Evaluation and current-response based identification of insulation degradation for high utilized electrical machines in railway application

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 δ) and capacitance (C) of the insulation system is recorded. With these non destructive diagnostic measurements, the correlation between the indicator values (tan δ, 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.