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P.W. Nussbaumer, M Vogelsberger, T. Wolbank:
"Induction Machine Insulation Health State Monitoring Based on Online Switching Transient Exploitation";
IEEE Transactions on Industrial Electronics (invited), 1 (2014).

Today´s variable speed drives are usually operated close to their maximum tolerable conditions. The fast switching of modern power electronic devices leads to high stress of the winding insulation. As a result, an insulation breakdown may lead to sudden breakdown and high economic loss. To avoid unpredictable downtimes and enable repair on demand, monitoring of insulation health state is getting more and more important. The paper proposes a method to monitor changes in the insulation health state by evaluating the machines high frequency properties. Deterioration of insulation condition is usually linked with a change of insulation capacity and thus also influences high frequency properties. Initiating a voltage step excitation of the machine by the switching of the inverter, the high frequency properties can be identified by measuring the resulting current response. This response is usually seen as current signal ringing and contains the machines high frequency information. Applying signal processing tools changes in the high frequency information is extracted and an insulation state indicator derived. The applicability of the method is verified by measurements on two test machines (5.5kW and 1.4MW) having different power rating as well as different insulation system.

http://dx.doi.org/10.1109/TIE.2014.2361114,

http://publik.tuwien.ac.at/files/PubDat_235607.pdf