Monitoring of thermal degraded AC machine winding insulation by inverter pulse excitation

The demand for AC machines in traction applications fed by voltage source inverters is increasing. These highly efficient drives working near and even above their rated values are expected to operate for many years or even decades. Thus, condition monitoring is gaining a more important role. With focus in this work on outages due to deteriorated machine winding insulation an online monitoring method is presented to detect changes in the insulation strength. With the proposed method the insulation system state is assessed before an actual short circuit occurs, without the need for additional equipment or disconnection of the drive. The inverter is used as a source of excitation, by applying pulse sequences with different duration to enable high frequency excitation and analysis in a range suitable for the insulation condition monitoring. The evaluation of a change in the electrical strength of the insulation is made by analyzing the transient current responses measured with the built-in sensors of the inverter.