Online variable speed drive machine monitoring

Electrical machines in VSDs have been reported to be less reliable than those fed direct -on-line from the grid mains, see Montanan. This is because the voltage supply imposes extra stresses when synthesised by a power electronic converter. Our focus here is on drive systems using voltage source converters (VSC) where the DC link is capacitively smoothed. Two-level pulse-width-modulation (PWM) is usually used in low-voltage drives for machine voltages up to 690 V,line-line. Multi-level converters may be used in MV drives for motors up to 11 kV, see Stone et al. (2014). In addition to voltage source converters, there are also current source converters with an inductively smoothed DC link for large motors rated at tens of MW. Harmonics and inter-harmonics in the inverter output and machine windings also increase machine stresses through electrical, thermal and mechanical effects. we will outline some major and common issues of electrical machines in VSDs or generator systems and describe the available techniques for monitoring the major fault mechanisms which are new to VSDs. The working principles of these techniques will be analysed to understand their capabilities and limitations.