Extraction of PD data from UWB measurements on motor stator bars fed by an IGBT inverter

The modem power converters adopted for adjustable speed drive applications are based on switching devices, such as IGBTs, characterised by very fast commutations and high switching frequencies. With such low switching times (fractions of μs), problems have become apparent on the long-term behaviour of the motor insulation. Indeed, the inverter output voltage is not pure sinusoidal but it presents sharp pulses in correspondence with the power devices commutations and it is characterised by a considerable harmonic content. Due to supplementary stresses caused by commutations and to space charge formation inside the insulation, the dielectric strength of the materials decreases as a result of the more intense partial discharge (PD) activity and the life expectancy of the machine is sensibly reduced. Since little is known on the PD behaviour of the insulating material under such distorted voltages, an experimental activity has been carried out by performing Ultra-Wide Band detection of partial discharges on specimen subjected to typical inverter output voltages. The analysis of the detected signals has been carried out by means of a suitable digital signal processing approach, based on the Wavelet Transform, capable of extracting the PD pulses occurring during the overvoltages caused by the commutations of semiconducting devices.