurements on form wound coil specimens have been done. These stator bars have been aged by different accelerated aging cycles until the failure of the insulation is determined by voltage exposure tests. After every aging cycle the dissipation factor (tan ) and capacitance (C) of the insulation system is recorded. With these non destructive diagnostic measurements, the correlation between the indicator values (tan , PI, C) and the degradation of the insulation strength until a breakdown can be investigated. Furthermore, an online insulation condition monitoring method for traction drives is proposed. In this method the inverter is used as a source of excitation with a simple switching pattern. The current transducer's transient response at step excitation is used to evaluate a change of the insulation system. The drive system containing machine, cabling and inverter can be represented with a complex network of electrical, also parasitic components like resistances, inductances and capacitances. Insulation degradation within the machine also changes these components as investigated at the accelerated aging tests, and thus the tendency can be used for the monitoring method.

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.

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