Influence of voltage excitation and current sensors on monitoring of stator winding insulation based on transient current step response

Stator insulation defects in traction drives machine windings can be analyzed by evaluating the current response after a voltage step excitation. The status of the stator insulation material regarding aging, is observed in a longer perspective. Because of the fact that insulation degradation of a stator winding system is a long developing process, a deviation of the parasitic electrical components (parasitic winding capacitances), can be monitored with the high frequency behavior of the transient current responses. Incipient stator insulation defects can be detected comparing the measured transient current response obtained when a drive was set into service with measurements taken during operation of the drive. Using the current sensors (transducers) already applied for the current control, no additional sensor hardware is necessary for the application of the method. The influence of voltage step shape and current transducer performance on the ability to detect parasitic insulation capacitance changes is investigated. Measurements are based on a silicon carbide voltage source inverter enabling high dv/dt rates up to 25kV/μs and a 1.4 MW induction machine designed for traction application having preformed coils with fiber-insulation and winding taps.